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CHEMICAL PROPERTIES OF σπ-BONDS
CHAPTER X
CHEMICAL PROPERTIES O F σπ-BONDS 1. ADDITION REACTIONS
THE theory of electronic bond charges provides scarcely any improvements on the ideas already held by organic chemists about the relationship between the distribution of π-electron density and the course of electrophilic, nucleophilic and radical addition reactions. It enables assessments to be made with great confidence, however, about this distribution, mainly because it explains the character of the interaction between σ- and π-electrons. In Chapter VIII we showed that electrostatic repulsion, the energy of which reaches some tens of kilocalories per one gramme-or-electron, takes place between the
4.1-270 4.Ecn £- = = 1-19 e. 3 + Ec* 3+1-270 170
Chemical Properties of σπ -bonds
171
into the bond with the C*. We will now suppose that X = CH 2 COOH. The electronegativity of the COT in the carboxyl group is equal to 1-620 (p. 121.). Then, from formula (2.1) we find that A% cn. = CC)
4Λ 2Ί0
' = 1-04 e, 2+1-620+1-270
and hence it follows that the value of the π-electron atmosphere on the Cn—X bond must be greater than in propylene. If X =CC1 3 , then ^C(CJT) — 0-84 e and consequently the ττ-electron atmosphere of the Cn—CC13 bond must be still larger, and of the C*=C W bond smaller. If the electronegativities of all the R's in X = CR 1 R 2 R 3 were known, it would be possible to arrange the substituents in order according to their capacity for attracting or repelling ^-electrons. The place of hydrogen in this series, of course, is not determined by the fact that its share in the C—X bond is equal to one σ-electron. Since the energy of the C—H ττ-bond is less than the energy of the C—C π-bond (p. 147) it may be assumed that it will occupy a place in this series between the substituents for which AQ(C*)> I e — somewhere between CH 3 and CH 2 COOH, but rather nearer to the former. In the second group of substituents X is an atom having a free pair of electrons in the valency layer. It is evident that the π-bond Cn—X is strengthened here on account of the conjugation of these electrons with the π-electrons of the ethylene bond. Since the affinity for the electron in halogens is greater than in oxygen, and more so than in nitrogen, it may be imagined that the share of the "free" electrons of the atom X in the bond C—X will be increased, but the interaction stability with the ^-electrons of the ethylene bond will decrease. Consequently, the static π-electron atmosphere of the C = C bond and its increase on account of polarization under the influence of an electrophilic reagent will be greatest when X is the group NR 1 R 2 , and least when X is halogen. It is evident that the role of the substituents R here will be the same as in the example discussed above, withX^CR^Ra. X = CRx = CR 2 R 3 and CR = O, and also X = C = C R , = C N , C 6 H 5 and so on, belong to the third group of substituents. It is evident that in this case the interaction is also located on the C"—X bond between the π-electrons, and when X = C R = O or C = N it is of a
172
Electronic Charges of Bonds in Organic Compounds
relatively more stable character. Here the substituents R exert an influence in relation to their electronegativities which in most cases may be predicted beforehand. Finally, the fourth group is made up of the highly electronegative +
+
+
substituents X = N R ^ R * , PR 1 R 2 R 3 , SRxR2 and so on, which evidently attract to themselves both the a- and the π-electrons from the 0 = C bond.* We will consider certain facts relating to electrophilic and nucleophilic addition at C==C and 0 = 0 bonds [1, p.151 et aL; 2,/?.347 et seq]. As would be expected, in relation to what has been said earlier, when the number of CH 3 groups substituting hydrogen in ethylene is increased, the relative rate of addition of bromide (first stageaddition ofBr + )is increased, but in the case of vinyl bromide, crotonic acid and especially acrylic acid the rate of addition is decreased. The rate of addition of even a proton to vinyl chloride isreduced, although the addition still proceeds according to Markovnikov's rule. But + when N(CH 3 ) 3 , CC13, CN, COOH are introduced as X, proton addition takes place against Markovnikov's rule and in general these substituents activate not electrophilic but nucleophilic addition. The rate of nucleophilic addition increases in the order R
\>c=o< R \ >c=o< R \ >c=o<
CPLjCX
R/
W
H
\>c=o.
W
Methoxyl, CH 3 0, increasing the π-electron atmosphere of the O—C bond, and R (alkyl), repelling the π-electrons on the C = 0 bond, passivate the reaction compared with hydrogen. As substituent R, cyclopropyl exerts a passivating effect. The mechanism of this effect is difficult to explain [1, p. 221; 2, p.393], but if the already mentioned tf-electron donor properties of the cyclopropane ring are taken into account, it becomes understandable that the cyclopropyl group must decrease the electrophilic properties of the carbonyl carbon.** * Obviously the carbonium ion should also be included in this group of substiuents. ** The analogy in electronic structure which is traced between the double bond and the three-membered rings (cyclopropane, ethylene oxide, ethylenimine on a basis of similarity of some of the chemical properties, does not carry convic)
Chemical Properties of σπ-bonds
173
As regards the direction of radical addition reactions, in the addition for example, of hydrogen bromide to C H 2 = C H — X in the presence of peroxides with all X's addition to the methylene group takes place first. Consequently the distribution of the π-electron density plays no part in radical addition,* and the reaction direction is evidently determined mainly by the stability of the radical formed [1, p.195]. In order to estimate the relative energy stability—of the radicals which may be formed, for example, by the addition of Br to propylene, the considerations expressed in Chapter VIII are used, viz. a radical is more stable the greater the degree to which the (T-electron charges of the C—H bonds are increased, and this occurs when the latter are in the α-position to a single π-electron. From this point of view the formation of the radical CH3 —CH—CH 2 Br must be much more efficient energetically than the radical CH 3 -CHBr—CH 2 .
2.
SUBSTITUTION REACTIONS IN BENZENE DERIVATIVES
Activation Energy of Substitution Reactions at an Unsaturated Carbon Atom As was said in the previous chapter, the activation energy of substitution reactions at an unsaturated carbon atom, if these reactions pass through the stage of the formation of a transitional complex, is made up mainly of two parts: the energy necessary for overcoming the repulsion between the entering substituent and the given atom tion. In methylvinylketone the vinyl group passivates the carbonyl bond to nucleophilic addition, and this should be ascribed to the fact that owing to the direct interaction of the π-electrons of the C = C bonds and the C = 0 bonds the concentration of electrons round the carbon atom at which the addition occurs is increased. Thus, in methylvinylketone the carbonyl carbon atom is passivated mainly by the π-electrons, whereas in methylcyclopropylketone by the tf-electrons. Another example is as follows: in the catalytic hydrogenation of vinyl- and phenylcyclopropane, disruption of the C—C bond adjacent to the tertiary carbon atom takes place [2a ]. This can in no way, however, be considered an argument for the existence of π, π-conjugation between the three-membered ring and the system of π-bonds, because the direction of hydrogenation (and its rate) is determined, in all probability, by the energy efficiency (p. 145 et seq.) associated with the formation in the first stage of homologues of allyl and benzyl radicals. * Thus, radicals may be regarded as electroneutral or at least as weakly electrophilic agents [3].
174
Electronic Charges of Bonds in Organic Compounds
of carbon with its surroundings, and the energy of stretching of the bond at which the substitution occurs. If the carbon atom at which the substitution occurs is a donor of π-electrons, the first component of the activation energy will also depend on the static distribution of the π-electron charges at the bonds and on their possible displacement during the reaction. It may be foreseen that in electrophilic substitution reactions the increase in π-electron density at a given carbon atom, just as in addition reactions, will reduce, but in nucleophilic substitution reactions will increase, the "repellent" portion of the activation energy. As regards the role of the cr-electrons, the conclusion drawn for substitution reactions at a saturated carbon atom is also applicable to substitution reactions in the aromatic nucleus, because it is in the highest degree probable that they pass through a stage of formation of a "
\c<'' -^C-X+H. / νχ /
As was noted in the previous paragraph, it is possible to neglect the interaction of the radicals with the π-electrons and consider that the activation energies of radical reactions of substitution reactions in the aromatic nucleus are determined only by the value of the σ-electron charge of the C—H bonds at which the substitution takes place. In order to apply these conditions to electrophilic and nucleophilic substitution reactions in aromatic compounds, it is necessary to have some, albeit qualitative, data about the distribution of electronic charges in these compounds. or-Electron Bond Charges in Benzene Derivatives Calculations (see Table 23) give the following values for the electronic bond charges in benzene— AQG= 1-91-1-96,AQG — 0-95-0-91, ^ C H = 2-09-2-04 and A%H = 0-05-0-09 e. Thus, in benzene a kind of expulsion of cr-electrons from the carbon nucleus on to the C—H bonds takes place, which is also characteristic of other flat rings, especially cyclopropane. Hence the low electronegativity of
Chemical Properties of σπ-bonds
175
benzene carbon Ec = 1-08 (p. 112). Consequently, all the substituents X in C 6 H 5 X (except, perhaps, cyclopropyl, its homologues and analogues, and also metals which have an electronegativity much lower than hydrogen) are acceptors of σ-electrons and lower the σ-electron charge of the C—C and C—H bonds of the benzene ring. Even phenyl as a substituent of hydrogen in benzene can exhibit this property, since the central C—C bond in diphenyl is capable of absorbing from each benzene ring more cr-electrons than the C—H bond in benzene. Another aspect of the question is how is this decrease in cr-electron charges distributed over the ring, for example in mono-substituted benzenes C 6 H 5 X? If it were a question of a noncyclic compound of the type H H H H Λ.
I
\~^
i
I
I
v_^2
^3
i
I
^4
r i
· · ·
H H H H the picture of the change in electronegativities and electronic charges would be clear (see Figs. 7 and 8). A typical case is where the electronegativity of the X atom is greater than the electronegativity of hydrogen, Ex > 1. Then as a result of the entry of X into the hydrocarbon in place of the hydrogen some portion of the electronic charge will be transferred from the Cx—C2 and Cx—H bonds to the Cx—X bond, the electronic screening of the Cx atom to the side of the C 2 atom will decrease, but its electronegativity will increase. In turn the electronegativity of the C 2 atom to the side of the C 3 atom will increase (though as a result of damping of this effect, to a smaller extent than with the C x atom), but to the side of the C x atom it will decrease, and so on. It is evident then that the electronic charges of the C—H bonds are distributed in the order: AQ^ < ^Ο,Η ^ ^C 3 H and so on. It is more difficult to formulate such a series when the chain of carbon atoms is closed, as in H
H
X - C /
x
>C 4 H.
c—
H
176
Electronic Charges of Bonds in Organic Compounds
It is clear that ^4£aH < ^c 3 H' but it is impossible to say anything about the relative values of the electronic charges A^^ and .4C4H> whilst there are no ways of estimating the relative values of the sums of electronegativities ECi+ EQ 4 and 2ECs because, according to formula (2.4), the greater the sum of the electronegativities of the atoms combined with a given carbon atom, the smaller the electronic charges of the C—H bonds which it forms. Therefore, if ECt + ECt < < 2EQ3, then Αζ 3 π < Αζ 4 Η 5 and vice versa. In order to solve this problem it is possible to employ data about radical substitutions reactions in monosubstituted benzenes. Since orientants of both the first and second groups activate the nucleus with regard to radical substitution reactions, and since these reactions proceed more easily in the ortho- and para- than in the meta-position, apart from the nature of the orientant, it may be concluded that Α£,Η <
A£4H·
ττ-Electron Bond Charges in Benzene Derivatives What has already been said about the effect of X on the π-electron charges of C = C and C—X bonds in C H 2 = C H X may be repeated with regard to the effect of substituents X in C 6 H 5 X on the ττ-electron charges of bonds in the benzene ring. Thus, substituents of the second, third and fourth groups (p. 171 et seq.), compared with hydrogen, attract to themselves both the σ- and the π-electrons, in spite of their mutual repulsion. At the same time there occurs a reduction of the ττ-electron charges of bonds in the ring.* Substituents of the first group, combined with the phenyl group through a saturated carbon atom, may attract π-electrons compared with hydrogen, and to a greater or smaller extent depending on the electronegativity of the R atoms and groups in Ο ρ Η ^ Ο Κ ^ Ι ^ . Let us now consider calculations of π-electron charges of the bonds in diphenyl and styrene, the results of which are shown in Fig. 14. We must take into account the fact that the authors of these calculations included π-electron charges of C—H bonds in the charges of the C—C bonds, and therefore the latter are high by about 0*05-0-1 e. This, however, does not prevent the conclusion being drawn that * That this is so, for example, in the case of diphenyl and styrene, is seen from the calculations of Ham, Ruedenberg and Bagdasar'yan (see Fig. 14).
177
Chemical Properties of an-bonds
substituents which attract π-electrons to themselves decrease the π-electron charges mainly round the carbon atoms in the orthoposition. It is possible to consider to the sum or half-sum of the πelectron charges of the adjacent C—C bonds. For such a π-electron charge round the carbon atom we will introduce the term AQ„. Scherr [7] and Bagdasar'yan [6] have discussed the relationship between the π-electron charges round the carbon atoms in aromatic compounds and their reactivity. Scherr suggests that the sum of the π-electron charges characterizes the "bonded valency" (this term was also suggested by Ruedenberg, for the designation of such a sum) of a given carbon atom. In his opinion, the smaller the "bonded
f
Λ
+02
„
Λ
,
> C
\ΟΜΛ^ ^ 1055
ν
A^»CH
103F °
ue
CH
* OUO
FIG. 14. Electron charges of C—C bonds in diphenyl and styrene from the results of Ham and Ruedenberg [5] inscribed above, and of Bagdasar'yan [6] written below the bonds.
valency" the more reactive is the atom with regard to electrophilic agents. Scherr confirms his view by reference to the direction of the oxidation, halogenation, nitration and sulphonation reactions of polynuclear aromatic compounds. There are indications, however that in naphthalene, for example, nucleophilic substitution takes place in the same direction as electrophilic [8, p.592]. Thus it is possible that here it is not a question of "bonded valency", but that the transitional complex with a-naphthyl is most advantageous energetically for the same reason that the phenyl group in the α-position to the positive and negative charges or free π-electron stabilizes the corresponding ions and radicals. Bagdasar'yan expresses the relative values of the π-electron charges of C—C bonds in C 6 H 5 X as follows: 2 13 >(
2 3 1 ) > - X ( I ) and<(
2 13
\ — X(II), 2 3 1
where X(I) is an orientant of the first, and X(II)ofthe second group.
178
Electronic Charges of Bonds in Organic Compounds
In order to check this hypothesis we cannot, unfortunately, have recourse to data on the interatomic distances because of their low accuracy (they were all obtained by the X-ray method), but the calculations of Bagdasar'yan, Ham and Ruedenberg (see Fig. 14) definitely indicate that even in the case of orientants of the first group the distribution of π-electron charges expressed by the formula C6H5X(II) should be expected.* Thus with increase in the electronegativity of substituent X the jr-electron charge of phenyl as a rule decreases, and the π-electron charges round the atoms are distributed in the order ^Cn(meta)
^ &Cn(para)
^ ^Cn(ortho)
'
It is possible, therefore, to come to the conclusion that the effect of the substituent X on the decrease (or on the increase) of both er- and π-electron charges is greatest in the ortho- and weakest in the meta-position. Relationship between the Electronegativity of a Substituent and the Activation— Passivation of the Nucleus With increase in the electronegativity of X, in so far as the σ electron charges of the C—H bonds decrease, the electrophilic substitution is activated, but in so far as at the same time the π-electron charges round the atoms decrease, it is passivated. Clearly the total effect will depend on the relationship between the effect of the o- and π-electron charges. If the effect of the σ-electron charges of the C—H bonds always prevailed, the activation would increase gradually with increase of Ex; if the effect of the π-electron charges round the carbon atoms prevailed, the reaction with increase of Ex would become gradually passivated. In reality neither occurs. It is evident that for some value of Ex the effect of the cr-electron and π-electron charges is counterbalanced, but with further increase in Ex the π-electron charges will have a greater effect, which will be accompanied by still greater passivation of the electrophilic * Bagdasar'yan's calculation of the π-electron charges of the C2—C3 and C3 -C 4 bonds in styrene somewhat contradicts such a conclusion, but the conclusion is confirmed by Ham and Ruedenberg's calculation of the π-electron charges of bonds in stilbene, and also by similar calculations by Scherr [7].
Chemical Properties of σπ-bonds
179
substitution reaction. If the relative effect of the π-electron charges on the activation energy increases with increase of Ex, it is logical to suggest that the relative effect of the π-electron charges will increase with decrease of 2s x , and since metals having an electronegativity EM < 1 can enter as X, the σ-electron charges of the C—H bonds in C6H5X may prove to be greater than in benzene, which in turn must involve the passivation of electrophilic substitution. Thus we come to the important conclusion that electrophilic substitution reactions must be passivated not only when X has large, but also when it has small electronegativities, as shown in Fig. 15. The relationship of activation to Ex will be different for different places in the benzene nucleus, and in Fig. 15 the more sloping (dashed) curve must belong to substitution in the meta-position, since the electronic charges in the meta-position, as already stated, are less sensitive to change in the electronegativity of X. The curve corresponding to a substitution reaction in the para-position must be more convex than for the meta-position. The fact that in the activation of sodium on phenyl- and benzylsodium the meta derivatives are formed may serve as confirmation of ur conclusion [4, p. 203]. Relationship between the Electronegativity of a Substituent and its Orientating Action From Fig. 15 it follows that substituents X, activating electrophilic substitution, direct it mainly into the ortho- or /?ara-position, but passivating substituents direct it into the raeta-position. This is observed in the great majority of reactions studied. Generally speaking, however, there are no grounds for considering that the curves must cut at points through which a horizontal corresponding to the activation energy of benzene passes, as shown in Fig. 15(a). A relationship between the curves such as that shown in Fig. 15(b) is also possible. According to Fig. 15(b) an electrophilic substitution reaction must occur for substituent X l5 albeit with passivation, but it is mainly in the ortho- (or para-) position that this takes place when X is halogen. It is also possible that for those substituents which as a result of polarization can easily supply π-electrons to the phenyl nucleus, the corresponding points on the abscissae axis in Fig. 15 will lie consider-
180
Electronic Charges of Bonds in Organic Compounds
ably more to the left than should be the case from their electronegativities determined from data for saturated compounds. There are no grounds also for suggesting that these curves are identical for all reactants, since there is no doubt that the nature of the reactant, and not its charge alone, also has an effect on the activation energy. Whatever the differences between the curves may be, they cannot, however, affect our general conclusions about the nature of the orientation. The study of the relationships between the activation of reactions and the electronegativities of the substituents is a task for special investigation. Our purpose consists only in showing how the theory
FIG. 15. Relationship of activation-passivation of electrophilic substitution reactions in CeH5X to the electronegativity of the substituent X. The unbroken line relates to substitution in the ör/Ao-position, the dashed line to substitution in the /neta-position.
of electronic bond charges enables, by a deductive method, an explanation of the regularities observed during electrophilic substitution in mono-substituted benzenes to be arrived at. Obviously the same method of reasoning may be extended to nucleophilic substitution. The picture here will be the reverse. Alkyl Substituents as Acceptors of σ-Electrons A substantially new development of ours is the evaluation of alkyl substituents as acceptors of σ-electrons. Usually measurements of the dipole moments of toluene [9, p. 110] and other homologues of benzene are adduced as evidence of the electron donor character
181
Chemical Properties of σπ-bonds
of methyl and other alkyl groups, but the existing interpretation of the dipole moments themselves may be subject to doubt because their components, indebted in their origin to an incomplete number of electronic bond charges, are commensurate with the dipole moments themselves [10]. As Pauling notes [11, p. 153], the donor Yield of meta-isomer (in per cent)
/
HO 100 90 80 TO 60 50 40 300
325
350
"
FIG. 16. Relationship of the yield of meta-isomer in the mononitration of mono-substituted alkylbenzenes [2, p. 297] to the electronegativity of the atoms or groups in Q H g C R ^ R a . The point corresponding to C e H 5 CH 2 COOC 2 H 5 is indicated by X.
character of a methyl group, considered in accordance with the data about dipole moments, is found to be in contradiction with the fact that the electronegativity of carbon is higher than that of hydrogen. Meanwhile the direction of the dipole moment in toluene is not difficult to explain if it is taken into consideration that the component of the dipole moment arising from the displacement of the σ-electron charges towards the methyl group may be considerably smaller
182
Electronic Charges of Bonds in Organic Compounds
than the component arising on account of the displacement of the π-electrons to the opposite side. Regarding the distribution of alkyl groups in order according to electronegativity, supporters of the theory of electron displacement come to opposite conclusions. On a basis of inductive effect only, i.e. taking into account only the cr-electrons, they conclude that the tertiary butyl group is capable of repelling electrons to a greater extent than methyl [11, p. 193]. A similar conclusion is, however, refuted by data about the frequencies of oscillations of O—H bonds in aliphatic alcohols [12, p. 160]. From our point of view alkyl groups as substituents in alkylbenzenes reduce the ortho-, paraand increase the meta- orientation during electrophilic substitution in relation to the sum of the electronegativities of the hydrogen and carbon atoms combined with a junction atom of carbon. This is seen, for example, from Fig. 16.* There are facts, however, which contradict the foregoing point of view. For example, in the nitration of phenylnitromethane more meta-isomer is obtained than in the nitration of phenyldimethylnitromethane [2, p. 309]. Here, however, before drawing far-reaching conclusions, it must be pointed out that in the latter compound there is no interaction between the hydrogens of the methyl groups and the oxygen atoms of the nitro-group. Let us now consider the following, not commonplace example, which confirms our view on the nature of alkyls as substituents — acceptors of σ-electrons in alkyl benzenes —and enables us again to illustrate the conclusions drawn earlier. Hydrogen Exchange Reactions Electrophilic substitution reactions generally conform to the same rules as deuterization reactions in acid deuterized solvent (for example in DF or DBr, with catalyst BF 3 or AlBr3), but nucleophilic substitution reactions are like deuterization reactions in a basic solvent (for example, in ND 3 , catalyst potassamide).* * It is curious that there also lies on the curve a point for X = CH2COOC2H5, by which the value calculated earlier for the electronegativities of Cn in esters is confirmed (p. 121). * The proposed mechanism of exchange is as follows: the ion N D 2 _ first adds on to the carbon at which the substitution takes place, and it is possible
Chemical Properties of σπ-bonds
183
From the point of view developed in the foregoing, electrophilic deuterization must proceed more quickly in the nucleus with an increase in the number of methyl groups as substituents, and nucleophilic substitution more slowly. This general conclusion is completely confirmed experimentally the rate of exchange of hydrogen in toluene (solvents DBr or DF) is greater than in benzene, and increases with increase in the number of methyl groups combined with the ring, whilst the exchange is not extended to the methyl groups, and does not take place at all with hexamethylbenzene. Nucleophilic exchange in toluene proceeds first in the methyl group, and then in the nucleus,* and the rate of exchange in toluene is four times less than in benzene. Nucleophilic substitution at the C—H bonds as a rule takes place in groups situated in the α-position to the phenyl group, and only under rigorous conditions in the /^-position. This is understandable, since if the aromatic ring pushes out the σ-electrons, the increased α-electron charges will in the first place be associated with Ca —H bonds, but to some extent also with C^ —H bonds. The fact that the hydrogens of the methyl groups in anisole, C 6 H 5 OCH 3 , and dimethylaniline, C 6 H 5 N(CH 3 ) 2 , may be subjected to nucleophilic substitution may serve to confirm such properties of the benzene
ring.
In the last two chapters our object has been to point out the possibilities of the theory of electronic bond charges in the interpretation of important and typical reactions of organic chemistry. We leave the reader to make a comparison with the theoretical ideas of other authors from the point of view of generality and conviction. We limit ourselves to the following observations. The basic feature of the views set forth here consists in the fact that in the discussion of the reactions of benzene derivatives the role not only of the π- but also of the σ-electrons is made clear. In many cases it is just the latter which determine the direction of sub-
that a hydrogen bond is formed between the hydrogen and nitrogen, cleavage of the NHD~ ion then occurring. This hypothesis as well as the factual material of this section, is taken from Shatenshtein's paper [13]. * This, however, must not be regarded as an indication that the or-electron charges of the C—H bonds in the nucleus are smaller than in the methyl group, since the π-electrons of the nucleus passivate the nucleophilic substitution reaction.
184
Electronic Charges of Bonds in Organic Compounds
stitution in aromatic compounds, and therefore the chemical interpretation of these reactions exclusively in the framework of π-electrons may not be correct in principle. We employ ideas of the static and dynamic displacement of π-electrons in almost the same way as the followers of the theory of electron displacements, but nowhere have we found it necessary to use ideas of inductive and inductometric effects, i.e. of static and dynamic polarity of bonds, although one must not deny the existence of these effects. It is possible, however, to conclude that their effect on the reactivity of organic compounds is overstated and by means of them an attempt has been made to explain what is caused by an incomplete number of cr-electron bond charges. REFERENCES 1. O.A. REUTOV, Teoreticheskiye problemy organicheskoi khimii (Theoretical Problems of Organic Chemistry). Izd-vo Mosk. un-ta, Moscow (1956). 2. T.I. TEMNIKOVA, Kurs teoreticheskikh osnov organicheskoi khimii (Course of Theoretical Principles of Organic Chemistry). Goskhimizdat, Leningrad (1959).
2a. B. A. KAZANSKII, M. YU. LUKINA and L. G. SAL'NIKOVA, Dokl. Akad. Nauk, 115. 301 (1957); B. A. KAZANSKII, M. Y U . LUKINA and I. L.
SAFONOVA
Izv. Akad. Nauk SSSR, otdel. khim. nauk, 102 (1958). 3. D.R. ANGOOD and G. H. WILLIAMS, Chem. Revs., 57. 123 (1957). 4. J.E. LEFFLER, The Reactive Intermediates of Organic Chemistry. New York (1956). 5. N.S. HAM and K. REVDENBERG, / . Chem. Phys., 29, 1215 (1958). 6. Kh.S. BAGDASAR'YAN, Zh. fiz. khim., 28, 1098 (1954). 7. C.W. SCHERR, / . Chem. Phys. 21, 1582 (1953). 8. B.PULLMANN and A. PULLMANN, Les Theories electronique de la Chimie Organique. Paris (1952). 9. A. REMIK, Elektronnyye predstavleniya v organicheskoi khimii (Electronic Concepts in Organic Chemistry). IL., Moscow (1950). 10. G.V. BYKOV, IZV. Akad. Nauk SSSR, otdel. khim. Nauk, 1435 (1956). 11. L. PAULING, Priroda khimicheskoi svyazi (Nature of the Chemical Bond). Goskhimizdat, Moscow-Leningrad (1947). 12. P. MIRONE, Ann. Chim., 48, 156 (1958). 13. A.I. SHATENSHTEIN, In the symposium, Voprosy khimicheskoi kinetiki, kataliza i reaktsionnoi sposobnosti (Problems of Chemical Kinetics, Catalysis and Reactivity), p. 699. Izd-vo Akad. Nauk SSSR, Moscow (1955).
CHEMICAL PROPERTIES O F σπ-BONDS 1. ADDITION REACTIONS
THE theory of electronic bond charges provides scarcely any improvements on the ideas already held by organic chemists about the relationship between the distribution of π-electron density and the course of electrophilic, nucleophilic and radical addition reactions. It enables assessments to be made with great confidence, however, about this distribution, mainly because it explains the character of the interaction between σ- and π-electrons. In Chapter VIII we showed that electrostatic repulsion, the energy of which reaches some tens of kilocalories per one gramme-or-electron, takes place between the
4.1-270 4.Ecn £- = = 1-19 e. 3 + Ec* 3+1-270 170
Chemical Properties of σπ -bonds
171
into the bond with the C*. We will now suppose that X = CH 2 COOH. The electronegativity of the COT in the carboxyl group is equal to 1-620 (p. 121.). Then, from formula (2.1) we find that A% cn. = CC)
4Λ 2Ί0
' = 1-04 e, 2+1-620+1-270
and hence it follows that the value of the π-electron atmosphere on the Cn—X bond must be greater than in propylene. If X =CC1 3 , then ^C(CJT) — 0-84 e and consequently the ττ-electron atmosphere of the Cn—CC13 bond must be still larger, and of the C*=C W bond smaller. If the electronegativities of all the R's in X = CR 1 R 2 R 3 were known, it would be possible to arrange the substituents in order according to their capacity for attracting or repelling ^-electrons. The place of hydrogen in this series, of course, is not determined by the fact that its share in the C—X bond is equal to one σ-electron. Since the energy of the C—H ττ-bond is less than the energy of the C—C π-bond (p. 147) it may be assumed that it will occupy a place in this series between the substituents for which AQ(C*)> I e — somewhere between CH 3 and CH 2 COOH, but rather nearer to the former. In the second group of substituents X is an atom having a free pair of electrons in the valency layer. It is evident that the π-bond Cn—X is strengthened here on account of the conjugation of these electrons with the π-electrons of the ethylene bond. Since the affinity for the electron in halogens is greater than in oxygen, and more so than in nitrogen, it may be imagined that the share of the "free" electrons of the atom X in the bond C—X will be increased, but the interaction stability with the ^-electrons of the ethylene bond will decrease. Consequently, the static π-electron atmosphere of the C = C bond and its increase on account of polarization under the influence of an electrophilic reagent will be greatest when X is the group NR 1 R 2 , and least when X is halogen. It is evident that the role of the substituents R here will be the same as in the example discussed above, withX^CR^Ra. X = CRx = CR 2 R 3 and CR = O, and also X = C = C R , = C N , C 6 H 5 and so on, belong to the third group of substituents. It is evident that in this case the interaction is also located on the C"—X bond between the π-electrons, and when X = C R = O or C = N it is of a
172
Electronic Charges of Bonds in Organic Compounds
relatively more stable character. Here the substituents R exert an influence in relation to their electronegativities which in most cases may be predicted beforehand. Finally, the fourth group is made up of the highly electronegative +
+
+
substituents X = N R ^ R * , PR 1 R 2 R 3 , SRxR2 and so on, which evidently attract to themselves both the a- and the π-electrons from the 0 = C bond.* We will consider certain facts relating to electrophilic and nucleophilic addition at C==C and 0 = 0 bonds [1, p.151 et aL; 2,/?.347 et seq]. As would be expected, in relation to what has been said earlier, when the number of CH 3 groups substituting hydrogen in ethylene is increased, the relative rate of addition of bromide (first stageaddition ofBr + )is increased, but in the case of vinyl bromide, crotonic acid and especially acrylic acid the rate of addition is decreased. The rate of addition of even a proton to vinyl chloride isreduced, although the addition still proceeds according to Markovnikov's rule. But + when N(CH 3 ) 3 , CC13, CN, COOH are introduced as X, proton addition takes place against Markovnikov's rule and in general these substituents activate not electrophilic but nucleophilic addition. The rate of nucleophilic addition increases in the order R
\>c=o< R \ >c=o< R \ >c=o<
CPLjCX
R/
W
H
\>c=o.
W
Methoxyl, CH 3 0, increasing the π-electron atmosphere of the O—C bond, and R (alkyl), repelling the π-electrons on the C = 0 bond, passivate the reaction compared with hydrogen. As substituent R, cyclopropyl exerts a passivating effect. The mechanism of this effect is difficult to explain [1, p. 221; 2, p.393], but if the already mentioned tf-electron donor properties of the cyclopropane ring are taken into account, it becomes understandable that the cyclopropyl group must decrease the electrophilic properties of the carbonyl carbon.** * Obviously the carbonium ion should also be included in this group of substiuents. ** The analogy in electronic structure which is traced between the double bond and the three-membered rings (cyclopropane, ethylene oxide, ethylenimine on a basis of similarity of some of the chemical properties, does not carry convic)
Chemical Properties of σπ-bonds
173
As regards the direction of radical addition reactions, in the addition for example, of hydrogen bromide to C H 2 = C H — X in the presence of peroxides with all X's addition to the methylene group takes place first. Consequently the distribution of the π-electron density plays no part in radical addition,* and the reaction direction is evidently determined mainly by the stability of the radical formed [1, p.195]. In order to estimate the relative energy stability—of the radicals which may be formed, for example, by the addition of Br to propylene, the considerations expressed in Chapter VIII are used, viz. a radical is more stable the greater the degree to which the (T-electron charges of the C—H bonds are increased, and this occurs when the latter are in the α-position to a single π-electron. From this point of view the formation of the radical CH3 —CH—CH 2 Br must be much more efficient energetically than the radical CH 3 -CHBr—CH 2 .
2.
SUBSTITUTION REACTIONS IN BENZENE DERIVATIVES
Activation Energy of Substitution Reactions at an Unsaturated Carbon Atom As was said in the previous chapter, the activation energy of substitution reactions at an unsaturated carbon atom, if these reactions pass through the stage of the formation of a transitional complex, is made up mainly of two parts: the energy necessary for overcoming the repulsion between the entering substituent and the given atom tion. In methylvinylketone the vinyl group passivates the carbonyl bond to nucleophilic addition, and this should be ascribed to the fact that owing to the direct interaction of the π-electrons of the C = C bonds and the C = 0 bonds the concentration of electrons round the carbon atom at which the addition occurs is increased. Thus, in methylvinylketone the carbonyl carbon atom is passivated mainly by the π-electrons, whereas in methylcyclopropylketone by the tf-electrons. Another example is as follows: in the catalytic hydrogenation of vinyl- and phenylcyclopropane, disruption of the C—C bond adjacent to the tertiary carbon atom takes place [2a ]. This can in no way, however, be considered an argument for the existence of π, π-conjugation between the three-membered ring and the system of π-bonds, because the direction of hydrogenation (and its rate) is determined, in all probability, by the energy efficiency (p. 145 et seq.) associated with the formation in the first stage of homologues of allyl and benzyl radicals. * Thus, radicals may be regarded as electroneutral or at least as weakly electrophilic agents [3].
174
Electronic Charges of Bonds in Organic Compounds
of carbon with its surroundings, and the energy of stretching of the bond at which the substitution occurs. If the carbon atom at which the substitution occurs is a donor of π-electrons, the first component of the activation energy will also depend on the static distribution of the π-electron charges at the bonds and on their possible displacement during the reaction. It may be foreseen that in electrophilic substitution reactions the increase in π-electron density at a given carbon atom, just as in addition reactions, will reduce, but in nucleophilic substitution reactions will increase, the "repellent" portion of the activation energy. As regards the role of the cr-electrons, the conclusion drawn for substitution reactions at a saturated carbon atom is also applicable to substitution reactions in the aromatic nucleus, because it is in the highest degree probable that they pass through a stage of formation of a "
\c<'' -^C-X+H. / νχ /
As was noted in the previous paragraph, it is possible to neglect the interaction of the radicals with the π-electrons and consider that the activation energies of radical reactions of substitution reactions in the aromatic nucleus are determined only by the value of the σ-electron charge of the C—H bonds at which the substitution takes place. In order to apply these conditions to electrophilic and nucleophilic substitution reactions in aromatic compounds, it is necessary to have some, albeit qualitative, data about the distribution of electronic charges in these compounds. or-Electron Bond Charges in Benzene Derivatives Calculations (see Table 23) give the following values for the electronic bond charges in benzene— AQG= 1-91-1-96,AQG — 0-95-0-91, ^ C H = 2-09-2-04 and A%H = 0-05-0-09 e. Thus, in benzene a kind of expulsion of cr-electrons from the carbon nucleus on to the C—H bonds takes place, which is also characteristic of other flat rings, especially cyclopropane. Hence the low electronegativity of
Chemical Properties of σπ-bonds
175
benzene carbon Ec = 1-08 (p. 112). Consequently, all the substituents X in C 6 H 5 X (except, perhaps, cyclopropyl, its homologues and analogues, and also metals which have an electronegativity much lower than hydrogen) are acceptors of σ-electrons and lower the σ-electron charge of the C—C and C—H bonds of the benzene ring. Even phenyl as a substituent of hydrogen in benzene can exhibit this property, since the central C—C bond in diphenyl is capable of absorbing from each benzene ring more cr-electrons than the C—H bond in benzene. Another aspect of the question is how is this decrease in cr-electron charges distributed over the ring, for example in mono-substituted benzenes C 6 H 5 X? If it were a question of a noncyclic compound of the type H H H H Λ.
I
\~^
i
I
I
v_^2
^3
i
I
^4
r i
· · ·
H H H H the picture of the change in electronegativities and electronic charges would be clear (see Figs. 7 and 8). A typical case is where the electronegativity of the X atom is greater than the electronegativity of hydrogen, Ex > 1. Then as a result of the entry of X into the hydrocarbon in place of the hydrogen some portion of the electronic charge will be transferred from the Cx—C2 and Cx—H bonds to the Cx—X bond, the electronic screening of the Cx atom to the side of the C 2 atom will decrease, but its electronegativity will increase. In turn the electronegativity of the C 2 atom to the side of the C 3 atom will increase (though as a result of damping of this effect, to a smaller extent than with the C x atom), but to the side of the C x atom it will decrease, and so on. It is evident then that the electronic charges of the C—H bonds are distributed in the order: AQ^ < ^Ο,Η ^ ^C 3 H and so on. It is more difficult to formulate such a series when the chain of carbon atoms is closed, as in H
H
X - C /
x
>C 4 H.
c—
H
176
Electronic Charges of Bonds in Organic Compounds
It is clear that ^4£aH < ^c 3 H' but it is impossible to say anything about the relative values of the electronic charges A^^ and .4C4H> whilst there are no ways of estimating the relative values of the sums of electronegativities ECi+ EQ 4 and 2ECs because, according to formula (2.4), the greater the sum of the electronegativities of the atoms combined with a given carbon atom, the smaller the electronic charges of the C—H bonds which it forms. Therefore, if ECt + ECt < < 2EQ3, then Αζ 3 π < Αζ 4 Η 5 and vice versa. In order to solve this problem it is possible to employ data about radical substitutions reactions in monosubstituted benzenes. Since orientants of both the first and second groups activate the nucleus with regard to radical substitution reactions, and since these reactions proceed more easily in the ortho- and para- than in the meta-position, apart from the nature of the orientant, it may be concluded that Α£,Η <
A£4H·
ττ-Electron Bond Charges in Benzene Derivatives What has already been said about the effect of X on the π-electron charges of C = C and C—X bonds in C H 2 = C H X may be repeated with regard to the effect of substituents X in C 6 H 5 X on the ττ-electron charges of bonds in the benzene ring. Thus, substituents of the second, third and fourth groups (p. 171 et seq.), compared with hydrogen, attract to themselves both the σ- and the π-electrons, in spite of their mutual repulsion. At the same time there occurs a reduction of the ττ-electron charges of bonds in the ring.* Substituents of the first group, combined with the phenyl group through a saturated carbon atom, may attract π-electrons compared with hydrogen, and to a greater or smaller extent depending on the electronegativity of the R atoms and groups in Ο ρ Η ^ Ο Κ ^ Ι ^ . Let us now consider calculations of π-electron charges of the bonds in diphenyl and styrene, the results of which are shown in Fig. 14. We must take into account the fact that the authors of these calculations included π-electron charges of C—H bonds in the charges of the C—C bonds, and therefore the latter are high by about 0*05-0-1 e. This, however, does not prevent the conclusion being drawn that * That this is so, for example, in the case of diphenyl and styrene, is seen from the calculations of Ham, Ruedenberg and Bagdasar'yan (see Fig. 14).
177
Chemical Properties of an-bonds
substituents which attract π-electrons to themselves decrease the π-electron charges mainly round the carbon atoms in the orthoposition. It is possible to consider to the sum or half-sum of the πelectron charges of the adjacent C—C bonds. For such a π-electron charge round the carbon atom we will introduce the term AQ„. Scherr [7] and Bagdasar'yan [6] have discussed the relationship between the π-electron charges round the carbon atoms in aromatic compounds and their reactivity. Scherr suggests that the sum of the π-electron charges characterizes the "bonded valency" (this term was also suggested by Ruedenberg, for the designation of such a sum) of a given carbon atom. In his opinion, the smaller the "bonded
f
Λ
+02
„
Λ
,
> C
\ΟΜΛ^ ^ 1055
ν
A^»CH
103F °
ue
CH
* OUO
FIG. 14. Electron charges of C—C bonds in diphenyl and styrene from the results of Ham and Ruedenberg [5] inscribed above, and of Bagdasar'yan [6] written below the bonds.
valency" the more reactive is the atom with regard to electrophilic agents. Scherr confirms his view by reference to the direction of the oxidation, halogenation, nitration and sulphonation reactions of polynuclear aromatic compounds. There are indications, however that in naphthalene, for example, nucleophilic substitution takes place in the same direction as electrophilic [8, p.592]. Thus it is possible that here it is not a question of "bonded valency", but that the transitional complex with a-naphthyl is most advantageous energetically for the same reason that the phenyl group in the α-position to the positive and negative charges or free π-electron stabilizes the corresponding ions and radicals. Bagdasar'yan expresses the relative values of the π-electron charges of C—C bonds in C 6 H 5 X as follows: 2 13 >(
2 3 1 ) > - X ( I ) and<(
2 13
\ — X(II), 2 3 1
where X(I) is an orientant of the first, and X(II)ofthe second group.
178
Electronic Charges of Bonds in Organic Compounds
In order to check this hypothesis we cannot, unfortunately, have recourse to data on the interatomic distances because of their low accuracy (they were all obtained by the X-ray method), but the calculations of Bagdasar'yan, Ham and Ruedenberg (see Fig. 14) definitely indicate that even in the case of orientants of the first group the distribution of π-electron charges expressed by the formula C6H5X(II) should be expected.* Thus with increase in the electronegativity of substituent X the jr-electron charge of phenyl as a rule decreases, and the π-electron charges round the atoms are distributed in the order ^Cn(meta)
^ &Cn(para)
^ ^Cn(ortho)
'
It is possible, therefore, to come to the conclusion that the effect of the substituent X on the decrease (or on the increase) of both er- and π-electron charges is greatest in the ortho- and weakest in the meta-position. Relationship between the Electronegativity of a Substituent and the Activation— Passivation of the Nucleus With increase in the electronegativity of X, in so far as the σ electron charges of the C—H bonds decrease, the electrophilic substitution is activated, but in so far as at the same time the π-electron charges round the atoms decrease, it is passivated. Clearly the total effect will depend on the relationship between the effect of the o- and π-electron charges. If the effect of the σ-electron charges of the C—H bonds always prevailed, the activation would increase gradually with increase of Ex; if the effect of the π-electron charges round the carbon atoms prevailed, the reaction with increase of Ex would become gradually passivated. In reality neither occurs. It is evident that for some value of Ex the effect of the cr-electron and π-electron charges is counterbalanced, but with further increase in Ex the π-electron charges will have a greater effect, which will be accompanied by still greater passivation of the electrophilic * Bagdasar'yan's calculation of the π-electron charges of the C2—C3 and C3 -C 4 bonds in styrene somewhat contradicts such a conclusion, but the conclusion is confirmed by Ham and Ruedenberg's calculation of the π-electron charges of bonds in stilbene, and also by similar calculations by Scherr [7].
Chemical Properties of σπ-bonds
179
substitution reaction. If the relative effect of the π-electron charges on the activation energy increases with increase of Ex, it is logical to suggest that the relative effect of the π-electron charges will increase with decrease of 2s x , and since metals having an electronegativity EM < 1 can enter as X, the σ-electron charges of the C—H bonds in C6H5X may prove to be greater than in benzene, which in turn must involve the passivation of electrophilic substitution. Thus we come to the important conclusion that electrophilic substitution reactions must be passivated not only when X has large, but also when it has small electronegativities, as shown in Fig. 15. The relationship of activation to Ex will be different for different places in the benzene nucleus, and in Fig. 15 the more sloping (dashed) curve must belong to substitution in the meta-position, since the electronic charges in the meta-position, as already stated, are less sensitive to change in the electronegativity of X. The curve corresponding to a substitution reaction in the para-position must be more convex than for the meta-position. The fact that in the activation of sodium on phenyl- and benzylsodium the meta derivatives are formed may serve as confirmation of ur conclusion [4, p. 203]. Relationship between the Electronegativity of a Substituent and its Orientating Action From Fig. 15 it follows that substituents X, activating electrophilic substitution, direct it mainly into the ortho- or /?ara-position, but passivating substituents direct it into the raeta-position. This is observed in the great majority of reactions studied. Generally speaking, however, there are no grounds for considering that the curves must cut at points through which a horizontal corresponding to the activation energy of benzene passes, as shown in Fig. 15(a). A relationship between the curves such as that shown in Fig. 15(b) is also possible. According to Fig. 15(b) an electrophilic substitution reaction must occur for substituent X l5 albeit with passivation, but it is mainly in the ortho- (or para-) position that this takes place when X is halogen. It is also possible that for those substituents which as a result of polarization can easily supply π-electrons to the phenyl nucleus, the corresponding points on the abscissae axis in Fig. 15 will lie consider-
180
Electronic Charges of Bonds in Organic Compounds
ably more to the left than should be the case from their electronegativities determined from data for saturated compounds. There are no grounds also for suggesting that these curves are identical for all reactants, since there is no doubt that the nature of the reactant, and not its charge alone, also has an effect on the activation energy. Whatever the differences between the curves may be, they cannot, however, affect our general conclusions about the nature of the orientation. The study of the relationships between the activation of reactions and the electronegativities of the substituents is a task for special investigation. Our purpose consists only in showing how the theory
FIG. 15. Relationship of activation-passivation of electrophilic substitution reactions in CeH5X to the electronegativity of the substituent X. The unbroken line relates to substitution in the ör/Ao-position, the dashed line to substitution in the /neta-position.
of electronic bond charges enables, by a deductive method, an explanation of the regularities observed during electrophilic substitution in mono-substituted benzenes to be arrived at. Obviously the same method of reasoning may be extended to nucleophilic substitution. The picture here will be the reverse. Alkyl Substituents as Acceptors of σ-Electrons A substantially new development of ours is the evaluation of alkyl substituents as acceptors of σ-electrons. Usually measurements of the dipole moments of toluene [9, p. 110] and other homologues of benzene are adduced as evidence of the electron donor character
181
Chemical Properties of σπ-bonds
of methyl and other alkyl groups, but the existing interpretation of the dipole moments themselves may be subject to doubt because their components, indebted in their origin to an incomplete number of electronic bond charges, are commensurate with the dipole moments themselves [10]. As Pauling notes [11, p. 153], the donor Yield of meta-isomer (in per cent)
/
HO 100 90 80 TO 60 50 40 300
325
350
"
FIG. 16. Relationship of the yield of meta-isomer in the mononitration of mono-substituted alkylbenzenes [2, p. 297] to the electronegativity of the atoms or groups in Q H g C R ^ R a . The point corresponding to C e H 5 CH 2 COOC 2 H 5 is indicated by X.
character of a methyl group, considered in accordance with the data about dipole moments, is found to be in contradiction with the fact that the electronegativity of carbon is higher than that of hydrogen. Meanwhile the direction of the dipole moment in toluene is not difficult to explain if it is taken into consideration that the component of the dipole moment arising from the displacement of the σ-electron charges towards the methyl group may be considerably smaller
182
Electronic Charges of Bonds in Organic Compounds
than the component arising on account of the displacement of the π-electrons to the opposite side. Regarding the distribution of alkyl groups in order according to electronegativity, supporters of the theory of electron displacement come to opposite conclusions. On a basis of inductive effect only, i.e. taking into account only the cr-electrons, they conclude that the tertiary butyl group is capable of repelling electrons to a greater extent than methyl [11, p. 193]. A similar conclusion is, however, refuted by data about the frequencies of oscillations of O—H bonds in aliphatic alcohols [12, p. 160]. From our point of view alkyl groups as substituents in alkylbenzenes reduce the ortho-, paraand increase the meta- orientation during electrophilic substitution in relation to the sum of the electronegativities of the hydrogen and carbon atoms combined with a junction atom of carbon. This is seen, for example, from Fig. 16.* There are facts, however, which contradict the foregoing point of view. For example, in the nitration of phenylnitromethane more meta-isomer is obtained than in the nitration of phenyldimethylnitromethane [2, p. 309]. Here, however, before drawing far-reaching conclusions, it must be pointed out that in the latter compound there is no interaction between the hydrogens of the methyl groups and the oxygen atoms of the nitro-group. Let us now consider the following, not commonplace example, which confirms our view on the nature of alkyls as substituents — acceptors of σ-electrons in alkyl benzenes —and enables us again to illustrate the conclusions drawn earlier. Hydrogen Exchange Reactions Electrophilic substitution reactions generally conform to the same rules as deuterization reactions in acid deuterized solvent (for example in DF or DBr, with catalyst BF 3 or AlBr3), but nucleophilic substitution reactions are like deuterization reactions in a basic solvent (for example, in ND 3 , catalyst potassamide).* * It is curious that there also lies on the curve a point for X = CH2COOC2H5, by which the value calculated earlier for the electronegativities of Cn in esters is confirmed (p. 121). * The proposed mechanism of exchange is as follows: the ion N D 2 _ first adds on to the carbon at which the substitution takes place, and it is possible
Chemical Properties of σπ-bonds
183
From the point of view developed in the foregoing, electrophilic deuterization must proceed more quickly in the nucleus with an increase in the number of methyl groups as substituents, and nucleophilic substitution more slowly. This general conclusion is completely confirmed experimentally the rate of exchange of hydrogen in toluene (solvents DBr or DF) is greater than in benzene, and increases with increase in the number of methyl groups combined with the ring, whilst the exchange is not extended to the methyl groups, and does not take place at all with hexamethylbenzene. Nucleophilic exchange in toluene proceeds first in the methyl group, and then in the nucleus,* and the rate of exchange in toluene is four times less than in benzene. Nucleophilic substitution at the C—H bonds as a rule takes place in groups situated in the α-position to the phenyl group, and only under rigorous conditions in the /^-position. This is understandable, since if the aromatic ring pushes out the σ-electrons, the increased α-electron charges will in the first place be associated with Ca —H bonds, but to some extent also with C^ —H bonds. The fact that the hydrogens of the methyl groups in anisole, C 6 H 5 OCH 3 , and dimethylaniline, C 6 H 5 N(CH 3 ) 2 , may be subjected to nucleophilic substitution may serve to confirm such properties of the benzene
ring.
In the last two chapters our object has been to point out the possibilities of the theory of electronic bond charges in the interpretation of important and typical reactions of organic chemistry. We leave the reader to make a comparison with the theoretical ideas of other authors from the point of view of generality and conviction. We limit ourselves to the following observations. The basic feature of the views set forth here consists in the fact that in the discussion of the reactions of benzene derivatives the role not only of the π- but also of the σ-electrons is made clear. In many cases it is just the latter which determine the direction of sub-
that a hydrogen bond is formed between the hydrogen and nitrogen, cleavage of the NHD~ ion then occurring. This hypothesis as well as the factual material of this section, is taken from Shatenshtein's paper [13]. * This, however, must not be regarded as an indication that the or-electron charges of the C—H bonds in the nucleus are smaller than in the methyl group, since the π-electrons of the nucleus passivate the nucleophilic substitution reaction.
184
Electronic Charges of Bonds in Organic Compounds
stitution in aromatic compounds, and therefore the chemical interpretation of these reactions exclusively in the framework of π-electrons may not be correct in principle. We employ ideas of the static and dynamic displacement of π-electrons in almost the same way as the followers of the theory of electron displacements, but nowhere have we found it necessary to use ideas of inductive and inductometric effects, i.e. of static and dynamic polarity of bonds, although one must not deny the existence of these effects. It is possible, however, to conclude that their effect on the reactivity of organic compounds is overstated and by means of them an attempt has been made to explain what is caused by an incomplete number of cr-electron bond charges. REFERENCES 1. O.A. REUTOV, Teoreticheskiye problemy organicheskoi khimii (Theoretical Problems of Organic Chemistry). Izd-vo Mosk. un-ta, Moscow (1956). 2. T.I. TEMNIKOVA, Kurs teoreticheskikh osnov organicheskoi khimii (Course of Theoretical Principles of Organic Chemistry). Goskhimizdat, Leningrad (1959).
2a. B. A. KAZANSKII, M. YU. LUKINA and L. G. SAL'NIKOVA, Dokl. Akad. Nauk, 115. 301 (1957); B. A. KAZANSKII, M. Y U . LUKINA and I. L.
SAFONOVA
Izv. Akad. Nauk SSSR, otdel. khim. nauk, 102 (1958). 3. D.R. ANGOOD and G. H. WILLIAMS, Chem. Revs., 57. 123 (1957). 4. J.E. LEFFLER, The Reactive Intermediates of Organic Chemistry. New York (1956). 5. N.S. HAM and K. REVDENBERG, / . Chem. Phys., 29, 1215 (1958). 6. Kh.S. BAGDASAR'YAN, Zh. fiz. khim., 28, 1098 (1954). 7. C.W. SCHERR, / . Chem. Phys. 21, 1582 (1953). 8. B.PULLMANN and A. PULLMANN, Les Theories electronique de la Chimie Organique. Paris (1952). 9. A. REMIK, Elektronnyye predstavleniya v organicheskoi khimii (Electronic Concepts in Organic Chemistry). IL., Moscow (1950). 10. G.V. BYKOV, IZV. Akad. Nauk SSSR, otdel. khim. Nauk, 1435 (1956). 11. L. PAULING, Priroda khimicheskoi svyazi (Nature of the Chemical Bond). Goskhimizdat, Moscow-Leningrad (1947). 12. P. MIRONE, Ann. Chim., 48, 156 (1958). 13. A.I. SHATENSHTEIN, In the symposium, Voprosy khimicheskoi kinetiki, kataliza i reaktsionnoi sposobnosti (Problems of Chemical Kinetics, Catalysis and Reactivity), p. 699. Izd-vo Akad. Nauk SSSR, Moscow (1955).