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Conformations of highly hindered aryl ethers
63 Journal of Molecular Structure, 18 (1973) 63-74 0 Elsevier Scientific Publishing Company, Amsterdam
CONFORMATIONS XV.
MESOMERIC
OF HIGHLY MOMENTS
- Printed
HINDERED
in The Netherlands
ARYL
ETHERS
IN m-DMITROBENZENES*t
P. A. LEHMANN F. AND D. M. MCEACHERN B. Department of Chemistry, Centerfor Research and Advanced Stadies, National Polytechnic 14-740, Mexico 14, D.F. (Mexico)
Institute
A.P.
(Received
15 December
1972)
ABSTRACT
The dipole moments of fourteen m-dinitrobenzenes with different substituents (OMe, CN, CH3, CHO, Cl, CF,, COOMe, F, Br, NH2) in various positions were measured in order to study mesomeric interactions. Inclusion of conformational considerations permitted estimates to be made for the mesomeric moments of these groups at given positions. The values obtained could be applied with consistent results to a total of forty m-dinitrcbenzenes, including others previously reported. The mesomeric moment increments deduced ,compared with substituted benzenes possessing no nitro groups, were found to be only slightly larger than in non-nitro substituted benzenes (compared with substituted alkanes), which would indicate a reluctance of the extended 7r system of m-dinitrobenzene
to interact with other substituents present, in any way other than by direct extension of the x network. One apparent case of intermolecular electron formation was detected. The measured dipole moment a planar anti conformation for the methoxyl group.
donor-acceptor of o-iodoanisole
complex indicates
INTRODUCTION
In earlier work on the preferred conformations of highly hindered aryl ethers, determination of their dipole moments’-4 revealed the existence, in the * Portions of this work were presented & the Sixth Mexican Congress of Pure and Applied Chemistry heId in CuliacBn, SinaIoa, March lO-13,1971; cf. Rev. Sot. Quim. M&x., 15 (1971) 17. + Part XIV, P. A. Lehmann F. and A. Ciurlizza G., Reu. Latinomamer, Quim., in press.
64 dinitrophenoxy moiety, of a considerable electronic delocalization from the oxygen into the ring, equivalent to mesomeric moments of 1.2-1.3 D. Halogens, which are like the ether oxygen in that they have pairs of non-bondingp electrons capable of being delocalized upon demand through p-n interactions (Ingold’s f M effect)5, have also been shown to display analogous mesomeric moment@. In view of this it was of interest to determine the electron donating capabilities of halogens and other groups when substituted at different positions of m-dinitroarenes. The compounds whose dipole moments were determined are shown in Fig. 1.
“N-qO’
v-$&J;02q5J~
02Nyg2 ‘Me
24J720.03
2:
1.60~0.12
02N&N02
2 : 3.0 IA 0.05
complex)
TEDA
(4.19)
+$$02
(2.72)
r
(2.72)
p431=Ql5 (4.28)
E:
- CN
Me
p269foD9
270*
0.12
s
(2.75)
2.01*0.29 (291)
0.07
(5.67)
02+$N02
02N$$N02
6
2.: 2.72kO.34
4 : 522* -
(3.15)
::53
io.10
( 5.53 1
I,5 ‘. 2.50=
0.14
(2.55)
Fig. 1. %ructures of the compounds studied, -showing their measured dipole moments (D)* their calculated dipole moments (in parentheses) and the deduced mesomeric moments. Curved arrows not otherwiyz marked indicate a twist an$e of 45“.
65 EXPERIMENTAL
The compounds studied were obtained and purified as follows: 1. Synthesized 7 from sym-trinitrobenzene in 60 oA yield; recrystallized from MeOH to m-p. 102-104 “C. 2. Commercial product’ recrystallized from EtOH with charcoal treatment to m.p. 131-132 “C. 3. Commercial product9 recrystallized twice from aq. EtOH to m-p. 81-82 “C. 4. Commercial product’ recrystallized three times from HOAc to m-p. 121-122 “C. 5. Commercial product’ recrystallized from aq. EtOH with charcoal treatment, then from 2-heptanone to m-p. 86-87 “C. 6. Commercial product” recrystallized from aq. EtOH to m-p. 55-56 “C. 7. Commercial product’ ’ recrystallized twice from aq. EtOH to m.p. 98-100 “C. 8. Commercial product’ recrystallized twice from aq. EtOH to m.p. 141-143 “C. 9. Commercial product9 recrystallized with charcoal treatment from CHCl, to m-p. 74-76 “C. 10. Commercial product’ ’ recrystallized from EtOH to m.p. 183-187 “C. 11.Commercial product” recrystallized from EtOH with charcoal treatment to m-p. 88-91 C.. 12. Commercial product’ ’ recrystallized twice from EtOH to m-p. 184-186 “C!.
13. Commercial product9 recrystallized from EtOH to m-p. 146-147 “C. 14. Synthesizedi in 61 ‘A yield and recrystallized twicefromaq. CH,COCH3 to m-p. 218-221 OC. 15. Commercial product9 purified by careful fractional distillation after which it had m-p. 8-10 “C. All samples had a purity greater than 99.95 % determined by differential scanning calorimetry as described previously l. The dipole moments were obtained by the method of Halverstadt and Kumler” using an especially convenient pycnometer for the density measurements14. Further details are given in Part Il.
RESULTS
The data-from the dipole moment (DM) measurements (Table 1) were used to calculate the results (Table 2) by the procedure previously reported’.
66 TABLE
1
CONSTANTAND SPECIFICVOLUME
DIPOLEMOMENT~~EASURE~~:WEIGHTFRAC~~N~~~SUSDIELE~IC
I03UZ
El2
3,SDinitroanisole 0.000 0.361 0.692 1.076 1.486 1.821 2.161 2.507
2.271 2.275 2.278 2.282 2.286 2.289 2.292 2.296
2.276 (2.281)” 2.282 2.292 2.300 2.303 2.310 2.316
1.14521 I .14501 1.14489 1.14472 1.14444 l-14441 1.14408 1.14396 (4)
1.14496 (1.14472)” 1.14477 (1.14423) 1.14406 1.14384 1.14379 1.14345
Methyl 4-chloro-3,5-dinittobenzoate (7) 0.000 0.333 0.667 1.073 1.465 1.833 2.145 2.472
2.275 2.277 2.280 (2.284)’ 2.286 2.288 2.289 2.293
1.14506 1.14472 1.14454 1.14421 1.14400 1.14391 1.14370 1.14359
I-ChIoro-5-fluoro-2+dinitrobenzene (10) 0.000 0.342 0.677 1.078 1.490 I.837 2.157 2.507
2.276 (2.279)” 2.279 2.281 2.282 2.283 2.284 2.286
1.14486 1_14464 1.14458 1.14425 1.14400 1.14390 1.14370 1.14340
1-ChIoro-2,4_dinitronaphthaIene (13) 0.000 0.355 0.796 1.029 1.366 1.605 2.044 2.506
2.276 (2.280)a (2.280)’ 2.283 2.285 2.288 2.291 2.296
&I2
103w2
"12
(1.14481)J 1.14419 1.14416 1.14407 (1.14393)’ 1.14371 1.14346
0.000 0.332 0.710 1.081 1.479 I.834 2.199 2.514
2.267 2.267 2.267 2.268 2.269 2.269 2.270 (2.268)”
0.000 0.348 0.685 1.102 1.464 1.829 2.161 2.522
2.273 2.277 2.28 I 2.283 2.288 2.291 2.294 2.297
0.000 0.309 0.685 1.062 1.474 1.802 2.166 2.501
1.14521 1.I4509 1.14475 1.14460 1.14436 I.14427 1.14422 1.14394
I-Chloro-2,6_dinitrobenzene
‘512
“12
Dinitromesitylene (3)
3,5-Dinitrobenzonitrile (2)
(1)
2,6-Dinitrobenzaldehyde 0.000 0.343 0.333 1.073 1.465 1.791 2.153 2.601
10’02
“12
(5)
1.14501 I.14486 1.14458 I.14447 1.14420 1.14407 1.14397 1.14376
(2.276)9 2.280 2.282 2.284 2.286 2.287 2.289 2.290
1.1451 I.1445 1.144t 1.144: 1.144: 1.144: 1.144: 1.1441
4-Chloro-I-trifluorometh] 3,5-dinitrobenzene (6) 0.000 0.319 0.689 1.069 I.459 I.821 2.187 2.512
2.276 2.277 2.278 2.278 2.278 2.279 2.279 2.280
1.1451 1.144: 1.144; l-144! 1.144: 1.1441 1.1435 I.1431
4-Chloro-3,5-dinitrobenzonitrile (8) 1.14511 0.000 2.274 1.14483 0.364 2.274 1.14481 0.696 2.275 1.14448 1.071 2,274 1.14432 1.481 2.276 1.14417 1.826 2.275 1.14387 2.191 2.276 I.14379 2.512 2.275
I,S-DifIuoro-2,4-dinitrobe (9) 2.274 I.1445 0.000 (2.273)” 1.1445 0.330 2.277 1.14lt 0.676 (2.281)p 1.144 1.072 2.282 1.144: 1.471 2.281 1.144I 1.826 2.284 1.1431 2.156 2.284 1.143f 2.474
I-Bromo-5-fluoro-2,4-dinitrobenzene (11)
5-FIuoro-2,4_dinitroaniIin
0.000 0.315 0.669 1.076 1.496 1.841 2.182 2.518
2.269 2.270 2.272 2.274 2.275 2.275 2.277 2.277
1.14535 1.14521 1.14487 1.14466 1.14428 1.14409 1.14389 1.14367
2,2’-Difluoro-3,3’,5,5’-tetranitrobiphenyl (14) 0.000 0.324 0.687 1.104 1.501 1.858 2.184 2.554
2.272 2.273 (2.274)* 2.273 2.275 (2.276)a 2.276 2.275
1.14500 1.14483 1.14452 1.14428 (1.14393)a 1.14393 1.14366 1.14345
0.000 0.324 0.678 1.076 I .490 1.838 2.163 2.515
2.269 (2.273)” 2.282 2.289 2.296 (2.306)= 2.308 (2.314)”
1.145: 1.145t I.1441 1.1441 1.144: 1.144 I-144( 1.143’
2-Iodoanisole (15) 0.000 0.393 0.671 1.123 1.402 1.842 2.174 2.477
2.274 2.274 (2.274)” 2.277 (2.275)= 2.279 2.280 2.282
1.145: 1.144: 1_144 1.144: 1.144: 1.144( 1.143’ 1.143
67 TABLE 2 DIPOLE
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
MOMENT
2.272 2.266 2.279 2.276 2.274 2.277 2.275 2.274 2.275 2.276 2.270 2.270 2.275 2.272 2.274
RESULTS
0.87320 0.87323 0.87329 0.87341 0.87338 0.87329 0.87342 0.87330 0.87334 0.87346 0.87310 0.87321 0.87384 0.87339 0.87327
24.62 3.92 12.80 38.63 24.22 4.00 23.33 1.25 10.37 10.39 10.67 42.96 25.94 6.54 9.21
0.962
0.949 0.746 1.106 0.970 1.402 1.472 1.174 1.097 1.216 1.760 0.970 0.888 2.102 1.372
400.32 94.51 236.79 600.12 395.26 108.24 386.47 60.25 IS&.39 190.70 193.75 666.66 439.77 158.91 173.25
43.98
41.98 51.49 42.16 42.38 46.35 53.27 46.82 37.09 42.15 45.03 40.73 59.18 75.93 45.56
a a and /I are the slopes of the dielectric constant and density, respectively, versus
4.17*0.03 1.60f0.12 3.Olf0.05 5.22&O-07 4_15*0.07 1.74f0.12 4.03 &0.07 0.81 &to.26 2.72&O-34 2.69 &to.09 2.70f0.12 5.53f0.10 4.31f0.15 2.01 f0.29 2.50&0.14
mole fraction.
A!XUhSPTIONS
In order to estimate the DM for a particular compound the following assumptions were made: (i) The group moments (D) used were15: NO, = 3.98, OMe = 1.25 (55”), CN = 4.00, Me = -00.40, CHO = 2.76 (55”), Cl = 1.58, CF, = 2.56, COOMe = 1.83 (70”), F = 1.46, Br = 1.54, NH, = 1.53 (142”), I = 1.30, N, = 1 44r6 CMe -oo.4017. (ii) Nitro griu;s are twisted out of the plane of the ring by 90” when ortho to two methyl groups, a carbonyl oxygen, and iodine; they are twisted only 45” when ortho to methyl, chlorine and bromine. (iii) The mesomeric moment (MM) of a substituent is directed along the bond joining the substituent to the ring, taken as positive when directed into the ring. Comparison of the measured and calculated (in parentheses) DM’s, together with deduced MM’s and the preferred conformations are shown in Fig. 1.
DISCUSSION
The measured DM’s (D) could best be rationalized by considering the conformations and including the deduced MM’s shown in Fig. 1. The conformational assumptions where: (a) that nitro groups are coplanar with the ring when flanked
68 by hydrogen or fluorine; (b) that they are twisted 45” out of the ring plane when flanked by one methyl, chlorine or bromine substituent; and (c) that they are twisted 90” when flanked by two methyl groups, a carbonyl oxygen or iodine. These nominal values of twist are reasonable in view of X-ray crystallographic determinations on related compounds”. As beforele4, the group moment of a nitro substituent is 3.15 +0.83 cos% where 0 is the out-of-plane twist angle. When a nitro group has ortho to it a substituent with which it can hydrogen bond (i.e. OH or NH2), it is assumed that the preferred conformation is the one in which all the atoms involved in the hydrogen bonding are coplanar with the ring. Some of these results merit individual comment. In the anisole (1) the MM = 0.80 D is, as expected, smaller than that found for the other anisoles in which the methoxyl group is ortho and/or para to the nitro groups. Nevertheless, this relatively large MM argues for the existence of a methoxyl group which is coplanar with the ring. The measured DM’s of 2 and 8 cannot be explained on the basis of interaction moments. If the cyano group exhibits a -M effect, as has been deducedlg, then structures like A must make an important contribution in 2:”
Then in 8, structure B should make an even greater contribution:
8
B
and inclusion of the MM’s deduced from 5 for Cl and for CN from 2 should lead to a calculated DM of 1.44 or greater, whereas that measured was only 0.8 I_ Alternatively, one might consider that the cyano group might, upon demand from the nitro groups, exhibit a +M effect due to the stability of forms such as C and D:
69
In this case a MM of + 1.60 would have to be present to explain the observed moment. Similarly in 8, analogous structures would be expected to contribute more importantly, especially if expansion of the halogen’s octet is considered possible” : %N+No~N+NO~+-$N%
;
W-&N%
$3
d
NO
io
CN
However, for 8 a calculated moment of 2.74 D is obtained without a MM contribution from Cl, and of 1.74 D with one of 1 .O D. Both are greater than the observed moment of 0.81 D, from which this type of mesomerism can be excluded. In view of the fact that neither of these alternatives is viable, we feel that the best explanation is as follows: 2 is an excellent acceptor in EDA complexes22, and it may be participating in the formation of such a complex with the benzene (solvent).
The measured DM of 1.58 D is in the range of values previously reported for similar intermolecular EDA complexes 23 _ Support for this comes from the fact that in 8 the chlorine partially satisfies the ring’s electron deficiency by its mesomerit contribution, rendering it a poorer acceptor and resulting in the smaller intermolecular DM of 0.8 1 D observed for it. A MM of -0.45 D1’ may well be present and partially account for the observed moment. For compound 3, the observed DM can be rationalized best by considering that the nitro groups, being flanked by two bulky methyl groups, are rotated 90” out of the ring plane. The moment calculated for this structure is 3.15 D. If the nitro groups are twisted only 45”, then the calculated value of 3.56 D falls well outside the experimental limits. A MM of +0.35 D has been publishedlg, but even if present in 3, would not be detectable because of symmetry. For compound 4, best agreement was obtained for the conformation in which the aldehyde group is in the ring-plane, and the nitro group flanking it has been twisted completely out of this plane by steric interference from the carbonyl oxygen. An alternative conformation (aldehyde group out-of-plane, nitro groups coplanar) leads to higher calculated values, as do intermediate twist angles. No MM for the aldehyde group was detected. If in 5 the nitro groups are coplanar with the ring, a very high MM (+ 1.41 D)
70 would have to be present to explain the observed moment. By considering them twisted only through 45” (likely in view of the chlorine’s intermediate -size) a value of + 1.00 D is obtained, closer to that reported previously (+0.41 D)lQ and one that yields consistently rational agreement for other compounds in this series. For example, the application of a 45” nitro twist in 6 leads to a value of +0.16 D for the MM of the CF, group, compared with that of -0.2 reported earlierI 111 - the absence of nitro groups. Likewise, in the methyl benzoate 7, similar assumptions lead to excellent agreement, considering the ester group as having no MM, according to expectation. Rotation of the carboxymethyl group does not alter the calculated value, so that no preferred orientation for it can be deduced. For the difluoro compound 9, the small size of the fluorines should not cause out-of-plane twisting of the nitro groups, and the measured moment can be matched by inclusion of a MM for fluorine of +0.20, lower than that reported earlier (+0.41 D)l’. Using this value in compounds 10 and 11,together with a conformation in which the nitro group ortho to the chlorine or bromine has been twisted out of the plane by 45”, results in MM’s for chlorine and bromine of 0.60. Comparing these compounds with 5-8, in which the MM for chlorine is 1.00, it may be concluded that two nitro groups ortho to chlorine elicit from it a higher MM than when they are placed ortho and para to it. This is somewhat surprising since MO calculations24-26 have shown that a nitro group more effectively stabilizes a positive charge para to it than when ortho to it. However, they agree well with reported MM’s of f-O.41 for chlorine and 0.43 for bromine”. The amine 12 may be considered to exist preferentially in the conformation in which the amino group and the nitro group ortho to it are coplanar since a hydrogen bond may be expected to form between them. Using this conformation and the previous assumptions, good agreement is obtained with a MM of + 1.30 D for the amino group, somewhat larger that the + 1.02 D value reported previously (ref. 19). In the naphthalene compound 13, inclusion of an interannular moment’, as well as a 45” twisted nitro group, leads to agreement between calculated and measured DM’s only if a MM of + 1.00 D is used for chlorine. This is probably due to a larger contribution from the corresponding polar canonical forms than is present in the analogous benzene. In the biphenyl14, rotation of the Cr-Cr. bond (4) results in DM’s of 0.00, 1.54.and 2.18 D for # = 0” (anti), 90” (skew) and 180” (syn), respectively. The measured va!ue of 2.01 D corresponds to + = 135O,that is, 45” from the syn conformation: This is rather -unusual since dipole-dipole repulsion between the fluorines %ouId make (p = 45" the preferred twist conformation. In the 4 = 135” conformation the fiuorihes are approximately 3 A from each other, and a dipoleinduced dipole interaction; between them (L&d& forces) might be a-stabilizing factor ‘as was concluded-.&her for 2;2’-dichlo~obinhenylz7.
i a”
72 Finally, in compound 15 good agreement is found, without invoking any MM, for the anti conformation of the methoxyl group. This conformation has been substantiated for simiIar compounds in which the iodine is replaced by F, Cl and Brz8. Application of ihe MW’s to literature values A literature search” brought to light 20 additional dinitrobenzenes whose DM’s have been measured previously. These are presented in Fig. 2 together with another 8 (L21-L28) m-dinitrobenzenes measured earlier by us1 -4. As before, the measured values have been matched by calculated moments taking into consideration conformational preferences and deduced MM’s, and are in general accord with previous results.
CONCLUSIONS
The MM’s deduced from over forty m-dinitrobenzenes (Figs. 1 and 2) for different substituent groups at different locations are gathered in Table 3, together with values previously published. They are conveniently divided into three groups as shown: (a) halogens (b) oxygen and nitrogen substituents and (c) carbon sub-& stituents. A comparison of the halo-m-dinitrobenzenes with the halobenzenes themselves shows that the mesomeric interaction is only slightly larger in the former., As expected, the MM is large when the halogen substituent is ortho and/or para and zero when meta to them. In the I-X-2,4_dinitrobenzenes, where a comparison is possibIe between all four halogens, our results indicate that the MM’s are equal for Cl, Br and I. Our MM for F is smaller than that previously reported and may be due to secondary effects (field effect or p-x repulsions3’) acting in an opposing sense. The larger MM value deduced for 13 is expected on account of the greater number of canonical structures possible. This would indicate that p-x and/or d-z interaction is not extensive. In contrast to halogen substituents the MM’s found for oxygen and nitrogen substituents on m-dinitrobenzenes are considerably larger than previously reported for monosubstituted benzenes. This implies _that electron delocalization in these groups with their “pure” pmorbitals is much more extensive. As above mesomerism runs parallel to &ho-para/meta-directing abilities. In the third group of carbon substituents nq enhancement of MM was found over the monbsubstituted be-nes. The discrepancy found for the CF, group m_aywelf be within the experimentalerror: One apparent exception is the MM of Y 1.OOD attributable &qa phenyl stibstituent. However, this is in line with effective k-x i&era&ion as noted above atid tis f&nd in naphthaiene derivatives.
73 TABLE 3 SUMMARY
OF MESOMERIC
MO~IEN-R?
-N$JNo2
““a$
-“qN-&
R_~~
R
R R
-
F Cl Br I
-t- l-00(4) ? -
OMe
+1.22(2)
-l-z.oo(r)r, +2.30(l) t-2.500, +- 1.54h
OC&Is OH
+122(l) + l.oo(2) +1.10(l) -
f2.QtW) +1.00, i-1.80’ +1.30(l), +1.75(l) -1.00
NHz
NMez N3
CHz CI=S CMes CHO COOMe C=N
CsHr
-
?,0(2) T(2)
-
+0.20(6)b +0.60(2), +0.60(2) +0.60(l)
?+0.50(1) +0.80(l) O(1) O(1) -J-1.00
+1.00(l)=
AL
-
Lit.&
O(1) O(2) -
f0.41 -i-o.41 +0.43 -l-0.50
f0.41 -to.41 f0.43 j-o.so
f0.80(1)
-t-O.8
+0.96
f0.30 to.6 f1.02 Cl.66 -
j0.96
f0.20(1) f0.40(1) -+-0.16(l) -
t-o.35 -0.2 - 0.42
j-1.02 f1.66 j-o.35 -0.20
-0.45
9 The values calculated in columns 2,3 and 4 (our data) arc based upon monosubstituted benzene group moments. The vahtes quoted in the last two columns are the monosubstituted aIkyi-ary1 differences. A positive value represents a +M substituent, i.e. one that releases electrons into the ring; the total available MM is quoted in Debyes. b Number of compounds (Figs. 1 and 2) used to obtain the value is given in paren:heses. c Ref. 6, p. 395. d Ref. 19. ’ I-Chloro-2,4dinitronaphthaiene (13). ' This value for 2,4-dinitroanisole, apparently discrepant from the 1.31 D reported earlier by us’, is due to the fact that to facilitate comparison, it is here considered as acting along the O-C1 bond. * I-Methoxy-2,edinitronaphthalene (I_,%$). h 2,4,6,8-Tetranitrobenzofuran (~2g). ’ 2,CDinitronaphthol.
Overall, our work indicates that large MM’s are found only when J+G electron delocalization is possible. The smaller MM’s found for the halogen substituents indicate that in these, the p electrons are not as available for delocali&ion, because of hybridization effects. The above is undoubtedly a reflection of the great stability of the m-dinitrobenzene system (considered as an extended “a,-ene”) and its reluctance to engage in mesomeric interactions unless these result in a direct e&nsio&of the it network24- 26_
74 REFERENCES 1 2 3 4 5
P. A. LEHMANN F. AND D. M. MCEACHERN B., J. Mol. Strzrcture, 7 (1971) 253. D. M. MCEACHERN B. AND P. A. LEHMANN F., J. Mol. Structure, 7 (1971) 267. P. A. LEHMANN F. AND D. M. MCEACHERN B., J. Mol. Structure, 7 (1971) 277. D. M. MCEACHERN B. AED P. A. LEHMANN F., J. Mol. Structure, 11 (1972) 127. C. K. INGOLD, Srructztre and Mechanism in Organic Clzemistry, Cornell University Press. Ithaca, N.Y., 1953, pp. 73 ff. 6 L. E. SUTTON, in E. A. BRAUDE AND F. C. NACHOD (editors), Dipole Moments in Determination of Organic Structures by PhysicaI Methods, Academic Press, New York, 1955, pp. 373425. 7 H. GILMAN (editor), Organic Syntheses ,Collective Volume I, Wiley, New York, 1932, p. 219. 8 Aldrich Chemical Company, Inc., Milwaukee, Wise., U.S.A. 9 Eastman Kodak Company, Rochester, N-Y., U.S.A. 10 Pierce Chemical Company, Inc., Rockford, Ill., U.S.A. 11 K and K Laboratories, Inc., Plainview, N-Y., U.S.A. 12 H. ZAHN AND H. ZUBER, Chem. Ber., 86 (1953) 177. 13 I. F. HALVER~TADTAND W. D. KUMLER, J. Amer. Chem. Sot., 64 (1942) 2988. 14 P. A. LEHMANN F. AND D. M. MCEACHERN B., Rev. Iber. Ed. Qzdm., 3 (1969-1970) 92._ 15 C. P. SMYTH, Dielectric Behavior andStrzrcture, McGraw-Hill, NewYork, 1955, p- 253 and 314. 16 V. I. MINKIN, 0. A. OSIPOV AND Y. A. ZHDANOV, Dipole Moments in Organic Cizenzistry, Plenum Press, New York, 1970, p. 91. 17 M. T. ROGERS,J. Amer. Chem. Sot., 77 (1955) 3681. 18 J. TROX-~ER,Can. J. Chenz., 37 (1959) 1487 and 39 (1961) 1638. 19 Ref. 16, p. 205. 20 Ref. 15, p_ 317. 21 V. BALIAH AND M. UMA, Natzrr,vissenschaften,45 (1958) 512. 22 R. FOSTER, Organic Charge-Transfer Complexes, Academic Press, London and New York, 1969, p. 114. 23 G. BRIEGLEEAND J. CZEKALLA, Z. Efek0ochem., Ber. Bunsenges. Plzys. Chem. 58 (1954) 249 and 59 (1955) 184. 24 P. CAVENG, P. B. FISCHER,E. HEILBRONNER,A. L. MILLER AND H. ZOLLINGER, Helv. Chinz. Acta, 50 (1967) 548. 25 H. UEDA, N. SAKABE, J. TANAKA AND A. FURUSAKI, Bull. Chem. Sot. Jap., 41 (1968) 2866. 26 H. HOSOYA, S. HO~~YA AND S. NAKAGURA, Theor. Chim. Acta, 12 (1968) 117. 27 G. C. HAMPSON AND A. WEISSBERGER,J. Amer. Chem. Sot., 58 (1936) 2lll. 28 W. F. ANZILO~~I AND B. C. CURRAN, J. Amer. Cizem. Sot., 65 (1943) 607. 29 A. L. MCCLELLAN, Tables of Experimental Dipole Moments, W. H. Freeman and CO., San Francisco, 1963. 30 A. R. KATRITZ~Y AND R. D. TOPSOM, Angew. Chenz. fntl. Ed. Engl., 9 (1970) 87.
CONFORMATIONS XV.
MESOMERIC
OF HIGHLY MOMENTS
- Printed
HINDERED
in The Netherlands
ARYL
ETHERS
IN m-DMITROBENZENES*t
P. A. LEHMANN F. AND D. M. MCEACHERN B. Department of Chemistry, Centerfor Research and Advanced Stadies, National Polytechnic 14-740, Mexico 14, D.F. (Mexico)
Institute
A.P.
(Received
15 December
1972)
ABSTRACT
The dipole moments of fourteen m-dinitrobenzenes with different substituents (OMe, CN, CH3, CHO, Cl, CF,, COOMe, F, Br, NH2) in various positions were measured in order to study mesomeric interactions. Inclusion of conformational considerations permitted estimates to be made for the mesomeric moments of these groups at given positions. The values obtained could be applied with consistent results to a total of forty m-dinitrcbenzenes, including others previously reported. The mesomeric moment increments deduced ,compared with substituted benzenes possessing no nitro groups, were found to be only slightly larger than in non-nitro substituted benzenes (compared with substituted alkanes), which would indicate a reluctance of the extended 7r system of m-dinitrobenzene
to interact with other substituents present, in any way other than by direct extension of the x network. One apparent case of intermolecular electron formation was detected. The measured dipole moment a planar anti conformation for the methoxyl group.
donor-acceptor of o-iodoanisole
complex indicates
INTRODUCTION
In earlier work on the preferred conformations of highly hindered aryl ethers, determination of their dipole moments’-4 revealed the existence, in the * Portions of this work were presented & the Sixth Mexican Congress of Pure and Applied Chemistry heId in CuliacBn, SinaIoa, March lO-13,1971; cf. Rev. Sot. Quim. M&x., 15 (1971) 17. + Part XIV, P. A. Lehmann F. and A. Ciurlizza G., Reu. Latinomamer, Quim., in press.
64 dinitrophenoxy moiety, of a considerable electronic delocalization from the oxygen into the ring, equivalent to mesomeric moments of 1.2-1.3 D. Halogens, which are like the ether oxygen in that they have pairs of non-bondingp electrons capable of being delocalized upon demand through p-n interactions (Ingold’s f M effect)5, have also been shown to display analogous mesomeric moment@. In view of this it was of interest to determine the electron donating capabilities of halogens and other groups when substituted at different positions of m-dinitroarenes. The compounds whose dipole moments were determined are shown in Fig. 1.
“N-qO’
v-$&J;02q5J~
02Nyg2 ‘Me
24J720.03
2:
1.60~0.12
02N&N02
2 : 3.0 IA 0.05
complex)
TEDA
(4.19)
+$$02
(2.72)
r
(2.72)
p431=Ql5 (4.28)
E:
- CN
Me
p269foD9
270*
0.12
s
(2.75)
2.01*0.29 (291)
0.07
(5.67)
02+$N02
02N$$N02
6
2.: 2.72kO.34
4 : 522* -
(3.15)
::53
io.10
( 5.53 1
I,5 ‘. 2.50=
0.14
(2.55)
Fig. 1. %ructures of the compounds studied, -showing their measured dipole moments (D)* their calculated dipole moments (in parentheses) and the deduced mesomeric moments. Curved arrows not otherwiyz marked indicate a twist an$e of 45“.
65 EXPERIMENTAL
The compounds studied were obtained and purified as follows: 1. Synthesized 7 from sym-trinitrobenzene in 60 oA yield; recrystallized from MeOH to m-p. 102-104 “C. 2. Commercial product’ recrystallized from EtOH with charcoal treatment to m.p. 131-132 “C. 3. Commercial product9 recrystallized twice from aq. EtOH to m-p. 81-82 “C. 4. Commercial product’ recrystallized three times from HOAc to m-p. 121-122 “C. 5. Commercial product’ recrystallized from aq. EtOH with charcoal treatment, then from 2-heptanone to m-p. 86-87 “C. 6. Commercial product” recrystallized from aq. EtOH to m-p. 55-56 “C. 7. Commercial product’ ’ recrystallized twice from aq. EtOH to m.p. 98-100 “C. 8. Commercial product’ recrystallized twice from aq. EtOH to m.p. 141-143 “C. 9. Commercial product9 recrystallized with charcoal treatment from CHCl, to m-p. 74-76 “C. 10. Commercial product’ ’ recrystallized from EtOH to m.p. 183-187 “C. 11.Commercial product” recrystallized from EtOH with charcoal treatment to m-p. 88-91 C.. 12. Commercial product’ ’ recrystallized twice from EtOH to m-p. 184-186 “C!.
13. Commercial product9 recrystallized from EtOH to m-p. 146-147 “C. 14. Synthesizedi in 61 ‘A yield and recrystallized twicefromaq. CH,COCH3 to m-p. 218-221 OC. 15. Commercial product9 purified by careful fractional distillation after which it had m-p. 8-10 “C. All samples had a purity greater than 99.95 % determined by differential scanning calorimetry as described previously l. The dipole moments were obtained by the method of Halverstadt and Kumler” using an especially convenient pycnometer for the density measurements14. Further details are given in Part Il.
RESULTS
The data-from the dipole moment (DM) measurements (Table 1) were used to calculate the results (Table 2) by the procedure previously reported’.
66 TABLE
1
CONSTANTAND SPECIFICVOLUME
DIPOLEMOMENT~~EASURE~~:WEIGHTFRAC~~N~~~SUSDIELE~IC
I03UZ
El2
3,SDinitroanisole 0.000 0.361 0.692 1.076 1.486 1.821 2.161 2.507
2.271 2.275 2.278 2.282 2.286 2.289 2.292 2.296
2.276 (2.281)” 2.282 2.292 2.300 2.303 2.310 2.316
1.14521 I .14501 1.14489 1.14472 1.14444 l-14441 1.14408 1.14396 (4)
1.14496 (1.14472)” 1.14477 (1.14423) 1.14406 1.14384 1.14379 1.14345
Methyl 4-chloro-3,5-dinittobenzoate (7) 0.000 0.333 0.667 1.073 1.465 1.833 2.145 2.472
2.275 2.277 2.280 (2.284)’ 2.286 2.288 2.289 2.293
1.14506 1.14472 1.14454 1.14421 1.14400 1.14391 1.14370 1.14359
I-ChIoro-5-fluoro-2+dinitrobenzene (10) 0.000 0.342 0.677 1.078 1.490 I.837 2.157 2.507
2.276 (2.279)” 2.279 2.281 2.282 2.283 2.284 2.286
1.14486 1_14464 1.14458 1.14425 1.14400 1.14390 1.14370 1.14340
1-ChIoro-2,4_dinitronaphthaIene (13) 0.000 0.355 0.796 1.029 1.366 1.605 2.044 2.506
2.276 (2.280)a (2.280)’ 2.283 2.285 2.288 2.291 2.296
&I2
103w2
"12
(1.14481)J 1.14419 1.14416 1.14407 (1.14393)’ 1.14371 1.14346
0.000 0.332 0.710 1.081 1.479 I.834 2.199 2.514
2.267 2.267 2.267 2.268 2.269 2.269 2.270 (2.268)”
0.000 0.348 0.685 1.102 1.464 1.829 2.161 2.522
2.273 2.277 2.28 I 2.283 2.288 2.291 2.294 2.297
0.000 0.309 0.685 1.062 1.474 1.802 2.166 2.501
1.14521 1.I4509 1.14475 1.14460 1.14436 I.14427 1.14422 1.14394
I-Chloro-2,6_dinitrobenzene
‘512
“12
Dinitromesitylene (3)
3,5-Dinitrobenzonitrile (2)
(1)
2,6-Dinitrobenzaldehyde 0.000 0.343 0.333 1.073 1.465 1.791 2.153 2.601
10’02
“12
(5)
1.14501 I.14486 1.14458 I.14447 1.14420 1.14407 1.14397 1.14376
(2.276)9 2.280 2.282 2.284 2.286 2.287 2.289 2.290
1.1451 I.1445 1.144t 1.144: 1.144: 1.144: 1.144: 1.1441
4-Chloro-I-trifluorometh] 3,5-dinitrobenzene (6) 0.000 0.319 0.689 1.069 I.459 I.821 2.187 2.512
2.276 2.277 2.278 2.278 2.278 2.279 2.279 2.280
1.1451 1.144: 1.144; l-144! 1.144: 1.1441 1.1435 I.1431
4-Chloro-3,5-dinitrobenzonitrile (8) 1.14511 0.000 2.274 1.14483 0.364 2.274 1.14481 0.696 2.275 1.14448 1.071 2,274 1.14432 1.481 2.276 1.14417 1.826 2.275 1.14387 2.191 2.276 I.14379 2.512 2.275
I,S-DifIuoro-2,4-dinitrobe (9) 2.274 I.1445 0.000 (2.273)” 1.1445 0.330 2.277 1.14lt 0.676 (2.281)p 1.144 1.072 2.282 1.144: 1.471 2.281 1.144I 1.826 2.284 1.1431 2.156 2.284 1.143f 2.474
I-Bromo-5-fluoro-2,4-dinitrobenzene (11)
5-FIuoro-2,4_dinitroaniIin
0.000 0.315 0.669 1.076 1.496 1.841 2.182 2.518
2.269 2.270 2.272 2.274 2.275 2.275 2.277 2.277
1.14535 1.14521 1.14487 1.14466 1.14428 1.14409 1.14389 1.14367
2,2’-Difluoro-3,3’,5,5’-tetranitrobiphenyl (14) 0.000 0.324 0.687 1.104 1.501 1.858 2.184 2.554
2.272 2.273 (2.274)* 2.273 2.275 (2.276)a 2.276 2.275
1.14500 1.14483 1.14452 1.14428 (1.14393)a 1.14393 1.14366 1.14345
0.000 0.324 0.678 1.076 I .490 1.838 2.163 2.515
2.269 (2.273)” 2.282 2.289 2.296 (2.306)= 2.308 (2.314)”
1.145: 1.145t I.1441 1.1441 1.144: 1.144 I-144( 1.143’
2-Iodoanisole (15) 0.000 0.393 0.671 1.123 1.402 1.842 2.174 2.477
2.274 2.274 (2.274)” 2.277 (2.275)= 2.279 2.280 2.282
1.145: 1.144: 1_144 1.144: 1.144: 1.144( 1.143’ 1.143
67 TABLE 2 DIPOLE
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
MOMENT
2.272 2.266 2.279 2.276 2.274 2.277 2.275 2.274 2.275 2.276 2.270 2.270 2.275 2.272 2.274
RESULTS
0.87320 0.87323 0.87329 0.87341 0.87338 0.87329 0.87342 0.87330 0.87334 0.87346 0.87310 0.87321 0.87384 0.87339 0.87327
24.62 3.92 12.80 38.63 24.22 4.00 23.33 1.25 10.37 10.39 10.67 42.96 25.94 6.54 9.21
0.962
0.949 0.746 1.106 0.970 1.402 1.472 1.174 1.097 1.216 1.760 0.970 0.888 2.102 1.372
400.32 94.51 236.79 600.12 395.26 108.24 386.47 60.25 IS&.39 190.70 193.75 666.66 439.77 158.91 173.25
43.98
41.98 51.49 42.16 42.38 46.35 53.27 46.82 37.09 42.15 45.03 40.73 59.18 75.93 45.56
a a and /I are the slopes of the dielectric constant and density, respectively, versus
4.17*0.03 1.60f0.12 3.Olf0.05 5.22&O-07 4_15*0.07 1.74f0.12 4.03 &0.07 0.81 &to.26 2.72&O-34 2.69 &to.09 2.70f0.12 5.53f0.10 4.31f0.15 2.01 f0.29 2.50&0.14
mole fraction.
A!XUhSPTIONS
In order to estimate the DM for a particular compound the following assumptions were made: (i) The group moments (D) used were15: NO, = 3.98, OMe = 1.25 (55”), CN = 4.00, Me = -00.40, CHO = 2.76 (55”), Cl = 1.58, CF, = 2.56, COOMe = 1.83 (70”), F = 1.46, Br = 1.54, NH, = 1.53 (142”), I = 1.30, N, = 1 44r6 CMe -oo.4017. (ii) Nitro griu;s are twisted out of the plane of the ring by 90” when ortho to two methyl groups, a carbonyl oxygen, and iodine; they are twisted only 45” when ortho to methyl, chlorine and bromine. (iii) The mesomeric moment (MM) of a substituent is directed along the bond joining the substituent to the ring, taken as positive when directed into the ring. Comparison of the measured and calculated (in parentheses) DM’s, together with deduced MM’s and the preferred conformations are shown in Fig. 1.
DISCUSSION
The measured DM’s (D) could best be rationalized by considering the conformations and including the deduced MM’s shown in Fig. 1. The conformational assumptions where: (a) that nitro groups are coplanar with the ring when flanked
68 by hydrogen or fluorine; (b) that they are twisted 45” out of the ring plane when flanked by one methyl, chlorine or bromine substituent; and (c) that they are twisted 90” when flanked by two methyl groups, a carbonyl oxygen or iodine. These nominal values of twist are reasonable in view of X-ray crystallographic determinations on related compounds”. As beforele4, the group moment of a nitro substituent is 3.15 +0.83 cos% where 0 is the out-of-plane twist angle. When a nitro group has ortho to it a substituent with which it can hydrogen bond (i.e. OH or NH2), it is assumed that the preferred conformation is the one in which all the atoms involved in the hydrogen bonding are coplanar with the ring. Some of these results merit individual comment. In the anisole (1) the MM = 0.80 D is, as expected, smaller than that found for the other anisoles in which the methoxyl group is ortho and/or para to the nitro groups. Nevertheless, this relatively large MM argues for the existence of a methoxyl group which is coplanar with the ring. The measured DM’s of 2 and 8 cannot be explained on the basis of interaction moments. If the cyano group exhibits a -M effect, as has been deducedlg, then structures like A must make an important contribution in 2:”
Then in 8, structure B should make an even greater contribution:
8
B
and inclusion of the MM’s deduced from 5 for Cl and for CN from 2 should lead to a calculated DM of 1.44 or greater, whereas that measured was only 0.8 I_ Alternatively, one might consider that the cyano group might, upon demand from the nitro groups, exhibit a +M effect due to the stability of forms such as C and D:
69
In this case a MM of + 1.60 would have to be present to explain the observed moment. Similarly in 8, analogous structures would be expected to contribute more importantly, especially if expansion of the halogen’s octet is considered possible” : %N+No~N+NO~+-$N%
;
W-&N%
$3
d
NO
io
CN
However, for 8 a calculated moment of 2.74 D is obtained without a MM contribution from Cl, and of 1.74 D with one of 1 .O D. Both are greater than the observed moment of 0.81 D, from which this type of mesomerism can be excluded. In view of the fact that neither of these alternatives is viable, we feel that the best explanation is as follows: 2 is an excellent acceptor in EDA complexes22, and it may be participating in the formation of such a complex with the benzene (solvent).
The measured DM of 1.58 D is in the range of values previously reported for similar intermolecular EDA complexes 23 _ Support for this comes from the fact that in 8 the chlorine partially satisfies the ring’s electron deficiency by its mesomerit contribution, rendering it a poorer acceptor and resulting in the smaller intermolecular DM of 0.8 1 D observed for it. A MM of -0.45 D1’ may well be present and partially account for the observed moment. For compound 3, the observed DM can be rationalized best by considering that the nitro groups, being flanked by two bulky methyl groups, are rotated 90” out of the ring plane. The moment calculated for this structure is 3.15 D. If the nitro groups are twisted only 45”, then the calculated value of 3.56 D falls well outside the experimental limits. A MM of +0.35 D has been publishedlg, but even if present in 3, would not be detectable because of symmetry. For compound 4, best agreement was obtained for the conformation in which the aldehyde group is in the ring-plane, and the nitro group flanking it has been twisted completely out of this plane by steric interference from the carbonyl oxygen. An alternative conformation (aldehyde group out-of-plane, nitro groups coplanar) leads to higher calculated values, as do intermediate twist angles. No MM for the aldehyde group was detected. If in 5 the nitro groups are coplanar with the ring, a very high MM (+ 1.41 D)
70 would have to be present to explain the observed moment. By considering them twisted only through 45” (likely in view of the chlorine’s intermediate -size) a value of + 1.00 D is obtained, closer to that reported previously (+0.41 D)lQ and one that yields consistently rational agreement for other compounds in this series. For example, the application of a 45” nitro twist in 6 leads to a value of +0.16 D for the MM of the CF, group, compared with that of -0.2 reported earlierI 111 - the absence of nitro groups. Likewise, in the methyl benzoate 7, similar assumptions lead to excellent agreement, considering the ester group as having no MM, according to expectation. Rotation of the carboxymethyl group does not alter the calculated value, so that no preferred orientation for it can be deduced. For the difluoro compound 9, the small size of the fluorines should not cause out-of-plane twisting of the nitro groups, and the measured moment can be matched by inclusion of a MM for fluorine of +0.20, lower than that reported earlier (+0.41 D)l’. Using this value in compounds 10 and 11,together with a conformation in which the nitro group ortho to the chlorine or bromine has been twisted out of the plane by 45”, results in MM’s for chlorine and bromine of 0.60. Comparing these compounds with 5-8, in which the MM for chlorine is 1.00, it may be concluded that two nitro groups ortho to chlorine elicit from it a higher MM than when they are placed ortho and para to it. This is somewhat surprising since MO calculations24-26 have shown that a nitro group more effectively stabilizes a positive charge para to it than when ortho to it. However, they agree well with reported MM’s of f-O.41 for chlorine and 0.43 for bromine”. The amine 12 may be considered to exist preferentially in the conformation in which the amino group and the nitro group ortho to it are coplanar since a hydrogen bond may be expected to form between them. Using this conformation and the previous assumptions, good agreement is obtained with a MM of + 1.30 D for the amino group, somewhat larger that the + 1.02 D value reported previously (ref. 19). In the naphthalene compound 13, inclusion of an interannular moment’, as well as a 45” twisted nitro group, leads to agreement between calculated and measured DM’s only if a MM of + 1.00 D is used for chlorine. This is probably due to a larger contribution from the corresponding polar canonical forms than is present in the analogous benzene. In the biphenyl14, rotation of the Cr-Cr. bond (4) results in DM’s of 0.00, 1.54.and 2.18 D for # = 0” (anti), 90” (skew) and 180” (syn), respectively. The measured va!ue of 2.01 D corresponds to + = 135O,that is, 45” from the syn conformation: This is rather -unusual since dipole-dipole repulsion between the fluorines %ouId make (p = 45" the preferred twist conformation. In the 4 = 135” conformation the fiuorihes are approximately 3 A from each other, and a dipoleinduced dipole interaction; between them (L&d& forces) might be a-stabilizing factor ‘as was concluded-.&her for 2;2’-dichlo~obinhenylz7.
i a”
72 Finally, in compound 15 good agreement is found, without invoking any MM, for the anti conformation of the methoxyl group. This conformation has been substantiated for simiIar compounds in which the iodine is replaced by F, Cl and Brz8. Application of ihe MW’s to literature values A literature search” brought to light 20 additional dinitrobenzenes whose DM’s have been measured previously. These are presented in Fig. 2 together with another 8 (L21-L28) m-dinitrobenzenes measured earlier by us1 -4. As before, the measured values have been matched by calculated moments taking into consideration conformational preferences and deduced MM’s, and are in general accord with previous results.
CONCLUSIONS
The MM’s deduced from over forty m-dinitrobenzenes (Figs. 1 and 2) for different substituent groups at different locations are gathered in Table 3, together with values previously published. They are conveniently divided into three groups as shown: (a) halogens (b) oxygen and nitrogen substituents and (c) carbon sub-& stituents. A comparison of the halo-m-dinitrobenzenes with the halobenzenes themselves shows that the mesomeric interaction is only slightly larger in the former., As expected, the MM is large when the halogen substituent is ortho and/or para and zero when meta to them. In the I-X-2,4_dinitrobenzenes, where a comparison is possibIe between all four halogens, our results indicate that the MM’s are equal for Cl, Br and I. Our MM for F is smaller than that previously reported and may be due to secondary effects (field effect or p-x repulsions3’) acting in an opposing sense. The larger MM value deduced for 13 is expected on account of the greater number of canonical structures possible. This would indicate that p-x and/or d-z interaction is not extensive. In contrast to halogen substituents the MM’s found for oxygen and nitrogen substituents on m-dinitrobenzenes are considerably larger than previously reported for monosubstituted benzenes. This implies _that electron delocalization in these groups with their “pure” pmorbitals is much more extensive. As above mesomerism runs parallel to &ho-para/meta-directing abilities. In the third group of carbon substituents nq enhancement of MM was found over the monbsubstituted be-nes. The discrepancy found for the CF, group m_aywelf be within the experimentalerror: One apparent exception is the MM of Y 1.OOD attributable &qa phenyl stibstituent. However, this is in line with effective k-x i&era&ion as noted above atid tis f&nd in naphthaiene derivatives.
73 TABLE 3 SUMMARY
OF MESOMERIC
MO~IEN-R?
-N$JNo2
““a$
-“qN-&
R_~~
R
R R
-
F Cl Br I
-t- l-00(4) ? -
OMe
+1.22(2)
-l-z.oo(r)r, +2.30(l) t-2.500, +- 1.54h
OC&Is OH
+122(l) + l.oo(2) +1.10(l) -
f2.QtW) +1.00, i-1.80’ +1.30(l), +1.75(l) -1.00
NHz
NMez N3
CHz CI=S CMes CHO COOMe C=N
CsHr
-
?,0(2) T(2)
-
+0.20(6)b +0.60(2), +0.60(2) +0.60(l)
?+0.50(1) +0.80(l) O(1) O(1) -J-1.00
+1.00(l)=
AL
-
Lit.&
O(1) O(2) -
f0.41 -i-o.41 +0.43 -l-0.50
f0.41 -to.41 f0.43 j-o.so
f0.80(1)
-t-O.8
+0.96
f0.30 to.6 f1.02 Cl.66 -
j0.96
f0.20(1) f0.40(1) -+-0.16(l) -
t-o.35 -0.2 - 0.42
j-1.02 f1.66 j-o.35 -0.20
-0.45
9 The values calculated in columns 2,3 and 4 (our data) arc based upon monosubstituted benzene group moments. The vahtes quoted in the last two columns are the monosubstituted aIkyi-ary1 differences. A positive value represents a +M substituent, i.e. one that releases electrons into the ring; the total available MM is quoted in Debyes. b Number of compounds (Figs. 1 and 2) used to obtain the value is given in paren:heses. c Ref. 6, p. 395. d Ref. 19. ’ I-Chloro-2,4dinitronaphthaiene (13). ' This value for 2,4-dinitroanisole, apparently discrepant from the 1.31 D reported earlier by us’, is due to the fact that to facilitate comparison, it is here considered as acting along the O-C1 bond. * I-Methoxy-2,edinitronaphthalene (I_,%$). h 2,4,6,8-Tetranitrobenzofuran (~2g). ’ 2,CDinitronaphthol.
Overall, our work indicates that large MM’s are found only when J+G electron delocalization is possible. The smaller MM’s found for the halogen substituents indicate that in these, the p electrons are not as available for delocali&ion, because of hybridization effects. The above is undoubtedly a reflection of the great stability of the m-dinitrobenzene system (considered as an extended “a,-ene”) and its reluctance to engage in mesomeric interactions unless these result in a direct e&nsio&of the it network24- 26_
74 REFERENCES 1 2 3 4 5
P. A. LEHMANN F. AND D. M. MCEACHERN B., J. Mol. Strzrcture, 7 (1971) 253. D. M. MCEACHERN B. AND P. A. LEHMANN F., J. Mol. Structure, 7 (1971) 267. P. A. LEHMANN F. AND D. M. MCEACHERN B., J. Mol. Structure, 7 (1971) 277. D. M. MCEACHERN B. AED P. A. LEHMANN F., J. Mol. Structure, 11 (1972) 127. C. K. INGOLD, Srructztre and Mechanism in Organic Clzemistry, Cornell University Press. Ithaca, N.Y., 1953, pp. 73 ff. 6 L. E. SUTTON, in E. A. BRAUDE AND F. C. NACHOD (editors), Dipole Moments in Determination of Organic Structures by PhysicaI Methods, Academic Press, New York, 1955, pp. 373425. 7 H. GILMAN (editor), Organic Syntheses ,Collective Volume I, Wiley, New York, 1932, p. 219. 8 Aldrich Chemical Company, Inc., Milwaukee, Wise., U.S.A. 9 Eastman Kodak Company, Rochester, N-Y., U.S.A. 10 Pierce Chemical Company, Inc., Rockford, Ill., U.S.A. 11 K and K Laboratories, Inc., Plainview, N-Y., U.S.A. 12 H. ZAHN AND H. ZUBER, Chem. Ber., 86 (1953) 177. 13 I. F. HALVER~TADTAND W. D. KUMLER, J. Amer. Chem. Sot., 64 (1942) 2988. 14 P. A. LEHMANN F. AND D. M. MCEACHERN B., Rev. Iber. Ed. Qzdm., 3 (1969-1970) 92._ 15 C. P. SMYTH, Dielectric Behavior andStrzrcture, McGraw-Hill, NewYork, 1955, p- 253 and 314. 16 V. I. MINKIN, 0. A. OSIPOV AND Y. A. ZHDANOV, Dipole Moments in Organic Cizenzistry, Plenum Press, New York, 1970, p. 91. 17 M. T. ROGERS,J. Amer. Chem. Sot., 77 (1955) 3681. 18 J. TROX-~ER,Can. J. Chenz., 37 (1959) 1487 and 39 (1961) 1638. 19 Ref. 16, p. 205. 20 Ref. 15, p_ 317. 21 V. BALIAH AND M. UMA, Natzrr,vissenschaften,45 (1958) 512. 22 R. FOSTER, Organic Charge-Transfer Complexes, Academic Press, London and New York, 1969, p. 114. 23 G. BRIEGLEEAND J. CZEKALLA, Z. Efek0ochem., Ber. Bunsenges. Plzys. Chem. 58 (1954) 249 and 59 (1955) 184. 24 P. CAVENG, P. B. FISCHER,E. HEILBRONNER,A. L. MILLER AND H. ZOLLINGER, Helv. Chinz. Acta, 50 (1967) 548. 25 H. UEDA, N. SAKABE, J. TANAKA AND A. FURUSAKI, Bull. Chem. Sot. Jap., 41 (1968) 2866. 26 H. HOSOYA, S. HO~~YA AND S. NAKAGURA, Theor. Chim. Acta, 12 (1968) 117. 27 G. C. HAMPSON AND A. WEISSBERGER,J. Amer. Chem. Sot., 58 (1936) 2lll. 28 W. F. ANZILO~~I AND B. C. CURRAN, J. Amer. Cizem. Sot., 65 (1943) 607. 29 A. L. MCCLELLAN, Tables of Experimental Dipole Moments, W. H. Freeman and CO., San Francisco, 1963. 30 A. R. KATRITZ~Y AND R. D. TOPSOM, Angew. Chenz. fntl. Ed. Engl., 9 (1970) 87.