Infrared study on electrolyte solutions in acetone part III. reactivity of acetone in aldolic condensation

Journal of Molecular Structure, 26 (1975) 289-295 @Elsevier Scientific Publishing Company, Amsterdam -

289 Printed in The Netherlands

INFRARED STUDY ON ELECTROLYTE SOLUTIONS IN ACETONE PART III. REACTIVITY OF ACETONE IN ALDOLIC CONDENSATION

JOLANTA

BUKOWSKA

and ZBIGNIEW

KBCKI

Laboratory of Intermolecular Interactions, Institute of Fundamental Problems of Chemistry, Warsaw University, Warszawa (Poland) (Received 10 June 1974)

ABSTRACT Infrared spectra of perchlorate, halide and nitrate solutions in acetone are studied. The CO and CCC band frequency changes are discussed in terms of the electronic structure of acetone molecules complexed with cation and anion. The correlation between the shift of the CO and CCC bands and the reactivity of acetone molecules in aldolic condensation reaction is given. An explanation of the anion effect in aldolic condensation is proposed.

INTRODUCTION

The mechanism of carbonyl compound condensation of the aldolic type assumes enol formation in the first stage of reaction. Such a mechanism has been postulated on the basis of halogenation, deuteration and racemisation rate measurements, carried out on various ketones. On the other hand, it was recently found [l, 21 that, in certain experimental conditions, the halogenation of simple aliphatic ketones omits the enol formation step. In this paper a different approach to reactivity of ketones is proposed. It is based on the interpretation of the spectral parameter changes in infrared spectra

of acetone

molecules

influenced

by ions.

Solutions

in which

aldolic condensation takes place as well as the chemically stable ones have been studied. There were two problems of interest; the correlation between spectral parameter changes and the reactivity of acetone molecules_ From this point of view the reactive solutions were compared with nonreactive ones. The second problem arose from the well known fact that some halides (e.g. AlC13, Al&, , TiC14, ZnClz ) catalyze aldolic condensation, although in perchlorate solutions in acetone, reaction does not usually take place [3]. Thus, since ClO, is the weakly interacting anion, compared with the halide, the effect of anion on infrared bands was studied. Some of our results have been described elsewhere [4-6]. Now, some new data are presented and their interpretation in terms of reactivity of

290

anhydrous acetone electrolyte solutions in aldolic condensation is proposed. The previous results are included in the discussion. EXPERIMENTAL

All reagents were purified and dehydrated. Details of purification and preparing the solutions are described elsewhere [4]. Zn(C104 )*, Co(C104 )z, Cu(C104 )z, Zn(N03 )* and Cu(N03 )2 solutions were obtained by the exchange reactions between appropriate halides and silver perchlorate (or nitrate) solutions. The spectra were recorded with attenuated total reflection (ATR) attachment to the Zeiss-UR-10 spectrophotometer, using a Ge reflection element. The angle of incidence 8, equal to 45”) si~i~~antly exceeded the critical angle, and ATR spectra showed no band distortion or frequency shift. RESULTS

AND DISCUSSION

The frequencies of two acetone bands: v1 (CO stretching) and v4 (CCC asymmetric stretching) were determined in various perchlorate, halide and aitrate solutions at several salt concentrations. In all the solutions investigated splittings of acetone bands were observed [4,6]. The position of the primary band, as well as that of the new one, did not depend on concentration of the electrolyte. The primary component remained at the same position as in pure acetone. These facts indicate the existence of so called spectroscopic complexes of acetone molecules with ions. Lit, Na’, Ag’, Ba2+, Sr2+, Mg2+, Co2+, Znz+ perchlorate, Li’ and Co2” bromide and zinc nitrate solutions were nonreactive. In ZnBr2, ZnC12, HgC12, Cu(C10, )* and Hg(C104 )2 acetone solutions, aldolic condensation proceeded. The spectra were always recorded immediately after preparing solutions, so that the bands of condensation products were avoided. Bands corresponding to enol tautomer were found neither in nonreactive nor reactive solutions. In order to explain the fact that some sahs catalyze condensation reactions but others do not, the relative frequency changes of v I (CO stretching) band were plotted vs. the relative frequency changes of v4 (CCC asym, stretching) band (Fig. 1). values for various Lewis acid complexes The A”oo kc0 and Av,,,f+oe with acetone calculated from the literature data [‘7--12) are also included in Fig. 1. From the Fig. it can be seen that all the solutions studied can be classified into three groups: (1) solutions of Nat, Li+, Ba2+, Sr2’ and Mg2” perchlorates, (2) solutions of Ag”, Co’” and Zn2+ perchlorates and CoBr2, and (3) solutions of Cu(Cl0, )z, Zh-KX2 and ZnBrz ; the complexes of Sk&&, AsCls, BF,, , TiC14 also belong to this group. The nonreactive solutions of

291

35.0 -

5

-0

9 \

0

25.0

-

2 a

15.0-

5.0 -

5.0

15.0

25.0

35.0

45.0

Fig. 1. The relative frequency changes of V, (CO) acetone band vs. the relative frequency changes of v4 (CCC) band in acetone solutions of electrolytes. The symbols of cations

(e.g. Na’, Ag’) denote the respective perchlorate solutions. ZnX, refers to ZnBr, and ZnCl, solutions. ZnCl, refers to the solid ZnCl, - acetone complex.

Zn(N03)2 were not taken into account in Fig. 1 since the splitting of the CCC band was not observed. Condensation reactions only take place in solutions of the third group. The points in Fig. 1 for CuClz and CU(NO~)~ solutions are placed between the second and the third group. In these solutions no products of aldolic condensation were detected, but they were unstable due to redox reactions. It can be concluded from Fig. 1 that for given values of Avccc/~ccc the relative shift of the CO band increases as the interaction between acetone molecule and ions becomes more covalent. Consider now the observed effects in terms of the changes in electronic redistribution of the acetone molecule. The alkali (Li +, Na ‘) or alkaline earth (Sr’+, Ba*‘, Mg*+) metal cations bonded to the lone electron pairs of the CO group oxygen decrease v1 frequency due to weakening of the CO bond. Thus, the hybridization of the carbonyl carbon atom becomes more tetrahedral in comparison to that of the isolated molecule, resulting in a significant increase in the CCC vibration frequency [ 131. Therefore, the small downward shift of the CO frequency and the large upward shift of the CCC frequency are observed as a result of interaction of the acetone molecule with alkali and alkaline earth metal perchlorates. Cations like Zn2+, Co*+, Cu2+, .Ag + considerably decrease the CO band frequency due to charge delocalization between the cation and CO group.

292

The comparatively small increase in the CCC frequency gives evidence that diminution of the CCC angle is probably partly compensated by induced shift of the CH bond electrons towards the carbonyl carbon atomThe solutions in which the greatest weakening of the CO bond for is detected are those in which condensation given values of Avccc/v,cc proceeds (straight line III in Fig. 1). All remaining salts for which given values cause a smaller decrease in the CO band frequency than %cc~%cc follows from straight line III, do not catalyze reactions. From the above discussion, the existence of acetone molecule complexes with acceptors such as cations or halide molecules is an essential condition for the condensation reaction to occur. The electronic structure of acetone molecules in their complexes is presumably perturbed in the following way. The CO bond is considerably weakened due to charge delocalization towards an electron acceptor involving delocalization of electrons from CC and hence CH bonds towards the carbonyl carbon atom, This favours the interaction of the carbonyl carbon of one molecule with the methyl group carbon of the other one leading to diacetone alcohol formation, according to

where A means an electron acceptor (e.g. CuL!+,ZnCl, , ZnBr, , BF, ). This model is consistent with the results of CNDO calculations [143, which showed that there is an electron displacement away from the carbonyl carbon and away from the methyl hydrogen atoms due to protonation of the acetone molecule. ANION EFFECT IN ALDOLIC

CONDENSATION

In earlier work [4,61, we found that halide ions only slightly enhance v z (CO) frequency decrease caused by cations if the LiC104 and LiBr or Co(C104 )2 and CoBr, solutions were compared. The significant effect of the anion was observed in Zn2+ and Cu’+ perchlorate, halide and nitrate solutions. The GO band frequencies in these solutions are collected in Table 1. The frequencies of new components in Zn(C104 )2 and Zn(N0, )2 solutions are nearly the same, but in the zinc halide solutions they are considerably lower.

293 TABLE

1

Frequencies of the u, (CO) band in acetone halides and nitrates Solute

“max (cm-’

solutions

of zinc and copper perchlorates,

)

Zn(ClO, 1,

1712 1688

-24

ZnWO,

1712 1687

-25

A

ZnC1,

;;;;

(1680)b

-42

(-32)b

ZnBrz

1’;;;

(1680)b

-42

(-32)b

Cu(CIC,

),

1712 1665

-47

CuPJO,

),

1712 1675

-37

1712 1688

-24

CUCI, a

aFrequencies in the solid adduct of CuCl, with acetone. bThe second component of the new band, distinguishable

only at low salt concentrations.

There are various reports in which the authors state that zinc halides in acetone are not fully dissociated [ 3, 151. Our previous results [ 5 3 concerning the integral intensities of v 1 and v3 acetone bands also indicate that Cl- and Br- anions penetrate into the solvation shell of Zn*’ cation. The charge in the anion-cation bond is expected to be delocalized towards the halide atoms, as found by N.Q.R. spectroscopy for HgClz -ketone donor-acceptor complexes [16] _ Thus, undissociated or partly dissociated zinc halide molecules having higher electron affinity than Zn’+ cations decrease the CO bond force constant to a greater extent than does the Zn’+ species. Reverse anion effect occurs in copper salt solutions. Cu(C104 )* causes the largest and CuCl* the smallest splitting of the CO band. It can be assumed that in Cu(C104 )2 acetone solutions, the solvation sphere of Cu*’ consists of the acetone molecules alone. In CuC$ solutions, (CuCl, - acetone,) [17] or (CuCL acetone2)2- [IS] complexes were supposed to exist. There is a lack of information concerning copper nitrate solutions in acetone. Thus, we recorded their infrared spectra in the 800-1800 cm-’ range. The spectra of Zn(N0, )* acetone solutions were also studied. The frequencies of the bands ascribed to NO3 species are collected in Table 2. l

294

TABLE 2 Infrared frequencies (in cm-’ ) of the nitrate species in the acetone solutions of zinc and copper nitrates

CW’Q 1% + acetone

Assignment

808

~6 (B,) bend. out of pIane

1007

Y, (A, ) stretch. N-0

1283 - 1300 sh.

vI (A, ) sym. stretch. NO,

1498

u+ (B, ) asym. stretch. NO,

The appearance of the stretching NO (~2) band at about 1000 cm’-’ in the infrared is clear evidence that the symmetry of nitrate species is lower than l&, 119]_ Thus, the existence of Me *+--N03- pairs in acetone solutions of Zn(N03 )* and Cu(N03 I2 seems to be proved.. Zn*’ is probably covafently bonded to NO;, the Zn-0 stretching vibration band being detected in the Raman spectra of Zn(NO& acetone solutions [ZO] . CL?*-NO; bond also seems to be covalent, since the reduction of Cu*+ to Cu” due to electron transfer from NOj takes place in Cu(NO,), acetone solutions [Zl]. The reason why Cl- and NO< anions in copper salt solutions influence CO band frequency in a reverse manner to that observed in the corresponding zinc salt solutions is not quite clear. Most probably it is due to the tendency of Cu2+ cation to be reduced to Cul+. It is worth noting that the reduction of Cu2+ takes place in CuC12 and Cu(N03 )2 solutions [Zl],but not in Cu(C104 I2 solution. The charge delocahzation between anion (CI- or NO, ) and Cu2” cation, which must occur if the reduction proceeds, decreases the electron affinity of Cu2’_ Consequently, the CO bond force constant of the acetone molecule is expected to be smaller in (Cu - acetone6)2+ complexes than in (CuCl, - acetone,) 2--x or (CU(NO,)~ acetone,,)24Y. According to the above discussion the following conclusion can be expressed. Influence of anions on the CO stretching force constant is above ah due to interaction of anion with cation Such an interaction, presumably covalent, changes the eIectron affinity of cation towards the oxygen lone electron pairs. This effect is responsible for the differences in reactivity of some perchlorate and halide (e.g. ZnCI, , ZnBr2 and Zn(CIO,),) solutions. The well known fact that some hahdes catalyze the condensation reaction cannot be ascribed to the interaction of halide anions with hydrogen atoms of the CH, group as suggested earlier [S] . Enolization of acetone molecules influenced by ions did not find any spectroscopic support, in nonreactive as well as reactive solutions_ In view of our results, the assumption about enol formation in the first stage of reaction seems not to be essential, at least in anhydrous acetone solutions of electrolytes.

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