Ultrasonic relaxation of the conformational equilibria in some substituted vinyl compounds

Advances

in Molecular

Relaxation

Processes

Elsevier Publishing Company, Amsterdam.

ULTRASONIC RELAXATION RIA IN SOME SUBSTITUTED

E. WYN-JONES,

Department Britain)

K. R. CROOK

of Chemistry

OF THE CONFORMATIONAL VINYL COMPOUNDS

AND W.

and Applied

193

Printed in the Netherlands

EQUILIB-

J. ORVILLE-THOMAS

Chemistry,

University

of Salford,

Salford

M5 4WT

(Ct.

CONTENTS

............................. ............................... III. Experimental ............................. IV. Discussion .............................. A. Vinyl ethers ............................ I.

Introduction

II. Theory

B. Methyl viny1 sulphide ....................... C. Vinyl ketones ........................... D. Substituted propenes; 2,3-dibromo (VIII) and 2.3-dichloropropene Summary ................................ References. ...............................

(IX)

.

.

193

.

194 194 197 197 199 199 200 202 202

. . . . . .

. .

1. 1NTRODUCTlON

This paper describes ultrasonic relaxation measurements of the conformational equilibria between the rotational isomers formed by internal rotationabout the single bond between the /3 carbon atom of a vinyl or substituted vinyl group and the atoms 0, S, C(sp’) and C(s$) in molecules such as vinyl ethers, vinyl sulphide, vinyl ketones and propenes. Most conformational studies in these molecules have been confined to determining the structure of the more stable conformer’-5, the relative population of the stable rotational isomers1*2*6 and the barriers opposing the rotation of the methyl group in methyl vinyl ethe?, methyl vinyl sulphide4 and methyl vinyl ketone5. There is, however, a paucity of data on the potential barriers opposing conformational changes between the stable rotational isomers. These latter barriers cannot be determined using the infrared torsional method because of the lack of information concerning the exact structure of the less stable isomer, which can be tram (Ia) or gauche (Ib) as shown for methyl vinyl ether.

lb

Advan.

gauche

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WYN-JONES,

K.

R.

CROOK,

W.

J.

ORVILLE-THOMAS

In this method of determining barriers, the exact form of the potential energy function is required. On the NMR time scale, only “time averaged” spectra’ are observed for these molecules and thus derivation of the potential barriers would be a very difficult procedure. Earlier work by de Groot and Lamb’ has shown that some of these compounds exhibit an ultrasonic relaxation consistent with the perturbation of a conformational equilibrium. These data, although of a preliminary nature, indicate that the full relaxation spectra for most molecules in the frequency range 15-65 MHz will occur at temperatures in the region -20 to - 100 “C. Since barriers opposing conformational changes in two-state equilibria can be obtained directly from experimental ultrasonic relaxation data regardless of the exact shape of the potential energy function, we decided to extend these earlier data and carry out a full analysis of the relaxation. The results are described in this communication.

II.

THEORY

When a two-state conformational equilibrium is perturbed by an ultrasonic wave, a relaxation centred at a certain characteristic frequency f, will occur. If f, is close to the experimental frequency range and the relaxation strength is large enough (ca. 10m4) the relaxation can be detected experimentally by making sound absorption measurements at different frequencies. It can be shown that the barriers opposing the conformational change from the less stable to more stable state can be obtained from the slope of the plot log (Tr) vs. T-i(K-i). z is the relaxation time related9 to .f, by eqn. (1).

III.

EXPERIMENTAL

The ultrasonic measurements were carried out with a partly transistorised pulse apparatus designed by Barlow 1’ . The preliminary measurements of de Groot and Lamb’ indicate the relaxation spectra of many of these compounds in the above frequency range will occur in the temperature range 230-170 “K. In order to carry out these measurements a new mechanical cell, shown in Fig. 1, was designed; the principle of this cell is as described in a previous paper’ ’ . The cell was designed so that it could be totally immersed in the thermostat bath. The liquid is kept in a brass tube of diameter 3.75 mm, the bottom end of which is sealed with the lower quartz rod, together with its seating arrangement including the levelling screws. The length of the copper tube is ca. 0.6 m. The upper half of the cell holds a moveable PTFE rod which is attached to the upper quartz rod Aduan. Mol. Relaxation

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ULTRASONIC

RELAXATION

IN

VINYL

195

COMPOUNDS

A+0

Fig. 1. Low temperature

ultrasonic

cell. A, B, quartz

rods;

C, levelling

screws;

D, test liquid.

and detecting crystal. A simple rack and pinion system is used to control the movement of this rod in the liquid, making a total path length of 8 cm. There is a large Mercer manometer which measures the movement of this rod to 0.01 mm. The upper half of the cell is separated from the lower half by a PTFE insulator; this allows the lower half to attain the required temperature whilst the upper half is kept at room temperature. The temperature of the test liquid is measured with a thermocouple arrangement. The relaxation data for the compounds methyl vinyl ether, I, ethyl vinyl ether, II, n-butyl vinyl ether, III, chloromethyl vinyl ether, IV, methyl vinyl Advan.

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sulphide, V, methyl vinyl ketone, VI, methyl-3-butene-2-one, VII, and 2,3-dibromopropene, VIII, were analysed in the usual manner” and the relaxation frequencies are listed in Table 1. Some compounds were measured as pure liquids, whereas others were in solution. The barriers for the less stable to more stable isomeric change are in Table 2. All compounds were purified commercial samples. TABLE

1

RELAXATION

FREQUNCIES

Temperature (“0

fe (MHz)

Methyl vinyl ether (I)

-a5 -80 --75 -70 -65 -60 -55 -50 -47.5 -42.5 -37.5 -24.8

Ethyl vinyl ether (II)

-87.5 - 80.0 - 75.0 - 70.0 - 65.0 -60.0 -55.0 -24.8

7.5 15.0 18.0 25.3 33.8 42.1 41.5 50.0

n-Butyl vinyl ether (III)

-75 -70 -65 -60 -55 -50

13.4 17.5 29.2 38.0 57.1 69.9

Chioromethyl

-87.5 -85 -80 -75 -70 -65 -60 -50 -24.1

18.9 19.4 22.2 24.9 32.1 44.9 50.2 63.0 80.5

Advan.

vinyl ether (IV)

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12.2 17.7 25.8 30.0 43.9 54.0 65.0 74.9 80.2 89.8 93 121.7

ULTRASONIC

RELAXATION

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COMPOUNDS

197

TABLE 1 (continued)

W)

f. (MHz)

Methyl vinyl ketone (VI)

-66 -58 -53 -47.5 -41 -35 -29

12 15 21 30 46 60 68

Methyl-3-butene-2-one

-89.5 -84 -79 -72 -65

9.5 11.6 15.2 21 30

Temperature

2,3-Dibromopropene

(VII)

(VIII)

21 27 35 42 47.5 54.5 62

7.1 7.5 8.1 9.1 11.3 14.0 18.0

TABLE 2 ACTIVATION

ENTHALPIES

FOR THE LESS STABLE

TO MORE STABLE

Molecule

AHt (kJlmole)

Methyl vinyl ether (I) Ethyl vinyl ether (II) n-Butyl vinyl ether (III) Chloromethyl vinyl ether (IV) Methyl vinyl sulphide (V) Methyl vinyl ketone (VI) Methyl-3-butene-2-one (VII) 2,3-Dibromopropene (VIII)

15.5 16.8 24.4 13.4 ca. 8 20.1 13.5 18.6

ISOMBRIC

TRANSITION

IV. DISCUSSION

A. Vinyl ethers

Microwave spectroscopy3, as well as analysis of the high resolution infrared spectra lV2 of some vibration rotation bands, has shown that the most stable conformer in methyl and ethyl vinyl ethers is the cis isomer. In these compounds two Advan.

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separate isomers were detected in the liquid phase’,‘, the less stable isomer being the one formed by internal rotation about the vinyl C-O bond2. NMR’,‘~ and dipole moment data6 also indicate that his second isomer has a gauche conformation. The NMR studies also indicate that the degree of resonance in these compounds increases in the order IV, I, II and III, this being attributed to an increase in the relative population of the cis isomer in the same order. The energy barriers for the gauche to cis isomerisation in these molecules are shown in Table 2. The main factors which affect these barriers are* (i) degree of resonance due to the possibility of planar structures; (ii) non-bonded steric repulsive forces; (iii) electrostatic repulsive forces, and (iu) electrostatic attractive forces between polarisable alkyl groups and electronegative parts of the molecule. If conjugation were the main factor influencing the barriers in this series, one would expect that as the degree of resonance increases, the double bond character of the vinyl C-O bond also increases, which in turn will hinder the internal rotation. The barriers in Table 2 refer to the less stable to cis conformational change and in order to correlate these results with the NMR data, enthalpy differences, AH”, are needed. It has been inferred’a13 that AH” increases in the order IV, I, II to III and for I and II values of ca. 2.4-3.2 kJ . mole-’ are quoted. This would indicate that the barriers opposing internal rotation are not consistent with the changes in the degree of resonance for these molecules. In an attempt to determine why the barriers to internal rotation vary from the chloromethyl to the n-butyl derivative, molecular models of all different conformers were constructed using Prentice-Hall framework models. If, in the “sickle-shaped”2 conformers, internal rotation about the vinyl C-O bond only is considered, it is hard to visualise how the barriers vary so much in this series. On the other hand, if we allow for internal rotation about the alkyl C-O and also the C-C bonds, then considerable steric crowding will occur in certain conformations. Although this type of internal rotation does not give rise to spectroscopically distinguishable rotational isomers, there is no reason to believe that it does not affect the transition state conformations. Indeed, if this type of steric crowding were to affect the transition state energies, then as the size of the alkyl group increases, more steric interaction occurs and consequently the barrier to internal rotation will also increase. This is consistent with the experimentally determined barriers. This type of steric crowding will produce a number of different transition state conformers whose energies are close and consequently the measured barrier is an “average” value corresponding to a spread of activation energies; the value of this “average” barrier will increase as the number of possible transition states increases. In the chloromethyl compound the molecular model study showed that the most likely stable form is the cis isomer with a “sickle-shaped” conformation and Advan.

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with the rc electrons,

IN

VINYL

lone pair electrons

199

COMPOUNDS

and electronegative

chlorine

atom furthest

apart. These molecular models also showed that the most favoured transition state conformer in this molecule is the one involving internal rotation of a rigid -0-CH,-CH,-Cl fragment about the vinyl C-O bond. Any internal rotation of this group would appear to produce severe electrostatic interaction. Thus the possibility of a favoured transition state in this molecule compared with a spread of transition states in the alkyl derivatives may account, in part at least, for the relatively low barrier to internal rotation observed in this molecule. In addition, the gauche isomer in this molecule is probably destabilised with respect to the gauche isomers of the other molecules in the series. B. Methyl vinyl sulphide (V) In this molecule the more stable isomer is the planar cis form and the exact conformation of the second isomer is not known. The population of this less stable isomer in the liquid state must be greater than ca. 5 ‘A since the isomer was detected in the infrared spectrum I4 . The ultrasonic relaxation spectrum occurs at very low temperature and only the high-frequency part of the spectrum was observed in the present work. A full frequency/temperature analysis was not possible. Using a reasonable pre-exponential factor in the rate equationg, the barrier opposing the less stable to cis conformational change is estimated to be ca. 8 kJ * mole-l. C. Vinyl ketones The microwave spectrum is the s-truns form IX.

of Vi has shown that the more stable conformer

“\,=,/” H’

‘c=,

Internal rotation in these molecules takes place about two sp’ hybridised carbon atoms and the barriers opposing the conformational change from the less stable isomer to the s-tranS form are, respectively, 20.1 and 13.5 kJ . mole-l for VI and VII. One possible explanation for this large difference is the relative destabilisation of the second isomer of VII with respect to VI owing to enhanced non-bonded methyl/methyl interaction. The interactions that would occur in the s-cis planar isomer of VII are very similar to the methyl/hydrogen syn-axial interactions that Advan.

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occur in the axial isomers of molecules 1,3-dioxanl‘j and 2-methyl-1,3-dithian16.

such as methyl cyclohexane’5, 2-methylHowever, comparison of the magnitude of these interactions in cyclic compounds as a function of non-bonded distances indicates that in the s-cis isomer of VII the destabilisation expected from the methyl/methyl interaction is less than 3 kJ . mole-’ ; this value will be even smaller if the conformation of this isomer is gauche. Thus other factors such as electronic and possibly relative destabilisation of the transition state must also account for the difference in barriers for these molecules. The order of magnitude of these barriers, however, compares well with those found about other sp2-sp2 hybridised carbon atoms in unsaturated aldehydesstl’. D. Substituted

propenes;

2,3-dibromo-

(VIZZ) and 2,3-dichloropropene

(IX)

a conformational equilibrium exists, in the According to Crowder’s,lg, liquid state, in both VIII and IX between a more polar and a less polar form. In both molecules the more polar form is also the more stable isomer. It is also thought that these two energetically different isomers have non-planar configuration of the heavy atoms. In these molecules a relaxation was observed in the dibromide VIII but not in the dichloride IX. The relaxation in VIII was consistent with the perturbation of a conformational equilibrium and the barrier to rotation for the less polar to more polar isomerism is 18.6 kJ 9mole-‘. It is very surprising that no relaxation was observed in the dichloride, even over a wide temperature. In our experience, in all previous ultrasonic studies on conformational equilibria in halogenated compounds, either a relaxation has been observed in both chloro and bromo derivatives, or no relaxation has been observed at all. In order to explain the present result it is necessary to consider the following conditions which govern the relaxation of a conformational equilibrium by an ultrasonic wave. (LZ)For the perturbation of the equilibrium to occur the quantity d In K = (AH”/RT’) dT- (AV”/RT) dP must not be zero. AH” and AV’ are the respective enthalpy and volume difference between the conformers, K is the equilibrium constant, and dT and dP represent the temperature and pressure perturbation accompanying the passage of the sound wave. (b) The equilibrium constant must not be too small. The exact limit is not known at present but relaxations have been observed in one-sided conformational equilibria with as little as 0.05 % of the less stable form present. (c) The experimental ultrasonic frequency must be close to the characteristic relaxation frequency. From the spectroscopic evidence it is clear that the two rotational isomers of both VIII and IX have different energies. Considering condition (b), it has been found that some of the vibrational modes of both polar and non-polar isomers can be identified in the infrared spectrum of IX showing that the population of Aduan. Mol. Relaxation

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the higher energy,

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less polar form is at least ca. 5 %. This in turn means that the

equilibrium constant is much greater than the limiting value of ca. 0.05 %. Finally, previous evidence indicates that the relaxation spectra arising from conformational isomerism in corresponding chloro and bromo derivatives occur in the same frequency range. This means that the most likely explanation for the absence of a relaxation in VIII is associated with condition (a) where dK = 0 because (AH”/RT2)

dT N (AV”/RT)

dP

which means that the perturbation of the conformational equilibrium arising from the compression and temperature changes accompanying the passage of sound waves have equal and opposite effects. Finally, if we consider the isoelectronic molecules methyl vinyl ether, X, and

ii”’

ii

H/c\o/CH3 H/c\o/cH3 X

methyl formate, XI, the methyl group in whereas the barriers bond are 4 and 6-8

XI

it has been found that the barriers hindering the rotation of the stable cis isomers are 3.1 and 1.1 kcal/mole respectively3, opposing rotation about the C-O bond adjacent to the double kcal/mole respectively2’. If we first consider the rotation of

the methyl groups the most important interactions opposing the rotation of this group in X are between one of the methylene hydrogen atoms and the hydrogen atom of the methyl group. The corresponding interaction in XI is between the methyl group and the lone pair electrons on the carbonyl oxygen. It is well known from a comparison of the conformational analysis of methyl cyclohexane and 521 that syn axial interactions between methyl and hydrogen are methyl-1,3-dioxan much greater than those between methyl and lone pair electrons. This probably accounts, to a great extent, for the difference between the above barriers. On the other hand, the barrier opposing internal rotation about the C-O bond is greater in Xl. From an inspection of the molecular models it is apparent that the most significant non-bonded interactions affecting this barrier are lone pair/lone pair repulsive forces in XI and lone pair/hydrogen in X. Again by reference to the conformational analysis of six-membered heterocycleszl and in particular the anomeric effect, it is clear that lone pair/lone pair interactions are very much greater than lone pair/hydrogen interactions. Thus the trends in the barriers to internal rotation in these molecules appear to be governed by various non-bonded interactions. Advan. Mol. Relaxation

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SUMMARY

The barriers opposing the conformational changes between the less stable and more stable isomers in a number of vinyl and substituted vinyl compounds have been derived from ultrasonic relaxation data. The trends in these data are discussed in terms of intramolecular interactions. In 2,3-dichloropropene the absence of ultrasonic relaxation is attributed to the possibility that the compression and temperature changes accompanying the sound waves have equal and opposite effects on the conformational equilibrium.

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