Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 102, No. I, February 1990, pp. 45-50.
9 Printed in India.
Conformations of a, [i-unsaturated ketones: An IR spectroscopic study
G V E N K A T E S H W A R L U a n d B S U B R A H M A N Y A M * Department of Chemistry, Osmania University, Hyderabad 500007, India MS received 11 October 1989
Abstract. The conformational analyses of substituted methyl styryl ketones and phenyl styryl ketones have been carried out using IR spectroscopy. The split in the C=O bands of these compounds is attributed to the existence of two conformations, viz. s-cis and s-trans, in equilibrium. The methyl styryl ketones exist predominantly in the s-trans form whereas phenyl styryl ketones exist in the s-cis form. In all the ketones studied the proportion of the
s-trans form increases with increase in the polarity of the solvent while that of the s-cis form decreases. This shows that the s-trams form is more polar than the s-cis. The field effects between the C--O and C--C groups are found to be electrostatic repulsions which play a dominant role in the determination of the relative stabilities of the s-cis and s-trans forms over the steric effects and the electrostatic attractions between the carbonyl oxygen and the/3- carbon.
Keywords. ~t, fl-unsaturated ketones; split in the C--O bands; rotational isomers; polarity of the conformers; n-electron repulsions.
1. Introduction
T h e c o n f o r m a t i o n a l a n a l y s i s o f ~t, f l - u n s a t u r a t e d c a r b o n y l c o m p o u n d s has been a s u b j e c t o f c o n s i d e r a b l e interest a n d was i n v e s t i g a t e d b y m a n y w o r k e r s ( E r s k i n e a n d W a i g h t 1960; N o a c k a n d J o n e s 1961; H a y e s a n d T i m m o n s 1968; Bowles a n d G e o r g e 1969). T h e relative stabilities o f the two c o n f o r m e r s viz. s - c i s a n d s - t r a n s a r e c o n s i d e r e d to be d e t e r m i n e d b y t w o factors (a) field effects b e t w e e n the C - - O a n d C = C g r o u p s a n d (b) steric effects b e t w e e n the s u b s t i t u e n t s p r e s e n t on the c a r b o n y l a n d e t h y l e n i c c a r b o n a t o m s .
A l t h o u g h the influence of the steric effects o n the relative s t a b i l i t i e s o f the two c o n f o r m e r s has fully been e x p l o r e d , the exact n a t u r e o f the field effects is n o t c l e a r l y k n o w n . T h e field effects are c o n s i d e r e d by E r s k i n e a n d W a i g h t (1960) a n d N o a c k a n d J o n e s (1961) as e l e c t r o s t a t i c a t t r a c t i o n s b e t w e e n C--O, C = C d i p o l e s a r i s i n g f r o m the r e s o n a n c e in the e n o n e m o i e t y (I, II) which stabilise the s - c i s form. H a y e s a n d T i m m o n s 0 9 6 8 ) a n d Bowles a n d G e o r g e (1969) are of the o p i n i o n t h a t t h e y a r e e l e c t r o s t a t i c r e p u l s i o n s b e t w e e n the n e l e c t r o n s o f the C = O a n d C - C b o n d s which d e s t a b i l i z e the s - c i s form.
It was, therefore, c o n s i d e r e d w o r t h w h i l e to p r o b e the exact n a t u r e o f the field effect a n d its influence o n the relative stabilities o f the s - c i s a n d s - t r a n s forms. T o a c c o m p l i s h
* For correspondence
45
46 G Venkateshwarlu and B Subrahmanyam
this
I
the methyl styryl ketones and
+•0-
II
phenyl styryl ketones with electron-releasing groups in the p-position of the styryl ring were chosen.
2. Experimental 2.1 Materials
The compounds were prepared by aldol condensation of substituted benzaldehydes and acetone or acetophenone in 1 : 1 molar ratios in alkaline media in ethanol (Vogel 1980). The solids separated were purified and recrystallised twice from methanol and were TLC-pure. The solvents used in the study were purified by methods available in literature and freshly distilled before use and only the middle fractions were collected.
2.2 Measurement of the spectra
The IR spectroscopic technique was chosen for this study as the vibrational frequencies of the C=O bands are useful not only in distinguishing between the two conformers but also for indicating the relative electron densities of the C=O bonds.
The IR spectra of equimolar solutions (M/20) of methyl styryl ketones in solvents of varied dielectric constants were recorded at slow speed on a Perkin-Elmer 337 and those of phenyl styryl ketones on a Shimadzu 400 grating double beam spectrophotometer at 28~ using matched NaCI cells of l mm path length. The relative proportions of the s-cis and s-trans forms were estimated from the relative apparent optical densities of the C---O bands of the two forms (Erskine and Waight
1960; Kossanyi 1965).
3. Results and discussion
All the compounds exhibited doublet C=O bands in all the solvents studied viz. CCI,, CHCI3 and CH2C! 2. The intensities of the lower frequency bands increased with increase in polarity of the solvent and those of the higher frequency bands decreased.
This pattern of intensity variation of the C--O bands is analogous to that observed in the case of dibenzylideneacetone (Venkateshwarlu and Subrahmanyam 1987), which was shown to be due to rotational isomerism.
The split in the C=O bands may, therefore, be attributed to the existence of two rotamers viz. s-cis (llI) and s-trans (IV) in equilibrium. From the fact that the more polar forms are stabilised in polar solvents and the less polar forms in non-polar solvents, the higher frequency bands were assigned to the s-cis and the lower frequency bands to the s-trans forms.
Of the two forms, viz. s-cis and s-trans, the latter is more polar than the former. For the C=O dipole in the s-trans form is reinforced by the C=C dipole whereas it is
Conformations of ct, fl-unsaturated ketones 47
R 0
H\C/C-.~O H\C/C\ R
C C
ph/ ~H ph/ ~H
R= CH 3 or Cell 5
III IV
opposed by it in the s-cis form (llI and IV). This is consistent with the calculated dipole moments of the s-cis (3.25 D) and s-trans (3"80 D) forms of the methyl styryl ketone (Bentley et al 1949).
The band assignments are also consistent with those made by Hayes and Timmons (1968) on the basis of solvent shifts of the C=O bands and by Erskine and Waight (1960) on the basis of the band separations and the intensity ratios of the C--O and C=C bands.
3.1 Conformations of the molecules and their stabilities
From the relative intensities of the s-cis and s-trans C--O bands, it is clear that the predominant conformer in methyl styryl ketone is the s-trans. It is however s-cis in phenyl styryl ketone (figures 1 & 2).
z
z I-.-
,or
sol
7,
zol.
i
Iol-
8 8 w
P.. r,-
o~ ~ . t D c o e ~
F R E Q U E N C Y (cm -I )
Figure 1. Figure 2.
|
I
qe !! _w~_
FREOUENCY (cm -I)
| |
Figure I. The C--O bands of methyl styryl ketones in CCI 4. Substituent in the p-position is (It) -N(CH3)2, (b) - O C H 3 and (e) -H.
Figure 2. The C=O bands of phenyl styryl ketones in CCI,. Substituent at the p-position is (It) -N(CH3)2, (It) - O C H j and (r -H.
48 G Venkateshwarlu and B Subrahmanyara
H~c/C~ o H~c/C~o
II C~ II
C~ p h / H
ph j H
VI
It is interesting to note here that cinnamaldehyde is exclusively stabilised in the s- trans form and the stability of this form is slightly decreased in methyl styryl ketone and largely in the phenyl styryl ketone.
The destabilisation of this form is likely to be due to the steric effects of the methyl or phenyl groups in the s-trans form. It was shown by Winecoff and Boykin (1972) that the steric effect due to the methyl group is very small and it is equal in both the s-cis and s-trans forms. The steric effect due to the phenyl group however is larger in the s-cis form than in the s-trans form (Perjessy 1969; Rajasekaran and Gnanasekaran 1987). If steric effects are important in any way in the relative stabilisation of the two forms, the methyl group should not have any effect on the stabilities and the phenyl group should destabilise the s-cis form. These groups, however, destabilise the s-trans form and stabilise the s-cis form as compared to cinnamaldehyde.
The lower stability of the s-trans form in phenyl styryl ketone was earlier attributed by Hayes and Timmons (1968) to the repulsions between the two phenyl rings in this form. If this were true the phenyl vinyl ketone which does not contain two phenyl rings should prefer the s-trans form. But Kossanyi (1965) showed that it exists predominantly in the s-cis form.
In the absence of steric effects, the stabilisation of s-cis form in the styryl ketones as compared to cinnamaldehyde may be attributed to field effects. It may be (i) the decrease in the n-electron repulsion of the C=O and C=C bands, or (ii) increase in the electrostatic attraction between the #-carbon atom and the carbonyl oxygen, both changes resulting from the polarisation of the C=O bond by the hyperconjugation of the methyl group (V) or the mesomeric effect of the phenyl group (VI).
The two factors (i) and (ii), however, can be distinguished by the introduction of electron-releasing substituents into the styry! ring. The mesomeric effect of these groups not only decreases the n-electron densities of the C=O and C=C bonds, but also neutralises the positive charge on the #-carbon atom (VII, VIII).
If the n-electron repulsions of the C=O and C=C bonds are responsible for the relative stabilities of the two forms, the electron releasing groups should stabilise the s- cis form as compared to the parent ketone as n-electron densities of C=O and C=C bonds are decreased by these substituents.
On the other hand, if electrostatic attractions are responsible for the relative stabilities of the two conformers electron-releasing groups should destabilise the s-cis
Conformations of ct,//-unsaturated ketones 49
R I
(H3C)2 ~ ~ C~H
R I
H ~ C ~ ' C ~ O -
~ C ~ H 1
(H3C) z N -/
R= CH 3 or C6H s
VII
VIIIform as compared to the parent ketone, as the positive charge on the//-carbon atom is neutralised by the mesomeric interactions of the substituents, which reduces the electrostatic attractions between the carbonyl oxygen and//-carbon atom.
The ratios of the intensities of the s-cis/s-trans carbonyl bands of both methyl styryl ketone and phenyl styryl ketone are lower than their substituted derivatives and are in the order N(CH3)2 > OCH3 > H(table 1). This indicates that electron-releasing substituents stabilise the s-cis form by decreasing the ~-electron repulsions between the C=O and C=C bonds.
The results of the present investigation suggest that the field effects between the C=O and C--C groups are electrostatic repulsions and they play a significant role in determining the relative stabilities of the s-cis and s-trans forms over the steric effects and the electrostatic attractions between the carbonyl oxygen and the//-carbon.
Table 1. The C : O stretching frequencies (cm- ~) of some ~,//-unsaturated ketones in different solvents.
vC=O
CCI 4 CHCI a CH2CI 2
Substituent s-cis s-trans s-cis s-trans s-cis s-trans
* Values in parentheses correspond to the ratios of the intensities of the C=O bands of s-cis and s-trans forms
Methyl styryl ketones
N(CHa) 2 1689 1666 1681 1661 1683 1663
(0.47)* (0-43) (0.40)
O C H 3 1693 1670 1685 1665 1687 1665
(0"38) (0"35) (0-32)
H 1697 1674 1689 1669 1692 1670
(0.36) (0.33) (0.28)
Phenyl styryl ketones
N(CH3) 2 1660 1638 1651 1626 1653 1630
(3'64) (2"90) (2'72)
O C H 3 1664 1643 1658 1632 1660 1635
(2"94) (2"80) (2"60)
H 1670 1648 1663 1639 1665 1641
(2.77) (2.56) (2.47)
50 G Venkateshwarlu and B Subrahmanyam Acknowledgements
The authors are thankful to Prof T Navaneeth Rao for his kind help and constant encouragement. One of us (GV) is thankful to the Department of Atomic Energy, Bombay, for a fellowship.
References
Bentley J B, Everard K B, Marsden R J B and Sutton L E 1949 J. Chem. Soc. 2957 Bowles A J and George W O 1969 J. Chem. Soc. (B) 810
Erskine R L and Waight E S 1960 J. Chem. Soc. 3425 Hayes W P and Timmons C J 1968 Spectrochim. Acta A24 323 Kossanyi J 1965 Bull. Soc. Chim. 704
Noack K and Jones R N 1961 Can. J. Chem. 39 2201 Perjessy A 1969 Chem. Zvesti 23 905
Rajasekaran A and Gnanasekaran C 1987 J. Chem. Soc. Perkin Trans. 2 263
Venkateshwarlu G and Subrahmanyam B 1987 Proc. Indian Acad. Sci. (Chem. Sci.) 99 419
Vogel A I 1980 Text-book of practical oroanic chemistry 4th edn (London: ELBS and Longmans) pp. 794 and 796
Winecoff W F and Boykin D W Jr 1972 J. Ory. Chem. 37 674
