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Conformations of dibenzylideneacetone: An IR spectroscopic study

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Conformations of dibenzylideneacetone: A n I R s p e c t r o s c o p i c s t u d y

G V E N K A T E S H W A R L U and B S U B R A H M A N Y A M Department of Chemistry, Osmania University, Hyderabad 5(10 007, India.

MS received 9 October 1987

Abstract. The conformational analysis of dibenzylideneacetone has been carried out using IR spectroscopy. Appearance of a triplet C=O band is attributed to the coexistence of three conformers viz s-cis, cis. s-cis, trans and nonplanar s-trans, trans in contrast Io the earlier findings which showed the existence of only two conformers. The relative proportions of the conformers are in the order s-cis, trans > nonplanar s-trans, trans >

s-cis, cis in less polar solvents and nonplanar s-trans, trans > s-cis, trans > s-cis, cis in more polar solvents.

Keywords. Dibenzylideneacetone; triplet C=O band; skew conformation; polarity of the conformers; stability of conformers.

1. Introduction

C o n f o r m a t i o n a l analysis of dibenzylideneacetone ( D B A ) assumed g r e a t e r import- ance in view of its ability to form a novel series of zerovalent transition metal complexes because of its existence in several isomeric forms (Takahashi e t a l 1970;

Moseley and Maitlis 1971; Ukai e t a l 1974). By virtue of the existence of two degrees of rotational f r e e d o m imparted b y two single bonds between the olefin and carbonyl group, three c o n f o r m a t i o n s viz s - c / s , c i s , s - c i s , t r a n s and s - t r a n s , t r a n s are possible for D B A (figure 1).

A n u m b e r o f a t t e m p t s have been m a d e earlier to study the c o n f o r m a t i o n s of D B A , using different m e t h o d s viz. m o l a r polarizabilities and K e r r constants ( B r a m l e y and Le Fevre 1962), dipole m o m e n t s (Bentley e t a l 1949; T s u k e r m a n e t a l 1968), N M R ( T a n a k a e t a l 1978) and U V spectroscopy (Hoshi e t a l 1986).

F r o m K e r r constants and dipole m o m e n t s study D B A was shown to exist in only one f o r m ( s - c i s , c i s ) . Subsequent study of dipole m o m e n t s by T s u k e r m a n e t a l (1968) indicated the existence of m o r e than one conformation which was s u p p o r t e d by the two C = O bands o b s e r v e d in the I R spectrum. By a study of N M R spectra and I N D O m o l e c u l a r orbital calculations T a n a k a e t a l (1978) c a m e to the conclusion that D B A contains a large p r o p o r t i o n of the s - c / s , c i s with a small p r o p o r t i o n of s-c/s, t r a n s form. Hoshi e t a l (1986) who studied the U V absorption spectra of D B A at r o o m t e m p e r a t u r e and at 101 K, concluded that the s-c/s, c/s

*To whom all correspondence should be addressed 419

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420 G Venkateshwarlu and B Subrahmanyam

o

0 H~.c~C~c~C~ph H~c~C~c/H

s-cis, cis s-cis,trans s- trans, trans

•Llcco=

3 " 0 - 3 . 5 3 - 4 - 4-1 3 - 8 - 4 - 7

Figure 1. The thrcc possible conformations of dibenzylidcneacetonc.

conformer is nonplanar at room temperature and attains planarity at low temperatures.

When the IR spectrum of DBA was recorded in our laboratory in connection with a study of its metal complexes three carbonyl bands were observed in contrast to the earlier report (Tsukerman et al I968). This prompted a detailed study of the conformations of this compound. To distinguish between the different conformers, the IR spectra were recorded in solvents of different dielectric constants. The relative proportions of the conformers were determined from the relative intensities of the bands.

2. Experimental

2.1 Materials

DBA was prepared by the aidol condensation in alkaline medium, from benzaldehyde and acetone in 2:1 molar ratio, in ethanol (Vogel 1970). The yellow solid obtained was recrystallized twice from ethyl acetate and thrice from methanol and was TLC pure, m.p. 114~ The solvents were purified and freshly distilled before use and the middle fractions only were collected.

2.2 Measurement of the spectra

The IR spectra of equimolar solutions (M/20) of the sample in solvents of varied dielectric constants were recorded on a Shimadzu-400 grating double beam spectrophotometer at 28~ using NaCI matched cells of 1 mm path length. The wavenumber scale was expanded by a factor of four and the spectra were calibrated by using the polystyrene film. Extensive overlap of the three carbonyl bands pre- vented the measurements of their integrated intensities. The relative populations of the different conformers were therefore estimated by measuring the apparent optical densities of the C = O bands as was done in many investigations earlier (Erskine and Waight 1960; Hayes and Timmons 1968; Subrahmanyam et al 1980).

The positions of the C--O bands together with their relative apparent optical den- sities are presented in table 1.

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different solvents.

A = t c = o ( X ) / ( ~ c : o ( l ) + t c - o ( I I ) + tc_o(lll))

CCt, CHCI3 CH_~CI2

C = O (1) 1676 (0.2107) 1671 (0.1750) 1674 (0.1644) C = O (11) 1655 (11.4676) 1653 (0.4270) 1654 (0.3920) C = O (11I) 1650 (0.3216) 1648 (0.3970) 1649 (0.4430)

3. Results and discussions

The IR spectrum of DBA in G E l 4 exhibited three C = O bands at 1676, 1655 and 1650 cm -~. The first two bands were reported earlier and the frequencies of the two agreed with the values reported. They are distinct and well-defined; the third, however, appeared as an inflexion on the second band. The relative intensities of the three bands were found to vary with the change in the polarity of the solvent showing that the three C = O bands correspond to the three different conformers in equilibrium. The s-c/s, c/s conformer may be considered as similar to the s-c/s, form of benzylideneacetophenone with an additional s-c/s ethylenic double bond in cross conjugation. The carbonyl frequency of this conformer is expected to be higher than that of the s-c/s form of benzylideneacetophenone as the C = O frequency is enhanced by the interaction through space, of the C = O and C = C ~- electrons (Erskine and Waight 1960). This becomes evident when the frequencies of benzophenone (1664 cm -~) and s-c/s benzylideneacetophenone (1670 cm -~) are compared. So the band at 1676 cm -~ is assigned to the s-co, c/s conformation.

The s-c/s, trans confoflner may be treated as the s-c/s form of benzylideneace- tophenone with an additional conjugation of an s - t r a m C = C bond. The frequency of this is expected to be lower than that of the s-c/s form of benzylideneacetophe- none as trans conjugation often produces a lowering of C = O frequency. This is evident from the frequencies of benzophenone (1664 cm -1) and s-trans benzy- lideneacetophenone (1648 cm-1). The s-trans conjugation thus causes a lowering of C = O frequency by 16 cm -l. If the same effect prevails in the s-c/s, trans conformer, the C = O frequency of this compound should be around 1654 cm -~

(1670 c m - ~ - 1 6 cm-~). The s-c/s, trans conformer may also be considered the

s-trans form of benzylideneacetophenone with an additional conjugation of the s-c/s

double bond which raises the frequency by 6 cm -j. Then it may have a frequency closer to that of the s-trans form of benzylideneacetophenone (1648 cm -~) with an upward shift of 6 cm -~. Either way, the carbonyl frequency of this conformer is nearly 1654 cm -I. The band observed at 1655 cm -~ in the spectrum of DBA is therefore assigned to this conformation.

The s-trans, t r a m conformer should have the C = O frequency which is lower by 16cm -t than that of the s-trans benzylideneacetophenone which becomes 1632 cm -1 (1648 cm -t - 16 cm-~). The absence of such a band in this region revealed the nonexistence of this conformer. It may be due to the sterically- hindered structure of this form. This is evident from the framework model of DBA

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422 G Venkateshwarlu and B Subrahmanyam 0

H~c~C~c~H II

p h/~'H Hf!~ Ph

skew

Figure 2. The skew conformation of the s - t r a n s , t r a n s form of DBA.

which showed significant steric interaction between the two / 3 - H atoms and possibility of 7r-electron repulsion between the two C = C bonds which are sufficiently close. From these considerations it is believed to have a skew conformation so as to minimise the steric and ~r-electron repulsions (figure 2).

In this conformation conjugation of the second s-trans C = C is greatly inhibited and the frequency of this conformer is expected to be closer to that of s-trans benzylideneacetophenone (1648 cm-I). The inflexion observed at 1650 cm -~ is therefore attributed to the nonplanar s-trans, s-trans (skew) conformation.

3.1 Solvent effect

The frequencies and intensities of all the C = O bands are markedly affected by the solvents. The frequencies of the bands in CH2Ci2 and CHCI 3 are lower as compared to those in CCla. The solvent shifts are in the order CH2CI2 < CHCI 3.

In CC14 the intensities of the C = O bands are in the order s-c/s, trans > nonplanar s-trans, trans > s-c/s, cis. The intensity changes of the bands are quite significant.

With increasing polarity of the solvent, there is a progressive decrease in the intensities of the C = O (s-c/s, c/s) and C = O (s-c/s, trans) bands while the intensity of the C = O (skew) band increases. It is of interest to note that on passing from CC14 to CH2CI2 the intensities of the bands of s-c/s, trans and skew are reversed (figure 3).

From this it is clear that the skew form is more stable in solvents of greater polarity while s-c/s, c/s and s-c/s, trans forms are more stable in nonpolar or less polar solvents. This is consistent with the fact that more polar forms are stabilized in more polar solvents and less polar ones in lower or nonpolar solvents.

Amongst the three conformers the s-c/s, cis form is the least polar as the two C = C dipoles oppose the C = O dipole. The skew form (or s-trans, trans) is the most polar form as the C = O dipole is reinforced by the two C = C dipoles. In the s-c/s, trans form the dipole of one C = C reinforces the C = O dipole and the dipole of other C = C is opposed to it. The dipole moment of this is expected to be midway between the dipole moments of the above two forms. This is evident from the dipole data (Bentley et al 1949) of the different conformers (figure 1).

3.2 Conformations o f the D B A and their stabilities

From the intensities of the C = O bands of the three conformers of DBA in CC14 it is clear that the populations of these conformers are in the order s-c/s,

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9 0

8 0

70

A

w U Z

s o

=E 09 40 z

n~

3 0

2 0

I 0

0

0 0

eL | |

f

l

I I I I I I I I I _

0 ~ 0 ~ 0 ~ 0 ~ 0 ~')

I~. ~ (M 0 I~ ~ ~1 0 I~- ~ (M

F R E Q U E N C Y ( C M - I )

Figure 3. The C = O bands of DBA in different solvents. (a) CCI.~. (b) CHCI~.

(c) CH,CI2.

trans > nonplanar s-trans, trans > s-cis, cis. The lowest proportion of the s-c/s, c/s form may be due to greater destabilization caused by stronger repulsion between 7r-electrons of the C = O and two C = C bonds. The ~r-electron density of the C = O in this form is higher which is evident from the high C = O frequency. Of the two forms viz. s-c/s, trans and nonplanar s-trans, trans the latter is less stable than the former, probably due to decrease in the resonance stabilization energy owing to the nonplanarity of this conformation.

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424 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

Acknowledgements

O n e of the a u t h o r s ( G V ) is t h a n k f u l to the D e p a r t m e n t o f A t o m i c E n e r g y , G o v e r n m e n t o f I n d i a , for the a w a r d of a fellowship. T h e a u t h o r s wish to express their grateful t h a n k s to Prof. T N a v a n e e t h R a o for his kind h e l p a n d c o n s t a n t e n c o u r a g e m e n t as well as to the H e a d of t h e D e p a r t m e n t o f C h e m i s t r y .

References

Bentley J B, Everard K B, Marsden R J B and Sutton L E 1949 J. Chem. Soc. 2957 Bramley R and Le Fevre R J W 1962 J. Chem. Soc. 56

Erskine R L and Waight E S 1960 J. Chem. Soc. 3425 Hayes .W P and Timmons C J 1968 Spectrochim. Acta A24 323

Hoshi T, Kawashima T, Ukubo J and Yamamoto M 1986 J. Chem. Soc., Perkin. Trans. !! 1147 Moseley K and Maitlis P M 1971 Chem. Commun. 982

Subrahmanyam B, Venkateshwarlu G. Patnaik D D and Padmavathi Reddy E 1980 Proc. Indian Acad.

Sci. (Chem. Sci.) 89 219

Takahashi Y, Itho T, Sakai S and Ishii Y 1970 Chem. Commun. 1065

Tanaka H, Yamada K and Kawazura H 1978 J. Chem. Soc., Perkin. Trans. II 23t Tsukerman S V, Orlov V D, Surov Y N and Lavrushin V F 1968 J. Struct. Chem. 9 53 Ukai T, Kawazura H, Ishii Y, Bonhet J J and Ibers J A 1974 J. Organometal. Chem. 65 253 Vogel A I 1980 Text-book of practical organic chemistry 4th edn (London: English Language Book

Society and Longmans) p. 794

References

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