Correlation of ultraviolet absorption frequences of cis and trans substituted cinnamic acids with hammett substituent constants

Journnl of Molecular Structure, 174 (1988) 251-254 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

251

CORRELATION OF ULTRAVIOLET ABSORPTION FREQUENCES OF CIS AND TRANS SUBSTITUTED CINNAMIC ACIDS WITH HAMMETT SUBSTITUENT CONSTANTS G.S.USCUMLIC, V.V.KRSTIE and M.D.MUSKATIROVIC Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of Beograd, Yugoslavia ABSTRACT The ultraviolet absorption spectra of a number of cis and trans-3- and 4-substituted cinnamic acids have been determined. Examination of the obtained spectra shows that there is a fair linear relation between the positions of the absorption bands for the electronic transitions of the carbonyl group and the Hammett substituent constants. The plot of the ama values of the cinnamic acids versus the rate constants for he corresponding acl2s with diazodiphenylmethanein ethanol gave a fair correlations. This relationship was discussed in view of the proportionalityof the energy of activation for the reaction of these acids with DDM in ethanol and the energy for the electronic transitions in the carbonyl group under the influence of UV radiation. INTRODUCTION A correlation between the structure and ultraviolet absorption spectra of a, B -unsaturated acids have been succesfully studied by many authors (ref. 1 and 2). However, the correlations of ultraviolet spectra with substituent constants present greater difficulties in so far as both accuracy of measurements and of interpretation (ref. 3). In our previous work (ref. 4 and 5) it has been shown that there is a good linear relation between the positions of the absorption bands for the electronic transitions of the carbonyl group of a, B -unsaturated carboxylic acids and the Taft polar substituent constants. In connection with a study of the transmission of polar effects through the double bond it se,emedof interest to investigate UV spectra of cis and trans-3- and 4-substituted cinnamic acids (Scheme 1). In this work, the position and the intensity of the ultraviolet absorption bands of the phenyl substituted cinnamic acids have been determined, and ~,,,a,, values were correlated with the Hammett substituent constants. The obtained data were used together with the data from previous work concerning the reactivity of the same acids in reaction with DDM to discuss relationship of the energy of activation and the energy of the electron transitions under the influence of UV radiation.

0022-2860/88/$03.50

0 1988 Elsevier Science Publishers B.V.

252

H

X=H, p-CH3, m-Cl, p-Cl, p-OCH3

X=H, p-CH3, m -Cl, p-Cl, p-OCH3, p-Br, m-NO2

Scheme 1. cis and trans substituted cinnamic acids EXPERIMENTAL The substituted trans cinnamic acids were synthesized by the piridine catalyzed reaction of malonic acid and the corresponding benzaldehyde in 95% ethanol (ref. 6). The cis acids were prepared by the saponification of the cis ethyl cinnamate prepared by the hydrogenation of the appropriate ethyl phenylpropiolate over Lindlar catalyst (ref. 6). The ultraviolet spectra were recorded in 95% ethanol on Unicam SP 600 spectrophotometerat room temperature. RESULTS AND DISCUSSION The ultraviolet absorption maxima of the cis and trans-3- and 4-substituted cinnamic acids are given in Table 1. TABLEI. Ultraviolet absorption maxima of cis and trans cinnamic acids cis Acids

trans Acids

Substituent

x

max

(nm) ~~~~~~~~~

x

max

(4

~max.10-3

H

266

17.50

270

34.16

4-CH3

268

6.50

280

9.52

4-OCH3

267

21.18

279

48.75

4-Cl

265

5.80

274

27.12

3-Cl

264

13.00

268

29.68

4-W

273

8.23

3-NO2

260

25.20

4-NO2

307

5.16

253

The obtained data show that the ultraviolet absorption maxima of the trans acids are consistently at longer wave lenghts and of higher cmaX then those of the corresponding cis acids (Table 1). This is consistent with the notion of a superior conjugating system in the trans compounds. The effect of the substituents on the electronic spectra of cinnamic acids causes the shift of the absorption maxima which is much more pronounced in the case of trans cinnamic acids. The obtained Xmax values were succesfully correlated with the Hammett substituent constants. The lines described by the equation 1. for trans cinnamic acids with r = 0.9103 and equation 2. for cis cinnamic acids with r = 0.9344 are given in Figure 1.

P 5

0 cis cinnamic acids 0 trans cinnamic acids

:: x' 300280:

0

260240 220 F

I

I

I

- 0.3 -0.2

I

I

-0.1

0

I

0.1

I

0.2

I

0.3

I

0.4

I

0.5

I

0.6

4

0.7 d

Fig. 1. Relationship between A,,, values for the electronic transitions of the carbonyl group of cinnamic acids and a values x = 275-18.62 (J

(1)

x = 266- 5.54 CI

(2)

The different slopes of two lines (eq. 1 and 2) emphasizes the differences in sensitivity of trans and cis cinnamic acids on the substituent effects. Besides this, in our previous work we have shown that there is a fair linear correlation between log of rate constants for the same acids with DDM in ethanol (ref. 7) with Hammett substituent constants. The plot of the Amax values for these acids versus the log kDDM gave also a fair correlation which

254

can be represented by equation 3. for trans cinnamic acids with r = 0.9111 and equation 4. for cis cinnamic acids with r = 0.9017 (Figure 2).

260240-

-0.2 -011

0

0.1

0.2

0,3 0.4

0.5

0.6

0.7 lo9 k,,,

Fig. 2. Relationship between hmax values for the electronic transitions of the carbonyl group of cinnamic acids and correscponding log kooM values A = 274-47 log kDoM

(3)

,I= 270- 8.6 log kODM

(4)

The existence of these two correlations has been interpreted as the evidence of the proportionalityof the energy of activation for the reaction of these acids with OOM in ethanol and the energy for the electronic transitions in the carbonyl group under the influence of UV radiation. REFERENCES 1 A.T.Nielsen, J.Org.Chem., 22

( 1957) 1539-1548.

2 C.N.R.Rao, Ultraviolet and Visible Spectroscopy : Chemical Applications, Butterworths, London, 2nd ed., 1967. 3 N.B.Chapman and J.Shorter, Advances in Linear Free Energy Relationships, Plenum Press, London and New York, 1972, pp. 131-135. 4 G.USCumlic, V.Krstit and M.MuSkatiroviC, Vesnik Slovenskega kemijskega drubtva, Ljubljana, 33 (1986), 187-189. 5 G.USCumliC, V.KrstiC and M.MuSkatirovic,Vesnik Slovenskega kemijskega drustva, Ljubljana, 33 (1986), 189-191. 6 R.Fuchs and J.J.Bloomfield,J.Org.Chem., 31 (1966), 3423-3425. 7 G.USCumliC, 4.t.hYugoslav Congress on Organic Chemistry, Beograd, 1987,