Indian Journal of Chemistry Vol. 41B. January 2002, pp. 206-210

Reversal of substituent effect on electronic absorption spectra of N-( 4-substituted phenyl)-benzamides in different solvents

Gordana S Uscumlic* & Slobodan 0 Petrovic

Department of Organic Chemistry. Faculty of Technology & Metallurgy . University of Belgrade Karnegijeva 4. YU- lIOOO I Belgrade. Yugoslavia

Received 14 March 2000;. accepted (revised) 16 March 2001

Absorption spectra of eleven N-(4-substituted phenyl)-benzamides have been recorded in ten solvents in the range 200-400 nm. The substituents at the phenyl nucleus are as follows: N (CH)h. OCH). CH). H. CI. Br. F. CN. CF). COCH) and N02. The effects of substituents on the absorption spectra of investigated compounds are interpreted by correlation of ab­sorption frequencies with simple and extended Hammett equation. When the electron-releasing substituents are attached to the nitrogen atom substituent effects are transmitted through the amide bond by the usual mechanism. However. the effect of electron withdrawing substituents appears to be quite opposite. The effect of solvent polarity and hydrogen bonding on the absorption spectra are interpreted by means of linear solvation energy relationships using a general equation of the form v = Vo + sn* + b!) + aa. where n* is a measure of the solvent polarity.!) is the scale of the solvent hydrogen bond acceptor basi­cities and a is the scale of the solvent hydrogen bond donor acidities. The results obtained for N-(4-substituted phenyl)­benzamides are compared with the results for N-(4-substituted phenyl)-2-phenylacetamides under the same experimental conditions.

A number of workers l.3 have reviewed and critically

examined correlation of substituent constants with ul­traviolet absorption frequencies. They observed that these correlations present many difficulties both in in­terpretation and accuracy of measurement. By means of dual substituent parameter (DSP) treatments using various combinations of (JR+' crRo, crR' and crl as well as (crR ° - crR +) and (crR' - crR 0) Brownlee and Topsom4 and Topsom5 demonstrated convincingly that previously claimed simple relationships between frequency shifts and substituent parameters were mostly unfounded when the substituents were part of the studied chromo­phoric system. When the substituents being varied were not part of the chromophore system however, it was shown that good correlations were possible.

In our previous works we have reported the correla­tions between ultraviolet absorption frequencies of 3-N-(4-substituted benzoyl)-5,5-dimethyl hydantoins6

and 3-N-( 4-substituted phenyl)-5-carboxy uracils 7 with substituent parameters. These results support a sugges­tion by Brownlee and Topsom4. The present investiga­tion is an extension of earlier works wherein we had examined the transmission of electronic effects of sub­stituents through the amide bond of N-( 4-substituted phenyl)-2-phenylacetamides. The effect of substituents on the IR spectral characteristics of the C=O group was investigated. We reported that when the electron-

releasing substituents are attached to the nitrogen atom, substituent effects are transmitted to the C=O group by the usual mechanism. However, the effect of strong electron-withdrawing substituents appears to be quite oppositeS.

Similar investigations have been performed with four selies of substituted maleinimide derivatives9

, 3-N-(4-substituted benzoyl)-5,S-dimethylhydantoins IO

and 1 ,3-bis-substituted-5,5-dimethylhydantoins II .

The structure and the characteristic properties of ami des has been extensively studied since the amide bond -CONH- often occurs in various natural products, synthetic polymers and organic intermediates and is a very important structural feature, especially in peptide chemistry I2.13. The fact that the C-N bond of the amide moiety has a high rotational barrier should reflect the partial double bond character of this bond. X-ray crystallographic studies l4 on one hand and the theoretical calculations 15 on the other showed the planarity of the amide group. These facts can be described by the resonance between the two canonical structure I and II.

R"" +/H ......... _--'1,_ c= N

.~ "" . . R'

II

USCUMLIC et al. : ABSORPTION SPECTRA OF N-(4-SUBSTITUTED PHENYL)-BENZAMIDES 207

Q /H /C-N

Q o y ~ X

X X

1 N(CH3)2 7 Br

2 OCH3 8 CF3

3 CH3 9 CN

4 H 10 COCH3

5 F 11 N02

6 CI

Yuzuri et al. reported '6 that the 'H and I3C chemical shifts of substituted benzanilides XC6H4CONHC6H4 Y could be correlated well with G( G 0) substituent constants. In contrast to the good 8'H and 8'3C vs G plots, the 8'5N vs G 0 plots did not give any good correlations 17

• This finding was interpreted by assuming competition between the local 1t-polarization within a strongly conjugated amide group and electron-donating mesomerism in the anilino group.

In this paper, the ultraviolet absorption spectra have been recorded for N-(4-substituted phenyl)­benzamides 1-11 in the region 200-400 nm in ten different solvents.

We wish to explain the effects of the substituents in the UV spectrum of N-( 4-substituted phenyl)­benzarnides III different solvents. The N­phenylbenzamide spectrum has been taken as reference, which has three absorption bands: one at

260-290 nm and the others at 230-260 nm and 206-226 nm in different solvents.

The results have shown that the lower energy band is sensitive to the substituent electronic properties. No correlations are found for the higher energy bands. Ultraviolet absorption frequencies of the lower energy band of N-(4-substituted phenyl)-benzamides 1-11 in ten solvents are given in Table I.

The data from Table I confirm that the positions of the ultraviolet absorption frequencies depend on the nature of the substituent on the benzene ring. Introduction of a substituent into the benzene ring predominantly leads to a bathochromic shift of the long-wavelength absorption maximum as compared to that of unsubstituted N-phenyl benzarnide, except in the case for CF3 substituent in five solvents and in one case with F substituent, which cause hypsochromic shifts.

The UV absorption by N-(4-substituted phenyl)­benzamides can be attributed essentially to the

resonance system I-II. Structure I can be expected to make prepoderant contribution to the resonance hybrid representing the ground state. Electron­withdrawing groups increase the contribution of structure I and destabilize structure II. Electron­releasing groups increase the contribution of structure II and destabilize structure I.

In order to explain these results the absorption frequencies (Table I) were correlated with the various sets of substituent parameters G . The plot of the absorption frequencies versus G p or G p + and Gpo

constants give correlations which show deviations from the Hammett equation'8 in all investigated solvents (Figure 1).

The results thus obtained show that in N-(4-substituted phenyl)-benzamides, if the effect of electron-releasing substituents is observed the

Table I - Ultraviolet spectral data (vmax x lOo

3cm·' ) of N-(4-substituted phenyl)-benzamides

Solvent N(CH3) 2 OCH3 CH3 H F CI Br CF3 CN COCH3 N02

Methanol 32.59 35.51 37.01 37.74 37.54 37.31 37.06 37.68 33.59 33.81 33 .64

Ethanol 32.49 35.46 36.94 37.51 37.40 37.17 36.87 37.48 33.59 33.17 32.88

Propan-1 -o1 32.30 35.57 36.90 37.59 37.46 36.87 36.79 37.53 33.53 33.48 33.12

Propan-2-o1 32.43 34.39 36.82 37.37 37.62 36.90 36.93 37.51 33.59 33.44 33.16

Butan-1-ol 32.17 35.16 36.76 37.34 37.17 36.95 36.73 37.42 33.43 33.42 33.08

Butan-2-ol 32.42 34.65 36.76 37.31 37.31 36.81 36.63 37.28 33.46 33 .37 33.00

2-Methy1-1-propano1 32.40 34.67 36.87 37.39 37.28 36.79 36.84 37.50 33.48 33.48 33.10

2-Methyl-2-propanol 32.65 34.86 36.82 37.51 37.39 36.90 36.65 37.67 33.56 33.26 32.88

Ethyl acetate 32.73 34.98 36.94 37.69 37.47 37.00 36.77 37.53 33.48 33.28 33.00

Diethyl ether 33.20 35.71 37.62 38.42 38.19 37.80 37.50 38.47 33.92 33.98 33.76

208 INDIAN J. CHEM ., SEC B, JANUARY 2002

38

37

36 .

35 . V mu

34

33

32

-2 -1.5 ·1 -0.5 o 0.5

cr p'

Figure 1 - Relationships between IDOla. and (J + substituent con­p

stants for N-(4-substituted phenyl)-benzamides 1-11 in ethanol

mechanism of the transmission of substituent effects is governed by competing resonance interactions between the nitrogen atom lone pair and carbonyl group electrons, as well as the electrons of the substituent (structure III). The opposite effect of the strong electron-withdrawing substituents attached to the nitrogen atom suggest that these substituents destabilize the ground and exited states of the molecule and decreased the absorption frequencies (structure IV). This suggested that the electronic behaviour of the nitrogen atom is somewhat different between derivatives with electron-releasing and electron-withdrawing substituents. We infer that this phenomenon is caused by the difference in the conjugational or migrating ability of lone pair

electrons on nitrogen atom of N-(4-substituted phenyl)-benzamides. These effects cannot be explained in terms of the generally adopted mesomeric structure II and so it is more appropriate when studying the substituent effects on the amide group to regard the group as an n-7t delocalized system. Reversal of substituent effect in investigated system was interpreted by assuming competi tion between the local 7t-polarization within a strongly conjugated amide group and electron-donating mesomerism in the anilino group.

Electron-donating substituents via their +R-effect transmitted through the anilide system, increases the electron density at nitrogen atom, decreases the n-7t* transition energy and produces bathochromic shifts on the long wavelength absorption maximum as compared to that of the unsubstituted N­phenylbenzamide.

Electron-withdrawing substituents, in contrast, can be expected to decrease the electron density on the nitrogen atom_ However, our results show that effect of these substituents also produced bathochromic shifts. Effect of electron-withdrawing substituents is opposite to the effect of local polarization in the am­ide group and causes similar effect on UV absorption maximums as electron-donating substituents.

These results are in accordance with our previous investigations of the reversal of substituent effects on C=O stretching vibrations in hydantoin derivatives lO

and N-( 4-substituted phenyl)-2-phenylacetamides8.

The correlations obtained from the data given in Table I using Swain-Lupton l9 and Yukawa-Tsun02o

equations also gave poor results. We have found that the best insight into the transmission of substituent effects was obtained using Taft's dual substituent parameter (DSP) method2 1 (Eqn 1) and it was possible

Table II - Results of correlations vm •• x 10.3 for N-(4-substituted phenyl)-benzamides with Eqn I

Solvent PI PR+ PR..JPI vox 10-3 R8 Sb nC

Methanol 1.39 2.64 1.89 37.57 0.963 0.60 8

Ethanol 1.35 2.57 1.90 37.39 0.958 0.58 8

Propan- I-ol 1.26 2.65 2.10 37.42 0.951 0.65 8

Propan-2-ol 1.55 2.76 1.78 37.27 0.955 0.65 8

Butan-I-ol 1.38 2.70 1.95 37.25 0.966 0.69 8

Butan-2-ol 1.20 2.63 2.19 37.20 0.958 0.59 8

2-Methy I-I-propanol 1.18 2.74 2.32 37.33 0.968 0.53 8

2-Methyl-2-propanol 1.18 2.63 2.22 37.36 0.966 0.52 8

Ethylacetate 0.99 2.61 2.64 37.48 0.960 0.56 8

Diethylether 1.24 2.77 2.23 38.22 0.967 0.54 8

' Correlation coefficient; bStandard errors of the estimate; cNumber of points in the set

USCUMLI C et ai. : ABSORPTION SPECTRA OF N-(4-SUBSTITUTED PHENYL)-BENZAMIDES 209

Table 111- Results of correlations Villa. X 10') for N-(4-substituted phenyl)-benzamides with Eqn 2

Substituent vo'lO') s· b· a" Rb SC nd

N (CH)h 34.03 -1 .63 -0.86 -0.03 0.875 0.22 7

OCH) 37.95 -3.06 -2.93 1.41 0.889 0.29 7

CH) 38.92 -2.43 -1.41 0.44 0.976 0.09 7

H 39.94 -2 .67 -1.71 0.42 0.971 0.12 7

F 39.30 -2.37 - 1.10 0.29 0.854 0.23 7

CI 39.51 -2.92 - 1.98 0.85 0.986 0.07 7

Br 38.88 -2.52 - 1.57 0.68 0.918 0.16 7

CF) 40.02 -3 .38 - 1.38 0.48 0.963 0.14 7

CN 34.59 -1 .49 -0.62 0.28 0.892 0.10 7

COCH) 35.46 -2.59 -1.68 0.85 0.855 0.22 7

N02 35.49 -2.79 -2.12 1.03 0.860 0.26 7

·Solvatochromic coefficients; bCorrelation coefficient; cStandard errors of the estimate; dNumber of points in the set.

to distinguish between the contribution from the inductive (PI) and the resonance (PR +) effects of substituents.

.. . (1)

The correlations obtained from Eqn 1 (Table II) for eight substituents, excluding strong electron­withdrawing substituents (CN, COCH3 and N02) gave results which can be considered satisfactory. The composition of the electronic effect from DSP analysis indicates that the main effect through which these substituents influence chemical shifts of absorption frequencies is the resonance effect while the inductive effect is less significant.

The similar analysis of 15N chemical shifts showed a very large contribution (more than 80%) of the resonance effect l 6

.

The blending constant being defined as PR +/PI in­crease with decreasing solvent polarity. This is attrib­uted to a increasing relative importance of cross­conjugation as the polarity of the solvent decreased. In the less polar solvents, the increased sensitivity of the investigated molecules to substituent effects is due to the greater stability of the resonance structure which, through direct conjugation impart an enhanced resonance effect from the substituent to the reaction center. These results show that the polarity of the sol­vents strongly affects the mechanism of the transmis­sion of substituent effects in investigated systems.

The effects of solvent polarity and hydrogen bonding on the N-( 4-substituted phenyl)-benzamides are interpreted by means of the linear solvation energy relationships (LSER) concept proposed by Kamlet and Taft22 using a general solvation Eqn 2 of the form:

v = Vo + sn* + aa + bl3 .. . (2)

where a, 13 and n* are solvatochromic parameters and a, band s are the solvatochromic coefficients. In Eqn 2 n* is an index of the solvent dipolarity/polarizability, which is a measure of the ability of the solvent to stabilize a charge or a dipole by virtue of its dielectric effect. The variable a is a measure of the solvent hydrogen-bond donor (HBD) acidity, and describes the ability of a solvent to donate a proton in a solvent-to-solute hydrogen bond. The variable 13 is a measure of the solvent hydrogen-bond acceptor (HBA) basicity, and describes the ability of a solvent to accept a proton in a solute-to-solvent hydrogen bond.

The correlation of the spectroscopic data were carried out by means of multiple linear regression analysis. It was found that Vrnax for N­phenylbenzamide in seven solvents for which the solvatochromic parameters are known (methanol, ethanol, propan-2-0I, butan-I-ol, 2-methyl-2-propanol, ethyl acetate, diethyl ether) shows satisfactory correlation with n* , a and 13 values23. The multiple linear regression Eqn 3 is:

Vrnax = 39.94 - 2.67 n* - 1.71 13 + 0.42 a .. . (3)

(R=0.9708, s=0.125, n=7) The results of the correlations for all N-(4-

substituted phenyl)-benzamides are presented in Table III.

The negative signs of sand b coefficients in the total solvatochromic equations (Table III) for all N­(4-substituted phenyl)-benzamides indicate a bathochromic shift with both increasing solvent polarity and solvent hydrogen bond acceptor basicity.

210 INDIAN J. CHEM., SEC B, JANUARY 2002

This suggests stabilization of the electron excited state (II) relative to the state (I) and that most of the solvatochromism is due to solvent polarity and basicity rather than solvent acidity (aJs = -0.157; b/s=0.640 for N-phenylbenzamide). The correlations obtained from Eqn 2 (Table III) gave results which can be considened satisfactory when substituents of moderate elecronic effects are presented in the benzene nucleus. The correlations for six out of eleven substituents display a correlation coefficient R that is less than 0.9. This suggests to the additional interactions of solvents with strong electron-donating and electron-withdrawing substituents. These results show that the solvent effects on UV absorbtion spectra of N-(4-substituted phenyl)-benzamides is very compJexed.

The results for N-phenylbenzamide (Eqn 3) are compared with the results for N-phenyl-2-phenylacetamide (Eqn 4) in same solvents and under the same experimental conditions.

Vmax = 41.11 - 3.03 n* - 0.72 ~ + 2.60 a (R=0.9884, s=0.171, n = 7)

... (4)

The positive sign of a coefficient in Eqn 4 indicate a hypsochromic shift with increasing solvent hydrogen bond donor acidity . This suggests that most of the solvatochromism in N-phenyl-2-phenylacetamide is due to solvent polarity and acidity rather than solvent basicity (aJs=-0.858; b/s=0.237 for N-phenyl-2-phenylacetamide).

These results show that the introduction of -CHr group between phenyl and carbonyl groups in benzamide, influences the polarization of amide chromophore and changes the amide bond sensitivity to solvent effects compared with corresponding acetamide.

Experimental Section

The N-(4-substituted phenyl)-benzamides were prepared by the Schotten-Baumann reaction of the acylation of 4-substituted anilines with benzoyIchloride24

.

The compounds were purified by repeated crystallization and are stable. All synthesized ami des have a very high purity (G.c.) and were identified by microanalysis, FT IR, HNMR and MS data.

Ultraviolet absorption spectra in the region 200-400 nm were measured on a Shimadzu 160 A spec­trophotometer. The spectra were run in spectroquality solvents using concentrations of 1 x 10-5 M.

Acknowledgement The anthors are grateful to the Ministry for Science

and Technology of the Republic of Serbia for financial support.

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