Separation of polar and steric effects in the oxidation of ortho- substituted benzaldehydes by N-bromobenzamide
K A L Y A N K B A N E R J I
Department of Chemistry, University of Jodhpur, Jodhpur 342 001, India MS received 28 Deedmber 1987; revised 25 March 1988
Abstract, The kinetics of the oxidation of twelve ortho-substituted benzaldehydes by N-bromobenzamide (NBB) to the' corresponding benzoic acids have been studied. The reaction is first order with respect to NBB, the aldehyde and hydrogen ions. The addition of benzamide has no effect on the reaction rate. (PhCONH,Br) ~ has been postulated as the reactive oxidismg species. The correlation of rates with the single substituent- parameter equations is poor. The correlation with Charton's equation of inductive, resonance and steric parameters is satisfactory. However, excellent correlations were obtained, when Charton's steric parameter was used along with Taft's cr t and ~/~ substi- tuent constants. The polar reaction constants [lave negative values. The reaction is subject to steric hindrance by the ortho-substituents.
Keywords. Ortho-effect; correlation analysis; benzaldehydes; N-bromobenzamidc; ox- idation.
1. Introduction
T h e c o r r e l a t i o n o f the rate a n d structure of o r t h o - s u b s t i t u t e d a r o m a t i c c o m p o u n d s is c o m p l i c a t e d b e c a u s e of the possible interaction of the substituent with the o r t h o - s i t e t h r o u g h the polar effects, p r o x i m i t y effects, h y d r o g e n b o n d i n g etc.
A t t e m p t s have b e e n m a d e to quantitatively separate and analyse the c o n t r i b u t i o n of various p a r a m e t e r s to the o r t h o - e f f e c t (Pavelich and Taft 1957; J o n e s 1979;
C h a r t o n 1969, 1971, 1975; A s l e m et al 1981)_ O f these, C h a r t o n ' s t r e a t m e n t ( C h a r t o n 197l, t975: A s l e m et al 1981) is c o n s i d e r e d the best m e t h o d b e c a u s e of its w i d e r applicability in explaining the n a t u r e o f the ortho--effect.
A kinetic study of the o x i d a t i o n of m e t a - and p a r a - s u b s t i t u t e d b e n z a l d e h y d e s by N - b r o m o b e n z a m i d d ( N B B ) has b e e n r e p o r t e d f r o m this l a b o r a t o r y (Banerji 1986).
T h e rates of o x i d a t i o n s h o w e d excellent correlation in T a f t ' s dual substituent- p a r a m e t e r ( D S P ) e q u a t i o n ( D a y a l et al 1972).
T h e kinetics o f the o x i d a t i o n of twelve o r t h o - s u b s t i t u t e d b e n z a l d e h y d e s by N B B is being" p r e s e n t l y r e p o r t e d . T h e rates were c o r r e l a t e d with v a r i o u s single p a r a m e t e r and multi p a r a m e t e r equations.
2. Experimental
2.1 Materials
o - M e t h y l t h i o - , o - c y a n o - , o - a c e t y l - b e n z a l d e h y d e s and m e t h y l o - f o r m y l b e n z o a t e were p r e p a r e d by r e p o r t e d m e t h o d s (Fusion and Daniels 1926; Stewart et al 1960;
397
Traynelis and Boragnaes 1972). Rest of the aldehydes were commercial products and were purified by either recrystallisation or distillation under reduced pressure.
NBB was prepared by the reported method (Banerji 1986).
2.2 Product analysis
The oxidation of the aldehydes results in the formation of the corresponding benzoic acids. The quantitative analysis of the product (Banerji 1986) leads to the isolation of the corresponding benzoic acids in 8 5 - 9 2 % yields.
2.3 Kinetic measurements
The reactions were carried out under pseudo-first-order conditions by keeping a large excess of the aldehyde over NBB. The reactions were followed iodometrically for up to 70% of the consumption of NBB. The pseudo-first-order rate constant, k l , was determined from the linear plots of log [NBB] vs. time. Duplicate kinetic runs indicate that the rate constants are reproducible to within _+ 4 % . The solvent was 1 : 1 (v/v) acetic acid-water. The other details have been described earlier (Banerji 1986).
3. R e s u l t s a n d d i s c u s s i o n
The oxidation of the substituted benzaldehydes lead to the formation of the corresponding benzoic acids
A r C H O + P h C O N H B r + H 2 0 ~ A r C O O H + P h C O N H 2 + HBr. (i) The reaction is first order with respect to the aldehyde, NBB and hydrogen ions (table 1). The experimental rate law, therefore, has the following form (2).
- d [ N B B ] / d t = k[NI3B] [ A r C H O ] [H + ] (2) Table 1. Rate constants of the oxidation of o-methoxybenzal-
dehyde by NBB at 298 K.
[Aldehyde] 103 [NBB] [H + ] 105 k~
moldm ~ moldm 3 moldm 3 s t
0.5 2.0 0.5 9.90
0.5 3-5 0.5 9-78
0.5 5-0 0.5 9.90
0.5 7.5 0-5 1(/.2
0.5 10-0 0.5 9.81
0.5 15-0 (1.5 9.78
0. l 5.0 0.5 2.0(I
0.2 5.0 (t.5 3.93
0-3 5.0 0.5 5-94
0-6 5.0 0.5 12.(/
0-8 5.0 0-5 15.7
1-0 5.(I 0.5 19.8
0-5 5.0 (t. 1 1.95
0-5 5.0 11.4 7-92
0-5 5.0 0-6 11.9
0.5 5-0 0-8 15.6
(I-5 5-0 1.0 19.7
T h e r e a c t i o n r a t e is n o t a f f e c t e d b y a d d e d b e n z a m i d e . T h u s , h y d r o l y s i s a n d / o r d i s p r o p o r t i o n a t i o n o f t h e N - h a l o g e n o a m i d e is n o t i n v o l v e d in t h e o x i d a t i o n p r o c e s s . T h e l i n e a r i n c r e a s e in t h e r a t e w i t h a c i d i t y s u g g e s t s t h a t N B B is p r o t o n a t e d t o g i v e a s t r o n g e r e l e c t r o p h i l e a n d o x i d a n t .
P h C O N H B r + H 3 0 + ' , ( P h C O N H 2 B r ) + + H 2 0 . (3) T h e s p e c i f i c r a t e c o n s t a n t s , k, f o r t h e t w e l v e b e n z a l d e h y d e s w e r e d e t e r m i n e d at v a r i o u s t e m p e r a t u r e a n d t h e a c t i v a t i o n p a r a m e t e r s w e r e e v a l u a t e d ( t a b l e 2).
A l i n e a r i s o k i n e t i c r e l a t i o n s h i p b e t w e e n l o g k at 293 K a n d 313 K ( r = i)-9992, s l o p e = 0 . 7 3 3 0 ) f o r t h e o x i d a t i o n o f b e n z a l d e h y d e a n d t h e ortho-substituted
b e n z a l d e h y d e s s h o w s t h a t all t h e c o m p o u n d s a r e o x i d i s e d b y t h e s a m e m e c h a n i s m ( E x n e r 1973). T h e v a l u e o f i s o k i n e t i c t e m p e r a t u r e is 385 K. A l i n e a r i s o k i n e t i c r e l a t i o n s h i p is c o n s i d e r e d as a n e c e s s a r y c o n d i t i o n f o r t h e v a l i d i t y o f l i n e a r f r e e e n e r g y r e l a t i o n s h i p s ( E x n e r 1973).
T h e r a t e s at 298 K w e r e a n a l y s e d s e p a r a t e l y in t e r m s o f T a W s p o l a r a n d s t e r i c p a r a m e t e r e q u a t i o n s ( C h a r t o n 1969). T h e r e s u l t s a r e e x p r e s s e d as (4) a n d (5). T h e v a l u e s o f ~r,, a n d E, w e r e t h o s e g i v e n b y C h a r t o n a n d J o n e s ( C h a r t o n 1969; J o n e s
1979).
l o g k/ko = - 4 . 3 7 ~,,, (4)
R = 0 . 9 5 6 1 ; s.d. = 0 . 8 3 ; n = 8,
l o g k/ko = 1.1)3 E , , (5)
R = 0.421)2: s.d. = 0 . 4 8 ; n = 11,
w h e r e R is t h e c o e f f i c i e n t o f c o r r e l a t i o n , s.d. is t h e s t a n d a r d d e v i a t i o n a n d n is t h e n u m b e r o f d a t a p o i n t s .
T h e r a t e s f o r N H C O M e a n d C O O M e c o m p o u n d s w e r e n o t c o n s i d e r e d f o r (5) a n d t h o s e o f t h e N H C O M e , C O O M e , C F 3 , C N a n d S M e w e r e n o t c o n s i d e r e d f o r Table 2 Rate constants and activation parameters of the oxidation of o-substituted benzaldehydes by NBB
lip k/dm ~' mol " s t Att* AS * AG*
Substituent 293 K 298 K 303 K 3118 K 313 K k] mol ' ] tool t K ' k] tool '
H ~' 1411 206 303 44(1 615 54.2 _+ (1.3 - 133 +0.9 94.1t _+ 11.1
NO, 3-17 5.12 8.57 12.8 20.6 68,5+0-8 - 116+2.6 11/3+__0-6
F 35.4 52.0 76.4 115 166 56.7+11.5 - 136_ + 1.7 97-4_+0.4
CI 11t-7 16-3 24.1 37-5 56-7 61-0+-0.8 - 131 +2-5 100-+_11.6
Br 9.86 16.0 75-5 41-3 61.11 67.5 +_0.6 - 1H)-+2.1 1//(I-+/I.5
1 7.12 10.5 15.8 22.1 32.6 55-2+_/).5 - 154-+ 1-7 101 _+0-4
OMe 291 396 5811 835 1100 49.4 +_ 1.0 - 144 -+ 3.2 92.3 -+ 0-8
Me 90.0 1311 186 265 370 61-5-+11.7 - 146 + 1.3 95.2-+1/.6
NHCOMe 66.0 98.5 147 2112 308 55.4 + 0.9 - 135 + 2.9 95.9 • (]-7
SMe 69.2 103 153 215 325 55.9-+1/.7 - 133-+2.2 95-8•
COOMe 8.77 12.9 19.6 27.1 404) 55.1 -+0-7 - 153+2.2 101 -+11.5
CN 4.25 6.911 11.3 17.3 29.1 70-1• -108-+3.0 102+0.7
CF~ 2-116 3.32 5.41 8.77 12.8 68.0_+0.9 - 121 +2.8 11/4_+1/.7
~Data of Banerji 11986)
(4), since the substituent constants were not available. The results of the correlation analyses show that the observed reactivity of the ortho-substituted benzaldehydes towards N B B is not compatible with either the size of the substituents or thei.r Taft's polar substituents constants.
Since the single s u b s t i t u e n t - p a r a m e t e r equations did not yield satisfactory correlations, the rate data were analysed using C h a r t o n ' s m e t h o d ( C h a r t o n 1975).
The rate constants were correlated with (6) and (7). In (6) and (7). crt, err and V are inductive, resonance and steric substituent constants respectively and the values used were those compiled by Aslem et al (1981).
The results of the analysis of the rate data at 298 K in terms of (6) are given in (8).
log k o r t h o = a O" t + ~ O" R + h (6)
log kortho = a c r t + r o'R + & V + h (7) log k = - 1.91o- t - 1 . 8 6 o ' R - 4 - 1 9 (8) R = 0"9173: s.d. = 0-29; n = 13"
In multiple linear regression using (6), the coefficient of correlation is p o o r and the standard deviation is high. T h e absence of significant correlation with (6) leads to the conclusion that the electric effects alone are not sufficient to account for the ortho-substituent effect in this reaction.
The correlation with (7) was p e r f o r m e d using the rate data obtained at 293 K, 298 K, 303 K, 308 K, and 318 K, assuming both orthogonal and planar c o n f o r m a - tions for o - N O : and o - C O O M e groups. It was observed that the correlation is consistent with the orthogonal conformation of both o - N 0 2 and o - C O O M e groups.
T h e coefficient of multiple correlation, R, varied between 0.95 and 0.97, and the standard deviation ranged from 0-17 to 0.22. The correlation obtained at 298 K is given in (9).
log k = - 2.07 o ' t - 1.91 o-R - 0-76 V - 3-71, (9) R = {).9746; s.d. = 0-17: n = 13.
The correlations are thus just satisfactory and not even good.
It m a y be recalled that the rates of oxidation of para-substituted b e n z a l d e h y d e s (Banerji 1986) showed excellent correlation with Taft's crl and cr~ substituent constants (Dayal et al 1972). T h e r e f o r e , the rates of oxidation of the ortho- substituted benzaldehydes were correlated in a triparametric equation/asing T a f t ' s o-t and ~r~ along with C h a r t o n ' s steric p a r a m e t e r , V. The values of T a f t ' s cr / and cr~
constants were those given by Ehrenson et al (1973) except that of the methylthio group. Ehrenson et al (1973) have reported that no single cr~ value for methylthio group could be agreed to and listed a large n u m b e r of possible values ranging from - 0 . 6 4 to - 0 . 9 6 . We tried all the values separately and found that the best results are obtained when a cr~ value of - 0 - 6 6 was assigned to the SMe group. T h e deletion of the datum of SMe does not adversely affect the significance of the correlation. T h e series of ortho-substituted benzaldehydes meets the minimal basic r e q u i r e m e n t s of substituents for analysis by Taft's DSP equation (Shorter 1982).
T h e b e h a v i o u r of o - N 0 2 and o - C O O M e groups are consistent with their orthogonal conformations.
T h e significance of the correlation was tested by m e a n s of an F-test (Wine 1966).
T h e confidence level of the F-test is > 99.9%. The confidence level for the significance of pz. p,~ and & terms was obtained by a Student's t-test. T h e confidence level of the t-test is > 9 9 . 9 % , indicating operation of significant inductive, resonance and steric effects.
T o test the significance of all the three substituent constants, multiple linear regression analyses were carried out with Taft's ~rl and o-~, Taft's o't and V and with cr~ and V. T h e absence of significant correlations [(10)-(12)] showed that all the three art, o'~ and V substituent constants are significant.
log k = - 2.03 o ' i - 0.96 o~,~ - 4.09, (10)
R = 0-9244; s.d. = 0-28; n = 13,
log k = - 4-28 o-t - 0.79 V - 3.32, (11)
R = 0-7972; s.d. = 0.44; n = 13,
log k = - 1 . 2 9 c r ~ - 1 . 4 0 V - 4 . 3 3 , (12) R = 0-8627; s.d. = 0.37; n = 13.
T h e r e is no significant collinearity between Taft's o9 and V, ~r~ and V, and b e t w e e n T a f t ' s art and ~r~ values ( r - 0.3766, 0.3884 and 0.2336 respectively) for the 13 substituents.
T h e regression coefficients of o-~ and ~ , terms are negative indicating that the electron-releasing groups accelerate the reaction and electron-withdrawing groups retard it. Similar results were obtained in the oxidation of para- and m e t a - substituted benzaldehydes also (Banerji 1986). T h e negative regression coefficient for the steric t e r m indicates that the reaction is subject to steric hindrance by the ortho-substituents. T h e contribution of the resonance effect to the total polar effect was calculated by using (13) (Charton 1975).
lOOx Ip ,l (13)
P R - - ipzt+tp
I
T h e contribution of the steric p a r a m e t e r (Charton 1975) to the total effect of the substituents, Ps, was d e t e r m i n e d by using (14).
I,t,I
xlOO (14)
=
ip, l+lp ,l+
T h e values of Pn and Ps are also recorded in table 3. T h e values of Pn ~ 3 8 % . T h e values of Pn for the oxidation of para-substituted benzaldehydes (Banerji 1986) range f r o m 54 to 6 0 % . This shows that the balance of inductive and resonance effects is different for the ortho- and para- positions, resonance effects being less p r o n o u n c e d in the f o r m e r case. This may be due to the twisting away of the aldehyde group f r o m the plane of the benzene ring. T h e value of Ps shows that the steric effect is considerable in this reaction.
T a b l e 3 T e m p e r a t u r e d e p e n d e n c e o f t h e r e a c t i o n c o n s t a n t s .
T e m p e r a t u r e
( K ) P/ O,~ $ R s.d, PR Ps
293 - 1 . 8 0 - 1.09 - 1.08 0 . 9 9 7 6 0 . 0 6 37-7 2 7 . 2
2 9 8 - 1 . 7 4 - 1-06 - 1.06 (I.9969 0 . 0 6 3 7 ' 9 2 7 . 5
303 - 1 . 6 9 - 1.03 - 1-04 0 . 9 9 5 5 0 . 0 7 3 7 - 9 2 7 . 7
3 0 8 - 1.62 - 1-02 - 1-04 0 - 9 9 4 6 0 - 0 8 3 8 . 6 2 8 . 3
3 1 3 - 1.57 - 0 . 9 9 - 1 . 0 2 0 - 9 9 3 6 0 . 0 8 3 8 . 7 2 8 . 5
The magnitude of all the reaction constants decreases with an increase in the temperature. This points to a decrease in the selectivity at higher temperatures.
Out of the two polar effects, the decrease in the inductive effect is p r o p o r t i o n a t e l y more. This is reflected in the gradual increase in the value of Pn. T h e contribution of the steric factor to the total effect of the o-substituent also increases gradually with the increase in temperature.
A perusal of the data recorded in table 2 showed that except o-methoxy- benzaldehyde, all the other benzaldehydes reacted at a rate slower than that of the parent compound. The resonance substituent constant for the O M e group has a large negative value ( - 1-02), while the inductive and steric substituent constants have small positive values (0.27 and 0.36 respectively). T h e r e f o r e , the rate- e n h a n c e m e n t caused by the resonance effect is more than the combined retardation in the rate due to the inductive and steric effects. Thus a net r a t e - e n h a n c e m e n t results.
The large negative polar reaction constants, a correlation involving o-~ values and a substantial deuterium isotope effect observed (Banerji 1986) in the oxidation of benzaldehyde point to a transition state in which the positive charge is highly localized on the aldehydic carbon atom. The transition state .thus approaches a carbocation in nature. This agrees with the hydride transfer mechanism proposed earlier (Banerji 1986). The presence of an o-substituent hinders the approach of the oxidizing reagent to the aldehydic group. This causes the steric retardation of the rate.
4. Conclusion
The following conclusions emerge from this study: ( i ) C h a r t o n ' s inductive and resonance substituent constants are not sufficient to account for the polar effects of the ortho-substituents in this reaction, (ii) inductive, resonance and steric effects play important roles in determining the ortho-effect in the oxidation, (iii) there is a strong resonance interaction between a developing positive charge and the o-group in the transition state, and (iv) the results support a hydride transfer mechanism.
Acknowledgement
Thanks are due to the CSIR, New Delhi, and the U G C , New Delhi, for financial support.
References
Aslem M H, Burden A G, Chapman N B, Shorter J and Charton M 1981 J. Chem. Soc., Perkin H 500 Banerji K K 1986 J. Org. Chem. 51 4764
Bloor J E 1964 Tetrahedron 20 861
Charton M 1969 J. Am. Chem. Soc. 91 6649 Charton M 1971 Prog. Phys. Org. Chem. 8 235 Charton M 1975 J. Org. Chem. 40 407
Dayal S K, Ehrenson S and Taft R W 1972 J. Am. Chem. Soc. 94 9113 Ehrenson S, Brownlee R T C and Taft R W 1973 Prog. Phys. Org. Chem. 10 1 Exner O 1973 Prog. Phys. Org. Chem. 10 411
Fusion R C and Daniels J 1926 J. Am. Chem. Soc. 48 1093
Jones R A Y 1979 Physical and mechanistic organic chemistry (Cambridge: Cambridge University Press) Pavelich W A and Taft R W 1957 J. Am. Chem. Soc. 79 4935
Shorter J 1982 Correlation analysis of organic reactivity (Chichestor: Research Studies Press) p. 50 Stewart J H, Klundt I K and Peacock K 1960 J. Org. Chem. 25 913
Traynelis V J and Borganaes D M 1972 J. Org. Chem. 37 3824
Wine R L 1966 Statistics for scientists and engineers (New Delhi: Prentice Hall) p. 253, 293
