Equilibria and kinetics of reactions between carboxylic acids and the carbinol base of crystal violet in apolar aprotic solvents: Relative strengths of m- and o-substituted benzoic acids in toluene

Article abstract of DOI:10.1002/(SICI)1099-1395(199706)10:6<466::AID-POC916>3.0.CO;2-N

Equilibria and kinetics of proton-transfer reactions in toluene between a set of m- and o-substituted (methyl, methoxy, chloro, nitro and hydroxy) benzoic acids and the carbinol base (colourless) of Crystal Violet were studied spectrophotometrically. The data on equilibrium and kinetic parameters were critically analysed. The rate constant for the forward step of the acid-carbinol equilibrium was found to be an appropriate criterion for determining acidities in toluene. Toluene-phase acidities were found to be significantly more susceptible than aqueous-phase acidities to the structure of the substituents, particularly for the m-substituted acids. Quantitative analysis of the substituent effect on toluene-phase acidities using the Fujita-Nishioka model for the extended Hammett equation showed that the predominant factor contributing to the 'ortho effect' for benzoic acid-dye carbinol reactions in toluene is the proximity polar effect rather than the steric effect for most of the o-substituents

Full text of DOI:10.1002/(SICI)1099-1395(199706)10:6<466::AID-POC916>3.0.CO;2-N

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 466–471 (1997)
EQUILIBRIA AND KINETICS OF REACTIONS BETWEEN CARBOXYLIC
ACIDS AND THE CARBINOL BASE OF CRYSTAL VIOLET IN APOLAR
APROTIC SOLVENTS: RELATIVE STRENGTHS OF m- AND
o-SUBSTITUTED BENZOIC ACIDS IN TOLUENE
SUSANTA K. SEN GUPTA* AND UDAI ARVIND
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
Equilibria and kinetics of proton-transfer reactions in toluene between a set of m- and o-substituted (methyl, methoxy,
chloro, nitro and hydroxy) benzoic acids and the carbinol base (colourless) of Crystal Violet were studied
spectrophotometrically. The data on equilibrium and kinetic parameters were critically analysed. The rate constant for
the forward step of the acid–carbinol equilibrium was found to be an appropriate criterion for determining acidities in
toluene. Toluene-phase acidities were found to be significantly more susceptible than aqueous-phase acidities to the
structure of the substituents, particularly for the m-substituted acids. Quantitative analysis of the substituent effect on
toluene-phase acidities using the Fujita–Nishioka model for the extended Hammett equation showed that the
predominant factor contributing to the ‘ortho effect’ for benzoic acid–dye carbinol reactions in toluene is the proximity
polar effect rather than the steric effect for most of the o-substituents © 1997 by John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 10, 466–471 (1997) No. of Figures: 2 No. of Tables: 3 No. of References: 18
Keywords: carboxylic acids; Crystal Violet carbinol base; apolar aprotic solvents; benzoic acids; toluene
Received 28 August 1996; revised 28 December 1996; accepted 21 January 1997
INTRODUCTION
useful as reference acids for determining the basicities of
various aliphatic and aromatic amines, N-heterocyclics and
diaryl- and triarylguanidines in several apolar aprotic
solvents. Results of equilibria and kinetics of organic
base–indicator acid associations have been reviewed.
Davis and co-workers, the pioneers in the area, developed
an indirect method for determining the relative strengths of
The solution phase acidity/basicity of a solute reflects its
intrinsic proton-donating/accepting power and the effect of
the solvent, which is often clouded by its own acidity/
basicity, dielectric constant, polarizability and various
solute–solvent interactions. An apolar aprotic solvent
E =0·0–0·3) of low dielectric constant (␧<6) offers
1
2
N
T
(
1
9 aliphatic and 47 aromatic carboxylic acids in benzene by
significant advantages for investigating proton-transfer
processes as specific solute–solvent interactions would be
greatly minimized and the proton transfer is expected to be
direct. Despite these advantages, there have been far fewer
investigations on rate equilibria of proton-transfer reactions
in apolar aprotic solvents than in protic and dipolar aprotic
solvents. Moreover, since many organic reactions are
carried out in hydrocarbons and halocarbons, the measure-
ment of acid/base properties in such solvents is of
significant interest. However, the determination of the
strength of an acid/base in such a solvent which cannot by
itself act as a base/acid requires the use of an appropriate
reference base/acid. Indicator acids such as 2,4-dinitro-
phenol, picric acid, 2,4,6-trinitro-m-cresol, Bromophenol
Blue and Bromophthalein Magenta E have been found
studying the equilibrium of association for the carboxylic
acids with 1,3-diphenylguanidine, the reference base, in the
presence of Bromophthalein Magenta E which competes for
1, 3, 4
the base.
A direct method for determining relative
acidities in benzene was devised by Palit and co-workers,
who introduced the carbinol base of Crystal Violet as a
reference base. Acid–dye carbinol base reactions in an
apolar aprotic medium are interestingly slow enough for
their kinetics to be monitored by ordinary spectrophoto-
metry. They studied both the equilibria and kinetics of
benzene-phase reactions of various carboxylic acids, phe-
nols, fluoro and nitro alcohols and mercury(II) chloride
5–7
(
Lewis acid) with the carbinol base of Crystal Violet or
8
Ethyl Violet.
Sen Gupta and co-workers applied the procedure for
evaluating equilibrium and kinetic parameters for toluene-
phase reactions of chelating acids for boron and the
*
Correspondence to: S. K. Sen Gupta
1997 by John Wiley & Sons, Ltd.
©
CCC 0894–3230/97/060466–06 $17.50

RELATIVE STRENGTHS OF m- AND o-SUBSTITUTED BENZOIC ACIDS IN TOLUENE
boron-complexed acids (hydrogen spiroborates) with differ- RESULTS AND DISCUSSION
467
ent triphenylmethane dye carbinols and utilized the data for
the selection of optimum solvent extraction–spectro-
photometric reagent systems for boron determination.
Carbinol bases (colourless) of triarylmethane dyes [Mala-
chite Green, Brilliant Green, Methyl Violet, Crystal Violet,
Ethyl Violet and Victoria Pure Blue BO (Dye-OH)] react
with acidic substrates [(HA): aliphatic and aromatic car-
boxylic acids, phenols, fluoro and nitro alcohols, hydrogen
spiroborates] in benzene, toluene and other apolar aprotic
9, 10
Further, they determined the relative toluene-phase basici-
ties of six triarlylmethane dye carbinols and correlated the
+
basicities with ␴ parameters of their p-alkylamino sub-
R
11
stituents. However, there has not been reported any
systematic study on the effect of substituent structures on
the strengths of acids in such apolar aprotic solvents,
particularly by any direct method. Since organic acids in
apolar solvents undergo aggregation and homoconjugation
to varying extents, it is judicious to consider a set of acids
of very similar chemical type to minimize complications
while interpreting the results. A spectrophotometric inves-
tigation on the equilibria and kinetics of the toluene-phase
proton transfer between a set of selected o- and m-
substituted [CH (+R, +I), OCH (ϪI<+R), Cl (ϪI>+R)
+
Ϫ
5–11
media to form coloured ion associates (Dye A ):
+
Ϫ
*
Dye-OH+HA)Dye A +H O
(1)
2
It has been argued that the water molecule formed is
hydrogen bonded to one of the species present in the
5–11
system. Thus, equation (1) can be rewritten as
+
Ϫ
*
D+HA)DH A
(2)
+
Ϫ
where D represents a dye carbinol and DH A the ion
associate.
3
3
and NO2 (ϪI, ϪR) benzoic acids and the carbinol of
Crystal Violet was undertaken with the objectives of
determining a scale of toluene-phase acidities and shedding
some light on the intrinsic-only factors for the ‘ortho
effect.’ The results obtained are critically discussed here.
Equilibrium results
The equilibrium data for the toluene-phase reaction of the
set of m-and o-substituted benzoic acids (HA) with the
carbinol base of Crystal Violet (D) were found to fit well the
equation
EXPERIMENTAL
+
Ϫ
[
DH A ]
K=
(3)
n
[
D][HA]
The chemicals used were either of analytical-reagent grade
or highly purified by standard procedures. Toluene was
chosen as the reference apolar aprotic solvent because of its
The values of n, the overall acid exponent, and logK, the
association constant {obtained from the linear log
[DH A ]/[D]) vs log [HA] plot} are given in Table 1.
relatively
low
toxicity.
4,4Ј,4Љ-Tris(dimethyl-
+
Ϫ
(
amino)triphenyl carbinol, which is colourless, was
employed as the reference base. The toluene solution of the
Ϫ5
carbinol base was prepared by basification of a 10
M
The overall acid exponent (n)
aqueous solution of Crystal Violet [4,4Ј,4Љ-tris(dimethyl-
amino)triphenyl chloride] with 2 NaOH and subsequent
As can be seen from Table 1, n is non-integral and usually
greater than unity. This is explained as being due to
overlapping reactions of the simple monomer acid (HA) and
acid–acid anion homoconjugate complex acids,
M
extraction into toluene in accordance with a standard
5
procedure.
H[A · · · (HA) ] mу1 (such complex acids, which are
m
Choice of acids. A set of benzoic acids substituted at the
m- and o-positions with groups of varying R (resonance)
and I (inductive) effects, CH (+I, +R), OCH (+R>ϪI),
stronger than the simple HA, are known to form in toluene-
1
like media ) with the carbinol base of Crystal Violet in
3
3
toluene. Hence n is a measure of the number of HA
molecules associated with a D molecule at equilibrium. As
one would expect, the value of n has been found to be larger
for stronger acids which allow a greater degree of
homoconjugation (see Table 1). The comparatively small
value of n for salicylic acid reflects strong intramolecular
hydrogen bonding in the salicylate anion which would
inhibit seriously its homoconjugation. It is worth pointing
out that the value of n for an acid is not strictly a constant
Cl (ϪI>+R) and NO (ϪI, ϪR), were selected to study
2
their structural influences (polar-ordinary and proximity and
steric) on the proton transfer from the —COOH group.
Salicylic acid was also included in the set to study the
enhancement of acidity due to chelation in toluene as
compared with water.
Procedure. The equilibrium and kinetics of the reaction
between the carbinol base of Crystal Violet, Dye-OH
11
7
Ϫ6
Ϫ5
Ϫ4
Ϫ2
and may depend on acid concentration, specific solvent
(
10 –10
M
), and an acid (HA) (10 –10
M
+
) in dry
7
Ϫ
and temperature. In fact, n is a composite parameter and
equals of fϪr, where f and r are the individual acid
exponents for the forward and reverse steps of the
equilibrium [equation (2)], respectively [see equation (10)].
toluene, producing a coloured ion associate, Dye A , were
measured with a Shimadzu 160 A UV–VIS recording
spectrophotometer at 610 nm at 28±0·1 °C. The molar
absorptivity of Crystal Violet cation (Dye ) in toluene at
8 °C was found to be 5·9ϫ10 l mol cm
+
4
Ϫ1
Ϫ1
2
.
Association constant (K). As n is not a true constant, the
©
1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 466–471 (1997)

4
68
S. K. SEN GUPTA AND U. ARVIND
Table 1. Values of association constant (logK), overall acid exponent (n), individual rate constants
(
logk , logk ) and individual acid exponents ( f, r) for substituted benzoic acids–Crystal Violet
1 Ϫ1
carbinol base reactions in toluene at 28·5±0·1 °C
pKa
LogK
n
Logk1
LogkϪ1
(±2%)
f
r
Acid
(water) (±2%) (±3%) (±2%)
(±3%) (±3%)
Benzoic acid
4·20
3·91
4·27
4·09
4·09
2·94
3·83
2·17
3·49
3·00
1·85
1·96
1·74
2·73
2·29
4·11
3·96
5·38
5·24
4·80
0·98
1·19
0·93
1·13
1·11
1·23
1·29
1·33
1·30
1·12
1·75
1·99
1·46
2·70
2·09
4·72
3·99
7·25
7·91
5·55
Ϫ0·10
0·03
Ϫ0·28
Ϫ0·03
Ϫ0·20
0·62
0·03
1·87
2·67
0·75
1·39
1·67
1·37
1·58
1·48
1·70
1·57
2·05
2·21
1·57
0·41
0·48
0·44
0·45
0·37
0·48
0·28
0·72
0·91
0·45
o-Methylbenzoic acid
m-Methylbenzoic acid
o-Methoxybenzoic acid
m-Methoxybenzoic acid
o-Chlorobenzoic acid
m-Chlorobenzoic acid
o-Nitrobenzoic acid
m-Nitrobenzoic acid
o-Hydroxybenzoic acid
association constant (K) is simply an equilibrium concentra-
tion ratio and not a true thermodynamic constant. However,
the variation in n among the acids selected is small enough
of the substituent’s field-inductive effect on proton transfer
from the —COOH group. LogK is related to the pK for the
a
acids by
(
see Table 1) to employ logK values not unjustifiably as
logK=Ϫ4·76 pK +21·96
(4)
a
indices of toluene-phase acidities. A plot of logK (toluene)
vs pK (water) (Figure 1) yields two distinct straight lines,
one for the m-substituted benzoic acids only and the other
for the o-substituted acids.
The large enhancement (4·76-fold) of the susceptibility of
proton transfer to the nature of the substituent on changing
the medium from water (␧=78) to toluene (␧=2·38)
substantiates further that the influence of a m-substituent is
predominantly an electrostatic field effect exerted through
both the molecule and the surrounding solvent sheath.
a
m-Substituted benzoic acids. The orders of the strengths
of these acids in solvents as diverse as water and toluene are
exactly parallel (see Table 1), indicating the predominance
o-Substituted benzoic acids. The logK vs pK plot for
a
the o-isomers (Figure 1) can be represented by
logK=Ϫ1·77 pK +9·49
(5)
Comparison with that for the m-isomers [Figure 1; equation
4)], taken as the ‘normal’ behaviour, indicates clearly that
a
(
o-isomers are either too weak in toluene or too strong in
water. Further, the order of their logK values is not the same
as that of their pK values (see Table 1). An o-substituent
a
would influence the reaction centre of a substrate through a
number of intrinsic ‘effects’ such as resonance, steric and
intramolecular hydrogen bonding, besides the field-induc-
tive effect. However, in a solvating medium some of these
effects may be significantly affected. In water, steric
interference of an o-substituent (with a +R effect) bare or
hydrated with the hydrated —COOH group would inhibit its
resonance (acid-weakening) with the aromatic ring. Hydra-
tion would contribute to further enhancement of the acidity
by disrupting the chelated structure, if any, of the non-
dissociated acid, whereas in non-solvating toluene acidities
would be influenced primarily by the substituent’s intrinsic-
only factors. Thus, on changing the solvent from water to
toluene the, acidity of an o-isomer undergoes a considerable
reduction compared with that for the m-isomer. Obviously,
the logK values rank significantly better as indices for the
intrinsic strengths of o-substituted benzoic acids than their
Figure 1. Comparison of the strengths of (᭺) m- and (ᮀ) o-
substituted benzoic acids in toluene and water as measured by
pK values. The case of salicylic acid is an exception. Its
a
logK (toluene) and pK (water)
anomalously high acidity in water is preserved in toluene
a
©
1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 466–471 (1997)

RELATIVE STRENGTHS OF m- AND o-SUBSTITUTED BENZOIC ACIDS IN TOLUENE
469
1
2
(
see Table 1) and even in the gas phase. This must be due
Plots of the left-hand side of equation (11) vs log [HA] were
found to be linear for all the acids selected. LogkϪ1
(intercept) and f (slope) obtained by the least-squares
to the stabilization of the conjugate base, the salicylate
anion by the same strong intramolecular hydrogen bonding
involving the phenolic hydrogen and the carboxylate anion
in all the three media. Thus, the overall effect of an o-
substituent on proton transfer from the —COOH group in a
benzoic acid molecule is contributed to by field-inductive,
resonance, steric, cheltion (in both the undissociated acid
and its conjugate base), solvation and possibly other less
significant factors.
method and logk and r calculated using equations (9) and
1
(10), respectively, are given in Table 1.
The strength of an acid in an apolar aprotic solvent as
determined in terms of its association constant (K) with a
dye carbinol (when the n values do not differ widely) is thus
due to the interplay of at least four parameters, k , k , f
1
Ϫ1
and r.
Kinetic results
Individual acid exponents (f, r)
The toluene-phase reaction between a substituted benzoic
acid (HA) and the carbinol base (D) of Crystal Violet
The acid exponent f for the forward step can be interpreted
as the mean aggregation number of HA species, HA,
[
equation (2)] under the conditions employed, CHA ӷC (see
D
H(A · · · HA), H[A · · · (HA) ], etc., reacting with the dye
2
Experimental), follows first-order kinetics:
carbinol in the forward step, the acid reacting effectively as
an f-mer, (HA) . The f values, as expected, are generally
f
greater than unity and non-integral (see Table 1). Further,
the magnitude of f (1·4–2·2) shows an upward trend with
increase in the strength of HA.
The acid exponent r for the reverse step is much smaller
(0·3–0·9) (see Table 1) and can be interpreted as the number
2
·303
Xe
k=
log
(6)
ͩ
ͪ
Ϫ
t
X ϪXt
e
where k is the rate constant and X and Xt are the
e
+
absorbances of the ion associate (DH A ) at equilibrium
and at time t. The rate constant (k), however, depends upon
CHA , the total acid concentration. In an apolar aprotic
solvent, a carboxylic acid (HA) exists as an equilibrium
mixture of the monomer acid, HA, and the stronger
homoconjugate acid–acid anion complex acids,
H(A · · · HA), H[A · · · (HA) ], etc., the proportion of the
latter increasing with increase in CHA . Thus, the overall
proton-donating power of a carboxylic acid solution in such
solvents increases with increase in CHA . On the assumption
that both the forward and reverse steps of the equilibrium
of HA molecules released from the f-mer, (HA) , after the
f
equilibrium of its reaction with the dye carbinol has been
attained. The degree of homoconjugation in the ion
+
Ϫ
associate DH [A · · · (HA)nϪ1 ] (where n is the overall
+
Ϫ
acid exponent) symbolized as DH A would depend on the
Ϫ
+
relative affinity of the base A between the acids DH and
1
2
HA and differ from f. In fact, when HA is considerably
+
stronger than DH (the conjugate acid of the dye carbinol),
5–7, 11
n could exceed f and r and may even become negative.
Individual rate constants (k , k
)
Ϫ1
1
[
equation (2)] are influenced by the acid, equation (2) can be
As can be seen from Table 1, the magnitude for the rate
expressed as
constant of the forward step, k , is much greater than the
+
Ϫ
1
*
D+fHA)DH A +rHA
(7)
rate constant for the reverse step, kϪ1 , and both the
parameters vary considerably over the set of acids. How-
+
Ϫ
where DH A symbolizes the coloured ion associate
irrespective of the number of HA molecules associated with
a D molecule and f and r are the individual acid exponents
for the forward and reverse steps of the equilibrium,
respectively. If k and k represent the rate constants for the
ever, compared with kϪ1 , k for an acid is significantly more
1
sensitive to the strength of the acid. Further, the magnitudes
of k and f and also those of k and r follow a similar trend
1
Ϫ1
(
see Table 1), pointing to the catalytic role of the acid in
both the forward and reverse steps of the equilibrium
equation (2)].
1
Ϫ1
forward and reverse steps, respectively, it follows that
[
f
r
k=k [HA] +kϪ1[HA]
(8)
(9)
1
Log k scale of toluene-phase acidities
1
k1
K=
kϪ1
An acidity scale in terms of logk would be meaningful only
1
if the acids do not have significantly different f values. This
scale would be of greater intrinsic value than that in terms
of logK as the latter depends on the interplay of at least four
parameters (k , k , f and r) as against two (k and f) for the
former. The variation observed in f over the different acids
is ±0·42. However, it seems justifiable to infer that
neglecting the variation would not invalidate the general
n=fϪr
(10)
Algebraic manipulations of equations (8), (9) and (10)
lead to the relationship
1
Ϫ1
1
k
log
=f log [HA]+logkϪ1
(11)
ͩ
Ϫn
ͪ
conclusions reached on the basis of the logk
1
scale of
K+[HA]
acidities. As can be seen from Table 1, the logk acidity
1
©
1997 by John Wiley & Sons, Ltd.
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4
70
S. K. SEN GUPTA AND U. ARVIND
longer applies and an extended form must be used. An
13
equation proposed by Fujita and Nishioka has been found
to be obeyed for reactions of o-substituted benzoic acids
except those whose conjugate bases undergo significant
chelation (e.g. salicylic acid) and was used in this study to
estimate the relative contribution of various components of
the total effects of o-substituents on the toluene-phase
acidity of benzoic acids. The equation is
logKortho =␳␴ +␦E +fF+C
(15)
o
s
where Kortho is the equilibrium or rate constant of a reaction
for the o-substituted substrate, ␴ , Es and F are the
o
substituent parameters for the ordinary polar effect
1
3
(
␴ ϵ␴ ), the primary steric effect and the proximity polar
o
p
effect respectively, ␳, ␦ and f are the susceptibility constants
corresponding to ␴ , E and F, respectively, and C is the
o
s
intercept. It may be noted that the Fujita–Nishioka model
equation (15)], although obeyed by the substituents OCH3 ,
OC H5 and OC H5 capable of forming intramolecular
[
2
6
H-bonds with the undissociated carboxylic group, does not
contain any term representing this factor. This is corrobo-
rated by a chemometric analysis of o-substituent effects
which indicated that the chelation effect in the undissociated
acid is not as significant as to be necessary to express it
Figure 2. Comparison of the strengths of (᭺) m- and (ᮀ)
o-substituted benzoic acids in toluene and water as measured by
logk (toluene) and pK (water)
1
a
scale runs parallel to the logK scale for either isomer (o- or
m-). Further, the logk vs pK plot (Figure 2), like the logK
14
explicitly.
1
a
The logk1 data for the toluene-phase reactions of
substituted benzoic acids with the carbinol base of Crystal
Violet were correlated with equation (14) for the m-isomers
and with the equation (15) for the o-isomers by the method
of multiple linear regression analysis, and the levels of
significance of the correlation coefficient (R) and the
regression coefficients (␳, ␦, f) were examined statistically.
The calculations were carried out on a UPTRON DS-86
PCS computer and the results are discussed below.
vs pK plot, yields two separate straight lines, one for the m-
a
substituted acids:
logk =Ϫ8·27 pK +36·33
(12)
(13)
1
a
and the other for the o-substituted acids:
logk =Ϫ2·66 pK +12·99
1
a
However, the slope of the logk vs pK plot is considerably
1
a
greater than that of the corresponding logK vs pK plot
a
[
equations (4) and (5)]. Obviously, logk1 values are
Substituent parameters employed
significantly more sensitive than logK values to the structure
of the substituent and thus better indices of intrinsic
acidities.
Quantitative treatment of substituent effect on toluene-
phase strengths of benzoic acids by Hammett
methodology
Except for the o-OH group, which undergoes significant
chelation with carboxylate anion, all the other substituents
considered (m- and o-methyl, -methoxy, -chloro and -nitro)
were included in the regression analysis. The ‘ordinary
polar effect’ parameters ␴m [equation (14)] and ␴ (ϵ␴ )
o
p
The simple Hammett equation:
logK =␳␴ +C
(14)
x
x
Table 2. Substituent parameters (␴ , ␴ ϵ␴ , E and F) used for
m
p
o
s
correlation
where K is the equilibrium (or rate) constant of a reaction
x
for the substituted substrate, ␴x is the substituent (x)
constant pertaining to its electronic effects only, ␳, the
reaction constant, is a measure of the sensitivity of the
reaction to the electronic perturbation and C corresponds to
the equilibrium or rate constant for the unsubstituted
substrate, applies to m- and p-substituents only. However,
for o-substituents which exert, in addition to the ordinary
electronic effect (common to m- and p-substituents), various
proximity effects such as direct field effect, steric effect and
chelation effect to various extents in different physical-
organic systems, the simple Hammett equation (14) no
a
a
b
c
Substituent
␴m
␴o (ϵ␴p )
Es
F
Methyl
Methoxy
Chloro
Nitro
Ϫ0·07
0·12
0·37
Ϫ0·17
Ϫ0·27
0·23
Ϫ1·24
Ϫ0·55
Ϫ0·97
Ϫ1·01(Ќ)
Ϫ0·04
0·26
0·41
d
0·71
0·73
0·67
a
From Ref. 15.
From Ref. 13.
From Ref. 16.
b
c
d
s
For minimum dimension—perpendicular orientation. E for the maximum
dimension—coplanar orientation is Ϫ2·52 (ʈ).
©
1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 466–471 (1997)

RELATIVE STRENGTHS OF m- AND o-SUBSTITUTED BENZOIC ACIDS IN TOLUENE
471
Table 3. Correlation results of reactivity data with equations (14) and (15)
Result of F-test
on R
(level of significance) (level of significance)
Result of t-test on
␳, ␦ and f
a
a
a
b
c
d
Benzoic acid
Solvent
Log K
␳
␦
f
C
n
R
m-Substituted Toluene
o-Substituted Toluene
m-Substituted Water
Logk1
Logk1
pKa
8·28
2·27
0·99
1·00
—
Ϫ0·64
—
—
4·57
—
1·57
1·75
Ϫ4·21
Ϫ4·05
5
5
11
0·981
0·999
0·969
Better than 1%
Better than 5%
Better than 1%
Better than 1%
Better than 0·5%
Better than 5%
Better than 0·5%
Better than 0·05%
o-Substituted
Water
pKa
Ϫ0·33
1·28
13 0·979
a
Regression coefficient.
Intercept.
Number of points involved in the correlation.
Multiple correlation coefficient.
b
c
d
[
equation (15)] are from the compilations of McDaniel and
ACKNOWLEDGEMENTS
1
5
Brown and the proximity polar effect parameters, F
One of the authors (U.A.) gratefully acknowledges financial
support by the Council of Scientific and Industrial Research,
New Delhi, in the form of a research associateship.
[
equation (15)], are the Swain Lupton constants as
16
improved and extended by Hansch et al. The primary
steric effect parameters, E [equation (15)] are the Taft
constants as modified by Kutter and Hansch with H as
the reference substituent [E (H)=0] instead of CH (in
s
1
7
18
13
s
3
REFERENCES
original E values). For the non-symmetrical NO group, the
s
2
1
. M. M. Davis, Acid–Base Behavior in Aprotic Organic
Solvents, NBS Monograph No. 105. National Bureau of
Standards, Washington, DC (1968).
E value for the minimum dimension, i.e. perpendicular
s
orientation to the aromatic ring, was adopted since the
2
o-NO substituent was found to correlate best with this Es
2. J. E. Crooks and B. H. Robinson, Faraday Symp. Chem. Soc.
10, 29–40 (1975).
value rather than that for the maximum dimension corre-
sponding to its coplanar orientation with the ring, for
proton-transfer reactions of benzoic acids in water and
3. M. M. Davis and H. B. Hetzer, J. Res. Natl. Bur. Stand. (U.S.)
60, 569–592 (1958).
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1
3
benzene. The values of the parameters ␴ , ␴ (ϵ␴ ), E
m
o
p
s
(
1966).
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049–1059 (1967).
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Indian J. Chem. 14A, 19–23 (1976).
and F for methyl, methoxy, chloro and nitro substituents are
5
6
7
given in Table 2.
1
(
Results of correlation
The results of the correlation of logk (toluene) data with
8. S. K. De and S. R. Palit, J. Indian Chem. Soc. 45, 395–403
(1968).
1
equations (14) and (15) for the substituted benzoic acids and
the level of significance for the correlation coefficient (R)
and the regression coefficients (␳, ␦ and f) are given in
Table 3. Table 3 also includes correlation results of pKa
9
. S. K. Sen Gupta and O. Balasubramanian, Bull. Chem. Soc.
Jpn. 64, 1437–1439 (1991).
1
0. S. K. Sen Gupta, O. Balasubramanian and U. Arvind, Indian J.
Chem. 32A, 810–813 (1993).
(water) of the acids. The correlation results reveal that the
1
1. S. K. Sen Gupta and U. Arvind, J. Phys. Org. Chem. 6,
45–152 (1993).
strength of an o-substituted benzoic acid is sensitive more to
the proximity effects than to the ordinary polar effects of the
substituent (␦+f>␳). The present analysis demonstrates
further that the major factor contributing to the proximity
effect, usually referred as ‘ortho effect’, is the proximity
1
1
2. T. B. McMahon and P. Kebarle, J. Am. Chem. Soc. 99,
2222–2230 (1977).
13. T. Fujita and T. Nishioka, Prog. Phys. Org. Chem. 12, 49–89
(1976); T. Fujita, Anal. Chim. Acta 133, 667–676 (1981).
1
1
1
4. O. Pytela and J. Liska, Collect. Czech. Chem. Commun. 59,
005–2021 (1994).
5. D. H. McDaniel and H. C. Brown, J. Org. Chem. 23, 420–427
1958).
polar effect and not the steric effect (fF>␦E ) for methoxy,
s
2
chloro and nitro substituents (see Tables 2 and 3) on benzoic
acid–dye carbinol reactions in toluene. However, the
(
opposite holds true for the methyl substituent (␦E >fF). A
s
6. C. Hansch, A. Leo, S. H. Unger, K. H. Kim, D. Nikaitani and
similar conclusion is obtained from the correlation analysis
E. J. Lien, J. Med. Chem. 16, 1207–1216 (1973).
17. R. W. Taft, Jr, in Steric Effects in Organic Chemistry, edited by
M. S. Newman, p. 556. Wiley, New York (1956).
of pK data for 13 o-substituted benzoic acids where also the
a
steric effect is the dominant factor for the alkyl substituents
only.
18. E. Kutter and C. Hansch, J. Med. Chem. 12, 647–652 (1969).
©
1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 466–471 (1997)