General acid catalysis in benzoic acid-Crystal Violet carbinol base reactions in toluene

Article abstract of DOI:10.1002/kin.20628

Kinetics of benzoic acid-dye carbinol base reactions in an apolar aprotic solvent exhibit a complex dependence on the concentration of the acid, indicating the occurrence of general acid catalysis, the acid playing the dual role of the substrate and the catalyst. The complex kinetic behavior observed for 23 ortho- and meta-substituted benzoic acids has been traced to overlapping participations of the monomer acid, hyperacidic open chain homoconjugated complex (dimeric and trimeric) acids, and nonreactive cyclic dimer and trimer acids. It has been found that although the degree of proton transfer in the transition state is 50-60% when the acid acts exclusively in the monomeric form, it gets enhanced to as high as more than 90% when the role of homoconjugated complex acids is taken into consideration.

Full text of DOI:10.1002/kin.20628

General Acid Catalysis in
Benzoic Acid–Crystal Violet
Carbinol Base Reactions
in Toluene
SUSANTA K. SEN GUPTA, SANGEETA MISHRA, V. RADHA RANI, UDAI ARVIND
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
Received 13 June 2011; revised 14 October 2011; accepted 21 October 2011
DOI 10.1002/kin.20628
Published online 25 April 2012 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Kinetics of benzoic acid–dye carbinol base reactions in an apolar aprotic solvent
exhibit a complex dependence on the concentration of the acid, indicating the occurrence of
general acid catalysis, the acid playing the dual role of the substrate and the catalyst. The
complex kinetic behavior observed for 23 ortho- and meta-substituted benzoic acids has been
traced to overlapping participations of the monomer acid, hyperacidic open chain homocon-
jugated complex (dimeric and trimeric) acids, and nonreactive cyclic dimer and trimer acids. It
has been found that although the degree of proton transfer in the transition state is 50–60%
when the acid acts exclusively in the monomeric form, it gets enhanced to as high as more
than 90% when the role of homoconjugated complex acids is taken into consideration. ꢀ 2012
C
Wiley Periodicals, Inc. Int J Chem Kinet 44: 570–576, 2012
common pKa (H2O or DMSO) scale, which is sub-
jected to strong specific solvation effects. Among the
different bases tried, a triarylmethane dye carbinol
base, particularly Crystal Violet carbinol base, 4,4ꢁ,4ꢁꢁ-
tris(dimethylamino) triphenyl carbinol, has been found
to be an appropriate choice for the reference base. This
colorless carbinol base reacts with a variety of acids
(HA) ranging from simple carboxylic acids, phenols,
nitro-/fluoroalcohols to complex hydrogen spirobo-
rates in toluene and other apolar aprotic media, pro-
ducing a colored ion associate [2–19].
INTRODUCTION
Apolar aprotic solvents (ETN = 0.0 − 0.3) [1] offer
particular advantages for studying proton transfer re-
actions. In these solvents, specific solute–solvent in-
teractions are greatly minimized, if not eliminated and
proton transfer is expected to occur directly. Physic-
ochemical studies of acid–base reactions in an ap-
olar aprotic solvent between a series of acids and
a reference base are of significant aid in setting up
an acidity scale considerably more intrinsic than the
Timescales for these proton transfer reactions are in-
terestingly as long as several minutes, and the progress
of the reactions is conveniently monitored by ordi-
nary spectrophotometry. The reactions, in general, fol-
low pseudo–first-order kinetics with respect to the dye
carbinol base concentration. Their rate constants (k),
on the other hand, vary with acid concentrations (CHA).
Correspondence to: Susanta K. Sen Gupta; e-mail:
sksbhuchem@yahoo.co.in.
Contract grant sponsor: University Grants Commission, New
Delhi, India.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
c
ꢀ
2012 Wiley Periodicals, Inc.

GENERAL ACID CATALYSIS IN BENZOIC ACID–CRYSTAL VIOLET CARBINOL
571
The variation, however, did not follow any simple pat-
tern, indicating the occurrence of general acid cataly-
sis, an acid playing the dual role of the substrate and
the catalyst [2–4,7–14]. The complex k versus CHA
variation is not unexpected in view of the possibil-
ity of HA existing as a hyperacidic homoconjugated
complex acid, H(A· · ·(HA)m (m ≥ 1), a nonreactive
cyclic dimer (HA)2, etc., besides the monomeric HA
in an apolar aprotic medium [2–24]. The k versus CHA
variation has been probed in detail to determine the
constants of catalysis by such different acidic species
for a series of 23 meta- and ortho-substituted benzoic
acids in their toluene-phase reactions with Crystal Vi-
olet carbinol base, the reference base.
Correlations of both simple and composite catalytic
constants of the substituted benzoic acids with their
toluene-phase equilibrium reactivity parameters and
also with substituent parameters have been analyzed
with the focus on evaluating the contribution of ho-
moconjugated complex acids in catalyzing acid–base
reactions in apolar aprotic media. The results are dis-
cussed here.
H3C
CH3
N
C
N
H3C
H3C
N
OH
H3C
CH3
Figure 1 Structure of Crystal Violet carbinol base.
All stock solutions of Crystal Violet carbinol base
and meta- or ortho-substituted benzoic acids were pre-
pared by dissolving their weighed amounts in dry
toluene.
EXPERIMENTAL
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 relatively low toxicity. Crys-
tal Violet carbinol base, 4ꢁ,4ꢁꢁ,4ꢁꢁꢁ-tris(dimethylamino)
triphenyl carbinol (Fig. 1), which is colorless, was em-
ployed as the reference base.
Kinetics of the reaction between an acid, HA
(CHA ∼ 10−4–10−2 M, higher molarity order being
employed for relatively weaker acids and lower or-
der for stronger acids) and the colorless carbinol base
of Crystal Violet, Dye-OH (CD = 2.5×10−5 M), in dry
toluene, producing a colored ion associate, Dye+A−,
were monitored by absorbance measurements of the
ion associate at 610 nm in a Teflon-stoppered quartz
cuvette using a 160A Shimadzu UV–vis spectropho-
Crystal Violet carbinol base was prepared by
precipitating a 100-mL aqueous solution containing
∼100 mg of Crystal Violet dye [4,4ꢁ,4ꢁꢁ-tris (dimethy-
lamino) triphenyl chloride] with ∼2 M NaOH followed
by extraction of the carbinol base precipitated into
∼200-mL of toluene. The toluene extract was sepa-
rated and subjected to freeze–drying to obtain Crystal
Violet carbinol base as a powder (mp 180◦C). Elemen-
tal analysis results (C, 76.85%; H, 7.88%; N, 10.94%
as against C, 77.08%, H, 8.02%; N, 10.79% required
tometer at 28.0
0.1◦C. The sampling time for ab-
sorbance recordings during the reaction was every 5
to 10 s. Forty to sixty minutes was required to attain
the reaction equilibrium. The molar absorptivity (∈)
of Crystal Violet cation (Dye+) in toluene was found
3.3 × 104 dm3 mol−1 cm−1 at 28.0◦C and wavelength
610 nm.
for C25H31N3O); IR peak data in Nujol (3350 cm−1
,
bonded OH); 1H NMR signals in carbon tetrachloride
(2.90 ppm, s, 18H, 3N (CH3)2; 6.55 ppm, d, 6H, J =
10 Hz, aromatic H; 7.00 ppm, d, 6H, J = 10 Hz, aro-
matic H; 7.28 ppm, s, 1H, C OH) and freezing point
depression in benzene confirmed the structure of the
carbinol base (Fig. 1). Twenty-three substituted ben-
zoic acids, H; meta-Me, OMe, OPh, F, Cl, Br, I, COPh,
CF3, CN, NO2, ortho-Me, OMe, OEt, OPh, OAc, F, Cl,
Br, I, COPh, and NO2 benzoic acids, were employed
for the study.
RESULTS AND DISCUSSION
Acid–Dye Carbinol Base Reaction Kinetics
in Toluene
Substituted benzoic acids (HA) react with Crystal Vio-
let carbinol base (Dye-OH) in toluene to form colored
ion associates (Dye+A−):
+
−
ꢀ
ꢁ
A + Dye-OH
Dye A + H O
(1)
2
International Journal of Chemical Kinetics DOI 10.1002/kin.20628

572
SEN GUPTA ET AL.
Table II Equilibrium Parameters (log K , n) for
Toluene-Phase Reactions of Ortho-Substituted Benzoic
Acids with Crystal Violet Carbinol Base at 28.0 0.1◦C
It has been argued that the water molecule formed
(Eq. (1)) is hydrogen bonded to one of the species of
system and that they exist as one entity [25]. Taking
into consideration the possibility of HA existing not
simply as a monomer but effectively as a n-mer due
to formation of reactive open chain (homoconjugated
acid–acid anion complex acids) and nonreactive cyclic
acid multimers in toluene, the apolar aprotic solvent
[20–24], Eq. (1), can be reexpressed as
Substituent
log Ka
na
H
1.78
1.96
5.38
2.36
2.85
3.04
3.53
2.87
2.55
1.94
3.43
2.83
0.98
1.19
1.33
0.94
0.98
0.98
1.10
1.18
1.12
1.04
1.08
0.92
Me
NO2
F
Cl
Br
+
−
n
ꢀ
ꢁ
I
nHA + D
DH A
(2)
n
OMe
OEt
OPh
OAc
COPh
where D and DH+n An− represent the colorless carbinol
base of Crystal Violet and the colored ion associate, re-
spectively. The absorbance data for the reaction equi-
librium (Eq. (2)) for members of the present set of
meta- and ortho-substituted benzoic acids in toluene
fitted the expression
aRefs. [11,16]
ꢀ
ꢁ
DH+n A−n
[D][HA]n
other acids in apolar aprotic solvents [2–24]. This has
been ascribed to overlapping participations of open
chain dimeric and multimeric homoconjugated com-
plex acids H(A· · ·(HA)m (m ≥1) in the reaction with
the carbinol base (D) in apolar aprotic media [2–24].
The value of the acid exponent (n) for an acid de-
pends upon its concentration, temperature, and solvent
[10,17–24].
The time course of the absorbance data (At ) for
the reaction, Eq. (2), under the condition, CHA ꢂ CD
(vide the Experimental section) conforms excellently
to pseudo–first-order reversible kinetics for the acids
of the following set:
K =
(3)
where K is the association constant and n is an acid ex-
ponent. Values of log K and n for the 23 acids chosen
here were determined as the intercept and regression
coefficient, respectively, of the linear regression of log
((Aeq)/(b − Aeq)) on log [HA] (n = 7–8, R2 = 0.994–
0.996, F (481.41–969.36)-level significance on R bet-
ter than 0.1%), where Aeq and b are the absorbances of
DH+n A−n at 610 nm at equilibrium and when converted
totally from D (by the use of excess HA), respectively,
and [HA] ∼ CHA, are given in Tables I and II.
As seen, values of n are close to or greater than
unity. A similar behavior was observed for many
At = Aeq − (Aeq − A0) e−kt
(4)
where A0 is the absorbance at t = 0. The overall rate
constant, k, was determined from the exponential re-
gression of At on t(n = 15–20; R2 = 0.996–0.999,
F (1241.23–8246.17)-level of significance on R better
than 0.1% and t-level of significance on the regression
coefficient (k) better than 0.1%). The rate constant of
the relatively more reactive acids, meta- and ortho-
nitro benzoic acids for which the lower limit of CHA
was extended close to CD, however, was obtained by
an appropriate second-order procedure.
The rate constant (k) data, however, exhibit a com-
plex dependence on CHA (vide the Supporting Informa-
tion), indicating the occurrence of general acid catal-
ysis in the reaction (Eq. (3)), an acid playing the dual
role of the acid and the catalyst. The rate constant
(k) data obtained at varying acid concentrations, CHA
(∼10−4–10−2 M) and a fixed CD (2.5 × 10−5 M) when
examined for the pattern of k versus CHA variation, in
Table I Equilibrium Parameters (log K , n) for
Toluene-Phase Reactions of Meta Substituted Benzoic
Acids with Crystal Violet Carbinol Base at 28.0 0.1◦C
Substituent
log Ka
na
H
1.78
1.74
2.29
2.30
3.12
3.96
4.02
4.38
2.63
3.65
4.55
4.92
0.98
0.93
1.11
0.98
1.09
1.29
1.29
1.41
0.93
1.12
1.23
1.28
Me
OMe
OPh
F
Cl
Br
I
COPh
CF3
CN
NO2
aRefs. [11,17]
International Journal of Chemical Kinetics DOI 10.1002/kin.20628

GENERAL ACID CATALYSIS IN BENZOIC ACID–CRYSTAL VIOLET CARBINOL
573
fact, fits excellently an expression describing general
acid catalysis:
[29,30]. It is further seen unless CHA is high enough,
kα term is the dominant contributor to k (Eq. (5))
(Tables III and IV); CHA generally ranges within
10−4–10−2 M.
It is of interest to see how sensitive is log kα as a
toluene-phase reactivity parameter to substituent pa-
rameters. Results of correlation analysis of log kα
on substituent parameters σI, σR (Table V) for meta-
substituted acids according to Taft’s model [31]:
k = kαCHA + kβCH2A + kγ CH3A
(5)
for all the acids. kα, kβ, and kγ were obtained as regres-
sion coefficients from cubic regression of k on CHA
(n = 7–9; R2 = 0.957–.999, F (133.66–17369.14)-
level of significance on R better than 0.1%, and t-levels
of significanceon kα, kβ, and kγ betterthan0.1%, 0.1%,
and 0.5%, respectively). kα, kβ, and kγ (Tables III and
IV) represent constants for catalysis of toluene-phase
HA-D reactions (Eq. (2)) by monomeric, dimeric, and
trimeric HA species, respectively.
log kα = ασI + βσR + h
(6)
and on σI, σR, and v (Table V) for ortho-substituted
acids according to Charton’s model [32] is
Dimeric HA species in an apolar aprotic medium
may exist as a nonreactive cyclic dimer, (HA)2, as
well as a hyperacidic open chain homoconjugated
acid–acid anion complex acid, H(A· · ·HA), depend-
ing on the acid concentration used. kβ, thus, refers
to the net catalytic constant for the homoconjugated
dimer acid over the cyclic acid dimer. kβ can, in
fact, be positive or negative. kγ , similarly, refers to
the net catalytic constant for the reactive homocon-
jugated complex acid, H(A· · ·(HA)2) over the cyclic
trimer. Thus, the complex pattern of k versus CHA vari-
ation originates from overlapping participations of HA,
(HA)2, H(A· · ·HA), (HA)3, and H(A· · ·(HA)2) species
in toluene-phase HA-D reactions (Eq. (2)). A simi-
lar behavior has been observed for many other acids
in apolar (also dipolar) aprotic media while studying
not only reactions with dye carbinol bases [2–4,7–
14] but also ethyl diazoacetate–carboxylic acid in-
teractions, [26,27] the inversion of l-menthone [28]
and diazodiphenylmethane–carboxylic acid reactions
log kα = ασI + βσR + ψv + h
(7)
along with the best regression equations obtained are
given for both meta- (Eq. (8)) and ortho- (Eq. (9))
substituted benzoic acids in Table VI.
The field and resonance effects of the substituents
on the basis of their partial regression (β) coefficients
contribute to log kα in the ratio 66:34 (αꢁ = 0.88, βꢁ =
0.46) for meta- and 61:39 (αꢁ = 0.80, βꢁ = 0.52) for
ortho-substituted benzoic acids. It may be noted that
the addition of the steric effect parameter (v) to Eq. (9)
for log kα makes the correlation nonsignificant, The
t-level of significance on ψ (Eq. (7)) becomes as worse
as >80%.
Composite Catalytic Constant
As an alternative to Eq. (5), the following expression
in terms of a composite catalytic constant (kcat) and an
Table III Catalytic Constants (kα, kβ, kγ according to Eq. (5) and kcat with Exponent p According to Eq. (10))
Evaluated from k(min−1) versus CH A (M) for Meta-Substituted Benzoic Acids–Crystal Violet Carbinol Base Reactions
in Toluene at 28.0 + 0.1◦C
Meta
Substituent
kα
(102 L mol−1 min−1
kβ
(103 L2 mol−2 min−1
kγ
(105 L3 mol−3 min−1
k
cat
)
)
)
(102 Lp mol−p min−1
)
p
H
0.195
0.117
0.261
0.676
2.47
3.62
3.93
3.71
3.16
5.30
10.4
23.7
0.220
0.105
0.231
0.561
9.08
3.29
4.93
2.37
3.72
39.4
458
1.03
0.97
0.97
0.96
1.17
0.99
1.03
0.93
1.03
1.25
1.40
1.28
Me
OMe
OPh
F
Cl
Br
–
–
–
30.1
–
–
–
–
171
2250
–
–
–
–
–
–
–
–
–
–
–
–
I
COPh
CF3
CN
NO2
331
International Journal of Chemical Kinetics DOI 10.1002/kin.20628

574
SEN GUPTA ET AL.
Table IV Catalytic Constants (kα, kβ, kγ according to Eq. (5) and kcat with Exponent p According to Eq. (10))
Evaluated from k(min−1) versus CH A (M) for Ortho-Substituted Benzoic Acids–Crystal Violet Carbinol Base Reactions
in Toluene at 28.0 + 0.1◦C
Ortho
Substituent
kα
(102 L mol−1 min−1
kβ
(103 L2 mol−2 min−1
kγ
(105 L3 mol−3 min−1
k
cat
)
)
)
(102 Lp mol−p min−1
)
p
H
0.195
0.143
11.3
0.784
1.43
1.18
1.24
0.371
0.290
0.141
2.04
–
0.0554
9030
–
54.1
234
251
1.00
–
–
–
0.220
0.222
2280
1.51
19.5
99.83
36.2
0.798
0.390
0.728
17.2
1.03
1.09
1.51
1.12
1.18
1.15
1.36
1.11
1.06
1.47
1.23
1.56
Me
NO2
F
Cl
Br
–
–
−306
−625
–
I
OMe
OEt
OPh
OAc
COPh
–
−0.599
0.209
−432
–
200
1.07
78.5
84.9
Table V Substituent Parameters Employed for
Correlations of log kα and log kcat According to Models,
Eqs. (6) and (7)
and t-levels of significance on kcat and p better than
1% and 0.5%, respectively) are presented in Tables III
and IV. Meta- and ortho-substituent effects on log kcat
are found expressible according to Eqs. (6) and (7),
respectively, and the best regression results (Eqs. (11)
and (12)) are presented in Table VI. The steric factor
contribution to log kcat for ortho-substituted acids was
found not enough statistically significant. As estimated
from partial regression (β) coefficients, the relative
contribution of field and resonance effects on log kcat
is 61:39 (αꢁ = 0.82, βꢁ = 0.53) for meta- and 53:47
(αꢁ = 0.75, βꢁ = 0.66) for ortho-substituted acids.
Substituent
σIa
σRa
vb
H
0
−0.01
0.30
0.28
0.40
0.38
0.54
0.47
0.47
0.40
0.28c
0.40
0.57
0.67
0
0
0.52
0.32
0.32
0.32
0.32
0.27
0.55
Me
OMe
OEt
OPh
OAc
F
Cl
Br
I
COPh
CF3
CN
NO2
−0.16
−0.58
−0.57
−0.48
−0.23
−0.48
−0.25
−0.25
−0.16
0.15d
0.11
0.65
0.78
0.50(⊥)e
Catalytic Constants (log kα or log kcat
versus the Equilibrium Association
Constant (log K) Correlation
)
0.91
0.08
0.10
0.40
0.57(⊥)f
aRef. [33].
bRef. [34].
To see how far applicable is the Marcus theory
[38,39] for toluene-phase proton transfer reactions be-
tween benzoic acids and Crystal Violet carbinol base,
quadratic regression analysis of log kα and log kcat on
log K was carried for both meta- and ortho-substituted
benzoic acids. However, the results were statistically
nonsignificant in each case. On the other hand, lin-
ear regression of log kα and log kcat on log K yielded
statistically significant results (Table VII).
Furthermore, the values of the regression coeffi-
cient, the Brønsted exponent (b) [40,41] in log kα ver-
sus log Krelations for both meta- and ortho-substituted
benzoic acids, 0.59 and 0.53, respectively, satisfy the
condition: 0 < b < 1 (Eqs. (13) and (14) in Table VII)
and demonstrate the applicability of the Brønsted re-
lationship [40,41] to acid–dye carbinol base reactions
cTaft’s σF [35].
d Calculated as σp – σF [35].
eRefs. [34, 36].
f Calculated according to [37].
overall kinetic acid exponent (p):
k = kcat CHpA
(10)
fitted excellently k versus CHA data for the acids. Values
of kcat and p determined from nonlinear regression
analysis of k on CHA (n = 7–9, R2 = 0.987–0.999, F
(311.90–622.91)-level of significance on R better 0.1%
International Journal of Chemical Kinetics DOI 10.1002/kin.20628

GENERAL ACID CATALYSIS IN BENZOIC ACID–CRYSTAL VIOLET CARBINOL
575
Table VI Results of Regression Analysis of Catalytic Constants (log kα and log kcat) on Substituent Parameters
According to the Models Equations (6) and (7)
t-Level Significance
Best Regression
Equations
F and F-Significance
Level on R Better Than Coefficients Better Than
on the Regression
Model
(6)
n
R2
Meta-substituted
benzoic acids
log kα = 3.02 σI +
1.29 σR + 1.36 (8)
12 0.971 153.24
(0.1%)
0.1%
0.1%
0.1%
α: 0.1%; β: 0.1%
α: 0.1%; β: 0.1%
α: 0.1%; β: 0.5%
α: 0.1%; β: 0.1%
(6)
(7)
(7)
log k = 4.66 σI +
12 0.937
12 0.840
12 0.909
67.36
23.71
44.81
cat
2.45 σR + 1.26 (11)
log kα = 2.24 σI +
1.17 σR + 1.36 (9)
Ortho-substituted
benzoic acids
log k = 4.62 σI +
cat
3.24 σR + 1.89 (12)
Table VII Results of Linear Regression Analysis of log kα and log kcat on log K for Meta- and Ortho-Substituted
Benzoic Acids in Toluene
F and F-Significance
on R Level
t-Level Significance
on the Regression
Coefficient Better Than
Best Regression
Equations
n
R2
Better Than
Meta-substituted
benzoic acids
log kα = 0.33 + 0.59 log K (13)
12
0.854
58.37
0.1%
0.1%
log k = 0.81 log K (15)
12
12
0.701
0.849
25.76
56.08
0.1%
0.1%
0.1%
0.1%
cat
Ortho-substituted
benzoic acids
log kα = 0.33 + 0.53 log K (14)
log k = 0.96 log K (16)
12
0.774
37.59
0.1%
0.1%
cat
in toluene. The transition state of the reaction is thus
sufficiently dipolar where 50–60% proton transfer has
occurred. Furthermore, evidence to the dipolar nature
for the transition state is afforded by the observation
of increasing log kα with increasing dielectric constant
(εr ) of apolar aprotic solvents. On going from toluene
(εr = 2.38) [1] to chlorobenzene (εr = 5.62) [1], log
kα (at 28.0 0.1◦C) increases from 2.39 to 2.70 for
meta- fluorobenzoic acid and from 1.89 to 2.24 for
ortho-fluorobenzoic acid.
On the other hand, the regression coefficient (b), the
Brønsted exponent in log kcat versus log K relation,
is much larger for both meta- and ortho-substituted
acids, 0.81 and 0.96, respectively, (Eqs. (15) and (16),
Table VII). The large increase in the Brønsted expo-
nent (b) while changing the acid catalytic constant from
the monomer acid constant kα to the composite con-
stant, kcat (Table VII), is obviously a reflection of the
role of hyperacidic homoconjugated acid complexes in
enhancing the extent of proton transfer in the transi-
tion state of acid–base reactions in an apolar aprotic
solvent.
CONCLUSIONS
The complex kinetic behavior of benzoic acid–dye
carbinol base reactions in an apolar aprotic solvent
with respect to CHA variations has been traced to over-
lapping catalysis by the monomer acid, HA and hy-
peracidic homoconjugated complexes, H(A· · ·(HA)m)
with m ≥ 1. The dominant catalytic role is, however,
due to the monomer HA. Both the monomer acid cat-
alytic constant, kα, and the composite constant kcat con-
form significantly to the Brønsted relation for proton
transfer. The Brønsted exponent (b) for log kcat ver-
sus log K correlation indicates to a degree of proton
transfer as high as more than 90% in the transition
state of the reactions, highlighting the role of homo-
conjugated acid complexes in catalyzing the reactions.
Substituent effect analysis shows no significant con-
tribution of steric factors to the catalytic constants of
ortho-substituted benzoic acids. The field and reso-
nance components contribute as 66:34 and 61:39 pat-
terns to log kα and as 61:39 and 53:47 to log kcat,
respectively, for meta- and ortho-substituted acids.
International Journal of Chemical Kinetics DOI 10.1002/kin.20628

576
SEN GUPTA ET AL.
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Tables S1–S23 of the Supporting Information contain-
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)
versus acid concentration, CHA (M) data at 28.0
0.1◦C for reactions of 23 ortho- and meta-substituted
benzoic acids with Crystal Violet carbinol base (the
reference base) in toluene are available in the online
issue at www.wileyonlinelibrary.com.
The authors thank Mr. Satish Kumar for his assistance in
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