Kinetics of proton transfer between ortho substituted benzoic acids and the carbinol base of crystal violet in toluene. Ortho effect on the reactivity of benzoic acids in apolar aprotic solvents

Article abstract of DOI:10.1021/jp110851s

Apolar aprotic solvents are particularly advantageous for investigating the intrinsic ortho effect free from complications of specific solvent effects. A kinetic study for toluene-phase proton transfers between ortho F, Cl, Br, I, OMe, OEt, OPh, OAc, Me, NO₂, COMe, COPh, OH, NH₂, and H benzoic acids and crystal violet carbinol base has shown the forward rate constant (log k₊₁) is the most appropriate reactivity parameter in toluene. log k₊₁ (toluene) as compared to other reported reactivity parameters in benzene, toluene, or chlorobenzene has been found more sensitive to the ortho substituent effect. The regression results of the correlation of log k₊₁ (toluene) of the acids (except OH and NH₂ substituted ones) according to seven ortho effect models are all very significant, and the best result is given by Fujita-Nishiokas model. The overall analysis reveals that a substituents ortho effect pattern is a 58:24:18 ratio of its ordinary electrical, proximity electrical, and steric effects and that the proximity electrical effect is the major component to account for the peculiarity of the substituents ortho effect. The results further favor the transmission of this effect mainly through the molecular cavity. The effect may, however, be outweighed by the steric component for bulky enough substituents, e.g., Me. The enhanced strength exhibited by salicylic acid in toluene has been quantitatively described using Pytela-Lisas δ_HB ⁱ parameter. The abnormally high log k₊₁ observed for anthranilic acid in toluene has been ascribed to a very extensive homoconjugation in its acid-acid anion complex induced by the acids three hydrogen bond donors.

Full text of DOI:10.1021/jp110851s

ARTICLE
pubs.acs.org/JPCA
Kinetics of Proton Transfer between Ortho Substituted Benzoic Acids
and the Carbinol Base of Crystal Violet in Toluene. Ortho Effect on the
Reactivity of Benzoic Acids in Apolar Aprotic Solvents
Susanta K. Sen Gupta* and Sangeeta Mishra
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
S Supporting Information
b
ABSTRACT: Apolar aprotic solvents are particularly advantageous for
investigating the intrinsic ortho effect free from complications of specific
solvent effects. A kinetic study for toluene-phase proton transfers between
ortho F, Cl, Br, I, OMe, OEt, OPh, OAc, Me, NO2, COMe, COPh, OH,
NH2, and H benzoic acids and crystal violet carbinol base has shown the
forward rate constant (log kþ1) is the most appropriate reactivity param-
eter in toluene. log kþ1 (toluene) as compared to other reported reactivity
parameters in benzene, toluene, or chlorobenzene has been found more
sensitive to the ortho substituent effect. The regression results of the
correlation of log k (toluene) of the acids (except OH and NH2
þ1
substituted ones) according to seven ortho effect models are all very
significant, and the best result is given by FujitaꢀNishioka’s model. The
overall analysis reveals that a substituent’s ortho effect pattern is a 58:24:18
ratio of its ordinary electrical, proximity electrical, and steric effects and that
the proximity electrical effect is the major component to account for the
peculiarity of the substituent’s ortho effect. The results further favor the
transmission of this effect mainly through the molecular cavity. The effect
may, however, be outweighed by the steric component for bulky enough substituents, e.g., Me. The enhanced strength exhibited by
i
salicylic acid in toluene has been quantitatively described using PytelaꢀLiꢀska’s σHB parameter. The abnormally high log k
þ1
observed for anthranilic acid in toluene has been ascribed to a very extensive homoconjugation in its acidꢀacid anion complex
induced by the acid’s three hydrogen bond donors.
’ INTRODUCTION
ranging from simple aliphatic and aromatic carboxylic acids,
fluoro and nitro alcohols, and phenols to complex hydrogen
spiroborates corresponding to dicarboxy, hydroxyl carboxy, and
dihydroxy acids, and colorless carbinol bases derived from crystal
violet and several other triarylmethane dyes in different apolar
aprotic solvents are as long as several tens to several hundreds of
seconds.18ꢀ27 In fact, kinetics of all such reactions are conveni-
ently monitored by ordinary spectrophotometry of the colored
ion-pair product. A mechanism for such abnormally slow proton
transfers has been elucidated in terms of a fast equilibrium
between an acid and a dye carbinol base to form an intermediate
hydrogen bonded complex, followed by a rate limiting proton
transfer along the hydrogen bond to form a colored ion-pair
product under the combined influence of the monomeric acid
catalyst, the nonreactive cyclic dimeric acid inhibitor, and the
hyperacidic open chain dimeric and trimeric acid catalysts.27
Another example of such a slow minute scale proton transfer is
Ortho substituted benzoic acids and other benzene derivatives
behave peculiarly compared to their meta or para isomers.1,2 The
ortho substituent effect or the ortho effect on the acidity of
benzoic acid involves possible proximity interactions besides the
normal polar ones and is too complex to be correlated by simple
forms of the Hammett equation.3ꢀ16 For probing this peculiarity,
it seems particularly advantageous to use an apolar aprotic
solvent (dielectric constant, εr < 15; dipole moment, μ <
8.3 ꢁ 10ꢀ30 C m; and DimrothꢀReichardt’s normalized solvent
polarity parameter, ENT ca. 0.0ꢀ0.3)17 as the medium for reac-
tions of these acids. In such solvents, specific solvation due to the
solvent’s own acidity and/or basicity is reduced to a minimum.
However, acidity measurements in an apolar aprotic solvent
require a reference base. The colorless carbinol base of crystal
violet dye, has been found to be a very useful reference base for
determining the relative strengths of a variety of oxygen acids in
benzene, toluene, and other apolar aprotic solvents.18ꢀ27
A very remarkable feature of acidꢀdye carbinol base reactions
in an apolar aprotic solvent is their abnormally slow kinetics.
Time scales of proton transfers between a variety of oxygen acids
Received: November 13, 2010
Revised:
March 25, 2011
Published: April 19, 2011
r
2011 American Chemical Society
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the rate determining step in the reactions of aliphatic and
aromatic carboxylic acids with diazodiphenylmethane in different
classes of solvents.11,28ꢀ33 Further examples of slow proton
transfers are the reactions between bromophenol blue and
substituted pyridines in apolar aprotic solvents, time scales of
which, however, are much shorter, in the range of milliseconds.34
An interesting result of an earlier study of toluene-phase
acidꢀcrystal violet carbinol base reactions for a small set of four
ortho substituted benzoic acids is that compared to log K
(toluene) or pKa (water), log k (forward rate constant in
þ1
toluene) has a significantly higher sensitivity to the ortho effect.25
It may be noted that two other ortho effect studies based on the
equilibrium constant data for benzoic acids in benzene38 and
toluene40 have been reported.
The advantages of the virtual absence of specific soluteꢀ
solvent interactions, easy monitoring of kinetics, and the en-
hanced sensitivity of the forward rate constant toward the ortho
effect make studying proton-transfer kinetics in an apolar aprotic
solvent particularly appropriate for the analysis of the ortho effect
on the reactivity/acidity of benzoic acids.
Figure 1. Structure of crystal violet carbinol base.
Several correlation models based on the extended Hammett
methodology, e.g., the Charton models,6,35 the FujitaꢀNishioka
models7,36 and the PytelaꢀLiꢀska model,8 have been proposed toward
understanding the nature and composition of the ortho effect.
Despite a large number of equilibrium6ꢀ8,25,36ꢀ40 and kine-
tic6ꢀ8,11,12,25,27,33,41ꢀ43 studies complemented by spectroscopic14,44ꢀ48
and theoretical13,14,16,49ꢀ52 studies of the ortho effect in benzoic
acids and their derivatives, there are only three reports, to the
best of the authors’ knowledge, of kinetic studies for ortho
substituted benzoic acids in an apolar aprotic solvent.25,27,33
While the first two reports25,27 concern a limited number of
ortho substituted benzoic acids (five and two, respectively) but
with due considerations to complex effects of acid concentrations
on the rate constants of their reactions in apolar aprotic solvents,
the third one33 though involves as many as 27 ortho substituted
benzoic acids and neglects the pronounced dependence of the
rate constants of their reactions on acid concentrations.
The two limitations from using a small number of ortho
substituted benzoic acids and from neglecting concentration effects
of the acids in an apolar aprotic solvent need remedying for a proper
analysis of the specific solvent effect free ortho effect. The present
work is directed at this task. We report here the results of detailed
kinetics of toluene-phase reactions of a set of 15 ortho substituted
benzoic acids (including the unsubstituted one) with crystal violet
carbinol base for a range of acid concentrations. Kinetic data for six
acids of the set, viz., ortho Me, OMe, OH, Cl, NO2, and H benzoic
acids reported earlier,25 were reexamined, and the data for chloro,
hydroxy, and methoxy substituted acids needed revision. We report
further a comparison of the results of quantitative analysis of the
the reference apolar aprotic solvent because of its relatively low
toxicity. Crystal violet carbinol base, [4,40,400-tris(dimethyl-
amino)triphenyl]carbinol (Figure 1), which is colorless, was
employed as the reference base.
Crystal violet carbinol base was prepared by precipitating a
100 mL aqueous solution containing ∼100 mg of crystal violet
dye [4,40,400-tris(dimethylamino)triphenyl chloride] with ∼2 M
NaOH followed by extraction of the carbinol base precipitate
into ∼200 mL of toluene. The toluene extract was separated and
subjected to freeze-drying to obtain crystal violet carbinol base as
a powder (mp 180 ꢀC). Elemental analysis results (C, 76.85%; H,
7.88%; N, 10.94% as against C, 77.08%, H, 8.02%; N, 10.79%
required for C25H31N3O), IR peak data in Nujol (3350 cmꢀ1
bonded OH), H NMR signals in carbon tetrachloride (2.90
,
1
ppm, s, 18 H, 3 N(CH3)2; 6.55 ppm, d, 6 H, J = 10 Hz, aromatic
1
H; 7.00 ppm, d, 6 H, J = 10 Hz, aromatic H; 7.28 ppm, s, H,
CꢀOH), and freezing point depression in benzene confirmed
the structure of the carbinol base (Figure 1).
All stock solutions of crystal violet carbinol base and ortho
substituted benzoic acids were prepared by dissolving their
weighed amounts in dry toluene.
Kinetics of the reaction between an acid, HA (CHA
∼
10ꢀ4ꢀ10ꢀ2 M, higher molarity order being employed for
relatively weaker acids and lower order for stronger acids), and
the colorless carbinol base of crystal violet, dyeꢀOH (2.5 ꢁ 10ꢀ5
M), in dry toluene, producing a colored ion associate, dyeþAꢀ,
were monitored by absorbance measurements of the ion associ-
ate at 610 nm in a Teflon stoppered quartz cuvette using a 160 A
Shimadzu UVꢀvis spectrophotometer at 28.0 ( 0.1 ꢀC. The
sampling time for absobance recordings during the reaction was
every 5ꢀ10 s. A time of 40ꢀ60 min was required to attain the
reaction equilibrium. The molar absorptivity (ɛ) of crystal violet
cation (dyeþ) in toluene was found to be 3.3 ꢁ 104 dm3
molꢀ1 cmꢀ1 at 28.0 ꢀC.
ortho effect on the forward rate constant, log k (the most
þ1
appropriate rate coefficient found) of the reactions by seven
extended Hammett-type models.6ꢀ8,35,36 The substituents chosen
arebothsimpleandsymmetricones(Me, F, Cl, Br, andI), andthose
allowing more than one conformation (NO2, COMe, and COPh),
hydrogen bonding in the undissociated acid only (OMe, OEt, OPh,
and OCOMe), hydrogen bonding in the dissociated acid anion
(OH and NH2), or intramolecular cyclization (COMe and COPh).
’ RESULTS AND DISCUSSION
’ EXPERIMENTAL SECTION
AcidꢀDye Carbinol Base Reaction Kinetics in Toluene.
Ortho substituted benzoic acids (HA) react with crystal violet
carbinol base (dyeꢀOH) in toluene to form colored ion
The chemicals used were of either analytical reagent grade or
highly purified by standard procedures. Toluene was chosen as
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The Journal of Physical Chemistry A
ARTICLE
Table 1. 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
significance on R better than 0.1% and t-level of significance on the
regression coefficient, k, better than 0.1%). The rate constants of
relatively more reactive acids, o-nitrobenzoic and salicylic acids for
which the lower limit of CHA was extended close to CD, were
obtained by an appropriate second-order procedure.
substituent
log K
n
substituent
log K
n
H
1.78 a
1.96 b
5.38 b
2.36 a
2.85 a
3.04 a
3.53 a
2.87 a
0.98 a
1.19 b
1.33 b
0.94 a
0.98 a
0.98 a
1.10 a
1.18 a
OEt
2.55
1.94 a
3.43
4.32
2.83
4.81
5.34
1.12
1.04 a
1.08
1.80
0.92
1.10
2.22
The toluene-phase rate constant (k) data, though independent
of initial CD, exhibit a complex dependence on CHA (vide
Supporting Information) and conform very well to the following:
Me
NO2
F
OPh
OAc
2
k = kRCHA þ kβCHA þ kγCHA3, indicating the occurrence of
COMe
COPh
OH
general acid catalysis in the reaction (eq 2), an acid playing the dual
role of the acid substrate and the catalyst. kR, kβ, and kγ are the
coefficients of catalysis by the monomer acid, dimer (open chain
homoconjugated) acid, and trimer (open chain homoconjugated)
acid, respectively. In particular, kβ represents the net effect of
catalysis by the hyperacidic open chain dimer over inhibition by
the nonreactive cyclic dimer acid. This is expected in view of
increasing participation of hyperacidic homoconjugated complex
acids in an apolar (also a dipolar) aprotic solvent with increasing
Cl
Br
I
NH2
OMe
a Reference 40. b Reference 25.
associates (dyeþAꢀ)
nHA þ dye ꢀ OH h dyeþAꢀ þ H2O
ð1Þ
CHA.
27,29,30,38,59 Participation of such homoconjugated open chain
dimers has been noted for many other acids in apolar (also dipolar)
aprotic media while studying not only reactions with dye car-
binol bases18ꢀ27 but also ethyl diazoacetateꢀcarboxylic acid
interactions,60,61 the inversion of l-menthone,62 and diazodiphenyl-
methaneꢀcarboxylic acid reactions.29,30
where the acid exponent (n) takes into consideration the
possibility of HA existing effectively as a n-mer due to formation
of open chain and cyclic acid dimers in toluene, the apolar aprotic
solvent.38
It has been argued that the water molecule formed (eq1) is
hydrogen bonded to one of the species of the system and that they
exist as one entity.18ꢀ27,39,40,53ꢀ58 Equation 1 can be reexpressed as
Assuming both the forward and the reverse steps of the equili-
brium (eq 2) are influenced by the acid, eq2 can be rewritten as
D þ fHA h DHþAꢀ þ rHA
where f and r are the individual acid exponents for the forward and
reverse steps of the equilibrium, respectively. f can be interpreted as
the weighted or effective mean aggregation number of HA species,
HA, H(A (HA)m) (m g 1) reacting with D in the forward step. r
is thus the number of HA molecules released from the effective
ð5Þ
nHA þ D h DHþAꢀ
ð2Þ
where D and DHþAꢀ represent the colorless carbinol base of crystal
violet and the colored ion associate, respectively. It has been
reported that absorbance data for the reaction equilibrium (eq2)
for 10 members of the present set of ortho substituted benzoic acids
in toluene fitted the expression:
3 3 3
f-mer, (HA)f after the equilibrium, eq 2, has been attained. If kþ1 and
½DHþAꢀꢂ
kꢀ1 represent the rate constants for the forward and reverse steps,
K ¼
ð3Þ
n
½Dꢂ½HAꢂ
respectively, it follows that
where K is the association constant.25,40 Values of log K and n for the
15 acids chosen here (Table 1) were determined as the intercept
and regression coefficient, respectively, of the linear regression of
log[Aeq/(b ꢀ Aeq)] on log [HA] (n = 7ꢀ8, R2= 0.994ꢀ0.996,
F(481.41ꢀ969.36)-level significance on R better than 0.1%), where
Aeq and b are the absorbances of DHþAꢀ at 610 nm at equilibrium
and when converted totally from D (by the use of excess HA)
f
r
k ¼ k ½HAꢂ þ k ½HAꢂ
ð6Þ
ð7Þ
þ1
ꢀ1
K ¼ kþ1=k
ꢀ1
n ¼ f ꢀ r
ð8Þ
Equations 6ꢀ8 lead to a useful relation:
ꢀ
ꢁ
respectively and [HA] = CHA
.
k
As seen, values of n are close to or greater than unity. A similar
behavior was observed for many other acids in apolar aprotic
solvents.18ꢀ27,38ꢀ40,54ꢀ58 This has been ascribed to overlapping
participations of open chain dimeric and multimeric homoconju-
log
¼ f log ½HAꢂ þ log k
ð9Þ
ꢀ1
K þ ½HAꢂꢀn
where K and n were already determined from equilibrium measure-
ments (Table 1). log kꢀ1 and f were determined as the intercept and
the regression coefficient, respectively, of the linear regression of the
left-hand side of eq 9 on log [HA] for each member of the set of ortho
substituted benzoic acids (n = 7ꢀ8, R2 = 0.996ꢀ0.998, F-
(761.45ꢀ2134.14)-level of significance on R better than 0.1%).
gated complex acids H(A (HA)m) (m g 1) in the reaction with
3 3 3
the carbinol base (D) in apolar aprotic media.18ꢀ27,38ꢀ40,54ꢀ58
The value of n for an acid depends upon its concentration,
temperature, and solvent.24,38,56ꢀ58
The time course of the absorbance data (At) for the reaction,
eq 2 under the condition CHA . CD (vide Experimental
Section), conforms very well to pseudo-first-order reversible
kinetics for all of the acids of the set:
Values of the other two kinetic parameters, log k and r, were
þ1
þ1
obtained using eqs 7 and 8, respectively. Values of log k , log kꢀ1, f,
and r for the acids are compiled in Table 2 along with their pKa
(H2O), log KBHA (benzene), and log k (chlorobenzene) (vide infra)
As seen from Table 2, kþ1. kꢀ1 and f. r.Valuesoff,asexpected,
are nonintegral and greater than unity. Furthermore, log kþ1 appears
At ¼ Aeq ꢀ ðAeq ꢀ A0Þeꢀkt
where A0 is the absorbance at t = 0. The overall rate constant,
k, was determined from the exponential regression of At on t
(n = 15ꢀ20; R2 = 0.996ꢀ0.999, F(1241.23ꢀ8246.17)-level of
ð4Þ
much more susceptible to substituent effects than log k .
ꢀ1
It may be noted that whereas the association constant, log K, for a
given n is determined by the interplay of four parameters, viz., k
,
þ1
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The Journal of Physical Chemistry A
ARTICLE
Table 2. Individual Rate Constants (log kþ1, log kꢀ1), and Acid Exponents (f, r) for Toluene-Phase Reactions of Ortho Substituted
Benzoic Acids with Crystal Violet Carbinol Base at 28.0 ( 0.1 ꢀC along with pKa (H2O) at 25 ꢀC, log KBHA (Benzene) at 25 ꢀC, and
log k (Chlorobenzene) at 30.0 ꢀC for the Acid
substituent
log k
log k
f
r
pKa b (water)
log KBHA b (benzene)
log k d (chlorobenzene)
þ1
ꢀ1
H
1.80
1.99 a
7.25 a
2.93
4.56
5.16
4.73
2.28
1.96
2.08
4.50
5.26
4.62
4.46
14.66
0.02
1.46
1.67 a
2.05 a
1.54
1.86
1.98
1.82
1.46
1.32
1.72
1.74
2.70
1.88
1.34
6.44
0.48
0.48 a
0.72 a
0.60
0.88
1.00
0.72
0.28
0.22
0.68
0.66
0.90
0.98
0.24
4.24
4.20
3.91
2.17
3.27
2.94
2.85
2.86
4.09
4.21
3.53
3.48
3.41 c
3.54
3.00
4.95
5.26
4.94
7.44
5.77
6.08
6.17
6.22
3.7
0.34
ꢀ0.004
3.13
Me
0.03 a
1.87 a
0.57
NO2
F
1.15
Cl
1.71
1.52
Br
2.12
1.64
I
1.20
1.68
OMe
OEt
OPh
OAc
COMe
COPh
OH
NH2
ꢀ0.59
ꢀ0.59
0.14
ꢀ1.16
ꢀ1.52
0.33
1.07
5.72
1.22
0.94
1.79
6.44
7.45
4.91
1.91
3.13
0.15
ꢀ0.35
9.32
a Reference 25. b Reference 38. c Corrected value of the keto acid form.68 d Reference 33.
k
ꢀ1, f, and r, the forward rate constant, log kþ1, for a given f is
where σl is simply an alternative symbol of σI of eqs 10 and 11. σd
is the intrinsic delocalized (resonance) electrical effect parameter
which represents the delocalized electrical effect in a system with
zero electronic demand. σe is the electronic demand sensitivity
parameter which adjusts the delocalized effect of a group to meet
the electronic demand of the system. σd and σe are related to σR
of eqs 10 and 11 as35 σR = 0.380σe þ σd. The steric effect is
described by the v parameter of eq 10.
independent of any other parameter. Moreover, log kþ1, as
compared to log K, has higher sensitivity to the effects of the
substituents (Tables 1 and 2). log kþ1 thus appears to be the most
appropriate parameter for a quantitative study of structural effects of
ortho substituents on chemical reactivities of benzoic acids in
toluene or other similar apolar aprotic solvents. Results of quanti-
tative analysis of the ortho effect on log kþ1 (toluene) on the basis of
a number of multiparametric models proposed by Charton, Fujitaꢀ
Nishioka, and PytelaꢀLiꢀska are presented in the following.
Quantitative Analysis of the Ortho Effect on Toluene-
Phase Reactivities of Benzoic Acids Using Extended Ham-
mett-Type Models. The following well-known models pro-
posed for the quantification of structural effects of ortho
substituents on reactivities of benzoic acids and other benzene
derivatives have been employed to correlate the log kþ1 data set
in Table 2 using relevant substituent parameters by multiple
linear regression analysis.
(c) CRS model
log kortho ¼ Cσc50 þ Rσe þ Sv þ h
ð13Þ
is a simplified version of eq 12, where σl and σd are replaced by a
composite parameter, σc50, defined as σc50 = 0.50σl þ 0.50σd.
(3) FujitaꢀNishioka's model:
log kortho ¼ Fσoꢄp þ fF þ δEs þ h
ð14Þ
(1) Charton's model:6
where σo is considered equal to σp (i.e., σortho ꢄ σpara) andrepresents
a substituent’s ordinary electrical effect. F, the SwainꢀLuptonꢀ
Hansch field constant of a substituent,63,69 describes its proximity
electrical effect, while Es, the TaftꢀKutterꢀHansch parameter which
is based on the mean van der Walls radius of the substituent,3c,64 is
employed as its steric effect parameter.
log kortho ¼ RσI þ βσR þ Ψv þ h
ð10Þ
where kortho stands for reactivity or physicochemical data of
ortho substituted compounds. σI and σR are the localized
(field) and delocalized (resonance) electrical effect parameters,
respectively, of a substituent, while v is a monoparametric steric
effect parameter based on the van der Walls radius of the
substituent.
(4) FujitaꢀNishioka's reformulated model:
log kortho ¼ fF þ rR þ δEs þ h
ð15Þ
The reformulation of eq 14 follows from the relation7 σp =
0.92F þ R, where R is the substituents’s SwainꢀLuptonꢀ
Hansch resonance constant.63,69
(2) Charton's modified models:35
(a)
(5) PytelaꢀLiꢀska’s model:
ꢃ
log kortho ¼ RσI þ βσR þ Φv þ h
ð11Þ
i
i
i
log kortho ¼ Foσo þ Fsσs þ FHBσHB þ h
ð16Þ
where v of eq 10 is modified to a multiparametric steric effect
parameter, v*.
This model unlike the other models includes a substituent’s
intramolecular hydrogen bonding effect as a significant compo-
i
i
(b) LDRS model
nent of its ortho effect. The three parameters (σo , σsi, and σHB
)
were determined for 29 substituents by applying the method
of conjugated deviations to a large number of data sets of log
log kortho ¼ Lσl þ Dσd þ Rσe þ Sv þ h
ð12Þ
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The Journal of Physical Chemistry A
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Table 3. Substituent Parameters Employed for the Ortho Effect Analysis by Different Models, Equations 10ꢀ16
a
i c
d
d
d
d
a
i c
substituent
σoꢄp
σo
σI (or σl) d
σR
σd
σc50
σe
F h
R h
v i
v * d
Es
σsic
σHB
H
0
0.000
ꢀ0.150
1.400
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Me
ꢀ0.17
0.78
ꢀ0.01
0.67
0.54
0.47
0.47
0.40
0.30
0.28
0.40
0.38
0.30
0.28 e
0.35
0.24f
0.17
ꢀ0.16
0.10
ꢀ0.14 ꢀ0.15 ꢀ0.030 0.01 ꢀ0.18 0.52
0.52
ꢀ1.24
0.133
NO2
F
0.18
ꢀ0.48
ꢀ0.28
ꢀ0.27
ꢀ0.20
0.85 ꢀ0.077 0.65
0.13 0.57(^) j 0.35(^) l ꢀ1.01(^) 0.288
0.06
0.425
ꢀ0.48
ꢀ0.25
ꢀ0.25
ꢀ0.16
ꢀ0.58
ꢀ0.57
ꢀ0.48
ꢀ0.23
0.20
0.06 ꢀ0.041 0.45 ꢀ0.39 0.27
0.19 ꢀ0.011 0.42 ꢀ0.19 0.55
0.20 ꢀ0.018 0.45 ꢀ0.22 0.65
0.20 ꢀ0.057 0.42 ꢀ0.24 0.78
0.27
ꢀ0.46
ꢀ0.97
ꢀ1.16
ꢀ1.40
ꢀ0.55
ꢀ0.55
ꢀ0.55
ꢀ0.55
ꢀ0.88 j
0.198
0.430
0.489
0.521
0.257
0.212
0.356
0.143
Cl
0.23
0.576
0.55
Br
0.23
0.596
0.65
I
0.18
0.554
0.78
OMe
OEt
OPh
OAc
COMe
COPh
OH
ꢀ0.27
ꢀ0.24
ꢀ0.32
0.31
ꢀ0.211
ꢀ0.186
0.240
ꢀ0.55 ꢀ0.25 ꢀ0.064 0.29 ꢀ0.56 0.32
ꢀ0.55 ꢀ0.27 ꢀ0.070 0.26 ꢀ0.50 0.32
ꢀ0.51 ꢀ0.11 ꢀ0.083 0.37 ꢀ0.40 0.32
ꢀ0.24
0.25
0.36
0.48
0.57
0.405
0.14 ꢀ0.005 0.42 ꢀ0.11 0.32
0.05 ꢀ0.095 0.33
0.50
0.50
0.43 b
0.17 0.50(^)
0.12 0.50(^) k 0.50(^) l ꢀ0.88 j
0.50(^)
0.15 g
0.31
ꢀ0.37
ꢀ0.088
ꢀ0.62
ꢀ0.57 ꢀ0.22 ꢀ0.044 0.33 ꢀ0.70 0.32
0.32
ꢀ0.55
ꢀ0.61
0.392 0.695
0.235 0.153
NH2
ꢀ0.66
ꢀ0.788
ꢀ0.80
ꢀ0.68 ꢀ0.51 ꢀ0.13
0.08 ꢀ0.74 0.35
g
0.35
a Reference 7. b Reference 69. Reference 8. d Reference 35. Taft’s σF.69 f Reference 11, Calculated as σoꢄp ꢀ σI(ꢄσF).69 h Recalculated values.69
c
e
i Reference 70. j Calculated as per ref 7. k References 11 and 70. l Calculated as per refs 7 and 35.
i
kortho: σo describes the electrical effect and its values were
Benzoic acids with simple ortho substituents, H, Me, F, Cl, Br,
and I, which are monatomic or tetracoordinate symmetric top and
free from conformational and intramolecular hydrogen bonding
effects, are the straight choices for the correlation analysis.
The same is true for OMe, OEt, OPh, and OAc substituents
whose intramolecular hydrogen bonding effect in the undisso-
ciated acid is, however, not as significant as to be necessary to
express it explicitly.8
i
optimized according to an isoparameter relation between σo
and σm . σo correlates very well with the σI and σR constants. σsi
describes the steric effect and is not significantly related to any of
the known quantities of this type. σHBi describes the intramole-
cular hydrogen bonding between the oxygen atom of the
carboxylate anion and the ortho substituent, and it has nonzero
values for substituents such as OH, NH2, and NHCOCH3 etc.
Values of the parameters in eqs 10ꢀ16, viz., σI = σl, σR, σd, σe,
i
i
Ortho COMe and COPh benzoic acids cyclize intramolecu-
larly to an equilibrium mixture of open chain keto acid and cyclic
lactol tautomers with the keto acid content, nearly 25 and 100%,
respectively, in an apolar aprotic medium.65ꢀ67 While reacting
with crystal violet carbinol base, the equilibrium mixture would,
however, act entirely as the stronger acid form, the keto acid. The
three planar π-bonded substituents chosen, the above two (i.e.,
COMe, COPh) and NO2, show steric effects which may have
strong conformational dependence. Values of the steric effect
parameters calculated for the two extreme conformations, viz.,
coplanar ( , maximum dimension) with and orthogonal (^,
minimum dimension) to, the reaction site are substantially
σc50, σoꢄp, σo , F, R, v, v*, Es, σsi, and σHBi, of the substituents
i
selected and references of their sources are given in Table 3.
Coefficients of the substituent parameters and the intercepts
(h) in the above equations for log k data (Table 2) are
evaluated by the method of regression analysis.
Choice of log k Data Set for Correlation Analysis. As
shown already, the appropriate parameter for quantifying the
toluene-phase reactivity of an acid is its log k for a given f. As
þ1
þ1
þ1
seen from Table 2, excepting anthranilic acid, all the other acids have
their f around 2.0 ( 0.7. f of anthranilic acid is comparatively too
large, 6.44. Too large are also its log kþ1, log kꢀ1, and r (Table 2). In
fact, acids having one hydrogen bond donor center have their
f-exponent within 2.0 ( 0.7 (Table 2). It seems that the presence of
as many as three hydrogen bond donor centers in anthranilic acid
causes extensive enough homoconjugation in its hyperacidic open
different.7,11,64 log k data corresponding to these three sub-
þ1
stituents each (Table 2) have been found to correlate very well
with the steric parameter (^) of minimum dimension for the
orthogonal conformation rather than with the parameter ( ) for
the coplanar one. The same inference was also made for the ortho
NO2 group during proton-transfer studies of benzoic acids in
water and benzene.7,36 The data set of log kþ1 can thus justifiably
include the data corresponding to these three groups for
correlation by all these models, eqs 10ꢀ16.
chain complex, H(A (HA)m)27 at the acid concertrations
3ꢀ333
employed (10ꢀ2 to 10 M) and leads to such a high f-exponent
(6.44). It may be noted that at concentrations below 10ꢀ3 M, the
reaction of anthranilic acid is too slow to monitor, whereas the reac-
tion rate of salicylic acid is easily measured at concentrations as low
as 10ꢀ5 M. Due to the use of such low concentrations (∼10ꢀ5 M),
salicylic acid despite having two hydrogen bond donor centers has a
normal f-exponent (1.34) (Table 2). The log kþ1 data of anthranilic
The case of ortho OH substituent is an exception. It exerts
intramolecular hydrogen bonding effects in the undissociated
salicylic acid and more strongly in the salicylate anion. The effect
in the anion is strong enough as to be necessary to express it
explicitly as a hydrogen bonding effect parameter. This particular
case may thus be treated by PytelaꢀLiꢀska’s model which has a
separate σHBi parameter, eq 16.
acid was accordingly excluded from the log k data set while
þ1
correlating them in the light of the above models: eqs 10ꢀ16.
The ortho substituted benzoic acids, as already stated, have
different categories of substituents. It is thus worth examining the
justifiability of log kþ1 (toluene) of all these acids (Table 2) for
analysis by the ortho effect models.
Regression Analysis Results. Multiple linear regression
analysis of log k data of the ortho substituted benzoic acids
þ1
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The Journal of Physical Chemistry A
ARTICLE
Table 4. Results of Regression Analysis for log kþ1 (Toluene) by Different Ortho Effect Models
F-level significance on R
t-level (two-tail) significance on
model (eq)
(10)
best regression equation
na
13
R2
F
(better than)
regression coefficients (better than)
5:67σI þ 3:92σR þ 1:97v þ 1:79 ð17Þ
0.939
0.929
0.945
0.922
0.979
0.957
0.869
0.872
46.51
39.30
45.44
53.08
139.76
66.64
59.75
30.66
0.1%
R, 0.1%; β, 0.1%; ψ, 5%
R, 0.1%; β, 0.1%; φ, 10%
L, 0.1%; D, 0.1%; S, 5%
C, 0.1%; S, 2%
ꢃ
6:14σI þ 4:43σR þ 159v þ 1:85 ð18Þ
(11)
13
0.1%
5:49σl þ 3:38σd þ 2:17v þ 1:75
4:30σc50 þ 2:51v þ 2:13
ð19Þ
ð20Þ
(12)
12
0.1%
(13)
12
0.1%
3:60σoꢄp þ 2:81F ꢀ 0:93Es þ 1:63 ð21Þ
(14)
13
0.1%
F, 0.1%; f, 0.1%; δ, 0.5%
f, 0.1%; r, 0.1%; δ, 2%
Fo, 0.1%
6:20F þ 3:92R ꢀ 1:06Es þ 1:57
ð22Þ
ð23Þ
ð24Þ
(15)
13
0.1%
i
3:50σo þ 2:41
(16)
11
0.1%
i
i
(16)
12 þ
0.1%
Fo, 0.1%; FHB, 2%
3:50σo þ 3:49σHB þ 2:41
a n = 11 (ortho H, Me, OMe, OEt, OPh, OAc, F, Cl, Br, I, and NO2 benzoic acids); n = 12 (n = 11 plus ortho COMe benzoic acid); n = 13 (n = 12 plus
ortho COPh benzoic acid); n = 12þ (n = 11 plus salicylic acid)
(Table 2) was carried out using the correlation eqs 10ꢀ16. The
best regression equations thus obtained, eqs 17ꢀ24, and statis-
tics of the results are given in Table 4.
It is obvious that log kþ1 is a useful and sufficiently sensitive
parameter for analyzing the ortho effect on reactivities of benzoic
acids in toluene and other apolar aprotic solvents. The present
analysis relates to the ortho effect where complications due to
specific solvation and conformational dependence, intramolecu-
lar cyclization, or intramolecular hydrogen bonding in the
undissociated acid are minimal. The resulting pattern of ortho
effect components is obviously of more intrinsic nature than the
one based on the reactivity/acidity measurements of the acids in
a polar protic or a dipolar aprotic solvent.
As seen from Table 4, the regression results from all the
models are very significant. A comparison of statistics of the
results in eqs 17ꢀ24 in Table 4 shows eq 21 the most significant
one. Thus, FujitaꢀNishioka’s model, eq 14 appears to be the best
one for correlating log k (toluene). Its regression eq 21
þ1
explains 97.9% of the variance (Table 4). Further statistical
results from the models eq 14 and eq 21 show that the relative
contribution of the ordinary electrical (%F), proximity electrical
(%f), and steric (%δ) effects in the overall ortho substituent
effects on the basis of their partial regression (beta) coefficients
(0.70, 0.29, and ꢀ0.21) are in the ratio of 58:24:18. The ordinary
electrical effect is, obviously, the largest contributor to the ortho
effect. However, according to an earlier report25 based on the log
log kþ1 vs Other Reactivity Parameters in Apolar Aprotic
Solvents. As stated earlier, other reactivity parameters for
ortho substituted benzoic acids in such media have been
reported.33,38,40 These include log K (the association constant for
reactions with crystal violet cabinol base in toluene)40 (Table 1),
log KBHA (the association constant for reactions with 1,3-
diphenylguanidine in the presence of bromophthalein magenta
E, the indicator acid in benzene)38 (Table 2), and log k (the rate
coefficient for reactions with diazodiphenylmethane in
chlorobenzene)33 (Table 2). While correlating the data for these
reactivity parameters with FujitaꢀNishioka’s model, eq 14,
significant regression results are obtained for log K (toluene)
with σoꢄp and F only, and log KBHA (benzene) or log k
(chlorobenzene) with σoꢄp only. The correlations between log
k
þ1 data of ortho Me, NO2, Cl, OMe, and H benzoic acids, the
proximity electrical effect is the major factor contributing to the
ortho effect. As stated under the Introduction, while reexamining
log k data of these five acids, the values for two, viz., Cl and
þ1
OMe substituted acids, needed significant revision. On incor-
poration of these revised data, the ordinary electrical effect is
again found to be the largest contributor, doing away with the
confusion about the predominant component of the ortho effect.
Thus, the toluene-phase reactivity of an ortho substituted benzoic
acid (log kþ1) is virtually equally sensitive to the ordinary electrical
and total proximity effects (F ≈ f þ |δ|) of the substituents. The
analysis shows further that for most of the substituents the major
component of the total proximity effect is the proximity electrical
effect rather than the steric effect (fF > δEs). The opposite is,
however, found for the Me substituent (δEs > fF). Other exceptions
are I, COMe, and COPh substituents for which fF ≈ δEs.
k
þ1 (toluene) and log K (toluene), log KBHA (benzene), or log k
(chlorobenzene),
log k ¼ 1:50 log K ꢀ 0:73
ð25Þ
þ1
ðn ¼ 13; R2 ¼ 0:791; Fð49:63Þ-level significance on R
better than 0:1%Þ
The sign of the regression coefficients (F, f, and δ) in eq 21
corresponds to the reaction eq 2, being subject to both field-
assisted and steric acceleration by the ortho substituents. This is
exactly what one would expect in terms of the mechanism of the
reaction eq 2, the rate controlling step of which is HA catalyzed
formation of a benzoate anion as DHþAꢀ from a hydrogen
log k ¼ 1:47 log KBHA ꢀ 4:51
ð26Þ
þ1
ðn ¼ 10; R2 ¼ 0:733; Fð21:91Þ-level of significance on R
better than 0:5%Þ
bonded complex, D HA.27 This is favored not only by
3 3 3
log k ¼ 1:14 log k þ 2:68
ð27Þ
þ1
stronger electron-withdrawing substituents but also by bulkier
ones apparently due to a greater relief in the steric strain in the
more easily twistable acid anion during its formation from the
acid molecule.
ðn ¼ 12; R2 ¼ 0:766; Fð32:65Þ-level of significance on R
better than 0:1%Þ
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The Journal of Physical Chemistry A
ARTICLE
intramolecular hydrogen bonding effect in the salicylate anion
plays an overwhelming role in enhancing the reactivity (log k
)
þ1
of salicylic acid in toluene. In fact, as estimated by the partial
i
i
regression coefficients for σo and σHB from the results of the
models eq 16 and eq 24 (0.95 and 0.40), the percent contribution
of hydrogen bonding effect (σHBi) on log k is as large 30%.
þ1
This effect with its overwhelming role is, in fact, observed though
in different degrees in media ranging from highly polar protic
water to the solvent free gas phase.8,11,14,37
’ CONCLUSIONS
Ortho substituted benzoic acidꢀcrystal violet carbinol base
reactions in toluene follow general acid-catalyzed pseudo-first-
order reversible kinetics. The forward rate constant (log kþ1) of
the reactions is found to be the appropriate toluene-phase
reactivity/acidity parameter for the acids. The ortho effect on
log kþ1 was analyzed using a number of correlation models. The
regression results are all very significant, and the best regression
equation is given by FujitaꢀNishioka’s model. The findings are
consistent with the reaction mechanism involving the acid anion
formation from an acidꢀcarbinol hydrogen bonded complex in
Figure 2. A comparison of correlations of log kþ1 (toluene) on log K
(toluene), log KBHA (benzene), and log k (chlorobenzene).
show that as compared to log K (toluene), log KBHA (benzene),
or log k (chlorobenzene), log k (toluene) is distinctly more
þ1
sensitive to the ortho effect. The three correlations eqs 25ꢀ27
the rate determining step.27 log k is found to be equally
are compared in Figure 2.
þ1
log k (Toluene) vs pKa (Water). It is obvious that the
þ1
sensitive to a substituent’s ordinary electrical and total proximity
effects consisting of its proximity electrical and steric effects. A
substituent’s steric effect is generally subordinate to its proximity
electrical effect barring certain exceptions, e.g., Me and others.
Interestingly, log kþ1 (toluene) is ∼2.5 times as sensitive as pKa
(H2O) to structural effects of ortho substituents. The results
show further that, unlike the ordinary electrical effect, the
proximity electrical effect is transmitted mainly through the
molecular cavity rather through the solvent.
reactivity parameter log k is a very satisfactory and sensitive
þ1
measure of the strength of acids in toluene or other apolar aprotic
solvents. It is of interest to see the effect of changing the solvent
from apolar toluene to highly polar water on the sensitivity of
acidities to the ortho effect. On correlating the pKa (H2O) data
for the same 13 acids (employed in log k correlation) with
þ1
FujitaꢀNishioka’s model, eq 14,
pKa ðH2OÞ ¼ ꢀ 0:55σoꢄp ꢀ 1:94F þ 0:48Es þ 4:52 ð28Þ
The case of salicylic acid is an exception. Its enhanced acidity in
i
toluene was quantitatively described using PytelaꢀLiꢀska’s σHB
ðn ¼ 13, R2 ¼ 0:868Þ
parameter. The exceedingly high value observed for the toluene-
phase acidity of anthranilic acid and extensively homoconjugated
acid complex formation seem related to the acid’s three hydrogen
bond donors and the use of its relatively large concentrations.
with the F(19.76)-level of significance on R better than 0.1% and
the t-level of significance on F, f, and δ better than 10, 0.5, and
5%, respectively. A comparison between the results, eqs 21 and
27, shows readily that whereas the sensitivity to a substituent’s
ordinary electrical effect (F) increases as much as ∼6 times on
changing the solvent from water to toluene, the sensitivity to its
proximity electrical effect (f) increases ∼1.5 times only. This
favors strongly the transmission of a substituent’s proximity
electrical effect as a direct field effect mainly through the
molecular cavity rather through the solvent. The simultaneous
enhancement in the sensitivity of a substituent’s steric effects
indicates a greater ease in the relief of the steric strain during acid
to anion conversion in toluene as compared to H2O, seemingly
the anion being more easily twistable in toluene than in strongly
solvating water.
’ ASSOCIATED CONTENT
S
Supporting Information. Tables S1ꢀS15 listing the
b
experimental overall rate constant, k (minꢀ1), vs acid concentra-
tion, CHA (M), data at 28.0 ( 0.1 ꢀC for reactions of 15 ortho
substituted benzoic acids with Crystal Violet carbinol base (the
reference base) in toluene. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sksbhuchem@yahoo.co.in. Tel.: (91) 93 35 41 70 50.
The overall correlation,
log kþ1 ðtolueneÞ ¼ ꢀ 2:46pKa ðH2OÞ þ 12:18
ðn ¼ 13; R2 ¼ 0:756; Fð34:14Þ-level of significance on R
better than 0:1%Þ
ð29Þ
’ ACKNOWLEDGMENT
The financial support extended by University Grants Com-
mission, New Delhi, is gratefully acknowledged. We thank Mr.
Satish Kumar for his assistance in computer work.
shows log kþ1 (toluene) is ∼2.5 times as sensitive as pKa (water)
to the ortho effect.
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