14
VYAS, KOTHARI, AND BANERJI
1
1
3
molϪ sϪ ; r ϭ 0.9994) and at [Hϩ] Ն 1.2 mol dmϪ ,
where , represents the electronic demand of the re-
action site which is given by ϭ R/D, and D repre-
sents the delocalized electrical parameter of the di-
parametric LD equation.
For ortho-substituted compounds, it is necessary to
account for the possibility of steric effects and Char-
ton, therefore, modified LDR equation to LDRS [10],
eq. (9).
4
the form is kobs ϭ c[Hϩ]2 (c ϭ 4.22 Ϯ 0.02 ϫ 10Ϫ
2
1
dm6 molϪ sϪ ; r ϭ 0.9999). In view of this, we at-
tempted a correlation with eq. (5) for the entire range
of [Hϩ]; the results are given in eq. (6).
k
obs ϭ a ϩ b [Hϩ] ϩ c [Hϩ]2
(5)
k
obs ϭ 1.62 ϫ 10Ϫ4 Ϫ 9.0 Ϯ 3.5 ϫ 10Ϫ5 [Hϩ]
log k2 ϭ L
ϩ D ϩ R ϩ S V ϩ h (9)
1 d e
ϩ 4.35 Ϯ 0.07 ϫ 10Ϫ4 [Hϩ]2 (6)
R ϭ 0.9995; sd ϭ 7.0 ϫ 10Ϫ5; n ϭ 21
where V is the well known Charton’s steric parameter
based on Van der Waals radii [11].
The rates of oxidation of the ortho-, meta-, and
para-substituted alcohols show excellent correlations
with LDR/LDRS equations (Table VII). We have
used the standard deviation (sd), the coefficient of
multiple determination (R2), and Exner’s [12] para-
meter , as measures of goodness of fit.
The comparison of the L and D values for the sub-
stituted benzyl alcohols showed that the oxidation of
para-substituted benzyl alcohols is more susceptible
to the delocalized effect than to the localized effect.
However, the oxidation of ortho- and meta-substi-
tuted compounds exhibited a greater dependence on
the field effect.
A value of the intercept which is higher than the rate
3
constant at [Hϩ] ϭ 0.1 mol dmϪ and a negative
value of b render this correlation chemically insignifi-
cant. Equation (5) will be applicable if all the three
forms of the reactive species, viz. unprotonated,
singly protonated, and doubly protonated, participate
in the reaction simultaneously. This observation led
us to propose that at moderate concentrations of hy-
drogen ions the reaction follows an acid-independent
path and an acid-dependent path involving a singly
protonated form of a reactant as reactive species and
at higher hydrogen-ion concentration, the reaction in-
volves a doubly protonated form of the reactive
species.
All the three regression coefficients, L, D, and R,
are negative indicating an electron-deficient carbon
center in the transition state of the rate-determining
step. The positive value of adds a negative incre-
The observed solvent composition effect can be
explained on the basis of an increase in the acidity of
the medium with a decrease in the water concentra-
tion [9].
ment to according to eq. (8), thereby increasing
d
electron-donating power of the substituent and its ca-
pacity to stabilize a cationic species.
Correlation Analysis of Reactivity
The positive value of S indicates that the reaction
is subjected to steric acceleration by the ortho-sub-
stituent. This may be explained on the basis of high-
ground state energy of the sterically crowded alco-
hols. Since the crowding is relieved in the transition
state as well as in the product formed, the transition
state energy of the crowded and uncrowded alcohols
do not differ much and steric acceleration, therefore,
results.
A perusal of data recorded in Tables II and III re-
vealed that the formation constants, K, of ArCH2OH-
BBCP complexes are not very sensitive to the nature
of the substituent in the alcohol molecule. The rate of
decomposition, k2 , however, showed considerable
variation. We have attempted to correlate the rate and
structure in this reaction in terms of Charton’s [10]
LDR equation, eq. (7).
The percent contribution [10] of the delocalized
effect, PD is given by the following equation:
log k2 ϭ L
ϩ D
ϩ R
ϩ h
e
(7)
1
d
(
͉
͉
D
L
͉
͉
ϫ 100)
Here, is a localized (field and/or inductive) effect
1
PD ϭ
(10)
(
ϩ
͉
D
͉)
parameter, is the intrinsic delocalized (resonance)
d
electrical effect parameter when active site electronic
Similarly, the percent contribution of the steric para-
meter [10] to the total effect of the substituent, PS ,
was determined by using the following equation:
demand is minimal and represents the sensitivity
e
of the substituent to change in electronic demand by
the active site. The latter two substituent parameters
are related by eq. (8).
(͉
S
͉
ϫ 100)
PS ϭ
(11)
(͉
L
͉
ϩ
͉D͉ ϩ ͉S͉)
D ϭ e ϩ
(8)
d