3324 J . Org. Chem., Vol. 65, No. 11, 2000
Raju et al.
PhCH2Me3NBr2Cl h PhCH2Me3N+ + Br2Cl- (2)
The following equilibria may also exist in the solution.
quantitative description of structural effects on chemical
reactivities.
log k2 ) Lσl + Dσd + Rσe + h
(7)
Br2Cl- h Br2 + Cl-
(3)
(4)
Here, σl is a localized (field and/or inductive) effect
parameter, σd is the intrinsic delocalized (resonance)
electrical effect parameter when active-site electronic
demand is minimal, and σe represents the sensitivity of
the substituent to changes in electronic demand by the
active site. The latter two substituent parameters are
related by eq 8.
Br2 + H2O h HOBr + HBr
The probable oxidizing species in a solution of BT-
MACB are, therefore, chlorobromate ion, molecular bro-
mine, or hypobromous acid. The equilibria (3) and (4)
likely to be suppressed by the addition of BTMACl or
potassium bromide. No effect of BTMACl or bromide ion
on the reaction rate rules out any role of Br2 and HOBr
in the oxidation process. The oxidation of benzaldehyde
by bromine, in the pH range 1-4, is retarded by the
addition of bromide ions,11 whereas the oxidation by
BTMACB is unaffected by bromide ions. This also
indicates that in this reaction bromine is not the active
oxidizing species. For comparison, the oxidation of benz-
aldehyde by hypobromous acid was studied. The oxida-
tion was very slow initially but became faster later.
Similar results were obtained in the oxidation of acetal-
dehyde by HOBr.12 It has been attributed to the faster
oxidation of the aldehyde by bromine, formed by the
reaction of HOBr and bromide ion. Such a kinetic picture
is not obtained in the oxidation by BTMACB. Similar
results were obtained in the oxidation of aliphatic alde-
hydes by BTMACB.9 Thus, in the present reaction also
the reactive oxidizing species is the chlorobromate ion.
Solven t Com p osition Effect. The increase in the
rate of oxidation with an increase in the polarity of the
medium suggests that the transition state is more polar
than the reactants. The solvent effect was analyzed using
the Grunwald-Winstein equation.13
σD ) ησe + σd
(8)
Here, η represents the electronic demand of the reac-
tion site and is given by η ) R/D, and σD represents the
delocalized electrical parameter of the diparametric LD
equation.
For ortho-substituted compounds, the LDR equation
has been modified to the LDRS eq 917 to account for the
steric effects
log k2 ) Lσl + Dσd + Rσe + SV + h
(9)
where V is the well-known Charton’s steric parameter
based on van der Waals radii.18
The rates of oxidation of ortho-, meta-, and para-
substituted benzaldehydes show excellent correlations in
terms of the LDR/LDRS equations (Table 5). All three
series of substituted benzaldehydes meet the requirement
of a minimum number of substituents for analysis by
LDR and LDRS equations.17 We have used the standard
deviation (sd), the coefficient of multiple determination
(R2), and Exner’s19 parameter, ψ, as the measures of
goodness of fit.
The comparison of the L and D values for the substi-
tuted benzaldehydes showed that the oxidation of para-
substituted benzaldehydes is more susceptible to the
delocalization effect than to the localized effect. However,
the oxidation of ortho- and meta-substituted compounds
exhibited a greater dependence on the field effect. In all
cases, the magnitude of the reaction constants decreases
with an increase in the temperature, pointing to a
decrease in selectivity with an increase in temperature.
All three regression coefficients, L, D, and R, are
negative, indicating an electron-deficient carbon center
in the activated complex for the rate-determining step.
The positive value of η adds a negative increment to σd
(eq 8), reflecting the electron-donating power of the
substituent and its capacity to stablize a cationic species.
The negative value of S indicates that the reaction is
subject to steric hindrance by an ortho substituent.
The percent contribution of the delocalized effect, PD,
and the percent contribution of the steric parameter to
the total effect of the substituent, PS, were determined
by Charton’s method.18 They are recorded in Table 5. The
value of PD for the oxidation of para-substituted benzal-
dehydes is ca. 52%, whereas the corresponding values for
the meta- and ortho-sobstituted aldehydes are ca. 39%
and 49%, respectively. This shows that the balance of
localization and delocalization effects is different for
differently substituted benzaldehydes. The less pro-
nounced resonance effect from the ortho- position than
log k2 ) log k0 + mY
(5)
The plot of log k2 versus Y is linear (r2 ) 0.9912) with
m ) 0.53 ( 0.02. The value of m suggests that there is a
considerable charge separation in the transition state of
the reaction. However, the solvent composition may
simply affect the ground state stability of benzaldehyde.
Solvents of high polarity is likely to stablize the ionic
canonical form of benzaldehyde (eq 6).
Ph-CHdO T Ph-CH+-O-
(6)
Therefore, a reaction leading to the formation of a
cation is likely to be favored in solutions of high polarity.
Cor r ela t ion An a lysis of R ea ct ivit y. The effect of
substituents on reactivity has long been correlated with
the Hammett equation14 or with dual substituent-
parameter equations.15,1615,16 In the late 1980s, Charton17
introduced a triparametric LDR equation (eq 7) for the
(11) Pearlmutter-Hayman, B.; Weissmann, Y. J . Am. Chem. Soc.
1962, 84, 2323.
(12) Kaplan, L. J . Am. Chem. Soc. 1958, 80, 2639.
(13) Falnberg, A. H.; Winstein, S. J . Am. Chem. Soc. 1956, 78, 2770.
(14) J ohnson, C. D. The Hammett Equation; Cambridge University
Press: Cambridge, 1973; p 78.
(15) Dayal, S. K.; Ehrenson, S.; Taft, R. W. J . Am. Chem. Soc. 1972,
94, 9113.
(16) Swain, C. G.; Unger, S. H.; Rosenquist, N. R.; Swain, M. S. J .
Am. Chem. Soc. 1983, 105, 492.
(17) Charton, M.; Charton, B. Bull. Soc. Chim. Fr. 1988, 199 and
references therein.
(18) Charton, M. J . Org. Chem. 1975, 40, 407.
(19) Exner, O. Collect. Czech. Chem. Commun. 1966, 31, 3222.
Raju, V. Sitarama
