´
´
´
´
ˇ
614
NIKOLIC, USCUMLIC, AND JURANIC
chemical reactivity is discussed, as well as the effect of the molecular geometry on the reactivity
of the examined compounds. ꢀ 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 613–622, 2009
C
In the present work, the second-order rate constants
for the reaction of various 2-substituted cyclohex-1-
eneacetic and 2-substituted phenylacetic acids with
DDM in 11 aprotic solvents at 30◦C were determined.
To explain the kinetic results through solvent effects,
the second-order rate constants of the examined acids
in aprotic solvents, together with the second-order rate
constants for the same acids in three hydroxylic sol-
vents determined previously [15,16], were correlated
using the total solvatochromic equation [12], of the
form
INTRODUCTION
The reactivity of carboxylic acids with diazodiphenyl-
methane (DDM) is closely related to the acid molecular
structure and the present solvent. The main advantage
that makes this esterification convenient for examining
the solvent and structure influence on the carboxylic
acid reactivity, is that the catalyst is not necessary for
this reaction. It may vary in rate but takes place without
any additional support and in aprotic solvents it follows
the second-order kinetics [1,2]. The mechanism of this
reaction has been thoroughly examined [3–6], and it
was established that the rate-determining step involves
a proton transfer from the carboxylic acid to DDM
to form a diphenylmethanediazonium-carboxylate ion
pair, which rapidly reacts to give esters in the subse-
quent product-determining step (or ethers in the case
of hydroxylic solvents):
log k = A0 + sπ∗ + aα + bβ
(1)
where π∗ is the measure of solvent dipolar-
ity/polarizability, β represents the scale of solvent hy-
drogen bond acceptor basicities, α represents the scale
of solvent hydrogen bond donor acidities, and A0 is the
regression value of the solute property in the reference
solvent, cyclohexane [14]. The regression coefficients
s, a, and b measure the relative susceptibilities of the
solvent-dependent solute property (rate constants) to
the corresponding solvent parameter.
This paper demonstrates how the LSER method can
be applied to present and explain multiple interacting
effects of the solvent on the reactivity of 2-substituted
cyclohex-1-eneacetic and 2-substituted phenylacetic
acids in their reaction with DDM, and the influence of
the substituents of different nature at the C-2 position
in the ring for the reactions in the given solvent set. The
quantitative relationship between the molecular struc-
ture and the chemical reactivity has been discussed,
as well as the effect of geometry on the reactivity of
the examined molecules. The geometric data of the ex-
amined carboxylic acids, corresponding to the energy
minima in the applied solvents, were obtained using
semiempirical MNDO-PM3 energy calculations.
Ph2CN2 + RCOOH → Ph2CHN+2 + O2−CR
Our previous investigations of the reactivity of various
substituted and nonsubstituted unsaturated cyclic car-
boxylic acids with DDM in various solvents [7,8] and
the earlier work of Chapman et al. [9] have established
that the solvent effects are best interpreted in terms
of initial and transition state contributions of specific
and nonspecific solvent–solute interactions. The mul-
tiple linear regression analysis (MLRA) is very useful
in separating and quantifying such interactions on the
examined reactivity.
The first comprehensive application of the MLRA
to the kinetic phenomena was that of Koppel and
Palm [10], who listed regression constants for the sim-
ple Koppel–Palm equation [10] for various processes.
Shorter and coworkers [11] have applied the correlation
analysis to the solvent effects on the reaction between
DDM and benzoic acid.
In our recent papers [12,13], we examined the ef-
fects of a set of 11 aprotic and 3 protic solvents on the
reaction of various cycloalkenylcarboxylic acids and
cycloalkenylacetic acids with DDM by means of the
linear solvation energy relationship (LSER) concept
developed by Kamlet and Taft [14]. The correlation
equations obtained by stepwise regression for all the
examined acids showed that the total solvatochromic
equation can be used in its complete form, without sep-
arating of effects supporting the transition state (sol-
vent polarity and hydrogen bond donating ability) and
the ground state (hydrogen bond accepting ability).
MATERIALS AND METHODS
Cyclohex-1-eneacetic acid was prepared by the method
of Sugasawa and Saito [17] from cyclehexanone with
cyanoacetic acid and ammonium acetate. The obtained
nitrile was hydrolyzed with KOH to the acid: b.p. 138–
148◦C/17 mmHg (lit. [18] 139◦C/17 mmHg).
2-Substituted cyclohex-1-eneacetic acids were pre-
pared by a Reformatsky reaction with ethylbromoac-
etate and the corresponding 2-subsituted cyclohex-
anone, followed by saponification and dehydration of
the resulting hydroxy esters as reported previously
International Journal of Chemical Kinetics DOI 10.1002/kin