REACTION OF TRIETHYLAMMONIUM CARBOXYLATES WITH ETHYL ␣-HALOGENOACETATE
899
Table V Log k (XCH COOEt ϩ PhCOOH ϩ NEt Reactions)–Triple Solvent Parameter Regression [XCH COOEt] ϭ
2
2
3
2
[
PhCOOH] ϭ [NEt ] ϭ 0.025 M, Temperature 30ЊC, X ϭ Br or I
3
Set B: Aprotic Dipolar
Halogenated and Nitrogen-
Containing Solvents
Set A: Aprotic Dipolar
Ketonic Solvents
Set C: Protic Hydroxylic
Solvents
Correlation
n
r
s
n
r
s
n
r
s
Log k vs. Kirkwood function
7
0.704
0.162
10
0.846
0.171
6
0.943
0.392
2
ϩ ϩ Lassau and Jungers
(7)
(0.804)
(0.176)
(6)
(0.771)
(0.113)
(5)
(0.892)
(0.119)
log k(nϪ
3
Pr NϩMeI)
Log k vs. Kirkwood function
ϩ ϩ ET
9)
(9)
7
0.765
0.784
0.702
0.154
(0.180)
0.162
10
(6)
8
0.848
(0.836)
0.877
0.170
(0.105)
0.193
10
(9)
9
0.950
(0.738)
0.950
0.230
(0.193)
0.249
2
Log k vs. Kirkwood function
2
ϩ ϩ *
(7)
7
(7)
7
(0.771)
0.769
(0.791)
0.770
(0.181)
0.160
(0.178)
0.158
(5)
10
(6)
8
(0.998)
0.692
(0.844)
0.746
(0.014)
0.190
(0.104)
0.228
(8)
6
(5)
6
(0.888)
0.904
(0.886)
0.937
(0.164)
0.482
(0.121)
0.409
Log k vs. Lassau and Jungers
2
log k(nϪPr3NϩMeI) ϩ ϩ ET
Log k vs. Lassau and Jungers
2
log k(nϪPr3NϩMeI) ϩ ϩ *
(7)
(0.799)
(0.177)
(5)
(0.997)
(0.024)
(5)
(0.757)
(0.146)
a
Values in parentheses are for the reaction of ICH COOEt ϩ PhCOOH ϩ NEt .
2
3
3
Ϫ1
reactivity in DMF is due to anion desolvation [18,19],
which increases the nucleophilicity of the benzoate an-
ions.
times greater than in ethanol (k ϭ 0.015 dm mol
2
Ϫ
1
m ). The decrease in rate in chloroform is entirely
different for several reasons. It is obvious that triethy-
lammonium benzoate is not existing in a similar
Ϫ
ϩ
PhCOOH ϩ Et N !: PhCOO Et NH
fashion in these two solvents. The nucleophile
3
3
Ϫ
ϩ
Ϫ
EF PhCOO ϩ Et NH
(3)
(C
6
H
5
COO ) is made less potent in ethanol due to
3
extensive solvation through hydrogen bonding [9],
and in chloroform the acid–amine complex [20] may
have a pseudosymmetric structure. IR evidence has
been reported in support of the presence of the latter
structure in chloroform [21].
In dioxane,the rate was slower than in chloroform and
some precipitate was formed during the reaction
within an hour,even though dioxane has a lower di-
electric constant (2.21) compared to chloroform (4.7).
In tetrahydrofuran and ethyl acetate,some crystals
were thrown out from the reaction mixture after the
reaction was approximately 30% complete. However,
the rate constants were determined before separation
of the product began,wherever possible.
Generally,in alcoholic solvents,the k2 values are
much less than those in acetone. In methyl alcohol,the
k2 value is the highest among the alcohols. It has been
already observed in the reaction between triethylam-
monium benzoate and phenacyl bromide that the k2
values are abnormal in methyl alcohol [22].
It is evident from the results in Table II that the
reaction is 4–260 times faster in aprotic dipolar ke-
tonic solvents than in protic solvents. This may be due
to anion desolvation [18,19] in aprotic dipolar solvents
such as DMF. In hydroxylic protic solvents,the anion
is very readily solvated and this leads to much retar-
dation of the reaction. The nucelophile is made less
potent by extensive solvation via hydrogen bonding
by hydroxylic solvents such as ethanol. The rate con-
Effect of Solvent on the Reaction in Terms
of Solvent Parameters
With a view to checking the linear correlation between
the log k values and solvent parameters,the log k2
2
values were subjected to simple as well as multiple
linear correlations. The results are presented in Tables
III–V.
3
Ϫ1
stant in DMF is 4 times higher (k ϭ 0.780 dm mol
2
Ϫ
1
3
Ϫ
1
Ϫ1
m ) than in methanol (k ϭ 0.209 dm mol m )
2
for the ethyl bromoacetate reaction. In alcohols, k ap-
From these studies,we conclude that there are sev-
eral solvent properties that simultaneously influence
the rates,and many times one of these solvent prop-
erties may be a predominant one.
2
pears to remain constant except in methanol,which
3
gives a higher value. In chloroform (k ϭ 0.200 dm
2
Ϫ
1
Ϫ1
mol m ),a moderately polar solvent,the rate is 13