Correlation of the rates of solvolysis of benzoyl fluoride and a consideration of leaving-group effects

Article abstract of DOI:10.1021/jo0492259

The specific rates of solvolysis of benzoyl fluoride have been determined at 25.0 °C in 37 pure and binary solvents. Together with seven values from the literature, these give a satisfactory correlation over the full range of solvents when the extended Grunwald-Winstein equation is applied. The sensitivities to changes in solvent nucleophilicity and solvent ionizing power are very similar to those for octyl fluoroformate, suggesting that the addition step of an addition-elimination mechanism is rate determining. In the solvent-composition region where benzoyl chloride also shows bimolecular solvolysis, the appreciable k_Cl/k_F values are proposed as being primarily due to a more efficient ground-state stabilization for the fluoride.

Full text of DOI:10.1021/jo0492259

Cor r ela tion of th e Ra tes of Solvolysis of Ben zoyl F lu or id e a n d a
Con sid er a tion of Lea vin g-Gr ou p Effects
,
‡
§
Dennis N. Kevill* and Malcolm J . D’Souza
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862,
and Department of Chemistry, Wesley College, 120 North State Street, Dover, Delaware 19901-3875
dkevill@niu.edu; dsouzama@wesley.edu
Received May 7, 2004
The specific rates of solvolysis of benzoyl fluoride have been determined at 25.0 °C in 37 pure and
binary solvents. Together with seven values from the literature, these give a satisfactory correlation
over the full range of solvents when the extended Grunwald-Winstein equation is applied. The
sensitivities to changes in solvent nucleophilicity and solvent ionizing power are very similar to
those for octyl fluoroformate, suggesting that the addition step of an addition-elimination
mechanism is rate determining. In the solvent-composition region where benzoyl chloride also shows
F
bimolecular solvolysis, the appreciable kCl/k values are proposed as being primarily due to a more
efficient ground-state stabilization for the fluoride.
Benzoyl fluoride was the third compound with a
solvents, as proceeding by a mechanism involving ioniza-
tion to an acylium ion in the rate-determining step,
followed by its relatively rapid capture. For the parent
benzoyl chloride and ring-substituted derivatives without
highly electron-supplying substituents, a pathway in-
1
carbon-fluorine bond to be synthesized and the first to
2
be prepared by the halide-exchange route. It has been
the subject of several kinetic investigations, mainly
involving solvolysis3 or aminolysis.
-8
9-14
The solvolyses
eq 1) are subject to catalysis by added strong acids and,
(
volving nucleophilic attack on substrate has usually been
proposed. Several of the earlier reports3
,4,6a,16
are a little
with relatively high substrate concentrations, autoca-
talysis has also been observed. Since these acid catalysis
vague as regards mechanism but appear to favor a
concerted process, largely on the basis of large kCl/k
effects are not observed with other benzoyl halides, it is
F
usually proposed6
,7,15,16
that the protonation occurs on
ratios. Bender and J ones9 pointed out that the large
values can be rationalized in terms of a tetrahedral
intermediate that returns to reactants much more often
than it proceeds to products, such that the fission of the
carbon-fluorine bond will be rate determining. Several
fluorine, giving assistance to the breaking of the carbon-
fluorine bond in the rate-determining step. A parallel
explanation was proposed17 for the acid catalysis of the
hydrolysis of acetyl fluoride.
subsequent reports have given support to a mechanism
H O/ROH
6b,10,13
2
of this general type.
Other workers have considered
C H COF
8 C H COOH + C H COOR + HF
6 5 6 5
6
5
the mechanism to be near the borderline between con-
certed and stepwise and, therefore, difficult to de-
(1)
fine.8
,11,12,14,15
J encks and Song favored, for aminolysis,
12
Solvolyses of p-(dimethylamino)benzoyl fluoride were
a concerted pathway, involving characteristics of addi-
tion, without a stepwise mechanism being rigorously
excluded and, for solvolysis,8 an uncoupled concerted
pathway.
8
established, in highly ionizing and/or low nucleophilicity
‡
Northern Illinois University.
Wesley College.
§
A recent report18 concerning the solvolyses of octyl
(
(
(
(
(
1) Borodine, A. Ann. Chem. Pharm. 1863, 126, 58.
2) Banks, R. E.; Tatlow, J . C. Chem. Br. 1999, 35 (3), 21.
3) Bevan, C. W. L.; Hudson, R. F. J . Chem. Soc. 1953, 2187.
4) Swain, C. G.; Scott, C. B. J . Am. Chem. Soc. 1953, 75, 246.
5) Swain, C. G.; Mosely, R. B.; Bown, D. E. J . Am. Chem. Soc. 1955,
F
fluoroformate and chloroformate found the kCl/k ratio to
be only slightly greater than unity in 100% ethanol and
7
7, 3731.
6) (a) Satchell, D. P. N. J . Chem. Soc. 1963, 555. (b) Motie, R. E.;
Satchell, D. P. N.; Wassef, W. N. J . Chem. Soc., Perkin Trans. 2 1993,
087.
7) (a) Beguin, C.; Coulombeau, C.; Hamman, S. C. R. Acad. Sci.,
(12) Song, B. D.; J encks, W. P. J . Am. Chem. Soc. 1989, 111, 8479.
(13) (a) J edrzejczak, M.; Motie, R. E.; Satchell, D. P. N. J . Chem.
Soc., Perkin Trans. 2 1993, 599. (b) J edrzejczak, M.; Motie, R. E.;
Satchell, D. P. N.; Satchell, R. S.; Wassef, W. N. J . Chem. Soc., Perkin
Trans. 2 1994, 1471.
(
1
(
Ser. C 1972, 275, 1351. (b) Beguin, C.; Coulombeau, C.; Hamman, S.
J . Chem. Res., Synop. 1977, 178; J . Chem. Res., Miniprint 1977, 2107.
(14) Lee, B. C.; Sohn, D. S.; Yoon, J . H.; Yang, S. M.; Lee, I. Bull.
Korean Chem. Soc. 1993, 14, 621.
(15) Parker, R. E. Adv. Fluorine Chem. 1963, 3, 63.
(16) Kivinen, A. In The Chemistry of the Functional Groups: The
Chemistry of Acyl Halides; Patai, S., Ed.; Wiley: New York, 1972;
Chapter 6.
(17) Bunton, C. A.; Fendler, J . H. J . Org. Chem. 1966, 31, 2307.
(18) Kevill, D. N.; D’Souza, M. J . J . Chem. Soc., Perkin Trans. 2
2002, 240.
(8) Song, B. D.; J encks, W. P. J . Am. Chem. Soc. 1989, 111, 8470.
(9) Bender, M. L.; J ones, J . M. J . Org. Chem. 1962, 27, 3771.
(10) (a) Litvinenko, L. M.; Titskii, G. D.; Shpan’ko, I. V. J . Org.
Chem. USSR 1972, 8, 1016. (b) Oleinik, N. M.; Litvinenko, L. M.;
Sorokin, M. V. J . Org. Chem. USSR 1973, 9, 1715.
(11) Lee, I.; Shim, C. S.; Chung, S. Y.; Kim, H. Y.; Lee, H. W. J .
Chem. Soc., Perkin Trans. 2 1988, 1919.
1
0.1021/jo0492259 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/11/2004
7
044
J . Org. Chem. 2004, 69, 7044-7050

Correlation of the Rates of Solvolysis of Benzoyl Fluoride
00% methanol and to be somewhat below unity for
1
full range of solvents for the p-methoxy derivative and
an addition-elimination pathway for all but 97%
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) for the p-nitro
derivative. The parent compound, the p-methyl deriva-
tive, and the p-chloro derivative solvolyzed by either (or
both) of the mechanisms depending upon the properties
of the solvent. In particular, for the parent benzoyl
chloride, the mechanistic switch in binary aqueous
solvents was at approximately 70% methanol, 60% etha-
nol, and 50% acetone. For TFE-ethanol mixtures, it was
at about 70% TFE content.
The duality of mechanism for benzoyl chloride leads
to the need to exercise caution in the interpretation of
kCl/kF ratios for benzoyl halide solvolyses, since, for a
given solvent, the two halides may not be solvolyzing by
an identical pathway. Kinetic data recently became
available29 for the solvolyses of benzoyl bromide and three
para-substituted derivatives in a variety of solvents.
These data allow an extension of the consideration of
leaving group effects to a bromide ion leaving group.
In the present study, we consider the solvent variation
of the specific rates of solvolysis of benzoyl fluoride in
terms of linear free energy relationships (LFER) and
these analyses are then combined with a consideration
of leaving-group effects to arrive at a reasonable mech-
anism.
solvolyses in mixtures of water with ethanol, acetone,
dioxane, or 2,2,2-trifluoroethanol (TFE). These ratios
were considered to be consistent with the association step
of an association-dissociation mechanism being rate
determining. Similar results and conclusions had been
1
9,20
reported earlier for ethyl haloformate
and aryl halo-
formate21 solvolyses. In contrast, as mentioned above, for
both solvolyses and aminolyses of benzoyl halides and,
especially, acetyl halides, it has usually been found that
the chloride reacts considerably faster than the
fluoride7
,8,10,12,15-17,22
and this has usually been rational-
ized either in terms of a concerted process or an associa-
tion-dissociation pathway with dissociation being rate
determining.
The extended Grunwald-Winstein equation (eq 2)
treatment of octyl fluoroformate solvolysis led to a value
log(k/k ) ) lN + mY + c
(2)
0
T
Cl
for l, representing the sensitivity to changes in solvent
2
3,24
nucleophilicity (N
T
),
of 1.67 ( 0.07 and for m, repre-
senting the sensitivity to changes in solvent ionizing
power (YCl),2
5,26
of 0.76 ( 0.03. These values were found
18
to be similar to those for a variety of chloroformate esters
believed to solvolyze by the association-dissociation
pathway with association being rate determining. The
0
other entities in eq 2 are k and k representing, respec-
Resu lts
tively, the specific rates of solvolysis in a given solvent
and in 80% ethanol, and c, a constant (residual) term.
One might question the use of YCl for solvolyses of a
The specific rates of solvolysis of benzoyl fluoride at
25.0 °C are reported in Table 1. Of the values reported
in the table, 37 are determined in this study and 7 are
abstracted from reports5 of earlier determinations. Of
the total of 44 solvents used in the solvolysis studies,
F
fluoride, but Y values are not available and, in the
,8
absence of acid catalysis, 1-adamantyl fluoride (with the
recommended2
5,26
1-adamantyl component for poor leav-
2
7
23,24
25,26
ing groups) would solvolyze prohibitively slowly. How-
ever, for acid chlorides, if association is rate determining,
the solvent influence is for the movement of the electrons
within the carbonyl group toward the oxygen. Fortu-
nately, YCl appears to be a reasonable scale for this
process. Since this process will be essentially identical
for association of solvent with fluoroformates, one would
expect the YCl scale to also be a reasonable choice for
solvolyses of fluoroformates and acyl fluorides.
N
T
and YCl
values were both available for 41
solvents and these were utilized in the extended Grun-
wald-Winstein treatment using eq 2. The required YCl
values were not available for three of the aqueous dioxane
compositions used for the benzoyl fluoride solvolyses. The
N and YCl values are reported in Table 1, as are kCl/k
T F
ratios determined using available literature values
5
,8,30-35
for the specific rates of solvolysis of benzoyl chloride at
25.0 °C.
A recent analysis using eq 2 of data available for
solvolyses of benzoyl chloride and four para-substituted
derivatives28 indicated an ionization pathway over the
Discu ssion
F
The observation of kCl/k ratios of at least 38 (Table 1)
(
19) Green, M.; Hudson, R. F. J . Chem. Soc. 1962, 1055.
has to be reconciled with the values of close to unity,
frequently slightly below unity, observed for solvolyses
(20) Orlov, S. I.; Chimishkyan, A. L.; Grabarnik, M. S. J . Org. Chem.
USSR 1983, 19, 1981.
21) Queen, A.; Nour, T. A. J . Chem. Soc., Perkin Trans. 2 1976,
35.
22) Reference 15 contains order of magnitude errors for the k
ratio for solvolyses in 80% ethanol for the parent benzoyl halides
18-21
36
of haloformate esters.
This variance has suggested
(
9
differences in mechanism and a useful additional probe
will be to apply the extended Grunwald-Winstein equa-
(
F
/kCl
reported 1 order of magnitude low) and the p-methylbenzoyl halides
(29) Liu, K.-T.; Hou, I.-J . Tetrahedron 2001, 57, 3343.
(30) Liu, K.-T.; Chen, H.-I. J . Chem. Soc., Perkin Trans. 2 2000,
893.
(31) (a) Bentley, T. W.; Koo, I. S. J . Chem. Soc., Perkin Trans. 2
1989, 1385. (b) Bentley, T. W.; Carter, G. E.; Harris, H. C. J . Chem.
Soc., Perkin Trans. 2 1985, 983. (c) Bentley, T. W.; Harris, H. C. J .
Chem. Soc., Perkin Trans. 2 1986, 619.
(
, pp 81-115.
(
(
24) Kevill, D. N.; Anderson, S. W. J . Org. Chem. 1991, 56, 1845.
25) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17,
1
5
21.
(32) Archer, B. L.; Hudson, R. F. J . Chem. Soc. 1950, 3259.
(33) Brown, D. A.; Hudson, R. F. J . Chem. Soc. 1953, 3352.
(34) Lee, I.; Koo, I. S.; Sohn, S. C.; Lee, H. H. Bull. Korean Chem.
Soc. 1982, 3, 92.
(35) Crunden, E. W.; Hudson, R. F. J . Chem. Soc. 1956, 501.
(36) Kevill, D. N. In The Chemistry of the Functional Groups: The
Chemistry of Acyl Halides; Patai, S., Ed.; Wiley: New York, 1972;
Chapter 12.
(26) (a) Bentley, T. W.; Carter, G. E. J . Am. Chem. Soc. 1982, 104,
741. (b) Kevill, D. N.; D’Souza, M. J . J . Chem. Res., Synop. 1993, 174.
(
c) Lomas, J . S.; D’Souza, M. J .; Kevill, D. N. J . Am. Chem. Soc. 1995,
17, 5891. (d) Kyong, J . B.; Park, B.-C.; Kim, C.-B.; Kevill, D. N. J .
Org. Chem. 2000, 65, 8051.
1
(
(
27) Kevill, D. N.; Kyong, J . B. J . Org. Chem. 1992, 57, 258.
28) Kevill, D. N.; D’Souza, M. J . J . Phys. Org. Chem. 2002, 15, 881.
J . Org. Chem, Vol. 69, No. 21, 2004 7045

Kevill and D’Souza
TABLE 1. Sp ecific Ra tes of Solvolysis of Ben zoyl
a
F lu or id e in a Va r iety of P u r e a n d Mixed Solven ts a t 25.0
°
C, th e NT a n d YCl Va lu es for th e Solven ts, a n d a
Com p a r ison w ith Cor r esp on d in g Va lu es for Solvolysis of
Ben zoyl Ch lor id e (kCl/kF )
solventb
105k
/s-1 c
N
d
Y
Cl
e
F
T
F
(kCl/k )
0.148 ( 0.006f
3.16 ( 0.09
0.37 -2.52 524; 466
g
h
1
9
8
7
6
5
4
2
1
1
9
9
8
7
6
6
4
2
9
9
8
7
6
5
4
2
8
6
4
2
9
9
8
7
5
8
6
4
2
9
7
5
00% EtOH
0% EtOH
0% EtOH
0% EtOH
0% EtOH
0% EtOH
0% EtOH
0% EtOH
h
0.16 -0.94 52
f
g
h
6.53 ( 0.18
0.00
-0.20
-0.38
-0.58
-0.74
-1.16
-1.38
0.00 42; 45
h
9.95 ( 0.11
15.6 ( 0.5
25.5 ( 1.5
0.78 45
1.38 55
2.02 71
h
h
f
g
i
41.6 ( 1.3
2.75 128; 123
i
110 ( 6
4.09 336
4.57 611
j,k
j
00% H
2
O
180
h
00% MeOH
6.7% MeOH
0% MeOH
0% MeOH
0% MeOH
9.5% MeOH
0% MeOH
0% MeOH
0% MeOH
5% acetone
0% acetone
0% acetone
0% acetone
0% acetone
0% acetone
0% acetone
0% acetone
0% dioxane
0% dioxane
0% dioxane
0% dioxane
7% TFEp
1.23 ( 0.10
0.17 -1.17 347
f
l
l
g
3.63
0.12 -0.81 151
h
13.0 ( 0.9
25.6 ( 1.2
45.7 ( 1.4
-0.01 -0.18 62
h
-0.06
-0.40
-0.40
-0.54
-0.87
-1.23
0.67 50
1.46 44
h
g
h
i
f
l
l
42.7
1.50 45
71.1 ( 1.8
2.07 49
3.25 97
138 ( 6
i
160 ( 7
4.10 338
7.98 (( 0.08) × 10- -0.49 -3.19 687
3
m
n
F IGURE 1. Logarithmic plots of the specific rates of solvolysis
of benzoyl chloride and p-(dimethylamino)benzoyl fluoride
against the specific rates of solvolysis of benzoyl fluoride for
aqueous-ethanol compositions at 25.0 °C.
0.0586 ( 0.0011
-0.35 -2.22 293
f
g
h
0.409 ( 0.011
-0.37 -0.83 123; 164
g
f
1.32
-0.42
-0.52
-0.70
-0.83
-1.11
-0.46
-0.54
-0.84
-1.12
-3.30
-2.55
-2.19
-1.98
-1.73
-1.76
-0.94
0.17 83
h
m
5.58 ( 0.13
0.95 46; 45
f
g
7.95
1.73 102
2.46 154
3.77 359
i
16.2 ( 0.5
69.6 ( 1.3
0.524 ( 0.018
6.84 ( 0.10
19.3 ( 0.8
77.3 ( 1.3
fluoroformate specific rates of solvolysis in a series of
solvents.
i
o
107
38
108
The treatment in terms of the extended Grunwald-
Winstein equation leads to values of 1.58 ( 0.09 for l,
3.71 258
0
.82 ( 0.05 for m, and -0.09 for c. The multiple
q
3 i
0.00117
2.83 311 × 10
p
3 i
3 i
3 i
correlation coefficient (R) is 0.953, the F-test value is 186,
and the standard error of the estimate is 0.10. The plot
is not shown. The l and m values are very similar to those
for octyl fluoroformate.18 For the 26 solvents for which
specific rates are available for both benzoyl fluoride and
octyl fluoroformate, a direct comparison through an
LFER plot leads to good linearity with a slope of 0.95 (
0.05, intercept of 0.06 ( 0.07, correlation coefficient of
0.966, and F-test value of 338.
0% TFE
0.103 ( 0.001
0.484 ( 0.028
1.53 ( 0.09
2.85 5.46 × 10
2.90 2.32 × 10
2.96 1.28 × 10
0% TFEp
0% TFEp
0% TFEp
i
6.55 ( 0.20
3.16 831
r
3 h
3 h
0T-20E
0.0236 ( 0.0013
0.0547 ( 0.0011
0.102 ( 0.005
0.125 ( 0.011
0.0269 ( 0.0017
1.43 ( 0.10
1.89 4.65 × 10
r
0T-40E
0.63 1.76 × 10
r
h
s
0T-60E
0T-80E
-0.34 -0.48 648; 593
r
s
0.08 -1.42 576
0% HFIPp
-3.84
-2.94
-2.49
4.31
3.83
3.80
p
0% HFIP
0% HFIPp
5.21 ( 0.16
f
3 g
AcOH
HCO
0.000209
0.151
-1.78 -1.60 5.01 × 10
Since octyl fluoroformate is believed to solvolyze by an
addition-elimination pathway in all of the solvents
involved in the plots, the similarity in l and m values for
the two solvolyses, coupled with the good direct LFER
plot, gives rather strong evidence for an association-
dissociation mechanism.
f
3 t
2
H
-2.44
3.20 13 × 10
a
-3
Substrate concentration of 0.010 mol dm , unless otherwise
indicated. Unless otherwise indicated, the binary solvents are
b
on a volume-volume basis at 25.0 °C, with the other component
c
being water. With associated standard deviations; average of all
d
integrated first-order rate coefficients from duplicate runs. From
refs 23 and 24. e From refs 25 and 26. f From ref 5: values of 0.162
Solvolyses for which a unit mechanism is observed over
a wide range of solvents can be useful as a standard for
in 100% EtOH, 6.17 in 80% EtOH, 39.8 in 40% EtOH, and 0.398
in 80% acetone; six additional values are presented in the table.
an LFER investigation of possible solvent-induced changes
g
h
Using the values from ref 5. Value for benzoyl chloride from
31a
in mechanism in other solvolyses.
For example, the
i
ref 30. Value for benzoyl chloride (interpolated for 90% TFE and
8
solvolyses of p-methoxybenzoyl2
8,31a
and p-nitrobenzoyl
28
0% TFE) from ref 31. j Values from ref 8. A solvent deuterium
k
l
chlorides have been used in this way. We can also
consider the possible use of the solvolyses of benzoyl
fluoride as a similarity model.
isotope effect value (kH/kD) of 2.0 (ref 8). Interpolated value.
m
n
Value for benzoyl chloride from ref 32. Value for benzoyl
o
chloride from ref 5. Value for benzoyl chloride by interpolation
within data of ref 33. On a weight/weight basis; substrate
concentration 0.020 mol dm
p
A plot of log k for benzoyl chloride solvolysis against
log k for benzoyl fluoride solvolysis is shown in Figure 1.
The previously determined aqueous-ethanol solvent
range for a dominant ionization mechanism for benzoyl
-3
q
7
.
Extrapolated value based on 10 k
-
1
(
2
s ) values of 64.9 ( 2.2 at 82.4 °C, 7.64 ( 0.21 at 62.4 °C, and
.56 ( 0.20 at 50.0 °C. r T-E are TFE-ethanol mixtures; substrate
-
3 s
concentration 0.020 mol dm
ref 34. Value for benzoyl chloride (ref 31b) approximately ex-
trapolated from data at 9 °C (ref 35).
. Value for benzoyl chloride from
t
28
chloride solvolysis, with more than 60% water content,
gives a linear plot with a slope of 2.27 (correlation coeffi-
cient of 0.9978). The points for the less-aqueous solvents
lie above the plot, considerably above for 100-80%
ethanol, consistent with the previously indicated incur-
sion of an appreciable contribution from the addition-
elimination pathway. However, a good linear relationship
tion (eq 2) and compare the l and m values with those
previously obtained for octyl fluoroformate. A more direct
comparison can also be made by constructing the LFER
for a direct comparison of benzoyl fluoride and octyl
7
046 J . Org. Chem., Vol. 69, No. 21, 2004

Correlation of the Rates of Solvolysis of Benzoyl Fluoride
is not observed in the bimolecular range, associated with
the appreciably higher m value for benzoyl fluoride
solvolyses when eq 2 is applied. Indeed, the different
responses to changes in solvent ionizing power sug-
gest that benzoyl fluoride solvolysis will not be a good
similarity model for the bimolecular acyl chloride sol-
volyses.
When both of the halides are reacting by the same
mechanism, the kCl/k
ratio is usually taken as a measure
F
of the extent of bond breaking in the C-X bond at the
4
transition state of the rate-determining step. Application
of this argument to the bimolecular solvolysis would
require either a concerted displacement pathway or a
rate-determining role for the second step of an addition-
elimination pathway. This type of argument was used
by Swain and Scott4 in a consideration of RCl/RF ratios
for solvolyses in aqueous acetone. A range of values from
An LFER comparison with other acyl fluorides could
be more fruitful and the solvolyses of p-(dimethylamino)-
,39
benzoyl fluoride, previously studied in aqueous ethanol
8
6
2
and plotted against YCl
,
can alternatively be plotted
10 for triphenylmethyl halides to 10 for benzoyl halides
was observed and the results discussed in terms of the
relative amounts of C-X bond fission and C-O bond
formation at the transition state.
against log k for benzoyl fluoride solvolyses; this plot is
also shown in Figure 1. As with the benzoyl chloride plot,
a good linear plot (correlation coefficient of 0.9984 and
slope of 2.92) is observed at high water content. It starts
to curve upward at beyond 40% EtOH, at slightly lower
ethanol content than for benzoyl chloride (which is linear
as far as 50% ethanol content). The plots are consistent
with both of the solvolyses which are compared to the
solvolyses of benzoyl fluoride in Figure 1 undergoing a
solvent-induced change in mechanism. Since the solvoly-
ses of p-(dimethylamino)benzoyl fluoride were studied
An interesting result was that, for reaction of benzoyl
halides with hydroxide ion in 50% acetone, the kCl/k ratio
F
for the second-order rate coefficients was somewhat below
4
unity (0.71). However, values of unity or lower are not
always a feature of reactions under nonsolvolytic condi-
9
tions. Bender and J ones found extremely large leaving-
group effects in a consideration of the second-order rate
coefficients for the reaction of benzoyl halides with
8
only as far as 80% ethanol, the solvent range for the
morpholine in cyclohexane as solvent. The kCl/k
F
ratio was
bimolecular mechanism is insufficient for a correlation
analysis in this region to be possible. The slopes of greater
than unity for the linear regions of both plots are
consistent with the expected8 wider variation of specific
rate for the unimolecular mechanism involved in this
region relative to the corresponding bimolecular solvoly-
ses of benzoyl fluoride.
2600 and the kBr/kCl ratio was 25. The overall ratio
(k /k ) was about 10 . The authors proposed two ways by
I F
5
which the rather large rate differences could occur. One
involves the partitioning of the intermediate needing
consideration, with a considerable return to reactants
being able to compete effectively with progress to prod-
ucts. The other involved a ground-state effect with
differences in the efficiency of resonance stabilization of
the reactant. They concluded that both effects were of
importance but that data did not allow an assessment of
the relative importance of the two effects.
,28
F
In the comparison of kCl/k ratios for benzoyl chloride
and benzoyl fluoride, one must bear in mind that, for the
solvolyses of the chloride previously shown to proceed by
an ionization mechanism,28 the corresponding fluoride
would react much slower by this mechanism and the
There has been a consideration of the stabilization by
bromine and chlorine of a carbon-carbon double bond.
Application of ab initio MO calculations indicated that
the stabilization by chlorine was greater by 1.3 kcal/mol.
It was pointed out that this could translate into a factor
of about 10-fold in reactivity and modest values of about
this magnitude could be observed even if the addition
bimolecular mechanism (probably with general-base
catalysis8
,37,38
) will be the one in operation. For example,
O of the p-(dimethylamino)-
for the hydrolyses in 100% H
benzoyl halides, the kCl/k
ratio is very large (3 × 10 ).
Accordingly, the appreciable but somewhat lower values
observed in TFE-H O, TFE-EtOH, acetic acid, and
formic acid, the largest being 3.1 × 10 for solvolysis in
7% TFE, can be considered to reflect a comparison of
2
8
7
F
2
5
40
step was rate determining. A subsequent theoretical
study41 confirmed that substituent effects at the vinyl
carbon are modest and it was emphasized that they are
much less than at the acetyl carbon. For the acetyl carbon
the detailed nature of the interaction depends on the
nature of the substituent but calculation indicates that
there can be strong Coulombic interactions, due to the
9
ionization reaction of benzoyl chloride with bimolecular
reaction of benzoyl fluoride.
The value of 38 observed in 60% dioxane changes only
slightly as one replaces the dioxane component with
ethanol, methanol, or acetone. For all four of these binary
4
2
very pronounced polarization of the carbonyl group.
F
systems, the kCl/k value initially falls from the value in
the pure solvents or in solvents with an appreciable
organic component as increasing amounts of water are
present and then when the water content starts to move
above 40%, with increasing tendency toward ionization
reaction for the benzoyl chloride solvolysis,28 the trend
reverses as the values start to rise, culminating in a value
A theoretical evaluation of the effects of fluorine in
acetyl fluoride42 indicated a relatively small effect from
π-electron donation and a major contribution resulting
from the high polarity in the C-F bond, which Coulom-
bically stabilizes the C-O group, such that both the
C-X43 and C-O bonds are strengthened by the introduc-
tion of a more electronegative substituent. This results
2 F
of 611 at 100% H O. Our interest in the kCl/k ratio is
primarily concentrated in the less aqueous region where
both of the halides react by the bimolecular mechanism
and with the observation that, unlike for haloformate
ester solvolysis, the ratio does not fall to unity (or slightly
lower).
(39) Bunton, C. A. Nucleophilic Substitution at a Saturated Carbon
Atom; Elsevier: Amsterdam, The Netherlands, 1963; pp 160-162.
(
40) Hoz, S.; Basch, H.; Wolk, J . L.; Rappoport, Z.; Goldberg, M. J .
Org. Chem. 1991, 56, 5424.
(41) Wiberg, K. B.; Rablen, P. R. J . Am. Chem. Soc. 1993, 115, 9234.
(42) Wiberg, K. B.; Hadad, C. M.; Rablen, P. R.; Cioslowski, J . J .
Am. Chem. Soc. 1992, 114, 8644.
(43) Pauling, L. The Nature of the Chemical Bond; Cornell Univer-
sity Press: Ithaca, NY, 1939.
(
(
37) Kevill, D. N.; Foss, F. D. J . Am. Chem. Soc. 1969, 91, 5054.
38) Bentley, T. W.; Harris, H. C. J . Org. Chem. 1988, 53, 724.
J . Org. Chem, Vol. 69, No. 21, 2004 7047

Kevill and D’Souza
in larger effects for CH
3
COX than for CH
3
X or
SCHEME 1
dCHX.41,42 It was proposed that fluorine will act
44
CH
2
as a π-electron donor only when a full positive charge
can be stabilized. It was also shown42 that chlorine is a
rather poor π-electron donor. While the calculations sug-
gest that Bender and J ones could well be correct in their
assumption of an important ground-state stabilization,
their description of it as a resonance stabilization would
seem to imply π-electron donation, whereas the major
influence indicated by calculation is Coulombic in origin.
While differences in ground-state stabilization could
Since kCl/k
found to be in the range of 10 (for Ph
F
rate ratios for S
N
1 solvolyses have been
6
39
7
3
CX ) to 10 (for
8
p-Me NC H COX ), the relatively slow loss of fluoride
2 6 4
could lead to a situation where, for the fluoride, return
to reactant could be competitive for acyl fluorides but not
for acyl chlorides or haloformate esters. If partitioning
was important for benzoyl fluoride but not for octyl
fluoroformate, then one would expect this to be reflected
in the l and m values determined with eq 2. Changes
would be expected if the rate-determining step changes,
at least partially, from formation of the intermediate to
its progression to products. As already mentioned, the
differences in sensitivity values for the solvolyses of the
two substrates are very minor. This would suggest that,
if partitioning was important for the fluoride, the ratio
of return to product formation would have to be largely
independent of the identity of the solvent. This seems
rather unlikely since the fission of the C-F bond is
generally considered to be strongly influenced by the
solvation of the incipient fluoride ion.9 It appears that
the important characteristics of benzoyl fluoride solvoly-
sis, including the comparison with octyl fluoroformate
solvolysis, are best explained in terms of an addition-
F
explain appreciable kCl/k ratios for solvolyses of acyl
halides even if addition is rate determining, it is still
necessary to rationalize why large values can be observed
for the simple acyl halides (RCOCl, with R being alkyl
or aryl) but haloformate solvolyses usually show a kCl/k
F
ratio of close to unity. In this connection, it can be noted
that haloformate solvolyses are appreciably slower than
those of the related acyl halide with the non-carbonyl
oxygen removed. For example, for solvolyses in 89.1%
3
acetone at 25.0 °C, acetyl chloride reacts 9 × 10 times
faster than methyl chloroformate.45 This is usually
rationalized in terms of ground-state stabilization, in-
volving π-electron donation from the oxygen.36 It is
reasonable to suppose that a strong effect of this nature
could outweigh the alternative of stabilizing influences
involving the halogen, which is only a weak π-electron
donor and where the Coulombic influence will be reduced
by the interaction of the oxygen with the positive charge
of the R-carbon. This situation, expressed in terms of
π-electron donation, is illustrated in eq 3.
,15
elimination pathway, with fairly large kCl/k
F
ratios
resulting from ground-state stabilization, especially im-
portant for the fluoride. It is not possible, however, to
completely rule out influences from a possible return to
reactant from the tetrahedral intermediate.
The consideration of leaving-group effects can be
extended to benzoyl bromide by using a report by Liu
and Hou29 of the rates of solvolysis at 25.0 °C. High
reactivity at 25 °C and low solubility of substrate at lower
temperatures limited the range of solvent composition
to no more than 30% water content in binary mixtures
with ethanol, methanol, and acetone. The kBr/kCl ratios,
The possibility that the relatively large kCl/k
ratios for bimolecular attack arise from an increase in
F
rate
return to reactants for the fluoride (stronger C-X bond)
9
relative to that for the chlorides must also be considered.
28,30
in a comparison with benzoyl chloride solvolysis rates,
It is reasonable that such return would be more ap-
preciable for an acyl halide than for a haloformate ester.
If we assume deprotonation through general-base cataly-
sis in reaction with either alcohol or water (SOH), then
the intermediates can be written as in Scheme 1. These
formulations assume that the negatively charged oxygen
is not appreciably protonated. Either way, both types of
intermediate would have a considerable driving force for
ejection of a halide ion, as is indicated by the 109
difference in solvolysis rates for ethoxymethyl chloride
and butyl chloride, a minimum value for the influence
of the ethoxy group on ionization rates.46 The presence
of the additional alkoxy (or aryloxy) group in the inter-
mediate from the haloformate ester would give an ad-
ditional driving force to the large driving force already
present in the intermediate from the acyl halide. There
is evidence from 18O exchange for return to reactants
from intermediate during hydrolysis of benzoyl chloride
in 67-75% acetone.47 The amount is relatively small,
however, suggesting that only one in 19-26 intermedi-
ates returns to reactant. This would result in only a
minor perturbation of the concept that the association
step is rate determining.
show only a moderate variation with solvent composition.
The solvolyses in the above-mentioned binary mixtures
are to be expected to be bimolecular in character and in
1
00-80% methanol the ratio was 24-30 and the value
rose to 74-86 in 80-70% acetone. A value of 86 in 100%
ethanol fell to 43 in 90% ethanol and to 29 in 80%
ethanol. In 100% TFE, the mechanism will involve rate-
determining ionization, but the value of 34 remains in
the same range, with a slight increase to 57-71 in TFE-
ethanol mixtures with 20-60% ethanol content. It does
not appear that for these solvolyses the kBr/kCl value could
be helpful in assigning mechanism. The magnitudes of
the values for the ratio are consistent with values
48
previously tabulated for reactions of aliphatic and
49
aromatic acyl halides.
In considering the kBr/kCl ratio, it is possible to show
that the ratio is not only rather insensitive to changing
(
44) Wiberg, K. B.; Rablen, P. R. J . Org. Chem. 1998, 63, 3722.
(45) Ugi, I.; Beck, F. Chem. Ber. 1961, 94, 1839.
46) Streitwieser, A., J r. Solvolytic Displacement Reactions; McGraw-
Hill: New York, 1962; p 103.
47) Bunton, C. A.; Lewis, T. A.; Llewellyn, D. R. Chem. Ind.
(London) 1954, 1154.
(
(
7
048 J . Org. Chem., Vol. 69, No. 21, 2004

Correlation of the Rates of Solvolysis of Benzoyl Fluoride
TABLE 2. Com p a r ison of kCl/kF Ra tios for
p-Meth ylben zoyl Ha lid e, Ben zoyl Ha lid e, a n d
p-Nitr oben zoyl Ha lid e Solvolyses a t 25.0 °C
to a change in reaction mechanism, such as to an
ionization mechanism, which is certainly possible, or
some other factor is responsible for increasing the rate
of reaction of the protonated acyl fluorides. Indeed, such
an effect is to be expected if Coulombic interactions are
a
kCl/kF
solvent
p-MeC6H4COX
C6H5COX
p-NO2C6H4COX
an important influence in the determination of kCl/k
F
rate
1
8
4
1
1
8
7
5
00% EtOH
0% EtOH
0% EtOH
524
42
128
79
4.9
2.5
ratios. After protonation, the fluorine will carry a posi-
162
4
2
tive, not a negative charge and calculations support
what one would intuitively predict that placing a positive
charge next to the positive charge of the carbonyl carbon
has a large destabilizing ground-state effect, which can
lead to appreciably faster reactions. Indeed, the influence
of protonation can be considered as a logical extension
of the reduction in Coulombic stabilization, due to very
efficient hydrogen bonding, as the water content of mixed
solvents is increased.
b
b
00% H2O
611
1.2
00% MeOH
0% acetone
0% acetone
0% acetone
347
123
83
158
17.4
9.6
4.1
246
37.2
276
932
10.9 × 10
102
3
3
AcOH
HCO2H
5.01 × 10
3
13 × 10
a
Specific rate values from ref 5, unless otherwise stated. b From
ref 8.
the solvent, it is also only modestly changed by the
Con clu sion s
introduction of a para substituent into the two sub-
strates.2
8-30
With electron-supplying substituents, the
The solvolyses of benzoyl fluoride give a satisfactory
ratio range for the parents (24-86) rises slightly to
2-130 on introduction of a p-methyl group and this
extended Grunwald-Winstein correlation (eq 2) over a
wide range of N and YCl values, from 100% ethanol
T
4
value continues to rise modestly on changing to a
p-methoxy group (58-135). Introduction of the moder-
ately electron-withdrawing p-chloro substituent leaves
the range of values essentially unchanged (20-81).
(tending to favor bimolecular solvolysis) to 90% HFIP
(tending to favor ionization pathways). The sensitivities
to changes in N and YCl (l ) 1.58; m ) 0.82) are very
T
similar to those for octyl fluoroformate and several
chloroformate esters, which are believed to solvolyze by
an addition-elimination (association-dissociation) mech-
anism, with the addition step being rate determining.
The effect of para substituents upon the kCl/k
for two substituents and for the parent benzoyl halides
parent data also in Table 1) is shown in Table 2. Except
F
ratio
(
8
for the solvolyses in 100% H
by Swain, Mosely, and Bown. Large kCl/k
2
O, the data are from a study
The k /kF ratios, readily determined by using data
Cl
5
F
values are
available for the solvolysis of benzoyl chloride, are all
obtained for solvolyses of the p-methyl derivative because
the solvents are ones where the chloride would be
solvolyzing by the ionization mechanism. For the sol-
volyses of p-nitrobenzoyl chloride all of the solvolyses,
chloride and fluoride, will be by the bimolecular mech-
anism.28 The values are uniformly smaller than those for
the solvolyses of benzoyl halides and in 100% water a
value of only 1.2 is obtained. This is consistent with a
considerable ground-state solvation of the fluorine, with
partial transfer of negative charge to solvent molecules
such that the stabilizing intramolecular Coulombic forces
appreciably larger than unity, in contrast to values of
close to unity for the octyl haloformates. Several very
large values result from the solvolysis involving a solvent
for which the kCl value is measuring a superimposed
ionization pathway. However, appreciable values (>38)
are observed when both halides are believed to operate
by a bimolecular pathway. As water is added to the
organic component of binary mixtures with ethanol,
methanol, and acetone, the values fall from the 350-700
range to a value of about 40 and then steadily rise again
as the region is entered where ionization is known to be
dominant for benzoyl chloride solvolysis. With p-ni-
trobenzoyl halide solvolyses, it is known that the addi-
tion-elimination pathway will be dominant for both
are reduced. This approach predicts the fall in kCl/k
F
value as one proceeds from 100% ethanol to 100%
water. A similar trend is observed for the parent benzoyl
halides, except with more than 30% water (40% for
aqueous acetone) the trend reverses itself because of the
introduction of the ionization mechanism for benzoyl
chloride.28
halides, except in 97% HFIP, and the kCl/k
F
value
O.
continues to fall, reaching a value of 1.2 in 100% H
2
It is suggested that for the bimolecular pathway the
major influence of varying the halogen is a more pro-
nounced ground-state stabilization for the fluoride, rather
than increased return from a tetrahedral interme-
diate. For haloformates, the ground-state stabilization is
largely provided by the oxygen attached to the carbonyl
carbon.
An interesting aspect of acyl halide solvolyses is that
3
,6,7,15-17
only the fluorides are subject to acid catalysis.
This was very reasonably taken as evidence for proto-
nation at the fluorine because protonation at the carbonyl
oxygen would be expected to have similar consequences
for all acyl halides. Further consideration is then usually
in terms of the protonation assisting the rupture of the
C-F bond. If one is arguing against this bond being
broken, because the addition step is rate determining,
then it must be assumed either that the protonation leads
Using literature values, it is possible to consider
k
Br/kCl ratios as well. These show only modest variation
with either solvent composition or an introduction of
p-methoxy, p-methyl, or p-chloro substituents, with an
overall range of values of 20-135. However, high reactiv-
ity at 25 °C and low solubility at reduced temperatures
prevented the study of the solvolysis of the bromides in
aqueous-organic solvents with more than 30% water
content.
(
48) Kevill, D. N.; Kim, C.-B. J . Chem. Soc., Perkin Trans. 2 1988,
1
353.
(
49) Kevill, D. N.; Wang, W. F. K. J . Chem. Soc., Perkin Trans. 2
1
998, 2631.
J . Org. Chem, Vol. 69, No. 21, 2004 7049

Kevill and D’Souza
The acid catalysis frequently observed for solvolyses
of acyl fluorides does not necessarily arise because the
conversion to a better leaving group aids C-F bond
fission. It could involve a ground-state effect, with change
from Coulombic stabilization to Coulombic destabilization
leading to a considerably higher energy substrate and a
lower energy barrier to reaction.
Exp er im en ta l Section
Commercially available benzoyl fluoride (purity 99.5%) was
used as received. The determinations of specific rates of
solvolysis were as previously described.2
1,27
The multiple re-
gression analyses were carried out with the ABSTAT statisti-
cal package (Anderson-Bell, Arvada, Colorado).
J O0492259
7
050 J . Org. Chem., Vol. 69, No. 21, 2004