Correlation of the rates of solvolysis of methyl chloroformate with solvent properties

Article abstract of DOI:10.1039/a808929i

The specific rates of solvolyis of methyl chloroformate are very well correlated by the extended Grunwald-Winstein equation over a wide range of solvents; the pathway is believed to be predominantly addition-elimination, except that a positive deviation for solvolysis in 90% 1,1,1,3,3,3- hexafluoropropan-2-ol suggests an 80% contribution from an ionisation mechanism.

Full text of DOI:10.1039/a808929i

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150 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 150±151$
Correlation of the Rates of Solvolysis of Methyl
Chloroformate with Solvent Properties$
Dennis N. Kevill,*a Jong Chul Kimb and Jin Burm Kyongb
aDepartment of Chemistry and Biochemistry, Northern Illinois University, DeKalb,
Illinois 60115-2862, USA
bDepartment of Chemistry, Hanyang University, Ansan, Kyunggi-do 425-791, Korea
The specific rates of solvolyis of methyl chloroformate are very well correlated by the extended Grunwald±Winstein
equation over a wide range of solvents; the pathway is believed to be predominantly addition±elimination, except
that a positive deviation for solvolysis in 90% 1,1,1,3,3,3-hexafluoropropan-2-ol suggests an 80% contribution from an
ionisation mechanism.
The extended Grunwald±Winstein equation [eqn. (1)]
has been successfully applied to the solvolyses of several
chloroformate1,2 and chlorothioformate2,3 esters. In eqn. (1),
k and k0 are the speci®c rates of solvolysis of a substrate in
a given solvent and in 80% ethanol, respectively; l is
the sensitivity towards changes in NT, a scale of solvent
nucleophilicity based on the speci®c rates of solvolysis of the
S-methyldibenzothiophenium ion;4,5 and m is the sensitivity
towards changes in YCl, a scale of solvent ionising power
based on the speci®c rates of solvolysis of 1-adamantyl
chloride.6±8
by the nucleophilic center of the solvent molecules.
Incorporation of sulfur within the alkoxy (or phenoxy)
group favors an increased participation by the ionisation
mechanism in its competition with the addition±elimination
pathway.2,3
In the present study, the solvolyses of methyl chloro-
formate9,17,18 are investigated. First-order speci®c rates of
solvolysis measured at, or extrapolated to, 40.0 8C are
reported in Table 1 for a variety of pure and binary solvent
mixtures. Analysis of the data using eqn. (1) leads to a
fair linear correlation with values of 1.5220.11 for l,
logꢀk=k0† ˆ lNT ‡ mYCl ‡ c
ꢀ1†
The solvolyses of phenyl chloroformate have been
correlated using eqn. (1), with one correlation applying
over the full range of solvent composition studied,
including studies in solvents rich in the highly ionising
and poorly nucleophilic 1,1,1,3,3,3-hexa¯uoropropan-2-ol
(HFIP) or 2,2,2-tri¯uoroethanol (TFE). Appreciable sensi-
tivities to changes in both solvent nucleophilicity (l ˆ
1.6820.10) and solvent ionising power (m ˆ 0.5720.06)
were observed.1 The solvolyses of ethyl chloroformate2
could be successfully correlated using eqn. (1) with similar l
and m values (l ˆ 1.5620.09; m ˆ 0.5520.03), but only
in the less ionising and more nucleophilic solvents. In the
more ionising and least nucleophilic solvents (HCO2H,
100% and 97% TFE and 97%±50% HFIP), eqn. (1) could
be applied, but only with very di€erent sensitivity values
(l ˆ 0.6920.13; m ˆ 0.8220.16).
0.5520.07 for m, and 0.1120.11 for
c (correlation
coecient of 0.9584; n ˆ 21). Inspection of the plot showed
that there are serious deviations for 90% HFIP and formic
acid, which lie above and below the plot, respectively. The
deviation for 90% HFIP is consistent with a superimposed
contribution to the log (k/k0) values from an ionisation
mechanism. A considerably improved correlation (Fig. 1)
results when these two data points are omitted and values
are obtained for 19 solvents of 1.5920.09 for l, 0.5820.05
Two competing reaction pathways have been proposed.
There is strong independent evidence9±12 that phenyl chloro-
formate solvolyzes by an addition±elimination pathway
involving a tetrahedral intermediate [eqn. (2), or closely
related variant]. Evidence that this addition±elimination
pathway often operates for the solvolyses of haloformate
esters includes F/Cl leaving-group e€ects of close to
unity,13±16 consistent with the addition step being rate-
determining. The l values in the range 1.5±1.7 and m values
in the range 0.5±0.6 were considered1,2 to be typical values
for the operation of this mechanism.
The competing mechanism operating for ethyl chloro-
formate is believed to involve ionisation [eqn. (3)], but
incorporating, relative to 1-adamantyl chloride solvolysis,
an appreciable solvation of the developing carbocation
*To receive any correspondence.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
Fig. 1 Plot of log (k/k0) for solvolyses of methyl chloroformate
at 40.0 8C against (1.59 NT ‡ 0.58 YCl
)

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J. CHEM. RESEARCH (S), 1999 151
Table 1 Specific rates of solvolysis (with standard deviations)
of methyl chloroformate at 40.0 8C together with the appropriate
NT and YCl values
lower than the value one would estimate for an addition±
elimination pathway. The claim18 that the solvolysis of
methyl chloroformate proceeds by an ionisation process
in formic acid, the plausibility of which is based on the
approximately equal rate decreases reported as one moved
from isopropyl chloroformate formolysis to ethyl chloro-
formate formolysis to methyl chloroformate formolysis,
is rendered dubious based on the observation that these
70-fold decreases cause the MeOCOCl solvolysis to be
considerably slower even than the estimate for the addition±
elimination pathway. We propose that the formolysis
of MeOCOCl is proceeding by the addition±elimination
pathway, but do not have any good explanation for the low
speci®c rate values observed both in the present study and
previously.18
In conclusion, the solvolyses of methyl chloroformate are
indicated to proceed by a bimolecular addition±elimination
pathway in 20 of the 21 solvents considered. Only in
the most ionising±lowest nucleophilicity combination (90%
HFIP) is there evidence for a dominant ionisation pathway.
This behaviour di€ers from the analyses of the speci®c rates
of solvolysis of ethyl chloroformate, where the ionisation
pathway was dominant in formic acid, in 97% TFE, and
over the full range of HFIP±H2O mixtures investigated.2
1
b
NT
c
YCl
Solventa
105 k/s
100% EtOH
90% EtOH
80% EtOH
100% MeOH
90% MeOH
80% MeOH
90% Acetone
80% Acetone
70% Acetone
97% TFE
13.220.1d
37.120.1
0.37
0.16
0.00
0.17
0.01
0.06
0.35
0.37
0.42
3.30
2.55
1.98
1.73
3.84
2.94
2.49
2.44
1.38
0.94
0.34
0.08
2.52
0.94
0.00
1.17
0.18
0.67
2.39
0.80
0.17
2.83
2.85
2.96
3.16
4.31
3.83
3.80
3.20
4.57
0.63
0.48
1.42
52.720.3
52.120.7e
10323
16223
2.0220.02
7.1820.08
14.920.1
0.013620.0002
0.20020.005
3.9820.06
17.120.1
0.10620.008
0.43420.021
1.3120.03
0.10220.002f
21721g
1.8620.04
5.2420.08
9.3420.02
90% TFE
70% TFE
50% TFE
90% HFIP
70% HFIP
50% HFIP
100% HCO2H
100% H2O
60 T±40Eh
40 T±60Eh
20 T±80Eh
aVolume/volume basis at 25.0 8C, except for TFE±H2O and
HFIP±H2O mixtures, which are on a weight/weight basis. bFrom
ref. 5. cFrom refs. 7 and 8. dInterpolation within values of ref. 13
5
1
gives a value of 13.0Â10
range 15±35 8C (ref. 17) leads to a value of 51.3 Â10
fBy extrapolation, using the Arrhenius equation, of values of
1
s .
eExtrapolation of values in the
5 1
Experimental
s
.
Methyl chloroformate (Aldrich, 99%) was further puri®ed
by fractional distillation. Kinetic measurements were made
conductometrically using a TOA Electronics Ltd. (Japan) Model
CM-40 S instrument, with a cell constant of 0.915 cm 1. All runs
were performed in duplicate with at least 70 readings taken at
appropriate intervals over three half-lives and in®nity readings
taken after ten half-lives.
5
1
0.869 (20.013)Â10
s
at 60.0 8C and 0.307 (20.005) s at
5 1
s
50.0 8C; in ref. 18, a value in 99% HCO2H of 0.103Â 10
at
50 8C was reported. gFrom ref. 9. hT±E are TFE±ethanol mixtures.
for m and 0.1620.07 for c (correlation coecient of
0.9774). The log (k/k0) value calculated for the 90% HFIP
solvent then lies 0.75 units below the experimental value,
corresponding to an addition±elimination contribution to
the overall solvolyis of 18%.
Received, 16th November 1998; Accepted, 19th November 1998
Paper E/8/08929I
It is noteworthy that the speci®c rates of solvolysis in
97% TFE and in 70 and 50% HFIP lie nicely on the
plot (Fig. 1) governed by l and m values which are within
the range expected for an addition±elimination pathway.
Further, as one might anticipate, the changeover from a
dominant addition±elimination mechanism to a dominant
ionisation mechanism is at solvents of considerably lower
nucleophilicity and/or greater ionising power than what was
the case for the solvolyses of ethyl chloroformate.
References
1 D. N. Kevill and M. J. D'Souza, J. Chem. Soc., Perkin Trans. 2,
1997, 1721.
2 D. N. Kevill and M. J. D'Souza, J. Org. Chem., 1998, 63, 2120.
3 D. N. Kevill, M. W. Bond and M. J. D'Souza, J. Org. Chem.,
1997, 62, 7869.
4 D. N. Kevill and S. W. Anderson, J. Org. Chem., 1991, 56,
1845.
It is puzzling as to why the experimental log (k/k0) value
for solvolysis in HCO2H should lie 0.85 units below the
calculated line. In an attempt to shed light on this deviation,
we have given further consideration to values previously
reported,18 at 50.0 8C, for solvolyses of MeOCOCl and
EtOCOCl in 99% formic acid and in 35% acetone. Using
interpolated values of 0.91 for NT and 2.86 for YCl for
35% acetone and the corresponding values for HCO2H
from Table 1, together with the appropriate l and m values,
5 D. N. Kevill, in Advances in Quantitative Structure±Property
Relationships, ed. M. Charton, JAI Press, Greenwich,
Connecticut, 1996, vol. 1, pp. 81±115.
6 T. W. Bentley and G. E. Carter, J. Am. Chem. Soc., 1982, 104,
5741.
7 T. W. Bentley and G. Llewellyn, Prog. Phys. Org. Chem., 1990,
17, 121.
8 D. N. Kevill and M. J. D'Souza, J. Chem. Res. (S), 1993, 174.
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10 C. Csunderlik, R. Bacaloglu and G. Ostrogovich, J. Prakt.
Chem., 1975, 317, 81.
11 I. S. Koo, K. Yang, K. Kang, I. Lee and T. W. Bentley,
J. Chem. Soc., Perkin Trans. 2, 1998, 1179.
12 I. S. Koo, K. Yang, K. Kang and I. Lee, Bull. Korean Chem.
Soc., 1998, 19, 968.
13 S. I. Orlov, A. L. Chimishkyan and M. S. Grabarnik, J. Org.
Chem. USSR (Engl. Transl.), 1983, 19, 1981.
14 M. Green and R. F. Hudson, J. Chem. Soc., 1962, 1055.
15 A. Queen and T. A. Nour, J. Chem. Soc., Perkin Trans. 2, 1976,
935.
16 D. N. Kevill and J. B. Kyong, J. Org. Chem., 1992, 57, 258.
17 R. Leimu, Chem. Ber., 1937, 70, 1040.
18 E. W. Crunden and R. F. Hudson, J. Chem. Soc., 1961, 3748.
from Fig. 1 or the literature, one can estimate addition±
3
elimination pathway ratios (kHCO H/k35%A) of 5.4 Â10
2
3
for MeOCOCl solvolyses and 6.3 Â10
for EtOCOCl
solvolyses. This corresponds to only a 9% contribution to
the experimental value for the formolysis of EtOCOCl
(assuming a negligible e€ect of a 1% water content).
The corresponding estimated speci®c rate of formolysis
5
of methyl chloroformate is 1.32 Â10
s
1, 13 times the
reported18 experimental value. Indeed, the reported value is
threefold lower than the value we report (Table 1) for 100%
formic acid at the identical temperature, which is already