Correlation of the rates of solvolysis of the N,N-diphenylcarbamoylpyridinium ion

Article abstract of DOI:10.1002/(sici)1099-1395(199804)11:4<273::aid-poc1>3.0.co;2-a

Solvolyses of the N,N-diphenylcarbamoylpyridinium ion are subject to specific and/or general base catalysis, which can be eliminated by addition of perchloric acid or increased, especially in fluoroalcohol-containing solvents, by addition of pyridine. The uncatalyzed solvolyses in aqueous methanol and aqueous ethanol involve a weakly nucleophilically assisted (l=0.22) heterolysis and the solvolyses in the pure alcohols are anomalously slow.

Full text of DOI:10.1002/(sici)1099-1395(199804)11:4<273::aid-poc1>3.0.co;2-a

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 273–276 (1998)
Correlation of the rates of solvolysis of the N,N-diphenyl-
carbamoylpyridinium ion
Dennis N. Kevill,1* Michael W. Bond2 and Malcolm J. D'Souza2
1Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862, USA
2Department of Chemistry, Ball State University, Muncie, Indiana 47306-0445, USA
Received 15 August 1997; revised 2 October 1997; accepted 6 October 1997
ABSTRACT: Solvolyses of the N,N-diphenylcarbamoylpyridinium ion are subject to specific and/or general base
catalysis, which can be eliminated by addition of perchloric acid or increased, especially in fluoroalcohol-containing
solvents, by addition of pyridine. The uncatalyzed solvolyses in aqueous methanol and aqueous ethanol involve a
weakly nucleophilically assisted (l = 0.22) heterolysis and the solvolyses in the pure alcohols are anomalously slow.
1998 John Wiley & Sons, Ltd.
KEYWORDS: N,N-diphenylcarbamoylpyridinium ion; solvolysis
INTRODUCTION
changes in solvent ionizing power (YCl)5 values and c is
a constant (residual) term.
We have recently reported the influence of the solvent
upon the specific rates of solvolysis of several N,N-
disubstituted carbamoyl chlorides.1–3 Treatments in
terms of the extended (two-term) Grunwald– Winstein
equation4,5 indicate an appreciable variation in the
For studies of solvolyses with a relatively low degree
of nucleophilic participation by the solvent, it is
frequently helpful to introduce an initially positively
charged leaving group which leaves as a neutral mole-
cule.4,11 In particular, solvolyses of the 1-adamantyl-
dimethylsulfonium ion in a wide range of commonly
used solvolytic solvents show specific rates of solvolysis
log ꢀk=k0† ˆ lNT ‡ mYCl ‡ c
ꢀ1†
RX
‡
almost independent of solvent composition and a Y
scale, based on the specific rate values, hardly varies from
sensitivity (l) towards changes in solvent nucleophilicity
(NT),4 including values of 0.23 for solvolyses of N,N-
diphenylcarbamoyl chloride (1)1 and 0.61 for solvolyses
of N,N-dimethylcarbamoyl chloride (2).2 Carbamoyl
chloride solvolyses have usually been considered to be
unimolecular (SN1) in character6–9 and, consistent with
these earlier studies, the appreciable l values were
considered to involve a nucleophilic solvation of the
developing acylium ion. Kim et al.10 have proposed,
however, that 2 and other N,N-dialkylcarbamoyl chlor-
ides solvolyze by an SN2 mechanism but 1 by an SN1
mechanism. As regards the other terms in Eqn (1), k and
k0 are the specific rates of solvolysis of RX in the solvent
under consideration and in the standard solvent (80%
ethanol), respectively, m is the sensitivity towards
zero.12 Accordingly, solvent nucleophilicity scales based
‡
on solvolyses of R—X substrates can be set up
according the equation
log ꢀk=k0†
ˆ lNT ‡ c
ꢀ2†
‡
RꢁX
with l set at unity and c at zero for the standard
substrate.13,14
Pyridinium ion substrates of this charge type, with a
pyridine molecule leaving group, can be readily produced
by the reaction of a carbamoyl chloride with pyridine.15
Johnson and Rumon16 have presented evidence indicat-
ing that the hydrolysis of the N,N-dimethylcarbamoyl-
pyridinium ion (3) in pure water involves a direct
nucleophilic attack by the solvent, without prior acylium
ion formation. Similarly, it has been reported8a that, in
water, the N,N-diphenylcarbamoylpyridinium ion (4) is
very sensitive to attack by nucleophiles. In view of the
above observations, it is of interest to see whether the
*Correspondence to: D. N. Kevill, Department of Chemistry and
Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-
2862, USA.
1998 John Wiley & Sons, Ltd.
CCC 0894–3230/98/040273–04 $17.50

274
D. N. KEVILL, M. W. BOND AND M. J. D’SOUSA
solvolyses of 4 show a larger sensitivity towards changes
in solvent nucleophilicity than the rather low value of
0.23 associated with the solvolyses of 1.
RESULTS AND DISCUSSION
The specific rates of solvolysis of 4 in ethanol and
methanol, in several of their mixtures with water and in
100% water are presented in Table 1. Usually the
counterion is chloride but in a few instances it is
trifluoromethanesulfonate. The specific rates were found
to be essentially independent of the identity of the anion.
For 4 in 100% water, the ratio of the second-order rate
coefficient for reaction with hydroxide ion relative to the
first-order rate coefficient for reaction with water
molecules has been reported8a as 1.7 Â 109 dm3 molꢁ1
(earlier, in Ref.8a, a value for the second-order rate
coefficient one order of magnitude lower is presented; if
this value were correct, the value for the ratio is 1.7 Â 108
dm3 molꢁ1). Values of this magnitude lead to the
possibility of a contribution involving attack by the
conjugate base of a protic solvent even in neutral solution
Battye et al.19 found, however, that in hydrolysis of the
methoxycarbonylpyridinium ion, the second-order rate
coefficient for OHꢁ attack was negligible at pH values of
less than 5.4. Similarly, we find only a modest retardation
of the overall specific rate on addition of 5.7 Â 10ꢁ3 mol
dmꢁ3 perchloric acid to the solvolyses of 4, also reported
in Table 1. In accord with this observation, addition of
0.01 mol dmꢁ3 pyridine to the solvolysis in 50% ethanol
led to only an 18% increase in specific rate. Since
nucleophilic substitution by pyridine would be symme-
It has been proposed17 that a pyridine molecule leaving
group is considerably more solvated at the transition state
than a leaving Me2S molecule. However, a limited
amount of data for solvolyses of the 1-adamantylpyr-
idinium ion18 give relative rates at 190°C in acetic acid,
water, and 2,2,2-trifluoroethanol (TFE) of 1:1.4:2.2,
essentially identical with the ratios for the solvolyses of
the 1-adamantyldimethylsulfonium ion12 at 70.4°C of
1:1.7:2.6. Accordingly, we have continued to assume that
‡
the mY term can be neglected4 and that analyses of the
solvolyses of N-substituted pyridinium ions can be
carried out in terms of Eqn (2).
Table 1. Speci®c rates of solvolysis of the N,N-diphenylcarbamoylpyridinium iona at 62.5°C in aqueous ethanol and aqueous
methanol solvents and solvent nucleophilicity values
e
Solventb
Anion
105k(sꢁ1 c
)
105kA(sꢁ1 c,d
)
NT
100% EtOH
Clꢁ
OTfꢁ
Clꢁ
3.09 Æ 0.11
3.42 Æ 0.16
25.9 Æ 0.9
28.4 Æ 0.5
27.5 Æ 1.1
26.1 Æ 0.8
20.5 Æ 1.1
20.1 Æ 0.9f
20.3 Æ 0.5
17.2 Æ 0.4
14.5 Æ 1.3
13.9 Æ 0.9
6.66 Æ 0.16
24.0 Æ 0.6
30.0 Æ 0.9
26.9 Æ 1.0
21.2 Æ 1.0
14.9 Æ 0.8
‡0.37
90% EtOH
80% EtOH
23.0 Æ 0.7
25.9 Æ 1.9
‡0.16
Clꢁ
0.00
OTfꢁ
Clꢁ
70% EtOH
60% EtOH
50% EtOH
24.3 Æ 1.4
19.8 Æ 1.3
19.3 Æ 1.0
ꢁ0.20
ꢁ0.39
ꢁ0.58
Clꢁ
Clꢁ
OTfꢁ
Clꢁ
40% EtOH
100% H2O
16.0 Æ 1.2
13.1 Æ 0.8
ꢁ0.74
ꢁ1.38
Clꢁ
OTfꢁ
Clꢁ
100% MeOH
90% MeOH
80% MeOH
60% MeOH
40% MeOH
20% MeOH
‡0.17
ꢁ0.01
ꢁ0.06
ꢁ0.54
ꢁ0.87
ꢁ1.23
Clꢁ
22.0 Æ 1.8
29.0 Æ 1.4
26.3 Æ 1.7
20.3 Æ 1.7
14.7 Æ 0.8
Clꢁ
Clꢁ
Clꢁ
Clꢁ
a
Substrate concentration 0.0015– 0.0030 mol dmꢁ3
.
b Concentration on a v/v basis at 25.0°C.
c
With associated standard deviation.
d In the presence of 5.70 Â 10ꢁ3 M HClO4.
e
From Ref.4.
f In the presence of 0.0098 M pyridine, a value of 23.8 (Æ 0.7) Â 10ꢁ5 sꢁ1 was obtained.
1998 John Wiley & Sons, Ltd.
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SOLVOLYSIS OF THE N,N-DIPHENYLCARBAMOYLPYRIDINIUM ION
275
Table 2. Speci®c rates of solvolysis of the N,N-diphenylcarbamoylpyridinium iona at 62.5°C in 2,2,2-tri¯uoroethanol (TFE) and
aqueous TFE Solvents.
c
Solventb
Anion
103 HClO4
103 C5H5Nc
105k(sꢁ1 d
)
100% TFE
Clꢁ
3.89 Æ 0.27e
Clꢁ
6.1
12.3
24.6
134 Æ 12e
Clꢁ
178 Æ 11e,f
257 Æ 9e
Clꢁ
70% TFE
50% TFE
Clꢁ
20.1 Æ 1.9e,g
1.65 Æ 0.18h
38.7 Æ 1.3e,i
30.3 Æ 0.3e,j
33.5 Æ 1.6e,k
567 Æ 14e,f
586 Æ 10e,l
498 Æ 13e,f
867 Æ 52e
Clꢁ
5.72
Clꢁ
Clꢁ
OTfꢁ
Clꢁ
15.4
15.8
15.8
31.6
31.6
Clꢁ
OTfꢁ
Clꢁ
OTfꢁ
Clꢁ
850 Æ 21e
49.3 Æ 5.9e,m
9.37 Æ 0.31h
5.21 Æ 0.13h
4.45 Æ 0.18h
4.39 Æ 0.14h
3.62 Æ 0.30h
2.75 Æ 0.12h
3.44 Æ 0.18h
2.90 Æ 0.18h
3.26 Æ 0.08h
Clꢁ
0.18
0.36
0.36
0.72
1.43
1.43
2.86
5.72
11.44
Clꢁ
OTfꢁ
Clꢁ
Clꢁ
OTfꢁ
Clꢁ
Clꢁ
Clꢁ
a
Substrate concentration 0.003– 0.008 mol dmꢁ3
.
b Mixed solvents on a w/w basis; solvent nucleophilicity (NT) values ꢁ3.93 for 100% TFE, ꢁ1.98 for 70% TFE and ꢁ1.73 for 50% TFE (from Ref.
4).
c
In mol dmꢁ3
.
d Unless indicated otherwise, runs were performed in duplicate and average initial values or mean values are presented, accompanied by the standard
deviation.
e
Initial value.
f Single determination.
g Integrated value of 12.7 Æ 0.3 at 50% reaction.
h Mean value.
i
Integrated value of 28.7 Æ 1.1 at 50% reaction.
j
Freshly prepared (and different from above) batch of substrate.
k Integrated value of 18.9 Æ 0.6 at 52% reaction.
l
Integrated value of 320 Æ 3 at 51% reaction.
m
In the presence of 0.0033 MNEt4Cl, with integrated value of 31.6 Æ 1.1 at 48% reaction.
trical, nucleophilic catalysis cannot operate and, assum-
ing it is not a medium effect, this increase is to be
expected to arise from a combination of specific and
general base catalysis; it is established16,19,20 that
pyridines can exert a moderate to weak general base
catalysis in solvolyses of acylpyridinium ions.
Inspection of the data of Table 1 shows, consistent with
other studies12,21 with little or no nucleophilic participa-
tion by the solvent, very little variation of specific rate for
water and the aqueous alcohol solvents. In addition,
considerably lower rates are observed in the pure
alcohols, despite these being the most nucleophilic of
the solvents listed. This behavior strongly suggests that 4
does not solvolyze by the direct nucleophilic attack
mechanism proposed16 for 3.
catalysis by the conjugate bases of the solvents and the
addition of 5.7 Â 10ꢁ3 mol dmꢁ3 HClO4 now led to a 10–
13-fold reduction in specific rate for solvolyses in 70 and
50% TFE. In the absence of HClO4, the specific rates
decreased with extent of reaction and initial values are
reported; a reduction in conjugate base concentration is
expected as the pyridinium ion is produced during the
solvolyses:
‡
4 ‡ SOH ! Ph2NCOOS ‡ C5H5NH
C5H5NH ‡ OSꢁ „ C5H5N ‡ SOH
Ph2NCOOH ! Ph2NH ‡ CO2 ꢀwhen S=H†
ꢀ3†
‡
Addition of pyridine was found to lead to very large
increases in rate for both 100 and 50% TFE solvolysis.
While these observations indicate direct attack on 4
within the catalyzed pathways, it is possible that the
underlying non-catalyzed solvolyses could be domi-
nantly unimolecular (SN 1) in character. Consistent with
The identity of the anion was also found to be
unimportant in runs carried out in trifluorethanol (TFE)
and TFE–H2O mixtures (Table 2). An appreciable
catalytic effect was presumably due to specific base
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276
D. N. KEVILL, M. W. BOND AND M. J. D’SOUSA
the essentially identical rates in the presence of either
chloride or trifluoromethanesulfonate as counterion and
indicative of only a modest salt effect, addition to the
solvolysis in 50% TFE of a concentration of tetraethy-
lammonium chloride equal to that of the substrate led to
only a 27% increase in rate.
aqueous solution of perchloric acid was appropriately
diluted for its addition to runs in aqueous alcohol
solvents.
Acknowledgment
A correlation, using Eqn (2), of the specific rates of
solvolysis in water and in the nine aqueous ethanol and
aqueous methanol solvents containing 20% or more
water, for the runs in the presence of HClO4 (Table 1),
leads to values for l of 0.22 Æ 0.04, for the residual
(constant) term c of 0.03 Æ 0.05 and for the correlation
coefficient of 0.887. The low value for the correlation
coefficient is in part due to the limited range of NT values
(1.38 units) for these solvents. The l value is essentially
identical with that observed for solvolyses of 1,
suggesting a similarity in the mechanism. The specific
rate values which can then be estimated [using Eqn (2)]
for ethanol and methanol are 11 and 4.5 times those
actually observed (Table 1) and it is possible that the
lower experimental values could be associated with the
need to proceed to the solvent-separated ion– molecule
pair, so as to prevent internal return. In this regard, water
would be more efficient than a bulkier and less
electrophilic alcohol.22 The need to consider the separa-
tion is supported by the conclusion23 that, in the
borderline solvolyses of sec-alkylpyridinium ions, a
rate-determining SN1 cleavage in TFE changes to a
rate-determining ion– molecule pair dissociation in
1,1,1,3,3,3-hexafluoropropan-2-ol.
D.N.K. thanks Professor H. Mayr (Universita¨t Mu¨nchen)
for hospitality during the preparation of this paper.
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N,N-Diphenylcarbamoylpyridinium chloride was pre-
pared from pyridine and N,N-Diphenylcarbamoyl chlor-
ide (Aldrich, 98%) as described previously15; m.p. 108–
109°C (lit.8a m.p. 107.5–108.5°C). The trifluorometha-
nesulfonate salt of 4 was prepared as an oil by treatment
of a solution of the chloride salt in acetonitrile with an
equivalent amount of silver trifluoromethanesulfonate,
followed by filtration and removal of the solvent under
reduced pressure. The purification of the solvents and the
kinetic methods were as described previously.1 When
required, initial values for the specific rates were obtained
from approximately linear plots of integrated values
against extent of reaction. A standardized concentrated
1998 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 273–276 (1998)