Correlation of the rates of solvolysis of chlorodiphenylphosphine using the extended grunwald-winstein equation

Article abstract of DOI:10.1080/10426500903012478

Specific rates of solvolysis involving displacement of chloride from the trivalent phosphorus of chlorodiphenylphosphine (Ph2PCl, 1) are reported for ethanol, methanol, and aqueous binary mixtures incorporating ethanol, methanol, acetone, 2,2,2-trifluoroethanol (TFE), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The 25 solvents give an extended Grunwald-Winstein correlation with an l value of 1.25 0.09, m value of 0.46 0.06, and correlation coefficient (R) of 0.954. The reactions in aqueous acetone were unusually fast, and these data points lie above the plot. Specific rate values are also reported for TFE-ethanol mixtures. For five representative solvents, values were obtained at three additional temperatures and activation parameters are presented. Rather low enthalpies of activation (8.4 to 11.8 kcal mol-1) are accompanied by very negative entropies of activation (-33 to -46 cal mol-1K-1). The kinetic features observed are consistent with an SN2 reaction incorporating appreciable bond formation and bond breaking at the transition state, and they are very similar to those previously observed for diphenylphosphinyl chloride (Ph2POCl).

Full text of DOI:10.1080/10426500903012478

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Correlation of the Rates of Solvolysis
of Chlorodiphenylphosphine Using the
Extended Grunwald–Winstein Equation
a
b
Han Joong Koh & Dennis N. Kevill
a
Department of Science Education , Chonju National University of
Education , Chonju, Korea
b
Department of Chemistry and Biochemistry , Northern Illinois
University , DeKalb, Illinois, USA
Published online: 23 Mar 2010.
To cite this article: Han Joong Koh & Dennis N. Kevill (2010) Correlation of the Rates of Solvolysis of
Chlorodiphenylphosphine Using the Extended Grunwald–Winstein Equation, Phosphorus, Sulfur, and
Silicon and the Related Elements, 185:4, 865-874, DOI: 10.1080/10426500903012478
To link to this article: http://dx.doi.org/10.1080/10426500903012478
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Phosphorus, Sulfur, and Silicon, 185:865–874, 2010
Copyright ꢀ
Taylor & Francis Group, LLC
C
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903012478
CORRELATION OF THE RATES OF SOLVOLYSIS OF
CHLORODIPHENYLPHOSPHINE USING THE EXTENDED
GRUNWALD–WINSTEIN EQUATION
1
2
Han Joong Koh and Dennis N. Kevill
1
Department of Science Education, Chonju National University of Education,
Chonju, Korea
Department of Chemistry and Biochemistry, Northern Illinois University,
2
DeKalb, Illinois, USA
Specific rates of solvolysis involving displacement of chloride from the trivalent phos-
phorus of chlorodiphenylphosphine (Ph2PCl, 1) are reported for ethanol, methanol, and
aqueous binary mixtures incorporating ethanol, methanol, acetone, 2,2,2-trifluoroethanol
(TFE), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The 25 solvents give an extended
Grunwald–Winstein correlation with an l value of 1.25 ± 0.09, m value of 0.46 ± 0.06, and
correlation coefficient (R) of 0.954. The reactions in aqueous acetone were unusually fast,
and these data points lie above the plot. Specific rate values are also reported for TFE–ethanol
mixtures. For five representative solvents, values were obtained at three additional temper-
atures and activation parameters are presented. Rather low enthalpies of activation (8.4 to
−
1
1
mol
1.8 kcal mol ) are accompanied by very negative entropies of activation (−33 to −46 cal
−1
−1
K ). The kinetic features observed are consistent with an SN2 reaction incorporating
appreciable bond formation and bond breaking at the transition state, and they are very
similar to those previously observed for diphenylphosphinyl chloride (Ph2POCl).
Keywords Chlorodiphenylphosphine; solvent ionizing power; solvent nucleophilicity;
solvolysis
INTRODUCTION
There have been several studies of the kinetics of nucleophilic substitution
reaction at a tetrahedral phosphorus atom,1 and an SN2 displacement at phosphorus
–4
[
SN2(P)] is the usually proposed mechanism. Many of the studies have involved
solvolysis reactions, and we have previously reported kinetic studies of the solvolyses
of diaryl, dimethyl,6 and cyclic7 phosphorochloridates [(RO)2POCl], N,N,N ,N -
5
ꢁ
ꢁ
8
tetramethyldiamidophosphorochloridate [(Me2N)2POCl],
and diphenylphosphinyl
Received 13 March 2009; accepted 1 May 2009.
Presented, in part, at the 17th International Conference on Phosphorus Chemistry (ICPC-17), April 15–21,
007, Xiamen, China, and briefly summarized in the proceedings of the conference: H. J. Koh, S. J. Kang, and D. N.
2
Kevill, Phosphorus, Sulfur, and Silicon, 183, 364 (2008). H.J.K. thanks Chonju National University of Education
for the granting of a sabbatical leave during the time that this research was carried out. Acknowledgment is made
to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support
of this research.
Address correspondence to Dennis N. Kevill, Department of Chemistry and Biochemistry, Northern Illinois
University, DeKalb, Illinois, 60115-2862, USA. E-mail: dkevill@niu.edu
8
65

866
H. J. KOH AND D. N. KEVILL
9
10
chloride [Ph2POCl]. Bentley and Ebdon have reported on the kinetics of the solvolysis
of ethyl phenylphosphonochloridate [Ph(EtO)POCl]. Our studies have incorporated
correlations of the specific rates (first-order rate coefficients) using an extended form of
the Grunwald–Winstein equation1
1–13
(Equation 1).
log (k/k0) = lNT + mYCl + c
(1)
In Equation (1), k and k0 represent the specific rates of solvolysis of a substrate in the
solvent under consideration and in the standard solvent (80% ethanol), respectively, l is the
1
2–14
sensitivity towards changes in solvent nucleophilicity (NT),
m is the sensitivity towards
1
5,16
changes in solvent ionizing power (YCl, for a chloride-ion leaving group),
and c is a
constant (residual) term.
There have been far fewer studies of the rates of nucleophilic substitution reactions
at tricoordinated phosphorus. This is largely due to a tendency towards a fast reaction. In
particular, phosphorochloridite esters [(RO)2PCl] react extremely rapidly, and, for exam-
◦
17
ple, the ethanolysis of diethyl phosphorochloridite could be followed only at −65 C.
Reactions of chlorodialkylphosphines and chlorodiarylphosphines, although still fast, react
17
considerably slower, and Halmann has studied the ethanolyses of the diethyl, dipropyl,
and diphenyl derivatives.
Halmann experienced several difficulties in his studies. In the ethanolyses, both
chlorodiethylphosphine and chlorodiphenylphosphine exhibited a rapid increase in con-
ductivity followed by a slower subsequent increase. On rather indirect grounds, it was
assumed that the true solvolysis rate was the subsequent slow increase for the diethyl
derivative but the initial fast reaction for the diphenyl derivative.
In individual runs, the dipropyl derivative reacted with good first-order kinetics,
but the value for the rate coefficient was found to increase with increasing initial substrate
concentration. The addition of sulfuric or perchloric acid caused considerable rate increases,
and it was suggested that this acid catalysis was one cause for the dependence of the
17
specific rate of solvolysis on the substrate concentration. A very important observation
with the dipropyl derivative was that contrary to what one might intuitively expect, based on
substitutions at carbon, increasing the proportion of acetone in ethanol–acetone mixtures
led to increases in reaction rate. After several half-lives, the conductivity level started to
drop, and it was found that eventually the chloride-ion concentration was very low. It was
suggested that a product, initially formed from the attack of acetone on the chlorophosphine,
slowly incorporated the ejected chloride ion.
17
In the ethanolysis of chlorodiethylphosphine, it was suggested that an initial for-
18
mation of Et2POEt was followed by an Arbusov-type rearrangement to triethylphosphine
oxide (Et3PO). The replacement of chloride by hydroxide is also followed by a rearrange-
1
ment to the isomeric phosphine oxide (Equation 2):
P
O
H
P
O
(2)
H
For the present study, we have chosen chlorodiphenylphosphine (1) as the substrate.
Using a rapid-response conductivity technique, the kinetics of solvolysis can be conve-
niently followed using a low concentration of substrate, thus avoiding the dependence on

SOLVOLYSIS OF CHLORODIPHENYLPHOSPHINE
867
substrate concentration observed for the dipropyl derivative by Halmann.17 This choice
9
has the advantage of allowing a comparison with the previously reported solvolyses of
diphenylphosphinyl chloride 2, a substrate which has a double bond to an oxygen rather
than the lone pair of electrons of chlorodiphenylphosphine.
Ph
Ph
Ph
Ph
P
Cl
P
Cl
O
(1)
(2)
RESULTS
◦
The specific rates of solvolysis of 1 were determined at 25.0 C in 29 solvents. The
solvents consisted of ethanol, methanol, binary mixtures of 2,2,2-trifluoroethanol (TFE)
with ethanol, and binary mixtures of water with ethanol, methanol, acetone, TFE, and
1
,1,1,3,3,3-hexafluoro-2-propanol (HFIP). These values are reported in Table I, together
12,14
15,16
with the solvent nucleophilicity (NT)
comparison purposes, the specific rates, at 25.0 C, for the corresponding solvolyses of
diphenylphosphinyl chloride (2) are also reported in Table I.
and solvent ionizing power (YCl)
values. For
◦
Reported in Table II are specific rate values for 1 recorded over a fourfold variation
◦
in initial substrate concentration, at 25.0 C, for ethanol, 80% ethanol, and 50% ethanol.
In Table III are the specific rate values at three additional temperatures for five of the
solvolyses reported in Table I and the enthalpies and entropies of activation, calculated
from the data at all four temperatures.
DISCUSSION
Since Halmann17 found unusual kinetics, with a fast phase followed by a slow phase
in the ethanolyses of chlorodiphenylphosphine (1), the substrate of this investigation, and
chlorodiethylphosphine, we were pleased to see that, over the full range of 29 solvents,
good first-order kinetics were obtained throughout each run. Since the reactions are quite
fast, an initial perturbation, which is completed before the kinetic study can begin, cannot
be rigorously excluded. However, after homogeneity was established, we found no evidence
for the two regions of very different kinetic behavior described by Halmann.
17
Another observation by Halmann was that in the ethanolysis of chlorodipropy-
◦
lphosphine at 0.0 C, the specific rate increased by a factor of five when the concentration
−3
−3
increased from 0.8 × 10 M to 46 × 10 M (a factor of 60). Similar results, but with
reduced increases in specific rate, were observed with 1M water within the ethanol. Ac-
cordingly, we felt it necessary to check for the operation of this effect in our studies with
−3
1
. Since the runs were usually carried out with a 6.5 × 10 M concentration of substrate,
we carried out (Table II) three determinations (each in duplicate) with the concentration
also halved and doubled. Within this range, no substrate concentration dependence of the
specific rates was observed in ethanol, 80% ethanol, or 50% ethanol.
An extremely surprising aspect of the present study can be seen in Table I, where
◦
at 25.0 C, the specific rate of solvolysis of 1 in a given solvent is, across the full range

868
H. J. KOH AND D. N. KEVILL
a
Table I Specific rates of solvolysis of chlorodiphenylphosphine (Ph2PC1) in a variety of pure and mixed
◦
solvents at 25.0 C, the NT and YC1values for the solvents, and (for comparison) the corresponding specific rate
ꢁ
values (k ) for diphenylphosphinyl chloride (Ph2POC1)
ꢁ
Solventb
10 k, s
2
−lc
NTd
YC1e
10 k ,s
2
−1f
1
9
8
7
6
5
1
9
8
7
6
9
9
8
7
6
9
9
8
7
5
9
9
7
5
8
6
4
2
00% EtOH
0% EtOH
0% EtOH
0% EtOH
0% EtOH
0% EtOH
00%MeOH
0% MeOH
0% MeOH
0% MeOH
0% MeOH
5%Acetone
0%Acetone
0%Acetone
0% Acetone
0% Acetone
0.809 ± 0.005
11.0 ± 0.1
0.37
0.16
0.00
−0.20
−0.38
−0.58
0.17
−2.52
−0.94
0.00
0.78
1.38
0.702
11.2
21.6
30.9
43.8
62.6
6.78
28.4
51.6
70.0
102
0.833
4.15
14.8
29.8
51.4
0.0233
0.275
1.50
20.2 ± 0.4
30.0 ± 0.3
43.0 ± 0.2
55.2 ± 0.6
2.02
6.24 ± 0.04
29.1 ± 0.2
−1.17
−0.18
0.67
1.46
2.07
−3.19
−2.39
−0.83
0.17
0.95
2.83
2.85
2.90
2.96
3.16
5.17
4.31
3.83
3.80
−0.01
−0.06
−0.40
−0.54
−0.49
−0.35
−0.37
−0.42
−0.52
−3.30
−2.55
−2.19
−1.98
−1.73
−5.26
−3.84
−2.94
−2.49
−1.76
−0.94
−0.34
0.08
50.8 ± 1.3
68.3 ± 0.5
82.5 ± 1.2
1.05 ± 0.01
4.97 ± 0.06
15.8 ± 0.2
33.8 ± 0.1
52.8 ± 0.9
g
7% TFE
0% TFE
0% TFE
0% TFE
0.0280 ± 0.0062
0.322 ± 0.005
1.54 ± 0.01
3.96 ± 0.01
16.7 ± 0.6
g
g
g
3.97
16.5
0.000591
0.0365
1.48
g
0% TFE
g
7% HFIP
0% HFIP
0% HFIP
0% HFIP
0.000616 ± 0.000006
0.0296 ± 0.0002
1.04 ± 0.01
3.74 ± 0.01
0.0721 ± 0.0013
0.327 ± 0.002
0.722 ± 0.002
1.00 ± 0.03
g
g
g
3.59
h
0T-20E
1.89
0.63
0.0689
0.321
0.706
0.954
h
0T-40E
h
0T-60E
−0.48
h
0T-80E
−1.42
aSubstrate concentration of 0.0065 mol dm
−3
.
b
◦
Unless otherwise indicated, on a vol/vol basis, at 25.0 C, with the other component water.
With associated standard deviations.
From refs. 12 and 14.
From refs. 15 and 16.
From ref. 9.
c
d
e
f
g
h
Solvent prepared on weight/weight basis.
T-E represent 2,2,2-trifluoroethanol-ethanol mixtures.
of solvents, very similar to that for 2. When the specific rate values in Table III for five
◦
◦
solvents at temperatures in the 0 C to 55 C range are compared with the corresponding
9
values for 2, the similarity remains, and, accordingly for each solvent, the entropies and
enthalpies of activation are also very similar for solvolyses of 1 and 2.
Although a labeling error by the supplier is highly unlikely, so also is the probability
very low for two substrates to show the same kinetic reactivity values over the full range
of solvents usually used in solvolysis studies. Accordingly, we checked the samples of 1
9
used in the present study and the sample of 2 that was previously used in the study of
its solvolyses. This can very easily be done qualitatively by infrared spectroscopy and,

SOLVOLYSIS OF CHLORODIPHENYLPHOSPHINE
869
Table II Specific rates of solvolysis of chlorodiphenylphosphine (Ph2PC1) in
◦
ethanol and ethanol–water mixtures (v/v) at 25.0 C
[Ph2PC1], Ma
10 k, s
2
−1b
Solvent
100% EtOH
80% EtOH
50% EtOH
0.541
0.818 ± 0.017
0.809 ± 0.005
0.818 ± 0.005
19.9 ± 0.4
1
2
.013
.178
0.541
1
2
.013
.178
20.2 ± 0.4
20.7 ± 0.1
0.541
56.5 ± 0.8
1
2
.013
.178
55.2 ± 0.6
56.5 ± 1.6
a
Concentration of stock solution in MeCN, in solution: 3·5 × 10−3M, 6·6 ×
−
3
−3
1
0
M, and 14.2 × 10 M.
b
The k values are the averages of three or more runs, and with associated
standard deviation.
−1
indeed, 2 showed a strong absorption at 1250 cm due to the stretching vibration of
the phosphorus–oxygen bond, and this absorption was absent in the spectrum of 1. Also,
although both 1 and 2 were only guaranteed to 98% purity, both gave C, H elemental
analyses in excellent agreement with the theoretical values. Further, a sample of 1 from
a second commercial source gave specific rates of solvolysis in ethanol and 80% ethanol
essentially identical to those obtained using the original sample.
Table III Specific rates of solvolysis of chlorodiphenylphosphine (Ph2PC1) in pure and mixed solvents at various
ꢂ
ꢂ=
temperatures and the calculated enthalpies (ꢀH ) and entropies (ꢀS ) of activation
ꢂ
b
ꢂ
b
ꢀ
H
298
ꢀS 298
−1 −1
◦
2
−1a
−1
Solvent
Temp. C
10 k,s
Kcal mol
cal mol
K
1
8
1
9
9
00% EtOH
0% EtOHc
00% MeOH
7% TFEd
0.0
0.207 ± 0.006
0.464 ± 0.002
1.50 ± 0.02
1
3
5.0
5.0
0.0
5.0
5.0
0.0
5.0
5.0
8.8 ± 0.1
−38.6 ± 0.3
4.90 ± 0.05
1
3
10.9 ± 0.2
8.4 ± 0.1
8.4 ± 0.1
−33.6 ± 0.4
−35.8 ± 0.2
30.2 ± 0.3
1.56 ± 0.01
1
3
3.93 ± 0.03
10.4 ± 0.1
35.0
0.0405 ± 0.0006
0.0761 ± 0.0007
0.114 ± 0.007
0.00146 ± 0.00007
0.00244 ± 0.00004
0.00431 ± 0.00015
8.8 ± 0.2
−45.3 ± 0.6
−42.7 ± 0.6
4
5
5.0
5.0
7% HFIPd
35.0
11.8 ± 0.2
4
5
5.0
5.0
a
Averages of three or more runs, with standard deviation.
b
◦
Values at 25.0 C, from Table I, are also used in the calculation; the activation parameter values are accompanied
by the standard error.
c
◦
On a volume–volume basis at 25.0 C, other component is water.
d
On a weight–weight basis, the other component is water.

870
H. J. KOH AND D. N. KEVILL
The similarity of the specific rates, independent of whether the phosphorus is trivalent
with a lone pair of electrons on the phosphorus or tetrahedral with the lone pair replaced
by a (essentially double) bond to oxygen, does not seem to be a general phenomenon
1
9
◦
in such a comparison. Dostrovsky and Halmann found that, at 0.0 C, the tetrahedral
diethyl phosphorochloridate [(EtO)2POCl] solvolyses in ethanol with a specific rate of
−5
−1
17
2
.3 × 10 s . Halmann subsequently reported that the trivalent diethyl phosphorochlo-
◦
ridite [chlorodiethoxyphosphine, (EtO)2 PCl] underwent ethanolysis at −65 C with a spe-
−4
−1
◦
cific rate of 2 × 10 s , an order of magnitude faster at a 65 C lower temperature.
◦
In a study of the solvolysis of chlorodipropylphosphine at 0 C, Halmann found that
the specific rate in 100% ethanol was about doubled upon addition of 5% acetone, and this
value doubled again upon increasing the acetone content to 16%. The chloride ion initially
liberated was found to have been consumed after several half lives for disappearance of
substrate. Halmann suggested the pathway shown in Equation (3):
Cl-
Me
(
CH ) CO
3
2
O
Pr2PCl
Pr2P C Me
O Cl
(3)
- Cl- )
Pr2P
CMe2
slow
(
The rate increases upon adding acetone were contrary to what would have been
predicted based on nucleophilic substitution at carbon, where dilution of water or an
alcohol with acetone is a standard procedure for moderating the rates of solvolysis. It
2
0
4
has been estimated that both SN1 and SN2 solvolyses would decrease in rate by 10 to
6
1
0 upon moving from water to moist acetone. We find in the present study, consistent
17
with Halmann, that the solvolysis rates in aqueous acetone are faster than one would
predict. Indeed, the specific rate in 95% acetone (5% water) is comparable to that in 100%
ethanol. This observation applies to the solvolyses of both 1 and 2, with the close similarity
in specific rate values continuing to apply for the 95–60% aqueous acetone solvents. The
initial interaction with acetone appears to be independent of whether a lone-pair of electrons
or a double bond to oxygen is present at the reaction site.
The enthalpies of activation are at quite low values, in the range of 8.4 to 8.8 kcal
−1
mol , in ethanol, methanol, 80% ethanol, and 97% TFE, with a somewhat higher value
−
1
of 11.8 kcal mol in 97% HFIP. In all five solvents, a very negative entropy of activation
−1
−1
(
−33 to −46 cal mol K ) is observed, consistent with the proposed bimolecular nature
of the substitution process (Table III).
Application of the extended Grunwald–Winstein equation [Equation (1)] to the data
leads to values for all of the 29 solvents of Table I of 1.31 ± 0.14 for l and 0.51 ± 0.09
for m, with a rather poor multiple correlation coefficient of 0.899. Omission of the four
8
TFE–ethanol points, often falling below the correlation line in application of Equation (1),
improved the multiple correlation coefficient to 0.954, and the sensitivity values are now at
values of 1.25 ± 0.09 for l and 0.46 ± 0.06 for m (Figure 1).
In Table IV, these values are compared to those for several solvolytic displace-
ments of chloride ion from tetrahedral phosphorus, including solvolyses of 2, of dimethyl
6
phosphorochloridate [MeO)2POCl] and phosphorochloridothionate [(MeO)2PSCl], of
21
diphenyl phosphorochloridothionate [(PhO)2PSCl], and of diphenylthiophosphinyl chlo-
ride [Ph2PSCl].22 The l and m values are similar in all cases, and they and the l/m ratio

SOLVOLYSIS OF CHLORODIPHENYLPHOSPHINE
871
◦
Figure 1 Plot of log (k/ko) for solvolyses of 1 at 25.0 C against (1.25NT+0.46YC1).
are consistent with a direct displacement (SN2) pathway across the full range of solvents
included in the studies.
When the phosphorus is bonded to sulfur, rather than bonded to oxygen or carrying
a lone pair of elections, the correlation coefficients tend to be higher, suggesting that the
perturbations involving the electrophilic center of the solvent molecules and the lone pair,
oxygen, or sulfur are least for sulfur. The presence of sulfur also leads to slightly lower l
values and slightly elevated m values, such that the l/m ratios of 2.6 to 2.7 for 1, 2, and
(MeO)2 POCl decrease to 1.6 to 2.1 for the three sulfur-containing substrates of Table IV.
This indicates that the introduction of sulfur results in an SN2 transition state with increased
bond-breaking and reduced bond-forming characteristics.

872
H. J. KOH AND D. N. KEVILL
Table IV Extended Grunwald–Winstein correlations of the kinetics of solvolytic displacement of chloride from
phosphorus
na
Temp. C
◦
lb
mb
Rc
l/m
Substrate
1
1
2
29
25
27
18
28
30
25
25.0
25.0
25.0
25.0
25.0
55.0
55.0
1.31 ± 0.14
1.25 ± 0.09
1.42 ± 0.10
1.27 ± 0.14
1.17 ± 0.07
1.29 ± 0.07
1.00 ± 0.04
0.51 ± 0.09
0.46 ± 0.06
0.54 ± 0.07
0.47 ± 0.08
0.56 ± 0.03
0.64 ± 0.02
0.64 ± 0.03
0.899
0.954
0.956
0.941
0.966
0.989
0.983
2.57
2.72
2.63
2.70
2.09
2.02
1.56
d
d,e
d,f
(
(
(
MeO)2POC1
MeO)2PSC1
PhO)2PSC1
d,f
g
d,h
Ph2PSC1
aNumber of solvents.
b
From Equation (1) and with associated standard error.
c
Multiple correlation coefficient.
d
With the TFE–EtOH data points excluded from the correlation.
e
f
From ref. 9.
From ref. 6.
From ref. 21.
From ref. 22.
g
h
CONCLUSIONS
The irregularities in the kinetics found by Halmann,17 in the form of the existence
of two regions of different kinetic behavior for each run and a substrate concentration
dependence for the observed specific rate of solvolysis, were not found in the present study.
Good first-order kinetics were observed throughout each run for a wide range of solvents at
◦ ◦
5.0 C and in runs at 0–55 C for five selected solvents. The enthalpies of activation were
2
−1
low (8.4 to 11.8 kcal mol ), but this effect was countered by very negative entropies of
−1
−1
activation (−33 to −46 cal mol K ). The highly negative entropies of activation suggest
a bimolecular (or higher molecularity) process.
A surprising feature was that the specific rates of solvolysis of 1 were extremely
similar to those previously measured for 2, despite 1 having a lone pair of electrons on
the phosphorus and the tetrahedral 2 having a double bond to oxygen at that position.
Careful checks were made (infrared spectra and elemental analyses) to firmly establish the
identities of 1 and 2. This is not a general phenomenon for compounds related in this way,
however, since studies of the ethanolyses of (EtO)2PCl and (EtO)2POCl, with the identical
structural relationship, showed the trivalent species to react by an order of magnitude faster
◦
17
at a 65 C lower temperature, with a very rough estimate, suggesting a difference of about
three orders of magnitude at a common temperature.
Since the specific rates of solvolysis of 1 and 2 are extremely similar, so also are the
activation parameters and the sensitivities obtained from an extended Grunwald–Winstein
equation treatment of the data. The values for l of 1.25 ± 0.09 and for m of 0.46 ± 0.06
9
(
l/m ratio of 2.72) are very similar to those previously reported for solvolyses of 2 and for
other solvolyses in which a chloride ion is displaced from phosphorus. If a double bond to
sulfur replaces that to oxygen, a slightly lower l value and slightly higher m value results
in the l/m ratio falling to 1.6 to 2.1 (Table IV), suggesting a looser, but still bimolecular,
transition state.
17
An earlier surprising feature was the observation by Halmann that, unlike for solvol-
yses involving displacement at carbon, addition of acetone to ethanol accelerated rather than

SOLVOLYSIS OF CHLORODIPHENYLPHOSPHINE
873
reduced the rates of solvolysis of the trivalent chlorodipropylphosphine. Similarly, we find
that the rates of solvolysis of 1 are faster than one would anticipate in the aqueous-acetone
solvents. This can be readily seen by the acetone–water points lying above the best fit plot
of Figure 1.
The overall situation that extended Grunwald–Winstein equation treatments of several
kinetic studies involving displacement of chloride from phosphorus, across the full range
of solvents, strongly suggests a bimolecular process with both bond formation and bond
breaking having progressed appreciably at the transition state remains unaltered, with
the solvolyses of 1, the first trivalent phosphorus substrate to be included in the study,
conforming to this pattern.
EXPERIMENTAL
The chlorodiphenylphosphine (Aldrich 98% or TCI 98%) was used as received.
14
Solvents were purified as previously described.
The kinetic experiments were carried out by allowing a conductivity cell containing
2.5 mL of solvent to equilibrate, with stirring, in a constant-temperature water bath. A
2 µL portion of a 1.0 mol dm stock solution of 1 in acetonitrile was then added.
1
1
−3
The monitoring of increases in conductivity with time and the calculation of the specific
23
rates (first-order rate coefficients) were as previously reported. The stability (lack of any
appreciable perturbation due to reaction of the substrate with the solvent) of the concentrated
stock solution of 1 in acetonitrile was checked by showing, for solvolyses in 100% ethanol
◦
at 25.0 C, that identical specific rates were obtained when the stock solution was allowed to
stand for 0.5, 30, 75, or 375 minutes prior to the addition of a 12 µL portion to the ethanol.
The multiple regression analyses were performed using commercially available sta-
tistical packages.
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