The leaving group dependence in the rates of solvolysis of 1,2-diphenylethyl system

Article abstract of DOI:10.1002/poc.1662

1,2-Diphenylethyl chloride undergoes solvolysis by S_N1 mechanism in aqueous organic solvents. The α-phenyl group of 1,2-diphenylethyl chloride enters into conjugation with the developing carbocationic centre. The β-phenyl group on the other hand was unable to extend its conjugation via neighbouring group participation due to steric inhibition of resonance in the formation of non-classical carbocation. 1,2-Diphenylethyl chloride thus behaves similar to 1-phenylethyl chloride in its solvolysis pattern. The solvolytic rate studies of chloride and methanesulphonate of 1,2-diphenylethyl alcohol in various aqueous organic solvents show that the dispersion observed in the Winstein- Grunwald plot is not due to a change in leaving group but due to the difference in solvation requirements of aromatic and aliphatic groups. Copyright

Full text of DOI:10.1002/poc.1662

Research Article
Received: 2 February 2009,
Revised: 11 October 2009,
Accepted: 13 November 2009,
Published online in Wiley InterScience: 23 February 2010
(
www.interscience.wiley.com) DOI 10.1002/poc.1662
The leaving group dependence in the rates of
solvolysis of 1,2-diphenylethyl system
a
b
c
D. Santhosh Kumar , K. P. Jayakumar and S. Balachandran *
1,2-Diphenylethyl chloride undergoes solvolysis by S 1 mechanism in aqueous organic solvents. The a-phenyl group
N
of 1,2-diphenylethyl chloride enters into conjugation with the developing carbocationic centre. The b-phenyl group
on the other hand was unable to extend its conjugation via neighbouring group participation due to steric inhibition
of resonance in the formation of non-classical carbocation. 1,2-Diphenylethyl chloride thus behaves similar to
1
1
-phenylethyl chloride in its solvolysis pattern. The solvolytic rate studies of chloride and methanesulphonate of
,2-diphenylethyl alcohol in various aqueous organic solvents show that the dispersion observed in the Winstein–
Grunwald plot is not due to a change in leaving group but due to the difference in solvation requirements of aromatic
and aliphatic groups. Copyright ß 2010 John Wiley & Sons, Ltd.
Keywords: kinetics; leaving group effect; LFERS; solvolysis; solvent effect
different types of substrate systems.
INTRODUCTION
Solvolytic reaction rates are strongly affected by the solvent
log ðk=k Þ ¼ Y
(2)
o
sim
polarity and the most ambitious attempt to correlate reaction
rates with empirical parameters of solvent polarity is that of
For systems containing one aromatic ring which is in
conjugation with the reaction centre, p-methoxybenzyl chloride
is the model substrate. The a-deuterium isotopic studies showed
[
1,2]
Winstein and co-workers.
Eqn (1))
Winstein–Grunwald equation
(
a
nucleophilic solvent participation for p-methoxybenzyl
[
24]
log k=ko ¼ mY
(1)
chloride.
The nucleophilic assistance in the solvolysis of
p-methoxybenzyl chloride involves a solvation of the developing
based on the ion pair mechanism of solvolytic reaction is the
well-known equation in this regard, where k and k are the rate
carbocation, rather than a process involving appreciable covalent
o
[25]
attachment
and thus p-methoxybenzyl chloride is a good
constants in the solvent concerned and in 80:20 ethanol/water
mixture. The coefficient m represents the sensitivity towards
changes in solvent ionizing power (m ¼ 1 for t-butyl chloride). Y is
the ionizing power of the solvent. The equation is applied to
different systems with success but when solvolysis rates for a
specific substrate in different binary mixtures are correlated, lines
[26]
choice as a model substrate
interpretation of solvent effects in various systems.
and is successfully used for the
[27–30]
When a system with two aromatic rings that contribute their
electrons to the carbocationic centre by resonance, such as
1
,1-diphenylmethyl chloride, is compared with p-methoxybenzyl
chloride, a significant dispersion is observed. The contributions
[
2–7]
with different slopes are observed
for each solvent system
[22]
from two aromatic rings are not twice as large as those of one.
and this phenomenon is usually called ‘dispersion ’. To minimize
the dispersion effect alternative equations were recommended;
It is concluded that the solvation effect contributed by the
aromatic rings is the major reason for dispersion and comparison
with a suitable compound with same number of aromatic rings
can eliminate the dispersion effect. We selected a system having
two aromatic rings, 1,2-diphenylethyl in which only one ring
[
2]
[
8–15]
equations with more number of independent terms
and
However,
the Winstein ion pair mechanism of solvolytic reaction is used as
[
3–7]
even changes in model substrate were suggested.
[
16]
the basic equation for the explanation for solvent effect.
Dispersion effect is more pronounced when substrates
containing aromatic rings are studied.
[
17,18]
The solvation of
*
Correspondence to: S. Balachandran, Department of Chemistry, N. S. S.
College, Cherthala, Alappuzha, Kerala 670 055, India.
E-mail: sbcnair@sify.com
aromatic rings involving charge delocalization from the benzene
ring to the carbocationic centre may differ significantly from the
[
19–21]
solvation of aliphatic substrates.
The solvation requirement
a
D. Santhosh Kumar
of systems with one aromatic ring such as 1-phenylethyl chloride
Department of Chemistry, Government Arts College, Thiruvananthapuram,
Kerala 695 014, India
is different from that of t-butyl chloride and it is the main reason
[
22]
for dispersion phenomena. If p-methoxybenzyl chloride, which
undergoes solvolysis by S 1 (D ) mechanism, is used for
þ A
comparison instead of t-butyl chloride a good correlation is
b
c
K. P. Jayakumar
N
N
N
Forensic Science Laboratory, Thiruvananthapuram, Kerala 695 014, India
[
22]
obtained for solvolytic rate data.
As further extension, a
S. Balachandran
[
23]
similarity model is proposed with a generalized equation (Eqn
Department of Chemistry, N. S. S. College, Cherthala, Alappuzha, Kerala 670
055, India
(
2)) where five sets of different Ysim values were presented for
J. Phys. Org. Chem. 2010, 23 783–788
Copyright ß 2010 John Wiley & Sons, Ltd.

D. S. KUMAR, K. P. JAYAKUMAR AND S. BALACHANDRAN
equations are employed, novel effects may not be detected
because of the tendency to correlate all the data moderately
[
39]
successfully. Even without the use of aromatic ring parameter
(
I) the dispersion effect can be minimized by comparing the
[
22]
standard, p-methoxybenzyl chloride
system having different leaving groups (Scheme 1).
with 1,2-diphenylethyl
Scheme 1. Solvolysis reaction of 1,2-diphenylethyl system, When X ¼Cl,
Y ¼ H, CH and OCH When X ¼OSO CH , Y ¼ H
3
3
2
3
directly extends its conjugation with carbocation centre
(
RESULTS AND DISCUSSION
Scheme 1).
Instead of considering various models with appropriate leaving
Solvolytic rate constants of 1,2-diphenylethyl chloride in 80%
aqueous acetone and 80% aqueous DMSO are given in Table 1.
For a unimolecular reaction, an increase in the rate of solvolysis is
expected in more polar solvents. The rate of solvolysis in aqueous
dimethyl sulphoxide (DMSO) is nine times more than that in
aqueous acetone having same water content (dielectric
constants of acetone and DMSO are 20.7 and 46.7, respectively).
The m value calculated using t-butyl chloride scale (Table 2) is
0.87 which is almost same as that reported for 1-phenylethyl
chloride (0.88) suggesting that there is no appreciable change in
the magnitude of the nucleophilic solvent assistance in the rate
determining step of the solvolytic reactions of these two
groups and aromatic rings which is labour intensive, a new scale
[
26]
is developed which includes the contributions due to changes
in leaving group and the number of aromatic rings (Eqn (3)).
log k=ko ¼ mY þ hI
(3)
where I is the aromatic ring parameter independent of the
leaving group and h is the sensitivity to solvent-induced changes
in aromatic ring factor. This equation is successfully applied to
[
40]
[
31–36]
various systems.
The above equation was also applied to systems having
aromatic ring that cannot enter into conjugation with the
[
37]
developing carbocationic centre
and even to systems which
substrates. The kinetic parameters are consistent with S
solvolysis (Table 1).
N
1
have no aromatic ring at all and the dispersion observed is due to
[
38]
steric crowding at the carbocationic centre. The introduction
of an adjustable parameter in linear equation thus has no
relevance to aromatic ring contribution but simply brings the
value of correlation coefficient to unity and the standard
deviation to zero. A problem with multiparameter equations is
that strong covariances are often observed between the given
parameters; hence, Bentley opined that when multiparameter
The rate of solvolysis of 1,2-diphenylethyl chloride in 80%
aqueous acetone is 26 times less than that of 1-phenylethyl
chloride in the same solvent. The enthalpy of activation is higher
ꢀ
1
by 11.6 kJ mol
11.5 J mol
and entropy of activation is lower by
ꢀ
1
ꢀ1 [41]
K . The substitution of b-hydrogen by phenyl
group can retard the rate by the reduction in hyperconjugative
contribution by a hydrogen atom; also the benzyl group is
Table 1. Kinetic data for the solvolysis of 1,2-diphenylethyl chloride (I) and its 1-p-tolyl-2-phenylethyl chloride (II),
-p-Anisyl-2-phenylethyl chloride (III) derivatives in 80% aqueous acetone along with (I) in 80% aqueous DMSO and 1-phenylethyl
chloride (IV) in 80% aqueous acetone
1
4
ꢀ1a
6
6
6¼
Temperature
(8C)
10 k s
DH 298 K
ꢀDS 298 K
DG 298 K
4
ꢀ1a
ꢀ1
ꢀ1
Substrate
I
10 k s
at 298 K
(kJ mol
)
(J mol
K
)
(kJ mol
)
80.0
0.191
9
9
05.0
0.0
4.0
0.519
0.739
2.02
0.000268
102.1 ꢁ 0.3
47.9 ꢁ 1.1
69.1 ꢁ 1.0
43.3 ꢁ 1.0
116.4 ꢁ 0.4
1
In 80% aqueous DMSO
70.0
0.330
0.838
1.98
80.0
90.0
94.0
0.00245
0.0223
90.3 ꢁ 0.2
92.5 ꢁ 0.2
110.8 ꢁ 0.3
105.4 ꢁ 0.3
2.77
II
45.0
0.249
0.432
1.23
50.0
60.0
70.0
3.42
III
0.0
1.60
2.96
9.67
5
.0
1
2
5.0
5.0
28.5
0.00711
75.5 ꢁ 0.3
90.5 ꢁ 0.3
40.9 ꢁ 1.2
59.4 ꢁ 1.2
87.7 ꢁ 0.4
28.3
b
IV
108.6 ꢁ 0.3
a
Rate constants were determined in duplicate by titrimetric method. Estimated uncertainty is less than ꢁ2%.
b
Calculated from data in Reference [41].
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 783–788

SOLVOLYSIS OF 1,2-DIPHENYLETHYL SYSTEM
electron withdrawing with s* value þ0.22. The steric effect of
a-alkyl substituent has much more influence in the retardation of
Table 2. m-value for the solvolysis of 1,2-diphenylethyl
chloride in aqueous acetone (% v/v) at 80 8C
[41–48]
[43,49]
rate of solvolysis in aralkyl systems.
It is reported
that
the substitution of the methyl group by bulky alkyl groups in
1-phenylethyl chloride retards the rate; a substitution of the
isopropyl group in the place of methyl retards the rate by
b
Y-value t-butyl
chloride scale
4
ꢀ1a
Solvent % acetone
10 k s
[
8,50]
2
0 times. The Charton’s steric parameter
upsilon(n) for the
50
60
70
80
12.6
1.40
0.80
0.13
ꢀ0.67
isopropyl group is 0.76 and that of the benzyl group is 0.70. The
steric effect due to benzyl is slightly smaller than the isopropyl
group and retardation of the order of 10–15 times is thus
expected if a benzyl group replaces the methyl. The steric effect
3.85
1.06
0.191
of the bulky groups favours S
strain
N
1 reaction by the relief of steric
during the formation of transition state but it
m-value calculated using the Eqn (1) is 0.87.
a
Estimated uncertainty is less than ꢁ2%.
[
42,46,48,51]
b
retards the rate of reaction in S 2 reaction by preventing the
Data from Reference [76].
N
backside nucleophilic attack. The backside attack by a nucleo-
phile or solvent is difficult in a sterically hindered substrate and it
is shown that when bulky groups replace the a-alkyl group in
1
-phenylethyl chloride the mechanism of the reaction shifts more
[
44]
N N N
towards S 1 side in S 1–S 2 spectrum. The steric inhibition of
a
4
ꢀ1
Table 3. Solvolysis rate constants (10 k s ) and Y-values in various aqueous organic solvents at 25 8C
p-methoxy
1,2-diphenyl
1,2-diphenyl ethyl
1-phenyl
1-phenylneo-pentyl
Composition
benzyl chloride ethyl chloride methane sulphonate neopentyl chloride
methane sulphonate Y-value
b
e
l
9
8
7
6
9
8
7
6
9
8
7
9
8
6
0% Acetone
0% Acetone
0% Acetone
0% Acetone
0.0304
0.661
2.82
0.0000412
0.415
3.65
17.4
63.1
—
0.0000286
—
0.108
0.479
—
ꢀ1.86
ꢀ0.67
0.13
b,c
c
e
h
i
i
l
0.000268
f
h
l
0.00122
0.00514
0.000146
0.000824
—
b,c
f
h
l
27.6
0.80
b
e
g
m
m
m
m
l
0% CH CN
0.402
0.000386
2.65
—
0.490
ꢀ1.23
ꢀ0.14
0.53
3
b
f
h
h
i
0% CH
0% CH
3
3
CN
CN
5.08
18.8
72.2
0.00186
0.00464
16.4
74.1
—
0.345
2.50
6.31
—
0.000156
0.000522
d
b
f
i
1.43
h
0% CH CN
0.0126
—
—
—
—
0.00194
—
—
—
—
—
—
1.15
3.46
1.00
3
b
b
0% Dioxan
0% Dioxan
0% Dioxan
0.022
0.269
—
ꢀ2.03
ꢀ0.83
0.01
l
l
—
—
—
—
b
j
l
0% CH
0% CH
0% CH
3
3
3
OH
OH
OH
16.5
73.7
526
ꢀ0.30
b
j
l
—
—
—
—
0.38
b
k
l
15.8
1.49
a
Rate constants were determined by spectrophotometric method for 1,2-diphenylethyl methanesulphonate in different aqueous
organic solvents except acetone. For all other substrates titrimetric method was employed. At least duplicate measurements were
taken and the estimated uncertainty was less than ꢁ2%.
b
Data from Reference [22].
c
Data reported in Reference [24] was different from Reference [22]. The value reported agrees more with the extrapolated value from
Reference [22].
d
Determined by titrimetric method.
From the extrapolation (Arrhenius plot) (Rate constant values (10 k s ) at 80, 90 and 105 8C for 90% aqueous acetone solvents were
e
4
ꢀ1
0
.0333, 0.0908 and 0.367, respectively, and for 90% aqueous acetonitrile 0.276, 0.737and 2.91, respectively].
f
Data from Reference [77].
g
Data from Reference [63].
h
4
ꢀ1
From the extrapolation (Arrhenius plot) of unpublished data (Rate constant values (10 k s ) at 80, 90,100 and 110 8C for aqueous
acetone solvents were 80% (0.0233, 0.0638, 0.163, 0.402), 70% (0.0878, 0.217, 0.543, 1.31), 60% (0.456, 1.18, 2.80, 6.72) and for aqueous
acetonitrile solvents were 80% (0.0994, 0.260, 0.648, 1.54), 70% (0.292, 0.749, 1.83, 4.28), 60% (0.977, 2.48, 5.97, 13.7), respectively)
from the depository of Department of Chemistry, University of Kerala, Thiruvanathapuram, India.
i
Data from Reference [44].
Data from Reference [78].
j
k
4
ꢀ1
Obtained from the extrapolation using the data in reference [78] (in 70% aqueous methanol 10 k s ¼ 7.24).
1
l
Data from Reference [77].
m
Data from Reference [23].
J. Phys. Org. Chem. 2010, 23 783–788
Copyright ß 2010 John Wiley & Sons, Ltd.
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D. S. KUMAR, K. P. JAYAKUMAR AND S. BALACHANDRAN
[
21,41,45–48,52]
resonance suggested for S 1 reaction
is a more
N
plausible explanation for the change in thermodynamic
parameters observed for the solvolysis of 1,2-diphenylethyl
chloride compared to 1-phenylethyl chloride. The m value and
the kinetic parameters observed for 1-phenylneopentyl chloride
which is analogous to 1,2-diphenylethyl chloride are explained
[
44]
due to the steric inhibition of resonance.
The 26 times rate
retardation when the a-methyl group in 1-phenylethyl chloride is
replaced by the benzyl group indicates that the polar effects are
also important for the solvolysis of 1,2-diphenylethyl chloride.
The electron donating para substituents in the 1-phenyl group
of 1,2-diphenylethyl chloride increase the rate of solvolysis to
considerable amount (Table 1). p-Methyl substituent increases
5
the rate by 84 times and p-methoxy substituent by 10 times
which is of the same order as that of 1-phenylethyl chloride
[
41,52–61]
which undergoes solvolysis by S
p-methyl substitution in 1,2-diphenylethyl chloride reduces
N
1 mechanism.
The
ꢀ1
the enthalpy of activation by 9.6 kJ mol which is comparably
higher than that of 1-phenylethyl chloride (3.34 kJ mol ).
Figure 1. Winstein–Grunwald
plot
for
the
solvolyses
of
ꢀ
1 [62]
1,2-diphenylethyl methanesulphonate (a,b,c) and 1,2-diphenylethyl
This is due to the steric inhibition of resonance as in the case
of 1-phenylneopentyl chloride where the reduction in enthalpy
chloride (d,e). (a) Acetonitrile (aq) (slope: 0.813, intercept: ꢀ2.603,
r ¼ 0.9966), (b) Acetone (aq) (slope: 0.821, intercept: ꢀ2.866,
r ¼ 0.9999), (c) Dioxane (aq) (slope: 0.625, intercept: ꢀ3.161, r ¼ 0.9945),
ꢀ
1 [44]
of activation is 7.5 kJ mol
.
The entropy of activation is
comparable to 1-phenylneopentyl
chloride (8.8 kJ mol ) and shows appreciable variation with
ꢀ1
ꢀ1
(
d) Acetonitrile (aq) (slope: 0.662, intercept: ꢀ6.621, r ¼ 0.9966) and
more by 4.2 J mol
K
ꢀ
1
(e) Acetone (aq) (slope: 0.784, intercept: ꢀ6.977, r ¼ 0.9974), where r is
ꢀ
1
ꢀ1
[44]
the correlation coefficient
1-phenylethyl chloride (20.9 J mol
K
). Nair et al.
suggested
that when smaller enhancements in the entropy of activation
were observed, the steric inhibition of resonance does not
totally eliminate the hyperconjugative effect, but that its
magnitude is reduced significantly. The changes in enthalpy
and entropy values of activation for the p-methoxy substituent
are similar to those of p-methyl substituent. All these show that
contribute to the delocalization to the carbocationic centre due
to the intermittent CH group.
2
Large dispersion effects were observed for Grunwald–
Winstein plots when substrates containing aromatic rings
[
17–20,22,40,69]
are studied.
The rate data of 1,2-diphenylethyl
1
N
,2-diphenylethyl chloride undergoes solvolysis reaction by S 1
chloride and methanesulphonate, which undergo solvolysis by
[
63]
mechanism with limited contribution of nucleophilic solvent
assistance. The present authors have reported earlier that
methanesulphonate of 1,2-diphenylethyl alcohol undergoes
same mechanistic path way,
in various aqueous organic
solvents, are presented in Table 3. The Grunwald–Winstein plots
on t-butyl chloride scale show dispersion (Fig. 1) similar to that of
[
63]
[22,40]
S
N
1 solvolysis in 90% acetonitrile.
The p-methoxy derivate
1-phenylethyl chloride.
When the log k of 1,2-diphenylethyl
of 1,2-diphenylethyl chloride {1-chloro-1-(4-methoxyphenyl)-2-
chloride in various aqueous organic solvents are plotted against
Ysim based on p-methoxybenzyl chloride all points fall on the
same straight line (Fig. 2, Series 2). If both aromatic rings
[
64]
phenylethane}
and methanesulphonate of 1,2-diphenylethyl
[
63]
alcohol in pure acetonitrile undergo elimination reaction by E1
mechanism.
When the phenyl group replaces the b-hydrogen of ethyl
tosylate, a rate acceleration of 3040 times is observed in
trifluoroacetic acid at 75 8C which is proposed to be due to
neighbouring group participation of the phenyl group in the
[
65–67]
transition state.
place of b-hydrogen in 1-phenylethyl chloride retards the rate by
6 times; thus the probability of neighbouring group participa-
tion is not observed for the present substrate. The studies
The substitution of the phenyl group in the
2
[
42,44,49,52,68]
reported from the present laboratory
showed that
the reduction in the rate of solvolysis when bulky groups replace
the methyl group is due to the steric inhibition of resonance. The
steric hindrance offered by the bulky groups prevents the
carbocation intermediate to assume a planar configuration. This
loss of coplanarity reduces the resonance between the
developing p-orbitals of cationic intermediate and p-system of
the aryl ring. The present authors have shown that this is true in
the case of dehydrohalogenation of cumyl chloride in pure
acetonitrile also. In 1,2-diphenylethyl chloride, the steric strain
developed by the presence of two bulky phenyl groups in
adjacent carbon atoms prevents the formation of planar
intermediate with neighbouring group participation. Even
though the system contains two phenyl rings, one ring cannot
Figure 2. Correlation of Ysim based on p-methoxybenzyl chloride with (1)
log k of 1,2-diphenylethyl methanesulphonate (slope: 0.762, intercept:
[
68]
ꢀ
2.420, r ¼ 0.9951, n ¼ 9) and (2) log k of 1,2-diphenylethyl chloride
(
slope: 0.725, intercept: ꢀ6.509, r ¼ 0.9938, n ¼ 8) where r is the corre-
lation coefficient and n is the number of data points. Solvent codes: A:
Acetone/water, An: Acetonitrile/water, D: Dioxane/water
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 783–788

SOLVOLYSIS OF 1,2-DIPHENYLETHYL SYSTEM
eliminated without the use of the extended Winstein equation
Eqn (3)) with aromatic ring factor (I), which has been proposed to
eliminate the dispersion due to the aromatic ring contri-
(
[
26,70–71]
[37,38]
butions
and also the steric and electronic factors.
In the present system, electronic and steric factors are important
in stabilizing the carbocation; the comparison with a suitable
standard substrate (p-methoxybenzyl chloride) eliminates the
dispersion. Linear plots are obtained when similarity model is
applied to 1,2-diphenylethyl methanesulphonate and chloride. It
suggests that the solvation of benzylic carbocation involving
charge delocalization to benzene ring is the contributing factor
for dispersion and not the leaving group, which further confirms
that 1,2-diphenylethyl system undergoes solvolysis by S
mechanism where the leaving group has no effect.
N
1
EXPERIMENTAL
Figure 3. Correlation of Ysim based on p-methoxybenzyl chloride with (1)
log k of 1-phenylneopentyl methanesulphonate (slope: 0.725, intercept:
1
,2-diphenylethyl alcohol was prepared by Grignard reaction
ꢀ
4.018, r ¼ 0.9956, n ¼ 7) and (2) log k of 1-phenylneopentyl chloride
between benzaldehyde and benzylmagnesium chloride. For
para-substituted derivatives, the corresponding para-substituted
benzaldehydes are used. The solid alcohols were repeatedly
washed with petroleum ether and crystallized from petroleum
ether–benzene mixture. The melting point of 1,2-diphenylethyl
(
slope ¼ 0.874, intercept: ꢀ7.434, r ¼ 0.9947, n ¼ 6) where r is the corre-
lation coefficient and n is the number of data points. Solvent codes: A:
Acetone/water, An: Acetonitrile/water, M: Methanol/water
[
72]
contribute to the dispersion, the nature of plot will be with slight
alcohol is 67 8C (lit.
m.p. 67–67.5 8C) and that of p-methyl
[73]
[
22]
dispersion as in the case of 1,1-diphenylmethyl chloride.
The
derivative is 69 8C (lit. m.p. 69–70 8C). The p-methoxy derivative
was purified by repeated fractional distillation under reduced
pressure. The fraction boiling between 139 and 141 8C at 38 mm
pressure was collected (yield 80%). Repeated crystallization from
n-hexane–benzene mixture gave the alcohol with m.p. 58 8C
good correspondence with a system of single aromatic ring
suggests that only one aromatic ring contributes to the
conjugation i.e., the possibility of neighbouring group participa-
tion by the aromatic ring to form a non-classical bridge head is
unable to be realized.
[
73]
(lit. m.p. 58.5–59.5 8C) The authenticities of the alcohols were
[
72,73]
The dispersion is present for both leaving groups, the chloride
and methanesulphonate in t-butyl chloride scale (Fig. 1). If the
reason for dispersion is a difference in solvation between
aromatic ring which conjugates to carbocationic centre through
established by elemental analysis, IR and NMR spectra.
The alcohols were converted into the corresponding chlorides
by the method using thionyl chloride. For 1,2-diphenylethyl
chloride, the fraction boiling at 147–148 8C at 5 mm pressure was
[
17–21]
[74]
p-electrons of the benzene ring
alone, a change in the
collected (lit.
146–149 8C at 5 mm pressure). The TLC analysis
leaving group from chloride to methanesulphonate will not
influence the nature of the plot. Linear plots are obtained, when
log k of 1,2-diphenylethyl methanesulphonate and chloride in
various aqueous organic solvents are plotted against Ysim (Fig. 2,
Series 1 and 2) indicating that the change in the leaving group
simply causes a change in the rate and not in the nature of
correlation. The disappearance of dispersion for both the leaving
groups, with comparable slopes for the plots, indicates that the
dispersion is due to the solvation of carbocation rather than the
initial state of the substrate molecule. Further confirmation is
obtained from the solvolytic rate data of 1-phenylneopentyl
methanesulphonate and chloride (Fig. 3, Series 1 and 2).
using n-hexane showed a distinct spot of chloride and a light spot
due to alcohol. It has been observed that during distillation at
reduced pressure, all these halides eliminate HCl and form
alkenes. The distilled and undistilled 1,2-diphenylethyl chloride
gave same value of rate constant for solvolysis. The p-substituted
chlorides were used for kinetic studies without distillation where
the dehydrohalogenation is found to be more. Brown reported
solvolysis of tertiary aralkyl halides without distillation when they
[
75]
eliminate HCl during distillation.
Since all the chlorides were
found to eliminate hydrogen chloride on keeping, they were kept
in dry ether at sub 0 8C and kept for a maximum period of
2 weeks.
[
63]
The methanesulphonate was prepared
by treating the
alcohol with methanesulphonyl chloride at 0 8C. The solid
methanesulphonate formed was unstable at normal tempera-
ture, recrystallized from n-pentane at refrigerated temperature,
dried under vacuum and stored at ꢀ10 8C under which condition
it was stable. The purity of the methanesulphonate was tested by
thin layer chromatography.
CONCLUSION
The 1,2-diphenylethyl system has two phenyl groups but only the
a-phenyl group takes part in the resonance stabilization of
the intermediate carbocation. The resonance contribution from
the b-phenyl group is prevented by the intermittent CH group
Kinetic measurements were conducted both by titrimetric
method and spectrophotometric method. In the titrimetric
method, the acid produced was estimated by NaOH using
lacmoid indicator in dry acetone. A conventional thermostat
with Jackson thermometer was used for temperature control.
In the spectrophotometric method, the increase in absorbance
due to the product formed is monitored using Hitachi 220A
2
and the possibility of neighbouring group participation via
non-classical carbocation is prevented by the steric inhibition
of resonance. The elimination of dispersion effect when
p-methoxybenzyl chloride is taken as a standard for comparison
showed that the 1,2-diphenylethyl system behaves similar to
the 1-phenylethyl system. The dispersion effect thus can be
J. Phys. Org. Chem. 2010, 23 783–788
Copyright ß 2010 John Wiley & Sons, Ltd.
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D. S. KUMAR, K. P. JAYAKUMAR AND S. BALACHANDRAN
double-beam spectrophotometer fitted with a thermostated cell
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