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Nucleophilicity of halide ions in the micellar pseudophase
Tetrahedron Vol 45, No 6. pp 1747 to 1754. 1989 Pnnted I” Great Bntam
0
NUCLEOPHILICITY
OF
IONS IN THE
HALIDE
HAHAD
HICELLAR
PSEUDOPHASE
A.AL-LOHEDAN
icing Saud University, P.O.Dox of Chemistry, Riyadh-11451, Saudi Arabia.
Dept.
0040-4020/89 $3 oO+ 00 1989 Pergamon Press plc
2455,
(Recewed in UK 15 December 1988)
or CiwiV~~n-buQl-+ni~fcnatela anl lbreepe&iVelyie SulfcMte and n-krtyl-4-m catalyzedbycaticslic~lesofcetyltr~~~~~cabranide ccnstanttithtsurfa(clacl or C!BBrre@ze&ivelyl.The inueaseofrate bant]canbeanalyzedin~ofthecarcentrationof substrate (la-b) anahalideialinthemicellerpB&@iaW,at%lthesecald~ratelxtlstantsinmicellaratni~~aresimilar.
Abstrab--reactiatofBr-
It is llar
known
that
systems.
the
small
with
the
This
volume
of
[surfactant]
useful ween
and
ionic
sites
which
are
approach
on
has
reactions meters
the
been
by
,
described
accurately
aqueous
is given
The
tration stant,
of
terms +
llized for
first-order
However,
for
VW
an
the
of
that
by mice-
reactants
variations
distribution
xi
This
is
rate
of
terms
rate
i-e
counterions
in
constants
. A very
hydrophilic
fractions,p
ions
compete
bet-
for
, of these sites 2,5 . This general
constant
acid15,
by Br-
and
reaction
of
Cl
Y-,
+
certainty. and
can the
model
interionic may
subscripts
not
W and
The
be eliminated
should
be kv
and
constant, for
become is
hydrophilic 1747
by
counterion the
ion-exchange
constant
provided
anions
for ion
(2)
using a reactive ion sur12-14 , so its concen-
overall
constant
observed
that/,
reaction
once
OH-,
is conshould
substrate
reactions
of
in micellized
e.g.
be H
(1)
.......
of N-alkylpyridinium - 17.18
very
where
X-,
.......
Y;
micellar
behaviour
In additlon,
a nucleophile
respectively.
constant
[surfactant]
predicted
with
ion
ion-exchange
#
pseudophase
increaslng
substitution
of
reagent
micellar
alkanesulfonic
that
affected
both
in these
to assume
known
inert
competion
ionic
the
bound.
not
an
[X,l[Y,l/[X$[Y;l
the
with
the
is approximately
K; =
in
increase
and
oseudophases
and
micellar
the
is
micellar
interionic
in which
are
example
in
by
problem
factant,
and
quantitatively
examin
surface
values
for
and
to
strongly
successfully to rate and equilibrium constants of many g-11 . However, the treatment involves several para-
Xn denote
treated
are
concentrating
micelles
whose
between,
from l-5
micelles
counterions
applied
competion
of
pseudophases
micellar
in aqueous
e.g./
be
reactions
primarily
approach
micellar
neutralized
bimolecular
Layer
often
used
and
of
arises
Stern
can
widely
aqueous
rates
effect
F-,
or
H+
is fully in mice-
CTACN16 RCO;
and
,value
1748
H. A. AL-LOHEDAN
of
k my increases
micellar upon
with
nucleophilic
and
increasing
bound12'13.
The
aim
[surfactant]
of
the
substitution
by Cl-
n-butyl-Gbromobenzenesulfonate
chloride
or
bromide
(CTACl
or
work
Br-
(la-b)
or
even
present
CTABr
upon
in
the
to
substrate
examine
the
is
fully
micellar
effects
n-butyl-4-nitrobenzenesulfonate
solution
respectively Scheme
when
is
of
cetyltrimethylammonium
(scheme
1)
1
S03-CH2-CH2-CH2-CH3
The
reaction
of
unreactive
la
X=
lb
X-Br
NO2
(la-b)
counterions
with
water
such
as
is
inhibited
Mysylate RESULTS
Reaction
in the
chloride
ion
city
of
charge the
Presence
anions
and
the
density22'23.
second-order
rate
the
are
and or
lb the to
ctant
for
in Micellar
(Figure
the
of
be a better
counterions
the
to
nucleophile
are
Also,
solutions tend
kv
constant
2).
than
extensively
and
k y
,
and
This
Cl-.
hydrated
have
.
as
water
Br-
and
Cl-
or
Br-
smoothly with At
of
I and
with
high
II)
and
n-butyl-4-
respectively.
Br-
with
n-butyl-4-
respectively(
for
no
substrate
are
of
common
halide of
in water,
Layer
surfa-
Br-*appears
understandable
Stern
la
increasing[CTACll
concentration
reaction
rich
with
with
addition
high
differences
in the
and
than
nucleophili-
anions
(Table
x 10-51i-1s-1
of
2).
better
reactions).
increase
1 and
limits
Cl-
these
increase
,
is
hydrophilic
of Cl-
7.2
reactions
ky
which 20
reduces
2.25 x 10-4m-1s-I
reactions
upon
(Figure
reach
of
and
bromide
reactions
reactions
effects In the
contribution
bonding
small
in our
x loo5 kl-l~-l
salt
rate
1 and
surfactant
values
4.5
with
for
of
hydrogen
10-5m-1s-1
constants
Solutions:
first-order
[CTABr]
ion
to
is made
the
micelles
minor
nucleophilicity
behaviour
x
cationic
DISCUSSION
because
for
by
it makes
is greatest
9.84
are
AND
The
this
rate
bromobenzenesulfonate
Reaction
see
constants
second-order
corrections
Salt: solvent,
effects
We
nitrobenzenesulfonate While
of
in hydroxylic
so
of
because normal
a
micelles24. Quantitative factant] the
Treatment
is
aqueous
ctively stants
and
(Scheme being
of
generally
KW
micellar 2) and
Rate
treated
and k:.
the
The
react
variation
assumption
pseudophases can
The
Effects: on
designated
in
each
micellized
that, by
pseudophase,
surfactant
of
rate
S,
is distributed
subscripts with
constant
W and
with
11 respe-
first-order
(detergent)
[sur-
between rate
is designated
conD,,
Scheme'2
and
its
monomeric ding
(eq
concentration surfactant 3 and 4 ).
is that (cmc),
of
the is
total the
surfactant equilibrium
concentration, constant
less
for
KS
1;s
= Cs,]/[S,][D,]
.
= CDT] - cmc
.....
[D,]
that
substrate
and
(4)
of
bin-
Halide tons in the mxellar pseudophase There
is also
observed
Provided the
a reaction
rate
constant that
first-order
with so we
water,
equilibrium
rate
which
neglect
makes
a major
contribution
k,, D to the 2
it.
is maintained
constant
1749
is given
between
ki
kc; + k.W
reactants
in the
pseudophases,
by25
KS CD,, 1
........
= 1 + X, ED,,]
002
004
Flgure I Reactbm
ol n-butyl
006
006 CCTABrj
[NOB;]
(5)
01
M
47 Nitrobmzm.sulfOna~
with
ET. 4 in CTABr,
A m 0.04 M CTABr + NaBr. . For rmctlons of n-butyl 4-bromobmzenewlfonate wth B; in CTABri 0 in 004 M CTABr t NaBr
abe
Ob4
Ob6
Ob6
0‘10
0’12
014
Oi6
0.16
pmCijhl FIgwe 2
Reactions
of n-tutyl 4-Nltdmzemw
A in CTACI t O.IM MCI with
These kW and
rate
constants
kH and
the
.
for mmtlom
Ci n CTACI, 0 in CTACI +O.IM
can
be written
concentration
kir = kil
=
nom of n-butyl
NaCi
in terms
of Cl-
wlrnci, A I" crack, 4-branobenzenesulfmote
and
of
Br-
the
second-order
in each
pseudophase
+,&X,1 k,,mi
rate eq.
constants 6 and
7.
. . . . . . . . . (6)
= kR [Xi
]/CD,]
= kn,4
. . . . . . . . . (7)
1750
H A
Table
I.
Reaction
of
AL-LOHEDAN
n-butyl-4-nitrobenzenesulfonate
in the
absence
of
surfactanta.
[Salt]H
avalues
Table
II.
of
Reaction
NaCl
NaBr
0.02
4.9
5.2
0.08
5.5
6.6
0.15
6.2
7.9
0.2
6.7
9.5
lo5 k ws-
of
at
25°C.
In water
105kW
n-butyl-4-bromobenzenesulfonate
= 4.6 s-l.
in the
absence
of
surfactanta.
avalues The
rate
constant,
rni, which The the
for
The
nonionic
bility
The
k9J the
when
i.e.
of
to
the
X-
a mass
0.15
2.52
2.95
0.2
2.72
3.5
k
-' at
ws
defined
ion
of
of
with
to
from
ky
= 1.85 s-l.
kW
concentration
written
as
a mole
ratio,
is p. rate
data
therefore
involves
and
micelles,
which
Br-
in the
water
cannot
or
be measured
but
that
KS
aqueous
with
= lo2 Pi-l, solution
for may
pseudophase
because binding
both
CTACl
we
of
on KS
and
pseudophase.
the
increase
and
estimation
depends
micellar
directly
comparison
surfactant
the
with
, 16
is fully
[surfactant]
and some 15,16
.
increasing
action
,
lo5
of of and
I:,, by
considered
reaction similar,
2-9,18
CTABr
However,
concentration micellar
bound
aqueous and nicellar 12-14,18 model K' = X
CX,
for
sulfonic
l/LX,
reactions 15 can
acids when
the
some
this
POSSI-
pseudoehase
can
1
reactive
analyzed
reactive
of nucleophile 12,14,15,26, but
1 (CD,]-[X,1
in be
ion
the be
ion
on
is OH-
the or
surfactant
or
distribution
f tted
....
.
' salting
data5~g*14~18.
is constant
between
the
ion,
In water
using
of Cl-
suggest
rate
by
between
KS
salts
25'C.
surfactant
halide
in CTACN
thatp
substrate
counterion
2.5
treatment
solutes
e.g.
increases
2.20
substrate
solutes
variation
assumotion
0.08
, IS
and
in fitting
surfactant,
2.10
constants,
addition
However,
1.95
concentration
substrates
unreactive
out"
the
0.02
k,,
of
binding
between but
of
NaBr
lo4
quantitative
estimation
NaCl
of
a reactive
distribution
[SaltIf!
to eq.
F-,
even of 8,
. . . . . . . . . . . . (8)
Hahde Ions m the muAlar pseudophase Tab1 e
Rate
III.
and
equilibrium
constant
for
1751
reaction
in
-1
CTABra.
Medium
Substrate
Kstl-T
CTASr
la
115
10.2
14.4
0.66
0.04 CTABr
+ NaBr
lo4 k,,s
105 k;l&'
k;IkW
la
130
10.3
14.5
0.66
CTABr
lb
120
7.5
10.5
1.1
0.04 CTABr + NaDr
lb
135
7.3
10.4
1.1
atlodelI assumes a constant p of 0.8. The fit assumes a salt effect on substrate binding, see text.
Table
Rate and equilibrium constant for reaction in CTACla
IV.
tledium
K;t:-1
Substrate
KsM-1
104k,,s-1
105kmtq-1 s-l 2 I
k; I klJ
CiACl
la
230
115
2.9
4.1
0.91
CTACl + 0.114 RaCl
la
230
130
30
4.2
0.93
CTACl
lb
215
125
2.1
2.9
0.4
CTACl + O.lM NaCl
lb
215
135
2.0
2.8
0.4
aUe used modelil which is a mass action treatment with variablef
Equation
6 predicts
counterions much
will
with
[surfactant]
constantprs
to micelles
mlcelles,
2
assumed with
,we
binding
of
but
treating
a wide
apply
increasing
ctions gives
in
over
relatively
In CTACl.
range
this
halide The
fraction,P
with
is
Ki
large,
it may
our of
fail
data
e.g. an
for
we
or
the
hydrophilic
anions
two
,
we
the
Br-,
binds
~111
less
binds
use of
this
applied
(i)
of
than
that as
a
Ejr-
/
is
is oft& Increases
to micellar
approximation
a constant,&
by vary
strongly
strongly
concentration
a model
not
approximation
which
in CTABr.{li)that,/
on
that
Thus
. as
ion
reactlons
is
neutralized
possibilities
halide
based 12,14
assumption
, so
for that
rea-
eq.5-7
: k\, LX;1
kv =
+ K,,~,P[D,l 1 +
kU (
[X;l-
kW
CX, 1 1 + k,,K,BCD,l
([X+1-[$,I) 1
subscripts,
T denotes
the
total
. . . . . . . .. .
XSIDnl
1 +
where
I1
such
groups
although/
which
consider
concentration,
simplest
lo3
ion Cl-,
surfactant on
head
[surfacta;t]
for
possibility ion
of mlcellar
increasing
satisfactory
9,11,27.
Therefore
the
if
probably
to cationic
constant
that
increase
+
KSIDnl
+ k,,K,
[X,1
KS [D,,]
concentration
of
X-.
19)
. . . . . . . . . (10)
. . . .. ....
111th fully
micellar
(11)
1752
H. A. AL-LOHEDAN
bound
substrate
eq.
12 simplifies
to
. . . . . . . . . . . . . . (12)
kv = KMB Reaction
in CTABr:
kv
with
[CTABr]
the
rate
data
the
variations However,
value
of,9
"Salting such our
the eq.
fitted
increase
the
but
binding
to
other
combination
ky
is
with
the
small
Therefore, 28
in 0.04
we
did
tl CTABr
of
a constant@
we
could
this
model
is not
the
that
He
to
with
limits
reached
on
the rate
using
salt
is
the
variation
III.
He
parameters
inconsistent
aqueous
pseudophase.
not
halide
systems
explain
the
ion
of
note
that
provided
with
that
too
data
is
KS
by
,
precedence
small
for
a constant
increase
There
and
our
is
to
=
for
to
for
explain
reactions
of
all
Cl-
in
the
we
100
micelles,
could
be
that
HaBr
following
(1 + b[NaBr]
6,9
cf.ref. for
of
assumed
fitted
to model
increased
eq.
13,i.e.with
the
reaction
values
constants
that
satisfactory
high
rate
provided
"salts-out"
data
Some
for
parameter variation of
should the
Micellar counterion
at
b,
...........
and
18.
reactions
in CTACl eq.13,
size
growth
explain
be
8 can of
k.V
Although
in CTABr
and
e.g.
given of,&
assumption
and
CTACl+NaCl
greater
(13)
CTABr in
than
+ NaBr.
terms
of
which
40M-'
when
the
such
hydrophylic
variation ion
the
be
if
combined
in
surfactant
added
sizes
given
at
toward
larger
depends
the
and
ions
as
one
balance would
interact OH-
or
ii)
limit
any
[CTACl]
we
eq.8.
the but
and
assumed
Equation
cannot
be the
reasonable
computer
X-
CTACl
counter-ion
in any
effective
counterions
although
CTAF14'15*31,
that
8 places
reached
because
upper
limit of 1 14 limit . In pra-
103M-1.
added
of
with
pseudophase
a simole
and
distribution upon
Cl-,
and
and
and
we
with
programme were
used
able
added
to
salt
to pre-
fit
all
(figure
2)
IV.
in Table
with
k+,
beyond
and
[CTACl]
added
CTAOH
(model lower
I;: >
with
more
of
[Cl-]
in aqueous
[X-l
with
with
.
concentration
with
micellar the
12
increases
observed
although
present
reaction
interaction,
gest
the halide
1 on ,&,
little
CTACl
CTAoAc
counterion
5 and
a distribution micelles
To
0 and
variation
the
in CTAFor,
:
for
apparently
increasing
vary
data
thatp
that
counterion
only
would
Equations the
than
that
is always
cticep
dict
i)
conclude
in CTACl
increases
formal
is
these
added
oseudophase,
rate
using
is smaller
is close
there
the
without
added
substrates
NaBr
aqueous
(midel
therefore
Reactions ,3
the
fit
of
fits
reasonable.
effect it
not
i)
in Table
is that
+ NaBr
constant,& of
assuming
from
(model
parameters
in other
KS
substrate
the
.
constant
b = 12M-I,
of
from
it
assuming
l),
terms
possibility
substrate
reaction
a constant,9
in
of
One
a constantp
For
12 with
1)
smallg'lO.
B-11.
effects.
of
I (figure the
be
are
effect5'g,
salt
terms
can
out"
an
Equation (figure
the F-.
is not
unreasonable
surfactant
attracting
counterions
and
it should
also
micelles, of
expect least
because there 29 . The larger
solution
surfactant-surfactant variation with
the
head
added
salt
increase
and
of micellar mlcellar
if
is
.
surfactant-
size
to be
groups
lar-
as with
Halide IOIISin the mvxllar pseudophase Comparison ctions that
of
is
binds
can
beacuse
to
fitted
and
CTACl:
with
profiles
for
The
rate
assumption
approximation
cationic
is
micelles,
to micellesg911a27.
Br-
surfactant
it
we
in CTACl
for
fails
use
surfactant
constant
satisfactory but
Therefore
reactions
of
an
for
a mass
profiles
(Table
ion
Cl-
for
as
Br-
binds
model
rea-
IIIand figurel)
such
which
action
IV and
(Table
p
to
which
less
strongly
treat
figureZ).The
the
values
ClThe
for
Br-
or
should
values
reaction
of in
dimensions5'31. tration
as
for
liter
KS are
The
second-order ratio
which
g93o.
Therefore,
we
assume the
pseudophases,
constant anion
be that
of
second-order
Stern
rate
The
values
of
kl for
those
of
kN
with
coincidental However,
of
because
overall 2,18 Stern-Layer
the
it
the
a different
reach
=
kinetic
groups
(eq.
as moles
upon
of the
of
total
with
IV).
our
of
of
reaction
Stern
Layer
close
concen-
7) where
nucleophile
per
These
reaction
molar
would
in
almost
rate
the
mice-
volume
ident ical
is probab 1Y of
the
aporoximately be
The
.
are
similarity
probably 18
of
. . . . . . . . . . . . . (14)
estimated
element
different
a molar volume of ca. 0.14L5, -ls-1 is given by I1
kz,
The
is
have
solution.
0.14 kM
micelle
the
as
Layer,
micellar
and
head
element
conclusion
little
SternLayer.
twice
affected
similarity
of
that
of
by choice
k2 m and
k,, is
both
substrate and halide ion should be located near the 2,?4 surface . There are many examole of reactions for which
constants
in constants
solvent
terms
volume
overall
because rate
differences
the
III
deoends
in
to micellar
our
, so
micellar
second-order
in
(Table
volume
volume
understandable, water
reactions
in water
k,i is expressed
constant,
k;
constants
kN,
is approximated the
rate
k,i and
conventionally,
which
by defining
to
second-order
model.
is written
compared
The
in both rate
such
constants.
aqueous
reactive
pseudophase
be
lles,
and
of
counterion
can
binding
similar
micellar
aqueous
constants
larger
the
a molar
kH the of
have
K;
M-l
Cl-
and
rate
of
and t$= 40 (Table IV) are reasonable in comparison with KdH =55M-l -1 12 K;,, = 8OM ion for formate and acetate ion , because less hydrophilic
for
as
in CTABr
be
this
strongly
than
as
Reaction
in CTABr
1753
or to
in micelles
are
probably
a different
are due
similar
to
location
the
of
to
those
in water
properties
the
two
of
reactants
the
and
the
micelle
in the
as
a
micelle18.
EXPERIMENTAL The Materials: -.benzenesulfonate
:
Kinetics by
the
255
nm
adding all in a
All
preparation and the
decrease for
reactions
or
HCl
substrate
cuvette
so
contain
Acknowledgement:
Suoport
University
so
that
less seconds.
Saud
were
was
in a reciProca1
King
of
at
surfactant
followed 252
nm
added the
than
of
there
this
is greatly
a solution
IieCH. The
work
rate
constant
contribution
concentration
3.1%
and a standard
by the
acknowledged.
n-butyl-4-bromomethods 5.21
.
in water
at 25°C
n-butyl-4-nitrobenzenesulfonate The
is no as
followed
spectrophotometrically
for
n-butyl-4-bromobenzenesulfonate.
reactions lcm
n-butyl-4-nitrobenzenesulfonate
of absorbance
IO-S/i HBr
solutions
of
nurification
from
in MeCN of
the
first-order
Research
did
not
reaction
to the
substrate rate
Center,
and change
with
reaction was
College
For
mixture
10 -5H
constant,
by
OH-.
ky
and
the
,
are
of Science,
1754
H
A. AL-LOHEDAN REFERENCES
1. Cordes,E.H., Pure Appl. Chem., 1978,50,617. 2. Romsted,L.S. in "llicellizations, solubilization and Ilicroemulsions", Mittal, K.L. ed..Plenum.New York.1977.Vo1.2.n.509. Bunton,C:A.,Catal.Rev.Sc~.,Eno.,197~;83,680. Chemistry of Surfactant", Mitta1,K.L. ed.,Plenum,New i: Bunton,C.A. in "Solution York,1979,Vol.2,p.519. Al-Lohedan,H.A.,Ph.D. thesis, University of California,Santa Barbara,l981. Gulf Scient.Res.Eath.Phys.Sci.,l977,A5;2),p.l91. i: Al-Lohedan,H.A.,Arab Martinek,K.,Yatsimirski,A.K.,Levashov,A.V.,Berezln,I.V., In ref.2, p.489. 7. a. Cuccovia,I.M.,Schroter,E.H.,~~onterio,P.I1.,Chaimovich,H.,J.Org.Chem.,1978,43, 2248. Bunton,C.A., Romsted,L.S., J.Phys.Chem.,1981.85,2123. 9. Al-Lohedan,H.A., Al-Lohedan,H.A., Bunton,C.A., Romsted,L.S., J.Org.Chem.,1982.47,3528. Chaimovich,H., J.Phys.Chem., 1979,83,1844. Quina,F.H.. Bunton,C.A., Romsted,L.S., J.Org.Chem., 19C2,7,1160. 12. Al-Lohedan,H.A., Tetrahedron,l987,Vol.43,No.2,345. Al-Lohedan,H.A., Gan,L.H., Meffot,J.R., Romsted,L.S., Savalli,G., J.Phys.Chem., ::: Bunton,C.A., 1981, D5,4118. Bunton,C.A., Romsted,L.S., Savelli,G., J.Am.Chem.Soc.,1979,101,1253. Romsted,L.S., Thamavit,C., J.Am.Chem.Soc.,1980,109,3900. ::: Bunton,C.A., Behaviour of Surfactants, Theoretical Romsted,L.S., in "Solution 17. Bunton,C.A., and Applied Aspects", Fendler,E.J., Iiittal,K.L., ed. Plenum Press, New York, 1982, vo1.:,0.975. Al-Lohedan,H.A., Bunton,C.A., iloffatt,J.R., J.Phys.Chem.,19&3,87,332. Bunton,C.A., !'lhala,N., J.Am.Chem.Soc., 1982,104,6654. it: Al-Lohedan,H.A., unpublished result . Al-Lohedan,H.A., of Practical Organic Chemistry", 3rd edition,Longman, Vogel,I., in "A Textbook 195C,p.&20. Toronto, Savedoff,L.G., Smith,S., Stevens,I.D.R., Gall,J.S., Tetrahedron 22. #instein,S.. Letter.,.1960,24. 23. Parker,A.J., J.Chem.Soc. A, 1966,220 Chem.Rev.,l969,68,1. 24. Stigter.0.. J.Phys.Chem.,l964,68,3603. 25. flenger,F.tI., Portnoy,C.E., J.Am.Chem.Soc.,l967,89,4968. 26. Bunt0n.C.A.. Franks0n.J.. R0msted.L.S.. J.Phvs.Chem..1980.84.:CD7. 27. BartetiE., 6amboa,C.,.Sepulveda,L;, J.Phys.Chem.,1980,84,i72; Gamboa,C., Sepulveda,L., Soto,R., Ibid, 1981,85,142?. 28. Preparation at hiqh concentration of Nader and CTABr limits our work. Effect" , !liley Interscience, New York,1983, 29. Tanford,C.H., in "THe Hydrophobic G-C. 2nd Ed., Chapter 30. Bunton,C.A., Carrasco,N., Nuang,S.K., Paik,C.H., Romsted,L.S., J.Am.Chem.Soc., 1978,100,5420. 31. Al-Lohedan,H.A., J.Phys.Chem., 1987,91,4524.
2:
E:
0
NUCLEOPHILICITY
OF
IONS IN THE
HALIDE
HAHAD
HICELLAR
PSEUDOPHASE
A.AL-LOHEDAN
icing Saud University, P.O.Dox of Chemistry, Riyadh-11451, Saudi Arabia.
Dept.
0040-4020/89 $3 oO+ 00 1989 Pergamon Press plc
2455,
(Recewed in UK 15 December 1988)
or CiwiV~~n-buQl-+ni~fcnatela anl lbreepe&iVelyie SulfcMte and n-krtyl-4-m catalyzedbycaticslic~lesofcetyltr~~~~~cabranide ccnstanttithtsurfa(clacl or C!BBrre@ze&ivelyl.The inueaseofrate bant]canbeanalyzedin~ofthecarcentrationof substrate (la-b) anahalideialinthemicellerpB&@iaW,at%lthesecald~ratelxtlstantsinmicellaratni~~aresimilar.
Abstrab--reactiatofBr-
It is llar
known
that
systems.
the
small
with
the
This
volume
of
[surfactant]
useful ween
and
ionic
sites
which
are
approach
on
has
reactions meters
the
been
by
,
described
accurately
aqueous
is given
The
tration stant,
of
terms +
llized for
first-order
However,
for
VW
an
the
of
that
by mice-
reactants
variations
distribution
xi
This
is
rate
of
terms
rate
i-e
counterions
in
constants
. A very
hydrophilic
fractions,p
ions
compete
bet-
for
, of these sites 2,5 . This general
constant
acid15,
by Br-
and
reaction
of
Cl
Y-,
+
certainty. and
can the
model
interionic may
subscripts
not
W and
The
be eliminated
should
be kv
and
constant, for
become is
hydrophilic 1747
by
counterion the
ion-exchange
constant
provided
anions
for ion
(2)
using a reactive ion sur12-14 , so its concen-
overall
constant
observed
that/,
reaction
once
OH-,
is conshould
substrate
reactions
of
in micellized
e.g.
be H
(1)
.......
of N-alkylpyridinium - 17.18
very
where
X-,
.......
Y;
micellar
behaviour
In additlon,
a nucleophile
respectively.
constant
[surfactant]
predicted
with
ion
ion-exchange
#
pseudophase
increaslng
substitution
of
reagent
micellar
alkanesulfonic
that
affected
both
in these
to assume
known
inert
competion
ionic
the
bound.
not
an
[X,l[Y,l/[X$[Y;l
the
with
the
is approximately
K; =
in
increase
and
oseudophases
and
micellar
the
is
micellar
interionic
in which
are
example
in
by
problem
factant,
and
quantitatively
examin
surface
values
for
and
to
strongly
successfully to rate and equilibrium constants of many g-11 . However, the treatment involves several para-
Xn denote
treated
are
concentrating
micelles
whose
between,
from l-5
micelles
counterions
applied
competion
of
pseudophases
micellar
in aqueous
e.g./
be
reactions
primarily
approach
micellar
neutralized
bimolecular
Layer
often
used
and
of
arises
Stern
can
widely
aqueous
rates
effect
F-,
or
H+
is fully in mice-
CTACN16 RCO;
and
,value
1748
H. A. AL-LOHEDAN
of
k my increases
micellar upon
with
nucleophilic
and
increasing
bound12'13.
The
aim
[surfactant]
of
the
substitution
by Cl-
n-butyl-Gbromobenzenesulfonate
chloride
or
bromide
(CTACl
or
work
Br-
(la-b)
or
even
present
CTABr
upon
in
the
to
substrate
examine
the
is
fully
micellar
effects
n-butyl-4-nitrobenzenesulfonate
solution
respectively Scheme
when
is
of
cetyltrimethylammonium
(scheme
1)
1
S03-CH2-CH2-CH2-CH3
The
reaction
of
unreactive
la
X=
lb
X-Br
NO2
(la-b)
counterions
with
water
such
as
is
inhibited
Mysylate RESULTS
Reaction
in the
chloride
ion
city
of
charge the
Presence
anions
and
the
density22'23.
second-order
rate
the
are
and or
lb the to
ctant
for
in Micellar
(Figure
the
of
be a better
counterions
the
to
nucleophile
are
Also,
solutions tend
kv
constant
2).
than
extensively
and
k y
,
and
This
Cl-.
hydrated
have
.
as
water
Br-
and
Cl-
or
Br-
smoothly with At
of
I and
with
high
II)
and
n-butyl-4-
respectively.
Br-
with
n-butyl-4-
respectively(
for
no
substrate
are
of
common
halide of
in water,
Layer
surfa-
Br-*appears
understandable
Stern
la
increasing[CTACll
concentration
reaction
rich
with
with
addition
high
differences
in the
and
than
nucleophili-
anions
(Table
x 10-51i-1s-1
of
2).
better
reactions).
increase
1 and
limits
Cl-
these
increase
,
is
hydrophilic
of Cl-
7.2
reactions
ky
which 20
reduces
2.25 x 10-4m-1s-I
reactions
upon
(Figure
reach
of
and
bromide
reactions
reactions
effects In the
contribution
bonding
small
in our
x loo5 kl-l~-l
salt
rate
1 and
surfactant
values
4.5
with
for
of
hydrogen
10-5m-1s-1
constants
Solutions:
first-order
[CTABr]
ion
to
is made
the
micelles
minor
nucleophilicity
behaviour
x
cationic
DISCUSSION
because
for
by
it makes
is greatest
9.84
are
AND
The
this
rate
bromobenzenesulfonate
Reaction
see
constants
second-order
corrections
Salt: solvent,
effects
We
nitrobenzenesulfonate While
of
in hydroxylic
so
of
because normal
a
micelles24. Quantitative factant] the
Treatment
is
aqueous
ctively stants
and
(Scheme being
of
generally
KW
micellar 2) and
Rate
treated
and k:.
the
The
react
variation
assumption
pseudophases can
The
Effects: on
designated
in
each
micellized
that, by
pseudophase,
surfactant
of
rate
S,
is distributed
subscripts with
constant
W and
with
11 respe-
first-order
(detergent)
[sur-
between rate
is designated
conD,,
Scheme'2
and
its
monomeric ding
(eq
concentration surfactant 3 and 4 ).
is that (cmc),
of
the is
total the
surfactant equilibrium
concentration, constant
less
for
KS
1;s
= Cs,]/[S,][D,]
.
= CDT] - cmc
.....
[D,]
that
substrate
and
(4)
of
bin-
Halide tons in the mxellar pseudophase There
is also
observed
Provided the
a reaction
rate
constant that
first-order
with so we
water,
equilibrium
rate
which
neglect
makes
a major
contribution
k,, D to the 2
it.
is maintained
constant
1749
is given
between
ki
kc; + k.W
reactants
in the
pseudophases,
by25
KS CD,, 1
........
= 1 + X, ED,,]
002
004
Flgure I Reactbm
ol n-butyl
006
006 CCTABrj
[NOB;]
(5)
01
M
47 Nitrobmzm.sulfOna~
with
ET. 4 in CTABr,
A m 0.04 M CTABr + NaBr. . For rmctlons of n-butyl 4-bromobmzenewlfonate wth B; in CTABri 0 in 004 M CTABr t NaBr
abe
Ob4
Ob6
Ob6
0‘10
0’12
014
Oi6
0.16
pmCijhl FIgwe 2
Reactions
of n-tutyl 4-Nltdmzemw
A in CTACI t O.IM MCI with
These kW and
rate
constants
kH and
the
.
for mmtlom
Ci n CTACI, 0 in CTACI +O.IM
can
be written
concentration
kir = kil
=
nom of n-butyl
NaCi
in terms
of Cl-
wlrnci, A I" crack, 4-branobenzenesulfmote
and
of
Br-
the
second-order
in each
pseudophase
+,&X,1 k,,mi
rate eq.
constants 6 and
7.
. . . . . . . . . (6)
= kR [Xi
]/CD,]
= kn,4
. . . . . . . . . (7)
1750
H A
Table
I.
Reaction
of
AL-LOHEDAN
n-butyl-4-nitrobenzenesulfonate
in the
absence
of
surfactanta.
[Salt]H
avalues
Table
II.
of
Reaction
NaCl
NaBr
0.02
4.9
5.2
0.08
5.5
6.6
0.15
6.2
7.9
0.2
6.7
9.5
lo5 k ws-
of
at
25°C.
In water
105kW
n-butyl-4-bromobenzenesulfonate
= 4.6 s-l.
in the
absence
of
surfactanta.
avalues The
rate
constant,
rni, which The the
for
The
nonionic
bility
The
k9J the
when
i.e.
of
to
the
X-
a mass
0.15
2.52
2.95
0.2
2.72
3.5
k
-' at
ws
defined
ion
of
of
with
to
from
ky
= 1.85 s-l.
kW
concentration
written
as
a mole
ratio,
is p. rate
data
therefore
involves
and
micelles,
which
Br-
in the
water
cannot
or
be measured
but
that
KS
aqueous
with
= lo2 Pi-l, solution
for may
pseudophase
because binding
both
CTACl
we
of
on KS
and
pseudophase.
the
increase
and
estimation
depends
micellar
directly
comparison
surfactant
the
with
, 16
is fully
[surfactant]
and some 15,16
.
increasing
action
,
lo5
of of and
I:,, by
considered
reaction similar,
2-9,18
CTABr
However,
concentration micellar
bound
aqueous and nicellar 12-14,18 model K' = X
CX,
for
sulfonic
l/LX,
reactions 15 can
acids when
the
some
this
POSSI-
pseudoehase
can
1
reactive
analyzed
reactive
of nucleophile 12,14,15,26, but
1 (CD,]-[X,1
in be
ion
the be
ion
on
is OH-
the or
surfactant
or
distribution
f tted
....
.
' salting
data5~g*14~18.
is constant
between
the
ion,
In water
using
of Cl-
suggest
rate
by
between
KS
salts
25'C.
surfactant
halide
in CTACN
thatp
substrate
counterion
2.5
treatment
solutes
e.g.
increases
2.20
substrate
solutes
variation
assumotion
0.08
, IS
and
in fitting
surfactant,
2.10
constants,
addition
However,
1.95
concentration
substrates
unreactive
out"
the
0.02
k,,
of
binding
between but
of
NaBr
lo4
quantitative
estimation
NaCl
of
a reactive
distribution
[SaltIf!
to eq.
F-,
even of 8,
. . . . . . . . . . . . (8)
Hahde Ions m the muAlar pseudophase Tab1 e
Rate
III.
and
equilibrium
constant
for
1751
reaction
in
-1
CTABra.
Medium
Substrate
Kstl-T
CTASr
la
115
10.2
14.4
0.66
0.04 CTABr
+ NaBr
lo4 k,,s
105 k;l&'
k;IkW
la
130
10.3
14.5
0.66
CTABr
lb
120
7.5
10.5
1.1
0.04 CTABr + NaDr
lb
135
7.3
10.4
1.1
atlodelI assumes a constant p of 0.8. The fit assumes a salt effect on substrate binding, see text.
Table
Rate and equilibrium constant for reaction in CTACla
IV.
tledium
K;t:-1
Substrate
KsM-1
104k,,s-1
105kmtq-1 s-l 2 I
k; I klJ
CiACl
la
230
115
2.9
4.1
0.91
CTACl + 0.114 RaCl
la
230
130
30
4.2
0.93
CTACl
lb
215
125
2.1
2.9
0.4
CTACl + O.lM NaCl
lb
215
135
2.0
2.8
0.4
aUe used modelil which is a mass action treatment with variablef
Equation
6 predicts
counterions much
will
with
[surfactant]
constantprs
to micelles
mlcelles,
2
assumed with
,we
binding
of
but
treating
a wide
apply
increasing
ctions gives
in
over
relatively
In CTACl.
range
this
halide The
fraction,P
with
is
Ki
large,
it may
our of
fail
data
e.g. an
for
we
or
the
hydrophilic
anions
two
,
we
the
Br-,
binds
~111
less
binds
use of
this
applied
(i)
of
than
that as
a
Ejr-
/
is
is oft& Increases
to micellar
approximation
a constant,&
by vary
strongly
strongly
concentration
a model
not
approximation
which
in CTABr.{li)that,/
on
that
Thus
. as
ion
reactlons
is
neutralized
possibilities
halide
based 12,14
assumption
, so
for that
rea-
eq.5-7
: k\, LX;1
kv =
+ K,,~,P[D,l 1 +
kU (
[X;l-
kW
CX, 1 1 + k,,K,BCD,l
([X+1-[$,I) 1
subscripts,
T denotes
the
total
. . . . . . . .. .
XSIDnl
1 +
where
I1
such
groups
although/
which
consider
concentration,
simplest
lo3
ion Cl-,
surfactant on
head
[surfacta;t]
for
possibility ion
of mlcellar
increasing
satisfactory
9,11,27.
Therefore
the
if
probably
to cationic
constant
that
increase
+
KSIDnl
+ k,,K,
[X,1
KS [D,,]
concentration
of
X-.
19)
. . . . . . . . . (10)
. . . .. ....
111th fully
micellar
(11)
1752
H. A. AL-LOHEDAN
bound
substrate
eq.
12 simplifies
to
. . . . . . . . . . . . . . (12)
kv = KMB Reaction
in CTABr:
kv
with
[CTABr]
the
rate
data
the
variations However,
value
of,9
"Salting such our
the eq.
fitted
increase
the
but
binding
to
other
combination
ky
is
with
the
small
Therefore, 28
in 0.04
we
did
tl CTABr
of
a constant@
we
could
this
model
is not
the
that
He
to
with
limits
reached
on
the rate
using
salt
is
the
variation
III.
He
parameters
inconsistent
aqueous
pseudophase.
not
halide
systems
explain
the
ion
of
note
that
provided
with
that
too
data
is
KS
by
,
precedence
small
for
a constant
increase
There
and
our
is
to
=
for
to
for
explain
reactions
of
all
Cl-
in
the
we
100
micelles,
could
be
that
HaBr
following
(1 + b[NaBr]
6,9
cf.ref. for
of
assumed
fitted
to model
increased
eq.
13,i.e.with
the
reaction
values
constants
that
satisfactory
high
rate
provided
"salts-out"
data
Some
for
parameter variation of
should the
Micellar counterion
at
b,
...........
and
18.
reactions
in CTACl eq.13,
size
growth
explain
be
8 can of
k.V
Although
in CTABr
and
e.g.
given of,&
assumption
and
CTACl+NaCl
greater
(13)
CTABr in
than
+ NaBr.
terms
of
which
40M-'
when
the
such
hydrophylic
variation ion
the
be
if
combined
in
surfactant
added
sizes
given
at
toward
larger
depends
the
and
ions
as
one
balance would
interact OH-
or
ii)
limit
any
[CTACl]
we
eq.8.
the but
and
assumed
Equation
cannot
be the
reasonable
computer
X-
CTACl
counter-ion
in any
effective
counterions
although
CTAF14'15*31,
that
8 places
reached
because
upper
limit of 1 14 limit . In pra-
103M-1.
added
of
with
pseudophase
a simole
and
distribution upon
Cl-,
and
and
and
we
with
programme were
used
able
added
to
salt
to pre-
fit
all
(figure
2)
IV.
in Table
with
k+,
beyond
and
[CTACl]
added
CTAOH
(model lower
I;: >
with
more
of
[Cl-]
in aqueous
[X-l
with
with
.
concentration
with
micellar the
12
increases
observed
although
present
reaction
interaction,
gest
the halide
1 on ,&,
little
CTACl
CTAoAc
counterion
5 and
a distribution micelles
To
0 and
variation
the
in CTAFor,
:
for
apparently
increasing
vary
data
thatp
that
counterion
only
would
Equations the
than
that
is always
cticep
dict
i)
conclude
in CTACl
increases
formal
is
these
added
oseudophase,
rate
using
is smaller
is close
there
the
without
added
substrates
NaBr
aqueous
(midel
therefore
Reactions ,3
the
fit
of
fits
reasonable.
effect it
not
i)
in Table
is that
+ NaBr
constant,& of
assuming
from
(model
parameters
in other
KS
substrate
the
.
constant
b = 12M-I,
of
from
it
assuming
l),
terms
possibility
substrate
reaction
a constant,9
in
of
One
a constantp
For
12 with
1)
smallg'lO.
B-11.
effects.
of
I (figure the
be
are
effect5'g,
salt
terms
can
out"
an
Equation (figure
the F-.
is not
unreasonable
surfactant
attracting
counterions
and
it should
also
micelles, of
expect least
because there 29 . The larger
solution
surfactant-surfactant variation with
the
head
added
salt
increase
and
of micellar mlcellar
if
is
.
surfactant-
size
to be
groups
lar-
as with
Halide IOIISin the mvxllar pseudophase Comparison ctions that
of
is
binds
can
beacuse
to
fitted
and
CTACl:
with
profiles
for
The
rate
assumption
approximation
cationic
is
micelles,
to micellesg911a27.
Br-
surfactant
it
we
in CTACl
for
fails
use
surfactant
constant
satisfactory but
Therefore
reactions
of
an
for
a mass
profiles
(Table
ion
Cl-
for
as
Br-
binds
model
rea-
IIIand figurel)
such
which
action
IV and
(Table
p
to
which
less
strongly
treat
figureZ).The
the
values
ClThe
for
Br-
or
should
values
reaction
of in
dimensions5'31. tration
as
for
liter
KS are
The
second-order ratio
which
g93o.
Therefore,
we
assume the
pseudophases,
constant anion
be that
of
second-order
Stern
rate
The
values
of
kl for
those
of
kN
with
coincidental However,
of
because
overall 2,18 Stern-Layer
the
it
the
a different
reach
=
kinetic
groups
(eq.
as moles
upon
of the
of
total
with
IV).
our
of
of
reaction
Stern
Layer
close
concen-
7) where
nucleophile
per
These
reaction
molar
would
in
almost
rate
the
mice-
volume
ident ical
is probab 1Y of
the
aporoximately be
The
.
are
similarity
probably 18
of
. . . . . . . . . . . . . (14)
estimated
element
different
a molar volume of ca. 0.14L5, -ls-1 is given by I1
kz,
The
is
have
solution.
0.14 kM
micelle
the
as
Layer,
micellar
and
head
element
conclusion
little
SternLayer.
twice
affected
similarity
of
that
of
by choice
k2 m and
k,, is
both
substrate and halide ion should be located near the 2,?4 surface . There are many examole of reactions for which
constants
in constants
solvent
terms
volume
overall
because rate
differences
the
III
deoends
in
to micellar
our
, so
micellar
second-order
in
(Table
volume
volume
understandable, water
reactions
in water
k,i is expressed
constant,
k;
constants
kN,
is approximated the
rate
k,i and
conventionally,
which
by defining
to
second-order
model.
is written
compared
The
in both rate
such
constants.
aqueous
reactive
pseudophase
be
lles,
and
of
counterion
can
binding
similar
micellar
aqueous
constants
larger
the
a molar
kH the of
have
K;
M-l
Cl-
and
rate
of
and t$= 40 (Table IV) are reasonable in comparison with KdH =55M-l -1 12 K;,, = 8OM ion for formate and acetate ion , because less hydrophilic
for
as
in CTABr
be
this
strongly
than
as
Reaction
in CTABr
1753
or to
in micelles
are
probably
a different
are due
similar
to
location
the
of
to
those
in water
properties
the
two
of
reactants
the
and
the
micelle
in the
as
a
micelle18.
EXPERIMENTAL The Materials: -.benzenesulfonate
:
Kinetics by
the
255
nm
adding all in a
All
preparation and the
decrease for
reactions
or
HCl
substrate
cuvette
so
contain
Acknowledgement:
Suoport
University
so
that
less seconds.
Saud
were
was
in a reciProca1
King
of
at
surfactant
followed 252
nm
added the
than
of
there
this
is greatly
a solution
IieCH. The
work
rate
constant
contribution
concentration
3.1%
and a standard
by the
acknowledged.
n-butyl-4-bromomethods 5.21
.
in water
at 25°C
n-butyl-4-nitrobenzenesulfonate The
is no as
followed
spectrophotometrically
for
n-butyl-4-bromobenzenesulfonate.
reactions lcm
n-butyl-4-nitrobenzenesulfonate
of absorbance
IO-S/i HBr
solutions
of
nurification
from
in MeCN of
the
first-order
Research
did
not
reaction
to the
substrate rate
Center,
and change
with
reaction was
College
For
mixture
10 -5H
constant,
by
OH-.
ky
and
the
,
are
of Science,
1754
H
A. AL-LOHEDAN REFERENCES
1. Cordes,E.H., Pure Appl. Chem., 1978,50,617. 2. Romsted,L.S. in "llicellizations, solubilization and Ilicroemulsions", Mittal, K.L. ed..Plenum.New York.1977.Vo1.2.n.509. Bunton,C:A.,Catal.Rev.Sc~.,Eno.,197~;83,680. Chemistry of Surfactant", Mitta1,K.L. ed.,Plenum,New i: Bunton,C.A. in "Solution York,1979,Vol.2,p.519. Al-Lohedan,H.A.,Ph.D. thesis, University of California,Santa Barbara,l981. Gulf Scient.Res.Eath.Phys.Sci.,l977,A5;2),p.l91. i: Al-Lohedan,H.A.,Arab Martinek,K.,Yatsimirski,A.K.,Levashov,A.V.,Berezln,I.V., In ref.2, p.489. 7. a. Cuccovia,I.M.,Schroter,E.H.,~~onterio,P.I1.,Chaimovich,H.,J.Org.Chem.,1978,43, 2248. Bunton,C.A., Romsted,L.S., J.Phys.Chem.,1981.85,2123. 9. Al-Lohedan,H.A., Al-Lohedan,H.A., Bunton,C.A., Romsted,L.S., J.Org.Chem.,1982.47,3528. Chaimovich,H., J.Phys.Chem., 1979,83,1844. Quina,F.H.. Bunton,C.A., Romsted,L.S., J.Org.Chem., 19C2,7,1160. 12. Al-Lohedan,H.A., Tetrahedron,l987,Vol.43,No.2,345. Al-Lohedan,H.A., Gan,L.H., Meffot,J.R., Romsted,L.S., Savalli,G., J.Phys.Chem., ::: Bunton,C.A., 1981, D5,4118. Bunton,C.A., Romsted,L.S., Savelli,G., J.Am.Chem.Soc.,1979,101,1253. Romsted,L.S., Thamavit,C., J.Am.Chem.Soc.,1980,109,3900. ::: Bunton,C.A., Behaviour of Surfactants, Theoretical Romsted,L.S., in "Solution 17. Bunton,C.A., and Applied Aspects", Fendler,E.J., Iiittal,K.L., ed. Plenum Press, New York, 1982, vo1.:,0.975. Al-Lohedan,H.A., Bunton,C.A., iloffatt,J.R., J.Phys.Chem.,19&3,87,332. Bunton,C.A., !'lhala,N., J.Am.Chem.Soc., 1982,104,6654. it: Al-Lohedan,H.A., unpublished result . Al-Lohedan,H.A., of Practical Organic Chemistry", 3rd edition,Longman, Vogel,I., in "A Textbook 195C,p.&20. Toronto, Savedoff,L.G., Smith,S., Stevens,I.D.R., Gall,J.S., Tetrahedron 22. #instein,S.. Letter.,.1960,24. 23. Parker,A.J., J.Chem.Soc. A, 1966,220 Chem.Rev.,l969,68,1. 24. Stigter.0.. J.Phys.Chem.,l964,68,3603. 25. flenger,F.tI., Portnoy,C.E., J.Am.Chem.Soc.,l967,89,4968. 26. Bunt0n.C.A.. Franks0n.J.. R0msted.L.S.. J.Phvs.Chem..1980.84.:CD7. 27. BartetiE., 6amboa,C.,.Sepulveda,L;, J.Phys.Chem.,1980,84,i72; Gamboa,C., Sepulveda,L., Soto,R., Ibid, 1981,85,142?. 28. Preparation at hiqh concentration of Nader and CTABr limits our work. Effect" , !liley Interscience, New York,1983, 29. Tanford,C.H., in "THe Hydrophobic G-C. 2nd Ed., Chapter 30. Bunton,C.A., Carrasco,N., Nuang,S.K., Paik,C.H., Romsted,L.S., J.Am.Chem.Soc., 1978,100,5420. 31. Al-Lohedan,H.A., J.Phys.Chem., 1987,91,4524.
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