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: