-
Indian Journal of ChemistryVol. 34A. June 1995, pp. 440-445
Micellar catalysis on the redox reactions of dialkyl sulphides
withchromium(VI)
B Sankararaj, S Rajagopal" & K PitchumaniSchool of
Chemistry. Madurai Kamaraj University. Madurai 625021
Received 12 September 1994; revised 19 December 1994; accepted
10 January 1995
Chromium(VI) oxidation of diethyl sulphide (DES),
diisopropylsulphide(DPS), di-n-butyl sulphide(DBS) and di-t-butyl
sulphide (DTBS) in 99% acetonitrile-l % watertv/v) follows second
order kinetics,first order in each reactant. A mechanism involving
the rate determining nucleophilic attack of sulphideon Cr of H2Cr04
followed by fast ligand coupling between 0 - and S+ to form
sulphoxide as the producthas been formulated. The anionic
surfactant, sodium dodecyl sulphate, catalyses and the
cationicsurfactant, cetyltrimethylammonium chloride, inhibits the
rate of oxidation which is explained by theproposal that the
reaction takes place both in aqueous and micellar phases. The
catalytic role of H + anddevelopment of positive charge on sulphur
due to the electron transfer from sulphide to Cr(VI) favour
thereaction in the anionic micelle and disfavour in the cationic
micelle.
The uncatalysed and picolinic acid (PA) catalysedchromium (VI)
oxidation of alkyl aryl and diarylsulphides in aqueous acetic acid
and aqueousacetonitrile have been extensively studied andreported
from this laboratory!" 5. Though a largenumber of reports are
available on the oxidation oforganic sulphides with particular
focus on thesynthetic applications and mechanisms of thereactions"
-14, micellar effect on the redox reactionsof organic sulphides has
not been studied so far exceptthe recent reports by Buntou'
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SANKARARAJ et al.: MICEllAR CATALYSED Cr(VI) OXIDATION OF
DIALKYL SULPHIDES 441
with R2S and estimating the unconsumed oxidant.The stoichiometry
ofthe reaction was found to be inthe ratio 2:3 for Cr(VI) and R2S.
To analyse theproduct of the reaction, Cr(VI) ana R2S were taken
inthc range of 5 x \0-4-2 X \0-3 mol dm "> in thepresence of 0.5
mol dm - 3 H+. After the completion ofthe reaction, the contents
were extracted withchloroform and dried over anhydrous MgS04•
Theproducts were analysed by TLC as well as IRspectroscopy by
comparing them with the authenticsamples of the corresponding
sulphoxide andsulphone. The major product of the reaction
wasidentified as diethyl suiph oxide when diethylsulphide was taken
as the substrate. Thus the overallreaction can be represented by
Eq. 1.
2H2Cr04 + 3R2S -+ 3R2SO + 2HCr02 +H20 ... (1)
Results and Discussion
Cr(Vl) oxidation of dialkyl sulphides in micellar freemedium
The Cr(VI) oxidation of all four dialkyl sulphides,diethyl
sulphide (DES), diisopropyl sulphide (DPS),dibutyl sulphide (DBS)
and ditertiarybutyl sulphide(DTBS) follow second order kinetics,
first order eachin the oxidant and sulphide. This is confirmed
fromthe linear plots of log optical density versus time,constant k\
values at different [Cr(VI)], linear plots ofk, versus [sulphide]
and from the unit slope observedfrom the log-log plot of k, and
[sulphide]. As differentacids, perchloric and sulphuric acids, have
to be usedin the anionic and cationic micellar media tocircumvent
the solubility problem, the oxidationreaction has been studied at
different [HCI04] and[H2S04] and the relevant z, data are given in
Table I.The redox reaction is catalysed by both HCI04 andH2S04, but
the catalysis is more in HCI04 than inH2S04• The order in H + is
unity in H2S04 and morethan one in HCI04. The rate of the reaction
is notaffected appreciably by the change in the ionicstrength of
the medium (Table 1).The x, values at theionic strengths of 0.5 mol
dm - 3 and 1 mol dm - 3 bykeeping all other conditions similar are
1.603 and1.602 x 10- 3 S-I respectively. The reaction is
highlysensitive to the change in solvent composition (Table2). The
increase in acetonitrile content of the mediumupto 30% decreases
the rate but further increaseincreases the rate. This peculiar
effect indicates thatdifferent mechanisms are operating in aqueous
andacetonitrile media. We have postulated in our earlierreport that
an electron transfer from sulphide toCr(VI) takes place in the rate
determining step in the
Table l-[H+]dependence ofCr(VI) oxidation of DES at 298
K[er(VI)]: 1.0 x 10~4moldm-3,[DES]: 1.5 x 1O-3moldm-3,
Solvent: 1% acetonitrile-99% water
[H+] (mol dm-3)kl x 105(s-I)[H+] (mol dm-3)kl x 105(S-I)(H2S04)
(HCI04)
0.01 1.76 0.050.05 3.74 0.100.10 6.68 0.200.20 16.9 0.500.40
69.6 0.500.50 96.5
tIn the presence of 0.5 mol dm - 3 NaCl04
4.097.35
22.5160.0160.0t
Table 2 Solvent effect in the Cr(VI) oxidation of
dialkylsulphides at 298 K
[Cr(VI)]: 1.0x 1O-4moldm-3,[DES]: 1.5 x 1O-3moldm-3,[H+]: 0.56 N
H2S04
kx 104 (s-I) % CH3CN9.65 407.47 506.60 605.20 70.UO 80
% CH3CNI5
102030
kXI04(s-l)4.025.327.40
19.082.2
Cr(VI) oxidation of aryl methyl and diphenylsulphides'. The
increase in k, value with the decreaseia acetonitrile from 30% to
I% leads to the inferencethat a charge separated transition state
is formedwhen the composi tion of water in the medi urn is
high.These results are similar to the observations made inthe
oxidation of organic sulphides withperoxodisulphate ion!",
peroxodiphosphate ion'",phenyliodosodiacetate-", periodate
ion?",Nsbrornoacetamide", and permanganate ion 8. Thissolvent
effect is in favour of the nucleophilic attack ofsulphides on the
chromium of the oxidant. The k2values for DES, DPS, DBS and DTBS in
1%acetonitrile-99% water (v/v) indicate that thereaction is more
controlled by steric effect rather thanpolar effect (Tables 3 and
4). If the polar effect is thepredominant factor operating in this
reaction, the k2values should be in the order DES < DBS < DPS
<DTBS. But the observed rate constants are just in theopposite
order i.e. in the order 'of steric substituentconstant, Es. Hence
the k2 values are analysed in termsof the Taft's equation 2 (ref.
22) and the correlation oflog k2
logk = logko + E, .... (2)
with E, is good. The results of the correlation analysisfor the
k2 values observed in perchloric acid andsulphuric acid media are
given in Eqs 3 and 4respectively.
-
442 INDIAN J CHEM, SEe. A, JUNE 1995
log k = log ko - 0.937 E,(r=0.982 n=4)logk = logko -0.817
E,(r=0.962 n=4)
These solvent and substituent effects lead us topostulate the
mechanism in Scheme I for the Cr(VI)oxidation of dialkyl sulphides
in aqueous mediumcontaining small % of acetonitrile.
... (3)
... (4)
o 0R2S + ~cr k2" SlOW.
HO OH
. .. (5)
HO OH\ /
O.Cr-O-Is:
R' R
Ligond coupling. RzSO + Ht=r~ . .. (6 )010- and S, fosl
,~.
Scheme 1
Thus the reaction is initiated by the rate
controllingnucleophilic attack of sulphide on chromiumfollowed by
fast ligand coupling of 0 - and S+ in thehypervalent intermediate
state yielding the productssulphoxide and Cr(IV). As the
intermediate Cr(lV) ismore reactive than Cr(VI)I,3, it oxidises
sulphidesrapidly to form sulphoxides. Ligand coupling is a
wellestablished phenomenon+' and we have postulatedligand coupling
mechanism in the picolinic acidcatalysed Cr(VI) oxidation of aryl
methyl anddiphenyl sulphides and N-alkyl
substitutedphenathiazines+". The acid catalysis of the reactionalso
supports the mechanism as protonation of theoxidant makes it more
electrophilic. The differentsolvent effect observed here cannot be
taken as aconclusive evidence for different mechanismsoperating in
aqueous and organic solvent media.However, kinetic methods will not
be able todistinguish the two types of rate determining steps inthe
Cr(VI) oxidation of organic sulphides-nucleophilic attack of
sulphide on chromium (SN2) orsingle electron transfer (SET) from
sulphide to Cr inH2Cr04. That the mechanism of oxidation may be
acontinuum between these two extremes - singleelectron transfer or
nucleophilic substitution-hasbeen suggested by Bruice in the
hydroperoxideoxidation of organic sulphides!". Further Prosshas
established that the SN2",SETcontinuum has general
significance->. RecentlyBaciocchi et al}4 proposed electron
transfermechanism for Ce(IV) oxidation of aryl alkylsulphides. The
fact that alkyl aryl sulphides aresignificantly more reactive than
dialkyl sulphides has
been taken as evidence for the operation of electrontransfer
mechanism. In the Cr(VI) oxidation oforganic sulphides, dialkyl
sulphides are morereactive than alkyl aryl sulphides whichis in
supportof nucleophilic substitution of the substrate on thechro~ium
of H2CrO.i6• The k2 values for diethylsulphide and ethyl phenyl
sulphide under similarconditions are 1.14 and 0.53 dm" mol-I
S-Irespectively. However, this controversy over themechanism of the
reaction will not affect ourdiscussion on micellar effect on these
redoxreactions.
Cr(Vl) oxidation of dialkyl sulphides in the presence ofionic
surfactants
The kinetics of Cr(VI) oxidation of dialkylsulphides in sodium
dodecyl sulphate (SDS)and cetyltrimethylammonium chloride
(CTAC)follow total second order-first order each in Cr(VI)and
sulphide. The first order dependence of thereaction on the
substrate is evident from the linearplots of k, versus [sulphide]
for all sulphides in thepresence of H2S04 as well as HCI04. Thus
them~hanism proposed in micellar free medium isapplicable to the
Cr(VI) oxidation of dialkylsulphides in anionic and cationic
micellar media also.The effect of changing [SDS] and [CTAC] onthe
rateconstant of the redox reaction is shown in Tables 3and 4
respectively.
Effect of anionic surfactant, SDS, on the Cr(VI)oxidation of
dialkyl sulphides+I t is interesting to notethat the redox
reactions of all dialkyl sulphides arecatalysed by the anionic
surfactant, SDS, but thecatalytic activity of the surfactant
increases withincrease in the hydrophobicity of the substrate.
Ifwecompare the ratio kmic/ kaq for the different sulphides,the
value increases in the order DES (1.9) < DPS (3.3)
Table 3-Effect of SDS on the Cr(VI) oxidation of
dialkylsulphides at 298 K
[Cr(VT)]: 1.0 x 10-4moldm-l,[sulphide]: 1.0 x 1O-3moldm-J[H+]:
0.5 mol dm-J HCI04; solvent: 1% acetonitrile-99% water
[SDS] k2 (drn ' mol-1 S-l)(mol dm - 3) DES DPS DBS
~OO 1.00 ~n O.W0.01 1.02 0.50 1.230.02 1.25 0.59 1.42O.~ I.~ O.N
I.~0.06 1.56 0.76 1.800.08 1.57 0.79 1.94O. JO 1.74 0.86 1.710.20
2.01 0.89 1.700.25 1.74 0.82 1.750.30 1.49
DTBS0.050.170.200.230.250.260.280.320.340.44
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SANKARARAJ et al.: MICELLAR CATALYSED Cr{VI) OXIDATION OF
DIALKYL SULPHIDES 443
Table 4-Effect of CTAC on the Cr(VI) oxidation of
dialkylsulphides at 298 K
[Cr(VI)]:1 x 10-4 mol dm-3; [sulphide]: I x 1O--3moldm-3;[H+] :
0.56 N H2S04: solvent = 1% acetonitrile-99% water
[CTAC] k2 (dm- mol "! S-I)(mol dm-3) DES DPS DBS
0.000 0.64 0.152 0.1960.002 0.60 0.136 0.3500.004 0.47 0.132
0.2700.006 0.37 0.125 0.2200.008 033 0.107 0.1850.010 0.29 0.060
0.1600.015 0.055 0.1200.020 0.15 0.049 0.0900.0250.0300_040
DTBS0.0220.0490.0420.0280.0270.0280.0310.0250.025
0.0780.071
Ii
< DBS (6.0) < DTBS (8.0) which is almost in theorder of
hydrophobicity of the sulphides. RecentlyBunton and coworkers, from
the study of micellareffect on the periodate ion oxidation of
organicsulphides!", have established that dialkyl sulphidesare
solubilized efficiently by SDS and the bindingconstant for dipropyl
sulphide is approximately 100dm'' mol".'. One can very well assume
that the bindingconstant of dialkyl sulphides increases in the
orderDES < DPS < DBS ~ DTBS.
As we observe micellar catalysis in this reaction,the binding
constants cannot be evaluated by kineticmethods. As the reactions
have been studied at high[H+], the active Cr(VI) species under the
presentexperimental conditions is the neutral chromicacid 17. The
neutral oxidant may be partitionedbetween aqueous and micellar
phases. The binding ofH2Cr04 to the SDS micelle has already
beenestablished by Panigrahi et al?7. The micellarcatalysis
observed in the present study is very similarto the influence of
SDS micelles on the oxidation ofalcohol by chromic acid but
contrary to the Cr(VI)oxidation of malic acid and 104 oxidation of
organicsulphides. These kinetic results can be explained bymeans of
pseudo phase ion exchange (PIE) model asapplied for the Cr(VI)
oxidation of alcohol. The PIEmodel treats the micellar and aqueous
phases as twodistinct phases and in the present case the
redoxreaction occurs in both phases. The rate accelerationobserved
in this reaction is due to the higher localconcentrations of both
reactants at the micelle-waterinterphase as compared to their
stoichiometricconcentrations and the change in polarity of
themedium with the addition of surfactants to theaqueous
system.
Though the reactions have beencarried out at high[H +] (0.5 mol
dm - 3) and most of the Cr(VI) is in thechromic acid form, H2Cr04
will be in equilibrium
with the ionised form, HCrO-4 as shown in Eq. (8).
H2Cr04 ~ HCr04- + H+ ... (8)The anion. HCrOi can be considered
as
nondistributed in anionic micellar phase because ofthe repulsion
from the negatively charged micellarsurface. The reaction at the
micelle-water interphasecan be discussed by considering the
exchangeequilibrium between H+ and Na + at the micellarsurface, Eq.
(9)
... (9)
The ion exchange equilbrium constant, ~a, can bedefined (Eq. 10)
for the above equilibrium reactionas
~a = [H;] [Na~] / [H~] [Na;] ... (10)
In Eq. (10) [H;], [Na;], [H;;] and [Na;;] represent thereactive
ions and the micellar counter ions in aqueousand micellar phases
respectively. The value of ~a isavailable from the previous studies
and is close tounityl "?". This observation leads to the
conclusionthat there is no difference between the
specificadsorption of these two ions onto the micellar surfaceand H
+ and Na + can be considered to be statisticallydistributed between
aqueous and micellar phases.The concentration ofH+ ion in the
micellar phase, [H;;]depends on ~a and the fraction of micellar
headgroups neutralised, ~= ([H;;] + [N +aml) / [Do].If ~a~ 1, [H~]
is given by Eq. (11)
[H~] = ([Ht] ~[Do])/([Ht] + [Nat)) ... (11)The value of ~ has
been determined as - 0.6 in the
previous studies using conductivity methods when[substrate] is
lowI6•27.
With this background on the PIE model, the kineticresults in the
anionic micellar phase can be analysedby assuming Scheme 2 as the
mechanism for theCr(VI) oxidation of dialkyl sulphides in
micellarphase.
Scheme 2
Thus the oxidation reaction takes place in theaqueous phase as
well as in micellar phase. Thereaction is micellar catalysed
because the localconcentration of the reactants in the small
micellarvolume is very high compared to the aqueous phase.
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444 INDIAN J CHEM, SEe. A, JUNE 1995
Again with the increase in [surfactant], concentrationof Cr(VI)
in the micellar phase, [H2Cr04]m alsoincreases which may also
partly be responsible for theincrease in kobs'Similar
explanationhas been given byDrummond and Grieser for micellar
catalysis in theH202 oxidation reactions-".
The acid catalysis of the reaction can be explainedusing Eq.
(II) that increase in [H+]increase[HmJ+ whichaccelerates the Cr(VI)
oxidation of sulphides in themicellar phase. It has already been
established in thepresent study as well as in our previous studies
thatCr(VI) oxidation of organic sulphides is ca talysed byH+ l·j\
The rate constant data given in Table 3 showthat increase in
[surfactant] increases the kobs value,but, at high [surfactant],
the rate constant attains alimiting value. As all the sulphides are
waterinsoluble, the reactions have been studied in aqueousphase
containing I% acetonitrile. Similar solventsystem has been chosen
by Blasko et al. in the 104+oxidation of organic sulphides in SDS
'arid CT ACsolutions 16. It is pertinent to point out that
anincrease in [surfactant] increases the micellarsolubilization of
the sulphide. Once all the sulphide issolubilized into the micelle,
an increase in [surfactant]increases the micellar counterions
(i.e., Na +) thatdisplace H+ ions out of the micellar surface i.e.,
at veryhigh [SDS], [H";;]decreases thereby decreasing the
rateconstant value.
The real picture on the catalytic activity of micellecan emerge
only if the solvent effect on this reaction istaken into account
(Table 2). The k., values given inTable 2 show that the increase in
acetonitrile contentof the medium decreases the rate upto 30%
andincreases slightly from 30% to 60%, but a furtherincrease in the
composition of the aprotic solventenhances the rate enormously. It
has been establishedthat addition of surfactants to the aqueous
phasedecreases the dielectric constant of the mediumappreciably and
the micellar medium may beassumed to have a dielectric constant
close to 50%aqueous organic solvent- medium-". Thus if wecompare
the kl values in I% and 50% acetonitrile, adecrease in k, value is
anticipated with the increase in[SDS]. Hence if the change in
dielectric constant withthe increase in [surfactant] is also taken
into account,the micellar catalysis (i.e., kmic/kllq) will be more
thanobserved. Thus SOS, catalyses the oxidationenormously by
concentrating the reactants at themicelle-water interface. The
catalytic activity ofmicelle increases with the increase in the
. hydrophobicity of the substrate because thepercentage of
reaction taking place in micellar phaseis a function of the
concentration of the sulphide in themicellar phase, [R2Sl\b. The
value of [R2S]mincreases
with the increase in the hydrophobicity of sulphide aswell as
the [surfactant]. Perez-Benito and Rodenas!?have observed similar
results in the Cr(VI) oxidationof water insoluble alcohols.
Effect of cationic micelle, CT A Con the oxidation ofdialkyl
sulphides-The rate constant data given inTable 4 indicate that the
effect of cationic surfactant,CT AC, on the oxidation is different
from that of theanionic surfactant. Again the effect of
increasing[CTAC] leads to different results with the change
insubstrate from less hydrophobic to morehydrophobic. An increase
of[CT AC] decreases the k2value in the case of DES and DPS,
increases initiallybut further increase in [surfactant] decreases
the rateconstant in DRS. However, in DTBS, increase in[CTAC]
increases the k2 value upto 0.002 mol dm - 3and then decreases upto
0.006 mol dm - 3 and thenremains almost constant. Further, a
decrease in k2value is comparatively rapid in DES than in DPS.
Theinhibition of the reaction n the presence of cationicsurfactant
should be explained by considering thefact that Cr(VI) oxidation of
dialkyl sulphides inmicellar free medium is catalysed by H +, the
co-ion ofthe surfactant. Bunton and coworkers 16 haveestimated the
binding constant of dipropyl sulphideto CT AC as 140 drn ' mol- 1.
Thus the organicsulphides have stronger binding towards
cationicthan anionic surfactants. Since the redox reactionsare
inhibited by the increase in [CT AC], the bindingconstants, KB, of
sulphides to the cationic surfactantcan be estimated kinetically
from the plot of l/kobsversus C where C is the [surfactant]
exceeding thecritical micellar concentration 16.29. The values of
KBfor DES, DPS and OBS respectively are 226, 137 and58 dm" mol- 1 •
These binding constant values and theinhibition of. the rate in the
presence of the cationicsurfactant led us to conclude that the
reaction occursin the bulk aqueous phase and the [sulphide]
availablein the aqueous phase increases in the order OES <DPS
< DBS. However, the situation is not so simplethat the KB values
estimated by kinetic methodcannot be taken as reliable as the
calculated KB valuesare in the opposite trend of the hydrophobicity
of thesubstrates. Here the major part of the redox reactiontakes
place in the micellar phase with residualreaction in the aqueous
phase. The overall rate of thereaction is inhibited because the
approach ofH+ to themicellar surface is prevented due to the
repulsionfrom the cationic surfactant. The inhibition becomesless
with the increase in the hydrophobicity as thepercentage of
reaction occurring in the micellar phasebecomes more with more
hydrophobic substrate. It isinteresting to point out that CT AC
catalyses theoxidation ofOPS and DTBS in the high percentage of
-
SANKARARA1 et af.: MICELLAR CATALYSED Cr(VI) OXIDATION OF
DIALKYL SULPHIDES 445
acetonitrile. Further the' oxidation of an organicsulphide
involves the electron transfer from sulphideto Cr(VI) and the build
up of the positive charge onsulphur will be un favourable for
interaction withcationic micellar head groups. This argument is
alsoin support of micellar catalysis observed in thepresence of
anionic surfactants where thedevelopment of positive charge on
sulphur in R2S isfavoured due to coulombic attraction. Thus
theconclusion from the above arguments is that similarmechanism is
operating in the anionic and cationicmicelles but the inhibition in
the latter is due torepulsion of H + from. the micellar surface
andunfavourable condition due to the development ofpositive charge
on sulphide after electron transfer tothe oxidant.
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