Indian Journal of ChemistryVol. 23A, May 1984, pp. 389-391

Kinetics of Oxidation of Benzhydrols by N-Bromosaccharin

V MANOHARANt & N VENKATASUBRAMANIAN*tDepartment of Chemistry, Vivekananda College, Madras 600004

Received I September 1982: revised 19 September 1983; accepted 14 December 1983

The oxidation of benzhydrol and a number of substituted benzhydrols by N-bromosaccharin (NBSA) follows total secondorder kinetics. first order each in benzhydrol and NBSA. The reaction does not exhibit any kinetic isotope effect for 0(-

deuterium substitution, but has a pronounced substituent effect (Hammett r = - 2.2). A mechanism involving apreequilibrium formation of a hypobromite ester which decomposes in a slow step via a phenonium ion accounts for all theexperimental observations.

Halogens, Nshaloamides and imides belong to thegroup of versatile agents used for the oxidation of awide variety of organic compounds 1 -3. N-Bromo-saccharin (NBSA) belongs to this group and has beenused both as an allylic brominating agent and as anoxidant". The kinetics and mechanism of oxidation ofbenzhydrol and a number of substituted benzhydrolsby NBSA has been presently studied and the results arereported in this paper.

Materials and MethodsN-Bromosaccharin was prepared" by the action of

bromine on sac:charin dissolved in aqueous sodiumhydroxide at DC. The benzhydrols were obtained by thereduction of the corresponding benzophenones.Benzhydrol-a-d was prepared by the method ofRangadurai et al. 5 and was characterised by PMR andmass spectral studies. The reactions were followedupto three half-lives by the iodometric estimation ofthe unreacted NBSA at various time intervals. Unlessotherwise stated, all reactions were carried out underpseudo-first order conditions with excess[benzhydrol]. The rate coefficients were reproduciblewithin ±3%.

Results and DiscussionThe kinetics of oxidation of benzhydrol by NBSA

was followed in binary solvent mixtures of acetic acidand water in the absence of added mineral acids. Thiswas necessitated because of the earlier observationsthat in 'aqueous acid solutions, particularly in thepresence of mineral acids like HCl04 0:' HCI,benzhydrol underwent self-ionisation to yieldbenzhydryl ether", The disappearance of NBSA withtime followed good first order kinetics as evidenced bya linear plot oflog(b -x) versus time for over 75%of the

t Department of Chemistry, Government Arts College. Nandanam,Madras.t IDL-Ni!ro Nobel Basic Research Institute. Bangalore 560003.

reaction. However the pseudo-first order constant wasfound to be dependent on the initial [NBSA].Increasing [NBSA] tended to decrease the magnitudeof the first order rate coefficient. This is indicative of aprior equilibrium involving NBSA and othernucleophilic species present (probably H20) and theformation of intermediate species with loweredreactivity. In view of this complication (which was notexplored further), all comparisons were made at thesame initial [NBSA] (Table I).

Effect oj varying [BenzhydrofJThe dependence of rate on [benzhydrol] was studied

over a wide concentration range of benzhydro I both inthe presence and absence of added Hg(I I) acetate. Thedata revealed first order dependence of rate on[benzhydrol]. The oxidation of benzhydrol by NBSAin the presence of Hg(II) acetate was faster than in itsabsence, as was also observed in the case of oxidationof aliphatic alcohols and phenylmethyl carbinols 7

.

This is indicative of the formation of a more reactive

Table I-Effect of Varying [N8SA] and [8enzhydrol] onOxidation Rate

[Temp. =30'; solvent=80% HOAc-20/,o H20; J1 =0.25 moldm "'(NaN03)]

[NBSA] [Benzhydrol] 103 kl(moldm -3) (moldm -3) (s -I)

0.001 0.02 1.150.002 0.02 1.120.004 0.02 0.6310.002 0.01 0.561 5.61"0.002 0.02 1.12 5.60'0.002 0.03 1.67 5.57"0.002 0.04 2.25 5.62"0.002 0.01 1.46 14.6b

0.002 0.02 2.89 14.5b

0.002 0.03 5.62 14.6b

(a) No Hg(OACh; and (b) in the presence of 0.02 moldm 3

Hg(OAC)2'

102 k2(dm+mol ::' s -I)

389

INDIAN J. CHEM., VOL. 23A, MAY 1984

species formed by the interaction ofNBSA and Hg(II).it is worth recalling that no such acceleration wasnoticed in the corresponding reactions of N-bromosuccinimide (NBS).

Kinetic isotope effectBenzhydrol-c-dwas oxidised almost at the same rate

as the protio isomer by NBSA. For example, under theconditions [alcohol] =0.01 mol dm -3; [NBSA]=0.002 moldm -3 and 30°C, 102 k2 values forbenzhydrol and benzhydrol-c-zi oxidations were,respectively 5.61 and 5.59 dm+mol "! s -I in 80% (vjv)aq. acetic acid and 4.21 and 4.18 dm 'mol :' s -I in 50%(v Iv) aq. acetic acid. This result is indeed surprisingbecause in the oxidation of aliphatic alcohols by Br;an isotope effect of 2.1 has been reported".

Structural effects on the oxidationThe results of oxidation of substituted benzhydrols

having substituents of varying electronegativities arepresented in Table 2. It is observed that the rate ofoxidation is quite sensitive to the nature of substituentsin the benzene ring, a p-CH3 producing a three-foldacceleration and a p-Br a two-fold retardation. AHammett plot of the reactivity pattern is shown inFig. I, the slope of which gives the Hammett p = - 2.2(correlation coefficient 0.99).

Actiiation parametersThe reaction between benzhydrol and NBSA has

been investigated in the temperature range 30 to 50.The Arrhenius parameters for the reaction in 801..HOAc-20~~ water as solvent are as follows: E; = 72.4k.lmol " ': dHt= 69.8 klrnol :": and dSt = 175 JK -I

mol :'.

Mechanism of oxidationThe two important results of the present

investigation are: (i) the complete absence of anyisotope effect; and (ii) the large negative reactionconstant (p = - 2.2). These observations, therefore,rule out the possibility of tx - CH bond cleavage in therate-determining step and indicate the generation of alargely cationic centre in the transition state of thereaction.

Table 2-Substituent Effect in the Oxidation of R - C6H4-CHOH -C6H5 by NBSA at 30° in Aq. Acetic Acid (80%,

{[alcohol] =0.02 moldm -3; [NBSA] =0.002 moldrn r '}

R I02k2 R I02k2(dm+mol r+s I) (drrr'mol -IS -I)

Hp-Fp-Cl

5.612.862.39

2.318.3515.2

390

Fig. 1=Hammett plot

An interesting mechanism that can be considered forthis reaction is an extension of the dissociativemechanism of Burton and Cheeseman" (Scheme I).

fast +-+ (Ph)2 C=N(-HBr

ho

(Ph)2C=O + Hfr,Scheme 1

Scheme 1 envisages a slow interaction between apreformed benzhydryl cation and the oxidant and hasan analogy in the mechanism of the Mann-Popereaction 9. I 0 (the oxidation of dialkyl sulphide todialkyl sulphoxide by chloramine- T), While thismechanism appears to be in accord with the twoimportant experimental observations (i and ii), itdemands a second order dependence on benzhydrol asagainst the first order dependence in the present study.Further, it is known that benzhydrol is converted intobenzhydryl ether in an aqueous acidic solvent(particularly mineral acid). However, benzhydryl etherwas not formed in the present study.

Another possibility with an activated aromaticsystem like benzhydrol is an ArSE type of reaction viaipso substitution (Scheme 2). The production of aresonance stabilised cation as illustrated would be inconsonance with the observed Hammett correlation.Also as the c-Cv- H bond is broken in a fast step, thiswould be in accordance with the kinetic isotope effectof unity.

MANOHARAN & VENKATASUBRAMANIAN: OXIDATION OF BENZHYDROLS

rQR+R

~~x- Br

~~++--+

BrH-C-OH H-C-OH H-C-OH

I I I

Ph Ph Ph

1fast

X : OH. ) N- etc. R~RPh : Ctf15 G~

C-O-H c=oI IPh Ph

Scheme 2

Yet another alternative that can be thought of is arecent proposal of Banerji and coworkers+l+-umechanistic variant of the Deno proposal'? for theoxidation of alcohols by molecular bromine. The latterproposal was based on the observation that theoxidation of propan-2-ol by Br 2 had a kinetic isotopeeffect of 2.5 and therefore involved the cleavage of theCt-CH bond in the slow step. The modification ofthe proposal by Banerji et al. was the assumption thatthis step was a fast one succeding the slow formation ofhypobromite ester.

+

RCH20H+H20Br~R-CH1-OBr+H;O (3)R-CH1-OBr->RCHO+H++Br- (4)

Banerji et al. also observed a substituent effect in theoxidation with a p value of - 1.53 with no substantialisotope effect. While being attractive, it should bestressed that it would be difficult to accomodateBanerji's modification in the reaction scheme for it isknown that step (3) can at best be only an equilibriumreaction which will have a mutually nullifyingsubstituent effect, as has been observed in otheresterification reactions of alcohols by mineral acids 13.

The Deno proposal itself, simulates a £2 type ofelimination reaction, which would demand a positive pvalue.

We therefore, wish to propose the mechanismshown in Scheme 3, which appears to satisfy all theexperimental observations. We wish to stress that thisproposal seems to be specific for the oxidation ofaromatic alcohols and may not be a universal one forthe oxidation of all alcohols. This mechanism is basedon the 'phenonium ion' proposal of Cram et al. for thereactions of p-phenylethyl systems 14 and recommendsitself convincingly because it would account for both theobserved absence of any significant isotope effect andthe substantial substituent effect. We also wish to pointout that an excellent correlation exists between therates of oxidation of benzhydrols by NBS and NBSA(Fig. 2), underscoring a. similarity between the twooxidations 15. It is difficult to choose between this

...H ....H .....H

P0°~~

Ph2)0+ ~2J-Br -- I ....

R R R

!~

Ph - C: 0 Ph-C-O

0 fast @---H+

R R

Scheme 3

H p-CH3

1·1R'~-<Q)

OH

..oJ'>CDZ

HO

o-s

3' 0·7+N

0·5

1·2 1'6 2·04+Log kl (N.BS)

Fig. 2-Plot of 4+log k2 (NBS) versus 2+logkl (NBSA)

scheme and the one based on ipso substitution. Furtherinvestigations are needed in this direction.

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organic chemistry, edited by W. Foerst (Academic Press,New York), 1964.

4 Bachhawat J M & Mathur N K, Indian J Chern, 9 (1971) 1335.5 Rangadurai A, Srinivasan V S, Thiagarajan V &

Venkatasubramanian N, Indian J Chern, 20B (1981) 898.6 Burton H & Cheeseman G N H, J chern Soc. (1953) 986.7 Manoharan V & Venkatasubramanian N, J Indian chern Soc

(accepted for publication).8 SwainCG, Wiles RA & Bader R F W, JAm chem Soc, 83(1961)

1945.9 Ruff F & Kuesman A. J chem Soc Perkin Trans ll, (1975) 509.

10 Ruff F, Kamoto K, Furnkawa N & Oae S, Tetrahedron, 32(1976) 2763.

11 Banerji K K & Mukherjee J, J org Chern, 46 (1981) 2323.12 Deno N C & Potter N H, J Am chem Soc, 89 (1967) 3555.13 K1aning U, Acta chem Scand, 11 (\957) 13\3.14 Cram D J, J Am chern Soc, 71 (\949) 3963.15 Ganesa Gopalakrishnan, Ph.D. Thesis, University of ·Madras,

1977.

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