Indian Journal of ChemistryVol. 28A, October 1989, pp. 904-905

Kinetics of pyridinium fluorochromate oxi-dation of benzyl ethers in acetic acid -

medium

A Jagathesan, K Nambi & S J Arulraj*( Post Graduate Department of Chemistry, The Autonomous St.

Joseph's College, Tiruchirapalli 620002and

K A Basheer AhamedPost-Graduate Department of Chemistry, Jamal Mohamed Col-

lege, TiruchirapalJi 620020

Received 14.$eptember 1988; revised 9 December 1988;accepted 6 February 1989

Oxidation of benzyl ethers by pyridinium fluorochrom-ate (PFC) in aq. acetic acid is first order each in [benzylether], [PFC] and [H +]. Decrease in dielectric constant ofthe medium increases the rate and added NaCl decreasesit. The order of reactivities among the substituted benzylethers is: jrmethoxy > jrmethyl > benzyl ether > P:chloro > jrnitro.

Kinetics of oxidation of primary and secondary alco-hols 1, n-glucose/, aliphatic alcohols! and substitutedbenzyl alcohols" by pyridinium fluorochromate(PFC) are well documented. However the kinetics ofoxidation of benzyl ethers by PFC has not been re-ported so far and hence the title investigation.

ExperimentalAll the aromatic ethers used namely, P:

methoxybenzyl, Jrmethylbenzyl, benzyl, p-chlorobenzyl and Jrnitrobenzyl ethers were of ARgrade and were distilled or recrystallised before use.PFC was prepared in the laboratory and its purity waschecked by the usual methods I.

The reaction was carried out under pseudo-first or-der condi tions ([benzyl ether] ~ [PFCJj in glacial acet-ic acid containing sulphuric acid, the latter beingadded to maintain [H +]at 0.09 mol dm - 3. The courseof the reaction was followed iodometrically and therate constants, computed from the linear (r > 0.98)plots of log (titre) against time by least squares meth-od, were reproducible within ± 3%. Stoichiometricinvestigations revealed that one mole of benzyl etherrequired one mole ofPFC to yield the correspondingbenzaldehyde (fully identified by converting into 2,4-DNP derivative and then comparing with an authen-tic sample) and benzoic acid.

Results and discussionAt fixed [H +] with] substrate] in excess, the plot of

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log (titre) versus time is linear, indicating first order in[PFC].Plots oflog kobs versus log [benzyl ether] are lin-ear with unit slopes for all the substrates and a plot ofkohs versus [benzyl ether] also is linear passing throughthe origin indicating a first order dependence on [sub-strate]. The reaction is acid-catalysed and proceeds

/ with a brief induction period at low temperature. Thedependence on [acid] is unity. These observationslead to rate law (1 )

- u{PFC]/dt = k[benzyl ether] [PFC] [H +] ... (1)

At fixed ionic strength and [H +],the rates of oxidationof benzyl ethers with PFC increase with the decreasein polarity of the solvent. For instance under the con-ditions: [benzyl ether] = 1 x 10 - 2mol dm - 3,[PFC]= 1.5 x 1O-3moldm-3,[H+]=0.09moldm-3,temp. = 313 K, 102kz increased from 0.52 to 1.30 forbenzyl ether when % HOAc (v/v) was changed from70 to 90. In other words a decrease in the dielectricconstant of the medium increases the rate. This is dueto the less polar character of the transition state com-pared to the reactants. The plot oflog kz versus 1/D islinear with a positive slope indicating that the reactionfollows an ionic mechanism between a positive ionand a dipole. '

Increase in [H +]increases the rate of oxidation. Forexample under the conditions: [benzyl ether] =1 x 1O-2moldm-3,[PFC]= 1.5 x 1O-3moldm-3andtemp. = 313 K, 104 kobs values were found to be 0.80,1.30,4.01 and 22.57 at 1O[H+] = 0.09, 0.18, 0.36 and0.72 mol dm - 3 respectively. A plot of (1 + log kobs)

against 1 +10g[H+] does not give an ideal slope of un-ity, showing that the reaction is simply an acid-cata- .lysed one and H + ions are not involved in the rate-de-termining step. Added sodium chloride decreases therate. The rate constants were measured at 313, 323and 333 K for benzyl ether- PFC reaction. The activa-

Table I-Effect of substituents on rate constants [benzyletherJ= 1 x Io-'mol dm-'; [PFCj= 1.5 x JO-'mol dm-3;

[H,S04J= O.OYM; solvent = HOAc (YO% v/v); temp. = 313 K

Substituents a 102k2(drrr'mot r ts :")

p-CHP -0.27 4.48

p-CH3 -0.17 1.29

p-H 0.00 1.02

p-Cl 0.24 0.83p-N02 0.78 0.39

CIS HI! CHOCH2 CIS HI! T

IH

+C"HI!CHOCH2C"He +

+ H20 +C. H a CHz ""FciIt" C. Ha CHz OH + H

Scheme 1

tion parameters are: l!.H+ =63.97 kJ mol-I; andl!.S+ = -77.49 J deg-'mol-'.

A plot of log ~ versus a for benzyl ethers is linear

NOTES

with a slope of - 0.52, revealing that the reaction pro-ceeds through carbonium ion mechanism. (Table 1).

A large increase in rate with increase in [H+] sug-gests that protonated Cr6+ may be involved' in therate-determining step. Thus a mechanism involvinghydride ion transfer in the rate determining step canbe postulated for the PFC oxidation" - 8 (Scheme 1).

While electron releasing substituents increase therate of oxidation, electron withdrawing substituentsdecrease it considerably suggesting that the forma-tion of carbonium ion is destabilized in the case ofelectron withdrawing substituents.

AcknowledgementThe authors thank the authorities of the Autono-

mous St.Joseph's College for providing facilities. Oneof the authors (K A B) thanks the authorities of JamalMohamed College for permission to carry out thiswork.

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