SPECTROSCOPIC STUDIES OF THE REVERSIBLE REACTION BETWEEN 2,2-DIPHENYL-1-PICRYLHYDRAZYL AND 2,4,6-TRI-t-BUTYLPHENOL

Abstract

A B S T R A C T T h e reaction between 2,2-diphenyl-1-picrylhydrazyl ( D P P H ) and 2,4,6-tri-t-butylphe~lol in benzene solution has been studied b y optical and electron spin resonance methods. I n f h e absence o f oxygen, equilibrium is reached with t h e products 2,2-diphenyl-1-picryll~ydrame and t h e tri-t-butylphenoxy radical. T h e equilibriuln constant for t h e reaction isT h e corresponding activatlon energies are 6.5 f 0.8 kcal/mole and 7.7 f 0.7 kcal/mole. T h e rate constant for t h e reverse reaction is approximately 2 l/(mole s ) a t 20 OC and t h e activation energy is 4.8 f 1 kcal/mole. I N T R O D U C T I O N2,2-Diphenyl-1-picrylhydrazyl (DPPH) reacts with a wide range of hydrogen donors including aliphatic thiols ( I ) , aromatic amines (2, 3), hydroaromatic co~npounds (4, 5), and phenols (6,7,8). The major product of these reactions is 2,2-diphenyl-1-picrylhydrazine (DPPH-H) and kinetic studies have shown that in reactions with phenols the rate-determining step involves hydrogen abstraction fro111 the OH group (7,8). Since DPPI-I is a free radical and the solvent e.g. benzene, is of low polarity, it is probable that the rate-determining step can be represented by eq. [I].The reactions with phenols are, however, quite sensitive to inductive effects of inetasubstituents, and changes in para-substituents can alter the rate constants a t 30 O C by factors as great as 20 000. IliIcGowan, Powell, and Raw (6) have compared the effect of substituents on the rate of hydrogen abstraction by DPPI-I from phenols with their effect on the rates of solvolysis of dimethylbenzyl chlorides and have suggested that DPPH inay abstract hydride ions rather than hydrogen atoms from phenols.The reactions of DPPH with aromatic amines (3), phenols (7), and acetic acid and ethanol (9) are retarded by the product DPPI-I-H. The retardation of the reactions with anlines and phenols has been explained in terms of the reversibility of the rate-determining step [:I.]. Pro11 and Sutcliffe (9) argue that the hydrogen abstraction process is not reversible when acetic acid and ethanol act as donors, and state that retardation is due solely to complex formation between DPPH and DPPI-I-H. If their explanation is correct it should also apply to the reactions of D P P H with anlines and phenols. The radical nature of the reaction between DPPI-I and phenols a d the reversibility of the primary step can be most simply studied if a stable free radical X -is produced in

Kinetic, Electron Spin Resonance, and Product Studies of the Reaction Between 2,2-Diphenyl-1-picrylhydrazyl and 2,4,6- Tri-t-butylbenzenethiol

“…The reaction is subject to a deuterium isotope effect of 2.0, showing that the hydrogen abstraction reaction 1 is a rate controlling process. The retardation observed on addition of DPPH-H affords evidence that reaction 1 is reversible (5,6). Retardation by the product DPPH-H in normal reaction mixtures is small provided that observations are limited to the initial stages of the reaction.…”

“…photolysis or lead dioxide oxidation of TBBT in benzene or carbon tetrachloride solution. This sulfur radical is only observed at the end of the DPPH-TBBT reaction and its concentration is relatively low during the main course of the reaction, in contrast to the high concentration of 2,4,6-tri-t-butylphenoxyl which is observed in the corresponding reaction of DPPH with 2,4,6-tri-t-butylphenol (6).…”

“…Benzene solutions of DPPH and TBBT were separately degassed in an apparatus similar to that used in earlier studies (6,12). The apparatus was immersed in a thermostat for 20 min, the solutions were mixed, and the mixture transferred to an optical cell of 1 cm pathlength.…”

“…One of the few reactions of DPPH to be thoroughly investigated is the reaction with 2,4,6-tri-t-butylphenol (6). Abstraction of the phenolic hydrogen by DPPH yields DPPH-H and the 2,4,6-tri-t-butylphenoxy radical; in the absence of air, the phenoxy radical does not enter into secondary reactions.…”

A Kinetic and Electron Spin Resonance Study of the Polymerization of Vinyl Acetate Retarded by 2,4,6-Tri-t-butylphenol

“…In earlier studies (1,2) it was established that the reaction of 2,2-diphenyl-I -picrylhydrazyl (DPPH) with hindered phenols could readily be followed by e.s.r. spectroscopy.…”