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Acidities and homolytic bond dissociation enthalpies of 4‐substituted‐2,6‐di‐ tert ‐butylphenols
Author(s) -
Bordwell F. G.,
Zhang XianMan
Publication year - 1995
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610080803
Subject(s) - chemistry , homolysis , substituent , medicinal chemistry , bond dissociation energy , benzene , phenol , dissociation (chemistry) , phenols , polar effect , bond cleavage , delocalized electron , standard enthalpy of formation , radical , organic chemistry , catalysis
Abstract Equilibrium acidities and estimates of homolytic bond dissociation enthalpies ( BDE s) in DMSO of the O–H bondsfor α‐tocopherol, 2,6‐di‐ tert ‐butylphenol, six 4‐substituted‐2,6‐di‐ tert ‐butylphenols, and ten related phenols are reported. The presence of a 2,6‐di‐ tert ‐butyl group in a phenol increases its acidity and makes substituent effects on the acidity caused by a para electron‐withdrawing group larger. The BDE s of the O–H bonds in 2,4,6‐tri‐ tert ‐butylphenol, 4‐methoxy‐2,6‐di‐ tert ‐butylphenol, 4‐methyl‐2,6‐di‐ tert ‐butylphenol and α‐tocopherol, estimated by combining their p K HA values with the oxidation potentials of the conjugate anions, E ox (A − ), according to the equation BDE = 1·37p K HA + 23·1 E ox (A − ) + C have been found to agree to within 2 kcal mol −1 with literature values (1 kcal = 4·184 kJ). Introduction of 2,6‐di‐ tert ‐butyl groups into phenol and six 4‐substituted phenols weakens the O–H bonds by amounts ranging from 3·6 to 10·3 kcal mol −1 . These effects are attributed to increases in ground‐state energies which introduce strains that are relieved when homolytic cleavage of the O–H bond forms an oxygen‐centered radical where the odd electron can be delocalized into the benzene ring.