Substituent–substituent interactions in carbon-13 substituent chemical shifts of phenols and phenoxides

Abstract

The non-additive behaviour of ¹³C shifts observed for 4-X-substituted phenols or phenoxides are evidence for large substituent–substituent interactions (up to 5 p.p.m.;i.e., 75% of the substituent shifts observed in monosubstituted benzenes). X-Induced para-substituent shifts on C(1) are analysed by non-linear Dual Substituent Parameter correlations or by proportionality relationships with the corresponding shifts in monosubstituted benzenes (provided that for phenoxides electron donor or acceptor groups are treated separately). Similar proportionality relationships are also established for ipso SCSs [on C(4)]. Extension of this data treatment to various families of para-substituted benzenes leads to a general approach to the effect of substituent interactions on SCS values. In the factorisation model proposed, deviations from additivity are expressed via the product of ipso and para-SCS values, derived from the corresponding monosubstituted benzenes. The strict additivity of SCS values in meta-substituted derivatives is obeyed for phenols, whereas for phenoxides minute substituent–substituent interactions are observed.