The influence of solvent on the inductive order of substituents from infrared measurements on 4-substituted camphors: a new model of inductive effects

Abstract

4-Substituted camphors (1)–(24) have been chosen as purely inductive models of substituent effects in order to study the effect of solvent on the inductive order of substituents. The probe was the carbonyl stretching vibration, the frequency of which was measured in (i) the gas phase, (ii) non-polar solvents (heptane and carbon tetrachloride), (iii) a hydrogen-bonding acceptor solvent (pyridine), (iv) a hydrogen-bonding donor solvent (hexafluoropropan-2-ol), and (v) an amphoteric solvent (methanol). Correlation analysis shows the consistency of data for gas-phase and apolar solvents, the deviation of hydrogen-bonding donor substituents (OH, NHCO₂Et, NH₂, CO₂H, CONH₂) in pyridine, and the deviation of hydrogen-bonding acceptor substituents (all except H, alkyls, C₆H₅, CHCH₂, halogens) in hexafluoropropan-2-ol. In methanol the two kinds of deviation are so attenuated that most substituents behave inductively, in the same way as in apolar solvents. This explains the apparent, but fallacious independence of the inductive scale of substituents (defined mainly in alcoholic or aqueous media) on the solvent.