A quantitative description of fundamental polar reaction types. Proton- and hydride-transfer reactions connecting alcohols and carbonyl compounds in the gas phase

Abstract

Four simple but fundamental gas-phase reactions involving proton and hydride additions or extrusions, and comprising a complete thermochemical cycle, have been studied by correlation analyses. The species of interest were alcohols (proton and hydride extrusions, to alkoxide anion and hydroxycarbenium ion, respectively), and aldehydes and ketones (proton and hydride additions, to give the same ionic products as formed from the alcohols). The heats of these reactions can be described accurately by linear two- or three-parameter equations. Residual electronegativity is a readily calculated property which reflects the ease of stabilisation of charge by inductive-field effects. Effective polarisability quantifies the intramolecular stabilisation of charge by interaction between the charge centre and induced dipoles in the substituents. The third parameter reflects hyperconjugation influences by C–H and C–C bonds. The statistical models are internally consistent among the four reactions in reflecting the various physical influences on the reaction enthalpies. Electronegativity is responsible for roughly the same amount of stabilisation in all four reactions. Substituent polarisability is more effective at stabilising positive charge than negative. This generalisation applies to saturated and unsaturated cations (protonated alcohols–ethers, and protonated ketones–aldehydes, respectively). The analyses also reflect the degree to which bonds hyperconjugate with the CO bond, as well as with adjacent charged, unsaturated centres.