Proton magnetic resonance spectra of protonated dialkyl ketones: (Dialkylhydroxycarbonium ions)

Proton magnetic resonance spectra of eight protonated dialkylketones (dialkylhydroxycarbonium ions) have been measured in HF ‐ SbF5. Due to restricted rotation about the partially double C ‐ O bond, which leads to synanti isomerism, symmetric ketones (R1 = R2) give cations with non‐equivalent alkyl groups. Unsymmetric ketones (R1 ≠ R2) give two isomeric cations which are spectroscopically distinguishable below 0°C. The free energy barrier to rotation is at least 17 kcal/mole. Assignments of the observed PMR signals to the one or the other isomer of dialkylhydroxycarbonium ions are given. The preference of the hydroxyl proton for being syn to an alkyl group increases in the order tert‐butyl < neopentyl, ethyl, isopropyl < methyl. For methyl tert‐butyl ketone, the isomer ratio is about 15. The preferred conformations about the OC ‐ Cα bond of the alkyl groups are discussed on the basis of observed chemical shifts and stabilities.