Chemistry and spectroscopy in strongly acidic solutions. Part IX: Configurational preference in protonated carboxylic acids. Kinetics of reversible alkyl oxocarbonium ion formation

Protonation of aliphatic carboxylic acids RCOOH in HF‐BF3 solution at low temperature leads to dihydroxycarbonium ions. The configurations as regards the groups R and H at the partly double magnified image bonds depend on the steric requirements of group R: In the case of RCH3, CH2Cl, CH3CH2 and (CH3)2CH both a cis‐ and a trans‐configuration, and in the case of R(CH3)3C two trans‐configurations probably exist. The rate constants of the dehydration of the protonated acids to acyl cations that takes place at higher temperatures proved to vary only slightly for RCH3, CH3CH2, (CH3)2CH and (CH3)3C. In agreement with previous work on protonated formic acid the rates were found to be much lower in HF‐BF3 than in sulfuric acid solution (as estimated from Deno's results), possibly due to a difference in molecularity of the reaction.