Mono‐ and dicarbonium ions of β‐dicarbonyl compounds (ketones, carboxylic ACIDS and esters)

A PMR spectroscopic study is described of the protonation of a series of β‐dicarbonyl compounds (Table I) in HF‐SbF5. At high acidity they form 1,3‐dicarbonium ions, except in the case of compounds VI and VII. At lower acidity monocations are formed: diketones give allylic type monocations, and keto esters, dicarboxylic esters and acids give alkoxyhydroxy and dihydroxy‐carbonium ions, respectively. Between these two groups there is a marked difference in spectroscopic behaviour of solutions containing comparable amounts of mono‐ and dications. Equilibration of mono‐ and dications is “slow” with the diketones and “fast” with the other compounds. For the compounds studied, the equilibrium constants KB* = [dication]/[monocation] [H2F+] vary from less than 10−2 for tri‐ and hexafluoroacetylacetone to more than 107 for ethyl acetoacetate. The monocation‐dication equilibrium systems are useful as indicators and buffers at high acidity; they are unreactive towards alkylcarbonium ions. A lower limit has been established for the basicity of isobutene, which turns out to be much more basic than benzene.