Hydrogen rearrangements in ionized alkanoic acids. An ion lifetime study

Abstract

Single and double hydrogen rearrangements involving β-bond cleavage have been studied in ionized butanoic, pentanoic and hexanoic acids specifically deuterium labelled at the hydroxy group and all carbon atoms of the side chains. The reactions have been studied in the range 6×10 −11-10 −5 s after ionization using the field ionization kinetic technique. At short ion lifetimes ( r<10 −10s) n-butanoic acid shows exclusively a single hydrogen rearrangement which specifically involves the γ-hydrogen and which is followed by β-bond cleavage. At longer ion lifetimes ( t>10 −8s) this rearrangement is preceded by hydrogen exchange between the hydroxylic hydrogen and the β- and γ-hydrogens as found previously. In ionized n-pentanoic and n-hexanoic acids, double hydrogen transfer with β-bond cleavage competes effectively with single hydrogen transfer at long ion lifetimes. At short ion lifetimes (< 10 −10s) single hydrogen rearrangement in n-pentanoic and n-hexanoic again involves predominantly the γ-hydrogen atom, the double hydrogen transfer involving those atoms from the γ- and δ-positions. In the acids extensive hydrogen exchange which is observed even on the picosecond time scale leads to participation of the β- and ϵ- (for n-hexanoic acid) positions in the hydrogen transfer reactions, although γ- and δ-hydrogens are predominantly involved even at the longest time (∼ 10 −5 s).