GAS-PHASE REACTIONS OF DICOORDINATED BORON CATIONS WITH ALCOHOLS

The intrinsic gas-phase reactivity of simple dicoordinated boron catio ns toward alcohols was studied using a dual-cell Fourier-transform ion cyclotron resonance device. Reactions of the ions CH3BCH3+, CH3OBOCH3 + and CH3(CH2)(2)OBOH+ occur at or near collision rate with simple alc ohols and are dominated by abstraction oh water or a hydroxyl group by the ion. These two reactions likely occur via the same proton-bound i ntermediate, analogous to the dissociation of protonated ethanol to et hylene and H3O+. The branching ratio depends primarily on the stabilit y of the alkyl cation formed in the hydroxyl abstraction reaction. Hen ce, hydroxyl abstraction dominates for longer chain alcohols. The prot on affinity of (CH3)(2)BOH was determined to be 179 kcal/mol through p roton affinity bracketing experiments. Using this value, the heat of f ormation of (CH3)(2)BOH2+ is estimated to be 86 kcal/mol. Hence, dehyd ration of ethanol by CH3BCH3+ is concluded to be exothermic by 21 kcal /mol. The same reaction for CH3OBOCH3+ to yield (CH3O)(2)BOH2+ is exot hermic by about 33 kcal/mol; a reaction yielding (CH3O)(HO)B(HOCH3)(+) as the final ionic product is significantly more exothermic, by about 54 kcal/mol. Relative to water abstraction, hydroxyl abstraction is t hermodynamically more favorable for the ion CH3BCH3+ than for CH3OBOCH 3+. Analogous to these water and hydroxyl abstraction reactions of alc ohols, competitive ethanol and ethoxy abstractions were observed when the boron cations were allowed to react with ethyl acetate.