GAS-PHASE REACTIONS OF DICOORDINATED BORON CATIONS WITH ALCOHOLS

摘要

The intrinsic gas-phase reactivity of simple dicoordinated boron cations 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 alcohols and are dominated by abstraction oh water or a hydroxyl group by the ion. These two reactions likely occur via the same proton-bound intermediate, analogous to the dissociation of protonated ethanol to ethylene and H3O+. The branching ratio depends primarily on the stability of the alkyl cation formed in the hydroxyl abstraction reaction. Hence, hydroxyl abstraction dominates for longer chain alcohols. The proton affinity of (CH3)(2)BOH was determined to be 179 kcal/mol through proton affinity bracketing experiments. Using this value, the heat of formation of (CH3)(2)BOH2+ is estimated to be 86 kcal/mol. Hence, dehydration of ethanol by CH3BCH3+ is concluded to be exothermic by 21 kcal/mol. The same reaction for CH3OBOCH3+ to yield (CH3O)(2)BOH2+ is exothermic 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 thermodynamically more favorable for the ion CH3BCH3+ than for CH3OBOCH3+. Analogous to these water and hydroxyl abstraction reactions of alcohols, competitive ethanol and ethoxy abstractions were observed when the boron cations were allowed to react with ethyl acetate. [References: 25]