but the exo/endo ratio is still only about 30. This low ratio stands in striking contrast to the one observed for solvolyses,8 which is ~105 at 25°, and it is concluded that neighboring group assistance is energetically less favored in the gas phase than in solution. Two reasons probably do account for this: firstly, neighboring group migration is usually associated with some loss of rotational entropy for the reactant on ring closure to the transition state.9 This entropy loss, which impedes reactivity, will inevitably be more pronounced in the gas phase than in solution. Secondly, since neighboring group assistance depends, among other factors, critically upon the charge development at the migrating center, the driving force for migration in the gas phase is reduced.

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