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14.6: Sn1 Reactions and Rearrangements Involving Primary Substrates

  • Page ID
    216040
  • Primary substrates normally do not follow Sn1 mechanisms because they do not form stable cations. However, a hindered primary substrate can be forced into an Sn1 mechanism if sufficient energy and time are allowed, for example boiling the substrate in a nucleophilic solvent such as ethanol.

    The nucleophile cannot do a backside attack, and the substrate cannot form stable cations. In this case the substrate will begin to rearrange as the leaving group departs. This avoids formation of a primary cation. As the leaving group departs, a positive charge begins to develop on the carbon bearing the leaving group, and the rearrangement process starts, all in unison. The reaction of neopentyl bromide with ethanol illustrates this point.

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    As the positive charge develops on the primary carbon while bromine leaves, the methyl group is migrating to an adjacent position to forma more stable cation.

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