SN1 mechanism: hydrolysis of 2-bromo-2-methylpropane

Tertiary halogenoalkanes react by SN1. The C–Br bond breaks first to give a stable tertiary carbocation, which is then captured by the nucleophile. Water is the nucleophile here, so a final proton transfer gives the alcohol.

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Nucleophilic substitution (SN1)

The C–Br bond breaks on its own to give a tertiary carbocation (the slow, rate-determining step). Water then attacks the carbocation, and loss of a proton gives 2-methylpropan-2-ol.

Overall reaction

1. 1Ionisation: loss of bromideheterolysis (rate-determining)

   2-bromo-2-methylpropane

   slow

   tert-butyl carbocation

   +

   bromide ion

   Electron movement

   1. The C–Br bond breaks heterolytically; both electrons go to bromine.

   This is the slow step. It happens readily here because the tertiary carbocation is stabilised by the electron-donating (+I) effect of three methyl groups and by hyperconjugation. A polar solvent also stabilises the ions as they form.

2. 2Water attacks the carbocationnucleophilic attack

   tert-butyl carbocation

   +

   water

   fast

   protonated alcohol (oxonium ion)

   Electron movement

   1. A lone pair on the water oxygen attacks the positively charged carbon.

   The carbocation is planar (sp²) with an empty p orbital, so the nucleophile can attack from either face. Attack by neutral water gives a positively charged oxonium ion.

3. 3Deprotonationproton transfer · reversible

   oxonium ion

   +

   water

   fast

   2-methylpropan-2-ol

   +

   H₃O⁺

   Electron movement

   1. A lone pair on a second water molecule removes a proton from the oxonium ion.
   2. The O–H electrons move back onto the oxygen, which becomes neutral.

   A fast proton transfer to the solvent gives the neutral alcohol.