Q1(c). Neopentyl tosylate: 1,2-methyl shift

Introduction. Neopentyl tosylate, (CH₃)₃CCH₂OTs, is primary but very hindered: the tert-butyl group blocks back-side attack, so SN2 is extremely slow. In a polar, weakly nucleophilic solvent (water, aqueous ethanol, formic acid) the substrate ionises slowly instead, and the carbon skeleton rearranges by a 1,2-methyl shift (Wagner–Meerwein) to a tertiary cation that the solvent captures.

Scheme: SN1 with rearrangement: the methyl moves with its bonding pair to the electron-poor carbon.

1. Ionisation (rate-determining). The C–OTs bond breaks heterolytically; TsO⁻ is an excellent, resonance-stabilised leaving group. An unassisted primary cation is very unstable, so the migration of a methyl group from the quaternary carbon assists the departure of TsO⁻ (the methyl is anti-periplanar to the C–O bond). Ionisation and migration are therefore nearly concerted.
2. 1,2-Methyl shift. A CH₃ group moves with its bonding electron pair from C-2 to C-1. C-1 becomes CH₂-CH₃ (an ethyl group) and the positive charge moves to C-2, which now carries two methyls and an ethyl: the 2-methylbutan-2-yl (tert-amyl) cation, a tertiary cation stabilised by hyperconjugation with eight C–H bonds.
3. Capture by solvent. Water attacks the planar cation; loss of H⁺ gives 2-methylbutan-2-ol, (CH₃)₂C(OH)CH₂CH₃.
4. By-products. The same tertiary cation also loses H⁺ (E1) to give 2-methylbut-2-ene (main alkene, Saytzeff) and some 2-methylbut-1-ene. No neopentyl alcohol is formed.

Conclusion. Solvolysis of neopentyl tosylate is SN1 with a 1,2-methyl shift: the hindered primary system ionises with methyl participation to the tertiary tert-amyl cation, and water captures it to give 2-methylbutan-2-ol (with 2-methylbut-2-ene as the E1 by-product).

Application. The same rearrangement makes neopentyl halides useless in SN2 and SN1 synthesis and appears in Friedel–Crafts alkylation (neopentyl chloride gives tert-pentylbenzene) and in terpene and steroid rearrangements.