Q7(a). Pinacol rearrangement; Wolff–Kishner beside a ketal

(i) Pinacol rearrangement of 1,1,2-triphenylpropane-1,2-diol

Scheme: The more stable cation forms; the group with the higher migratory aptitude (phenyl) moves.

1. Which OH leaves. Protonation and loss of water can occur at C-1 (the Ph₂C(OH) end) or at C-2 (the PhC(CH₃)(OH) end). Loss from C-1 gives the diphenyl-substituted cation Ph₂C⁺, stabilised by two phenyl rings; this more stable carbocation forms (rate-determining ionisation).
2. Which group migrates. C-2 carries Ph, CH₃ and OH. Phenyl has the higher migratory aptitude (it migrates through a bridged phenonium-like transition state) and moves to C-1 with its bonding pair.
3. The positive charge now sits on C-2, stabilised by the oxygen lone pair (oxocarbenium ion); loss of H⁺ gives the ketone.
4. Product: 1,1,1-triphenylpropan-2-one, Ph₃C-CO-CH₃ (C₂₁H₁₈O). (Methyl migration would give 1,2,2-triphenylpropan-1-one, PhCO-CPh₂CH₃, a minor product at most.)

(ii) Wolff–Kishner reduction with the ketal untouched

Scheme: C=O of the side chain becomes CH₂; the ethylene ketal survives strong base.

1. The substrate is a monoprotected diketone: the ring C=O is masked as an ethylene ketal (1,3-dioxolane), and the side chain carries an unprotected ketone (2-oxobutyl).
2. Hydrazine condenses with the unprotected ketone to give the hydrazone. KOH at high temperature (Huang-Minlon: in hot diethylene glycol) removes N–H protons; the diimide anion loses N₂ and the carbanion is protonated: C=O → CH₂.
3. Ketals are stable to base, so the protected ring carbonyl is not touched. Product: 6-butyl-1,4-dioxaspiro[4.5]decane (C₁₂H₂₂O₂), the ethylene ketal of 2-butylcyclohexanone.
4. Clemmensen reduction (Zn(Hg)/HCl) would not do this job: the acid would hydrolyse the ketal and reduce both carbonyl groups.

Conclusion. (i) 1,1,1-triphenylpropan-2-one: Ph₂C⁺ forms and phenyl migrates. (ii) 6-butyl-1,4-dioxaspiro[4.5]decane: only the unprotected ketone is reduced to CH₂; the ketal survives the basic conditions (and can later be removed with aqueous acid to give 2-butylcyclohexanone).

Application. Pinacol rearrangements build quaternary carbon centres (pinacolone, spiro ketones); ketal protection with base-mediated reduction is a standard strategy in steroid synthesis, where one carbonyl must be reduced beside another.