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Consider the following reaction sequence starting from cyclohexanone (C6H10O): C6H10O + MeMgBr, then H2O gives Q. Q + conc. HCl gives S (major). Q + 20% H3PO4 at 360 K gives R (major). R + HBr with benzoyl peroxide and heat gives U (major). R + (i) H2/Ni, (ii) Br2/hv gives T (major). Choose the correct option(s).

1. U is 1-bromo-1-methylcyclohexane and S is 2-chloro-1-methylcyclohexane
2. S is 1-chloro-1-methylcyclohexane and U is 2-bromo-1-methylcyclohexane
3. U is 1-bromo-1-methylcyclohexane and S is 1-chloro-1-methylcyclohexane
4. S is 2-chloro-1-methylcyclohexane and U is 2-bromo-1-methylcyclohexane

Correct answer: S is 1-chloro-1-methylcyclohexane and U is 2-bromo-1-methylcyclohexane

Solution

Q is 1-methylcyclohexanol (tertiary alcohol). Conc. HCl causes SN1 on tertiary alcohol giving 1-chloro-1-methylcyclohexane (S). H3PO4 at 360 K causes E1 dehydration of Q; the major alkene from tertiary alcohol follows Zaitsev, giving 1-methylcyclohex-1-ene (R). HBr/benzoyl peroxide = anti-Markovnikov (radical) addition to R, placing Br at C2 (the less substituted carbon), giving (1-methyl-2-bromocyclohexane). This is option B: S = 1-chloro-1-methylcyclohexane, U = 2-bromo-1-methylcyclohexane.

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