Question

A compound ${}^{\prime }{A}^{\prime }$ of molecular formula $C_4{H}_{10}O$ on treatment with Lucas' reagent at room temperature gives a compound ${}^{\prime }{B}^{\prime }$ . When the compound ${}^{\prime }{B}^{\prime }$ is heated with alcoholic $KOH$ , it gives isobutene. The compound ${}^{\prime }{A}^{\prime }$ and ${}^{\prime }{B}^{\prime }$ respectively are

• A. $2-$ methyl $-2-$ propanol and $2-$ methyl $-2-$ chloropropane.
• B. $2-$ methyl $-1-$ propanol and $1-$ chloro $-2-$ methylpropane.
• C. $2-$ methyl $-1-$ propanol and $2-$ methyl $-2-$ chloropropane.
• D. Butan $-2-$ ol and $2-$ chlorobutane.

Answer 1

Answer: (A)

The reaction involved is given as:

The Lucas' test is used to distinguish between primary, secondary and tertiary alcohols, where the $OH$ is replaced by $Cl$ . The reaction is a nucleophilic subst
itution reaction. The tertiary forms a stable carbocation, which reacts readily with the reagent while the primary reacts slowly upon heating. The anhydrous zinc chloride and concentrated hydrochloric acid are known as Lucas' reagent.
After the formation of alkyl chloride, it is treated with alcoholic $KOH$ , then $\beta -$ elimination happened and formed isobutene.