Question

An unknown alcohol is treated with the "Lucas reagent" to determine whether the alcohol is primary, secondary or tertiary. Which alcohol reacts fastest and by what mechanism:

• A. Secondary alcohol by $S_N2$
• B. Tertiary alcohol by $S_N2$
• C. Secondary alcohol by $S_N1$
• D. Tertiary alcohol by $S_N1$

Answer 1

Answer: (D)

$3^{\circ} $ alcohol reacts fastest giving white turbidity immediately on adding Lucas reagent.
Reaction takes place through carbocation formation involving $S_N1$ mechanism. Rate of reaction in this reaction is directly proportional to the stability of the carbocation formed.
$3^{\circ} R-OH$ (tertiary alcohol) forms $3^{\circ}$ carbocation and therefore, the reaction is fastest.