The Lucas test is used to distinguish between primary, secondary, and tertiary alcohols based on their reaction rates with Lucas reagent (a mixture of concentrated hydrochloric acid and zinc chloride). The test relies on the formation of an alkyl chloride, which is insoluble in water, causing the solution to turn cloudy.

Step 1: Understanding the Reactivity
Tertiary alcohols react the fastest because they form stable carbocations. The order of reactivity is: tertiary > secondary > primary.

Step 2: Mechanism Involved
Tertiary alcohols undergo S_N1 (substitution nucleophilic unimolecular) mechanism. Here's why:

• The hydroxyl group (-OH) is a poor leaving group. Protonation by HCl converts it to a better leaving group (H₂O).
• For tertiary alcohols, the carbocation intermediate is stable due to hyperconjugation and inductive effects.
• The rate-determining step is the formation of the carbocation, which is unimolecular (hence S_N1).

The reaction can be represented as:

Step 3: Why not S_N2?
Tertiary alcohols are sterically hindered, making nucleophilic attack difficult. Hence, S_N2 is slow for tertiary alcohols.

Final Answer: Tertiary alcohol by S_N1 mechanism.

Related Topics

Nucleophilic Substitution Reactions: These involve the replacement of a leaving group by a nucleophile. S_N1 is unimolecular and favored for tertiary substrates, while S_N2 is bimolecular and favored for primary substrates.

Carbocation Stability: Tertiary carbocations are more stable than secondary or primary due to hyperconjugation and inductive effects.

Formulae

Rate law for S_N1: $\mathrm{Rate}=k[R-X]$

Rate law for S_N2: $\mathrm{Rate}=k[R-X]\left[\mathrm{Nu}\right]$