Question

Which alcohol reacts fastest with the Lucas's reagent?

Answer 1

1. To distinguish between primary, secondary, and tertiary alcohols, we use the Lucas test. The Lucas reagent is a combination of dry anhydrous zinc chloride and strong hydrochloric acid.
2. Based on the reactivity of alcohol, the test separates the alcohols.
3. An alcohol and a Lucas reagent undergo a two-step reaction. Carbocation initially formed in the first stage and alkyl chloride in the second. Based on their reactivity or the stability of the intermediate, the Lucas reagent distinguishes between different types of alcohol.
4. $\mathrm{OH}$ group of alcohol attack on the acid and become protonated. A carbocation is then created by removing the water molecules. The chloride ion attacks this carbocation and produces alkyl chloride as a result.
5. A carbocation occurs during the reaction, hence the stability of the carbocation affects the rate of reaction. The rate of reaction will increase as the carbocation becomes more stable.
6. The following is the order of stability of carbocation $3^{°}>2^{°}>1^{°}$ because tertiary alcohol forms a very stable carbocation, the tertiary alcohol reacts more quickly as a result.

Therefore, tertiary alcohol reacts fastest with Lucas's reagent.