Reactivity of Alcohols with Concentrated HCl and Anhydrous ZnCl2

When alcohols react with concentrated hydrochloric acid (HCl) and anhydrous zinc chloride (ZnCl2), the alcohol undergoes an elimination reaction to form an alkene. This reaction is known as the Lucas test and is used to differentiate between primary, secondary, and tertiary alcohols based on their reactivity.

The Lucas Test

The Lucas test is based on the principle that the rate of reaction between an alcohol and HCl/ZnCl2 mixture depends on the structure of the alcohol. The reactivity order for alcohols in this test is as follows:

1. Tertiary alcohols react the fastest.
2. Secondary alcohols react moderately.
3. Primary alcohols react the slowest.

Explanation of the Answer

Among the given options, the correct answer is (b) 2-Methylpropan-2-ol. Let's break down the structure of each option and analyze their reactivity.

a) 2-Butanol
- It is a primary alcohol.
- It has four carbon atoms with the hydroxyl group attached to the second carbon.
- The reaction with HCl/ZnCl2 will be relatively slow compared to the other options.

b) 2-Methylpropan-2-ol
- It is a tertiary alcohol.
- It has four carbon atoms with a methyl group attached to the second carbon and a hydroxyl group attached to the third carbon.
- Tertiary alcohols react the fastest in the Lucas test.
- Therefore, it is the most reactive among the given options.

c) 2-Methylpropanol
- It is a secondary alcohol.
- It has three carbon atoms with a methyl group attached to the second carbon and a hydroxyl group attached to the first carbon.
- Secondary alcohols react moderately in the Lucas test.
- It is less reactive than option (b) but more reactive than option (a).

d) 1-Butanol
- It is a primary alcohol.
- It has four carbon atoms with the hydroxyl group attached to the first carbon.
- It will have the slowest reaction among the given options.

Conclusion

Based on the reactivity order in the Lucas test, the alcohol that will react fastest with concentrated HCl and anhydrous ZnCl2 is 2-Methylpropan-2-ol (option b). Tertiary alcohols generally have the highest reactivity in elimination reactions, followed by secondary alcohols, while primary alcohols react the slowest.

B