Question

HBr reacts fastest with

• A. 2-methyl propan-1-ol
• B. 2-methyl propan-2-ol
• C. propan-2-ol
• D. propan-1-ol

Answer 1

The Correct Option is B

To determine which alcohol reacts fastest with HBr, we need to understand the underlying mechanism and factors that influence the reactivity of alcohols with hydrogen bromide.

HBr is a strong acid, and it reacts with alcohols to form alkyl bromides. The reactivity of alcohols with HBr usually follows the order of the stability of carbocations. The more stable the carbocation, the faster the reaction.

Here is the order of stability for carbocations:

• Tertiary carbocations (more stable)
• Secondary carbocations
• Primary carbocations (less stable)

Let's examine the given options to identify the type of alcohols:

1. 2-methyl propan-1-ol: This is a primary alcohol. The carbocation formed here is primary, which is less stable.
2. 2-methyl propan-2-ol: This is a tertiary alcohol. The carbocation formed here is tertiary, which is highly stable.
3. propan-2-ol: This is a secondary alcohol. The carbocation formed here is secondary, which is moderately stable.
4. propan-1-ol: This is a primary alcohol. The carbocation formed here is primary, which is less stable.

Given the above information, 2-methyl propan-2-ol will react fastest with HBr because it forms a tertiary carbocation, which is very stable and facilitates a faster reaction rate compared to the other types of alcohols. This makes option 2-methyl propan-2-ol the correct choice.

In summary, the reactivity of alcohols with HBr is directly related to the stability of the intermediate carbocation formed during the reaction. Since tertiary carbocations are the most stable, tertiary alcohols like 2-methyl propan-2-ol react the fastest.