Question

HBr reacts fastest with
A.$2 - Methylpropan - 1 - ol$
B.$2 - Methylpropan - 2 - ol$
C. $propan - 2 - ol$
D.$propan - 1 - ol$

Answer 1

Hint: The carbocations are the group where the carbon atom possesses a positive charge and has three bonds attached to another group. The hybridization of the carbocation is $s{p^2}$ and its structure is trigonal planar. The reactivity of HBr depends on carbocation stability.

Complete step by step answer:
The carbocations can be differentiated on the basis of the carbon groups attached to the carbon possessing the positive charge.
(1) Methyl carbocation: When there is no carbon attached to the carbon having the positive charge, then the carbocation is known as methyl carbocation.
(2) Tertiary carbocation: When three carbons are attached to the carbon having a positive charge, then the carbocation is known as tertiary carbocation.
(3) Secondary carbocation: When two carbons are attached to the carbon having a positive charge, then the carbocation is known as secondary carbocation.
(4) Primary carbocation: When only one carbon is attached to the carbon having a positive charge, then the carbocation is known as primary carbocation.
The reaction of HBr depends on the stability of the carbocation. The more stable is the carbocation, the faster will be the reaction.
The stability of the carbocation is dependent on the number of carbon atoms joined to the carbon having the positive charge.
With the increase in the substituting group, hyperconjugation increases, and thus stability of the carbocation increases.
The stability order of carbocation is shown below.
Tertiary carbocation > secondary carbocation > primary carbocation > methyl carbocation
$2 - Methylpropan - 1 - ol$ forms a primary carbocation.
$2 - Methylpropan - 2 - ol$ forms a tertiary carbocation.
$propan - 2 - ol$ forms a secondary carbocation.
$propan - 1 - ol$forms a primary carbocation.
Thus, HBr will react fastest in$2 - Methylpropan - 2 - ol$.
Therefore, the correct option is B.


Note:
The stability of carbocation depends on the increasing resonance structure due to the delocalization of the positive charge present in the carbon. The deficiency in the carbon is decreased because of the delocalization and thus stability is increased.