Ncert Exercises & Solutions

The order of reactivity of following alcohols with halogen acids is:

1. (I) >(II) > (III)

2. (III) > (II) > (I)

3. (II) >(I) > (III)

4. (I) >(III) > (II)

Hint: Rate of reaction depends on the stability of carbocation

Halogenation of alcohol by haloacids is done by the

S_{N} 1

mechanism. This

S_{N} 1

mechanism involves carbocation intermediate formation.

The generation of carbocations with the present three alcohol substrates in terms of the reaction mechanism is as follows:.

Here 1° carbocation formation occurs through protonation of 1° -OH followed by a nucleophilic attack.

Here $2 ° carbocation formation$ occurs which is comparatively more stable than primary alcohol.

the rate of S_N1 mechanism will increase as stability of carbocation will increase

Similarly, the tertiary carbocation formed from tertiary alcohol is most stable so therefore here the rate of reaction by S_N1 mechanism is highest.

So, the correct option is second

Conclusion:- Higher the stability of carbocation intermediate higher will be the rate of halogenation of given alcoholic substrates by haloacids.