Question

HBr reacts fastest with:
A) 2-methyl propane-2-ol
B) Propane-1-ol
C) Propane-2-ol
D) Propane-1-ol

Answer 1

Hint: Tertiary alcohols react fastest with hydrogen halides and tertiary alcohol is a compound in which a hydroxy group, ‒OH, is attached to a saturated carbon atom which has three other carbon atoms attached to it.

Complete step by step solution:
In this question, we have been asked that among the four options which compound would react fastest with HBr. As we know, HBr is a hydrogen halide, and as we can see that all the options contain alcohol. All the reactions will be carried out through a nucleophilic substitution reaction pathway. In the case of $\mathop {SN}\nolimits_1 $ reaction, first, the leaving group separates from the compound to form an intermediate carbocation and then the nucleophile attacks the carbon center of the carbocation. Thus, the tertiary alcohol will produce the most stable carbocation as tertiary carbocation is always marked as the most stable. This reaction usually proceeds via $\mathop {SN}\nolimits_1 $ mechanism which involves a carbocation intermediate that can undergo a rearrangement. Methanol and primary alcohols will proceed via $\mathop {SN}\nolimits_2 $ mechanism since these two will have highly unfavorable carbocation.
For the $\mathop {SN}\nolimits_2 $, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest). In the case of a $\mathop {SN}\nolimits_2 $ mechanism, the attack of the nucleophile and the removal of the leaving group from the compound takes place simultaneously. Thus, the lower the steric hindrance, the higher the rate of reaction will be.
For the $\mathop {SN}\nolimits_1 $, since carbocation stability increases as we go from primary to secondary to tertiary, the rate of reaction for the SN1 goes from primary (slowest) << secondary < tertiary (fastest). So, the alcohol reactivity order will be: $\mathop 3\nolimits^o > \mathop 2\nolimits^o > \mathop 1\nolimits^o $ methyl. So, the tertiary alcohol reacts faster with hydrogen halide. 2-methyl propane-2-ol is tertiary alcohol and thus, reacts fastest with HBr. Propane-1-ol is alkyl alcohol which reacts to the slowest with HBr. Propane-2-ol is secondary alcohol and Propane-1-ol is a primary alcohol.

$\therefore$ Hence, Option (A) is correct.

Note:
In these types of problems, one has to remember that the rate of reaction would be the fastest in which the intermediate carbocation is the most stable. The two nucleophilic substitution reactions are decided by the options provided in the question. By the use of the proper mechanisms, only one can get the desired major product.