Question

n-butyl bromide can be prepared by treating \(n\) -butyl alcohol with \(\mathrm{KBr}\) and conc. \(\mathrm{H}_{2} \mathrm{SO}_{4} .\) This process cannot be applied to prepare tertiary butyl bromide because (a) in tertiary alcohol, C - OH bond cleaves easily. (b) conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) dehydrates tertiary alcohol to form an alkene. (c) tertiary alcohol does not react with \(\mathrm{KBr}\). (d) in primary alcohol, C - OH bond cleaves easily.

Short answer

Answer: The reaction does not work for tertiary alcohols because concentrated H2SO4 dehydrates tertiary alcohol to form an alkene instead of proceeding with the nucleophilic substitution with bromide ion.

Step-by-step solution

Understand the process for primary alcohols

When n-butyl alcohol (a primary alcohol) is treated with \(\mathrm{KBr}\) and concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\), n-butyl bromide is formed. The \(\mathrm{H}_{2}\mathrm{SO}_{4}\) acts as an acid to protonate the alcohol, forming a good leaving group (water) that is then replaced by bromide ion generated from the reaction of \(\mathrm{KBr}\) with the acid. The reaction proceeds via a nucleophilic substitution mechanism (SN2).

Recognize the problem for tertiary alcohols

In the case of tertiary alcohols, the reaction with concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\) leads to dehydration instead of substitution with bromide ion. Dehydration of tertiary alcohol proceeds via an E1 mechanism, forming an alkene as the final product.

Select the correct option

Based on the understanding of the reaction process for primary and tertiary alcohols, we can conclude that the right answer is (b), which states that the concentrated \(\mathrm{H}_{2}\mathrm{SO}_{4}\) dehydrates tertiary alcohol to form an alkene instead of proceeding with the nucleophilic substitution with bromide ion.

Key concepts

Nucleophilic Substitution (SN2)

Nucleophilic Substitution (SN2) is a fundamental reaction mechanism in organic chemistry, especially for converting alcohols to halides. In an SN2 reaction, a nucleophile, which is rich in electrons, attacks an electrophile—usually a carbon atom with a good leaving group attached. The key feature of SN2 mechanisms is their concerted, one-step process where the bond formation and bond breaking occur simultaneously.
Typically, SN2 reactions occur with primary alkyl halides because they have less steric hindrance, allowing the nucleophile to approach the electrophile more easily. For example, in the preparation of n-butyl bromide from n-butyl alcohol, the ucleophile ( looks to the Br- ion from KBr) attacks the carbon atom of the n-butyl group as the leaving group (a water molecule after protonation by H2SO4) departs, resulting in the substitution. The product is n-butyl bromide.
The rate of SN2 reactions is affected by several factors including the strength of the nucleophile, the nature of the leaving group, and the steric bulk of the substrate.

Dehydration of Alcohols

Dehydration of alcohols is a chemical reaction that involves the removal of a water molecule from an alcohol, resulting in an alkene. This is particularly common when alcohols react with concentrated acids like sulfuric acid ( H2SO4), which acts as a dehydrating agent.
Tertiary alcohols are especially prone to dehydration because the carbocation intermediate, formed during the reaction, is stable due to alkyl group stabilization. In the dehydration process, the acid catalyst protonates the hydroxyl group, converting it into a good leaving group (water). After the water molecule departs, a carbocation is formed, subsequently losing a proton to generate the double bond of an alkene.
This process explains why concentrated sulfuric acid does not support the SN2 substitution to form tertiary butyl bromide as it instead causes dehydration to form an alkene.

E1 Mechanism

The E1 mechanism is a two-step reaction where the leaving group departs first, forming a carbocation. This is followed by the loss of a proton to form a double bond. E1 reactions are common in the dehydration of tertiary alcohols.
The process begins when the alcohol's hydroxyl group is protonated by a strong acid, turning it into a better leaving group which is then expelled from the molecule, forming a carbocation. This carbocation is crucial for the reaction, as its formation is the rate-determining step, meaning the reaction rate depends on this single molecular event.

• Step 1: The departure of the leaving group, forming a carbocation.
• Step 2: Deprotonation, leading to the formation of a double bond.

E1 reactions typically occur with strong acids like concentrated sulfuric acid and require the substrate to form a relatively stable carbocation; hence, tertiary alcohols are prime candidates for undergoing E1 dehydration.

Alcohol Reactivity

Alcohol reactivity is greatly influenced by the structure of the alcohol and the reaction conditions. Primary, secondary, and tertiary alcohols react differently due to variations in their structures. The key differences in reactivity arise from the stability of their intermediates and their interactions with reagents like acids and halides.
Primary alcohols, like n-butyl alcohol, are more prone to participate in SN2 reactions because they form less stable carbocations, leading the reaction to prefer the direct attack by a nucleophile. In contrast, tertiary alcohols are susceptible to dehydration reactions due to the stability of the resultant carbocation.
The reactivity of alcohols also depends on the strength of the acid used and its ability to protonate the hydroxyl group, thereby facilitating leaving group departure and subsequent reaction events.

Primary vs Tertiary Alcohols

Primary and tertiary alcohols differ significantly in their chemical behavior due to their structural characteristics. Primary alcohols feature one alkyl group attached to the carbon bonded to the hydroxyl group, making them less hindered and more open to nucleophilic attack. This characteristic makes them suitable for SN2 reactions, as observed with n-butyl alcohol turning into n-butyl bromide.
On the other hand, tertiary alcohols have three alkyl groups attached to the carbon with the hydroxyl group, creating significant steric hindrance. Thus, these alcohols do not easily undergo SN2 reactions but are instead more liable to undergo dehydration under acidic conditions, such as forming an alkene in the presence of concentrated H2SO4.
Recognizing these fundamental distinctions is crucial for predicting the outcomes of alcohol-related reactions and choosing the appropriate transformation pathway.