Question

Which of the following reaction does not involve a carbocation as intermediate? (a) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{Br}_{2} \mathrm{AlBr}_{3}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{Br}_{2} \longrightarrow \mathrm{BrCH}_{2}-\mathrm{CH}_{2} \mathrm{Br}\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}+\mathrm{HBr} \mathrm{H}^{+}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CBr}+\mathrm{H}_{2} \mathrm{O}\) (d) Both (b) and (c)

Short answer

Reaction (b) does not involve a carbocation intermediate.

Step-by-step solution

Understand Reaction Mechanisms

To identify which reaction does not involve a carbocation, understand that a carbocation is a positively charged carbon atom attached to three other atoms/groups. Typically, carbocations form as intermediates in electrophilic addition, substitution, or rearrangement reactions.

Analyze Reaction (a)

The reaction involves the substitution of bromine on benzene in the presence of a Lewis acid, AlBr3. This is an electrophilic aromatic substitution reaction where a carbocation-like intermediate, sigma complex or arenium ion, forms.

Analyze Reaction (b)

This reaction involves the addition of Br2 to an alkene, forming a vicinal dibromide. It follows a electrophilic addition mechanism known as halogenation. The reaction does not form a discrete carbocation; instead, it involves a cyclic bromonium ion intermediate.

Analyze Reaction (c)

This involves the reaction of tert-butyl alcohol with HBr, leading to the formation of tert-butyl bromide. It proceeds via a carbocation intermediate because the OH group is protonated and leaves, forming a tertiary carbocation before bromide ion adds.

Determine the Correct Option

Since Reaction (b) does not form a carbocation but rather a cyclic bromonium ion, unlike Reactions (a) and (c) which involve carbocations, (b) is the correct answer.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a type of organic reaction where an atom, typically hydrogen, attached to an aromatic system is replaced by an electrophile. This kind of reaction preserves the aromaticity of the benzene ring, which is a crucial aspect of its stability.
In the process of electrophilic aromatic substitution, the aromatic ring, like benzene, acts as a nucleophile and attracts electrophiles. This is quite different compared to aliphatic electrophilic substitutions where the reaction might lead to the loss of original structure.
Steps in these reactions include:

• An initial formation of a sigma complex, also known as an arenium ion, through the interaction between the aromatic system and the electrophile.
• The loss of a proton, which results in the substitution product and restores the aromaticity of the system.

In reaction (a), this is exactly what happens, involving benzene and bromine in the presence of a Lewis acid, AlBr3.

Halogenation Mechanism

The halogenation mechanism is a specific type of electrophilic addition where halogens are added to alkenes or alkynes. This is different from aromatic systems, as it involves the temporary formation of a cyclic intermediate.
Take reaction (b) as an example, where bromine is added to ethylene. This forms a "vicinal" dibromide—two bromine atoms on adjacent carbon atoms. Instead of a carbocation, a cyclic bromonium ion forms first, which is a key distinction. This intermediate is cyclic due to the bridging nature of the halogen between the carbons.
This ring-like transient doesn't allow a free carbocation to wander, maintaining the stability of the reaction until the dibromide is formed.

Intermediate Structures

Intermediate structures are often crucial in determining how a chemical reaction proceeds. These short-lived species allow us to understand the mechanistic pathway a reaction takes.
In many reactions, such as electrophilic aromatic substitution, an intermediate like a sigma complex can explain how substitution occurs without significant disruption to the aromatic system's stability.
With reaction (b), the cyclic bromonium ion is the intermediate. This minimizes the potential for carbocation rearrangement due to its stable, ring-held nature. Such stability is key in controlling the course and outcome of halogenation reactions.

Reaction Mechanisms

Reaction mechanisms are vital as they detail the step-by-step sequence through which reactants transform into products. They unveil the invisible, transient players—intermediates—that drive the reaction forward.
Understanding the mechanism helps identify intermediates formed and dictate conditions under which reactions proceed optimally. For instance, knowing that a carbocation can rearrange helps predict possible products when tertiary carbocations are formed, like in reaction (c).
Reaction (a) follows a clear path typical of electrophilic aromatic substitution while reaction (b) exemplifies an electrophilic addition with a halogen. By revealing these mechanisms, organic chemists can predict and control chemical behavior for synthetic applications.

Tertiary Carbocations

Tertiary carbocations are highly stable carbocation intermediates due to the stabilizing effect of three alkyl groups donating electron density towards the positively charged carbon center. They are pivotal in many reactions, such as electrophilic substitution and addition in aliphatic systems.
In the context of reaction (c), the conversion of tert-butyl alcohol to tert-butyl bromide involves forming a tertiary carbocation after protonation and departure of the leaving group, hydroxide. This resulting tertiary carbocation is highly stable, allowing the reaction to proceed effectively giving rise to the substitution product.
The stability order for carbocations can be generalized as tertiary > secondary > primary, where tertiary carbocations are often seen in solvolysis and other substitution reactions due to this inherent stability.