Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reaction does not involve a carbocation as intermediate? (a) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{Br}_{2} \quad \mathrm{AlBr}_{3} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) (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)

Step-by-Step Solution

Answer

Reaction (b) does not involve a carbocation.

Step 1: Identify Reaction Types

Start by recognizing the types of reactions provided. Reaction (a) is an electrophilic aromatic substitution, reaction (b) is a halogenation of an alkene, and reaction (c) is an acid-catalyzed substitution.

Step 2: Determine Intermediate Formation for (a)

In reaction (a), benzene reacts with bromine in the presence of aluminum bromide, forming a carbocation intermediate as part of the electrophilic aromatic substitution.

Step 3: Analyze Intermediate for (b)

In reaction (b), ethylene reacts with bromine leading to the formation of a bromonium ion bridge, not a carbocation. The bromonium ion is the intermediate in this halogenation reaction.

Step 4: Examine Intermediate Formation for (c)

In reaction (c), tert-butyl alcohol reacts with hydrobromic acid. Initially, the alcohol is protonated, and water leaves to form a tert-butyl carbocation, which then reacts with bromide.

Step 5: Conclude Without Carbocation

Reactions (a) and (c) involve carbocation intermediates, while reaction (b) involves a bromonium ion instead of a carbocation. Therefore, reaction (b) does not involve a carbocation.

Key Concepts

Electrophilic Aromatic Substitution

Understanding electrophilic aromatic substitution is key when dealing with reactions that involve benzene rings. Benzene is a fascinating molecule due to its stability, attributed to its delocalized electrons in a conjugated pi system. This special structure makes benzene less reactive under normal conditions. However, in electrophilic aromatic substitution (EAS), benzene reacts with a strong electrophile. This reaction is an exception, showing how unique interactions can occur.

• In an EAS, the benzene ring temporarily sacrifices its aromaticity to form an arenium ion (a positively charged carbocation intermediate) during the transition state.
• This intermediate is then quickly stabilized as a proton is lost, restoring the aromaticity of the benzene ring.
• This sequence of reactions reflects the typical way benzene and its derivatives undergo substitution rather than addition.

In practical applications, this mechanism is crucial for introducing substituents like halogens, nitriles, or sulfonic groups into a benzene ring, leading to a vast array of aromatic compounds.

Halogenation

Halogenation is a reaction where a halogen is added to a molecule, such as in the halogenation of alkenes, which is common in organic chemistry. Unlike the electrophilic aromatic substitution, this reaction doesn’t form a carbocation.

• When ethylene (\( CH_2=CH_2 \)) interacts with a halogen like bromine, it undergoes a specific halogenation reaction known as bromination.
• The double bond in the alkene is a region of high electron density, which attracts the electrophilic bromine molecules.
• A cyclic bromonium ion is formed as an intermediate, which is different from the open-carbocation intermediate found elsewhere. This ion is more stable due to the ring formation, avoiding the creation of a positively charged carbon atom.

These reactions are particularly useful in producing vicinal dihalides, which have halogen atoms on adjacent carbons, broadening the scope for further chemical transformations.

Reaction Intermediates

Intermediates play a fundamental role in understanding how chemical reactions occur step-by-step. Familiarity with these transient species can help predict the course of reactions and the energy requirements.

• Chemical reactions often proceed through one or more intermediate stages before reaching the final product, a concept known as the reaction mechanism.
• In the electrophilic aromatic substitution, an arenium ion serves as a key carbocation intermediate, while in halogenation of alkenes, a bromonium ion represents the intermediate step.
• These intermediates are typically less stable than either the reactants or products and are not usually isolated, but they are crucial for understanding the energy transition landscape of the reaction.

By studying intermediates, chemists can optimize conditions for the desired reaction rate and selectivity, leading to more efficient synthetic methods.