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) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{Br}_{2} \quad \mathrm{AlBr}_{3} \quad \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}^{2} \overline{+\mathrm{Br}_{2} \stackrel{3}{\longrightarrow}}{\longrightarrow} \mathrm{BrCH}_{2}-\mathrm{CH}_{2} \mathrm{Br}\) (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

Option (b) does not involve a carbocation intermediate.

Step 1: Identify Reaction Types

First, identify the type of organic reactions described in each option. Option (a) is a Friedel-Crafts Bromination involving a Lewis acid (AlBr₃), option (b) is an electrophilic addition of Br₂ to an alkene, option (c) is a substitution reaction of an alcohol with HBr.

Step 2: Assess Carbocation Formation

Consider how carbocations are typically formed in reactions. They commonly occur in substitution reactions (like SN1) or rearrangements. In (a), the reaction proceeds through an electrophilic aromatic substitution where a carbocation intermediate (arenium ion) is formed on the benzene ring. In (b), bromine adds to the alkene forming a cyclic bromonium ion, not a carbocation. In (c), the alcohol reacts with HBr, forming a carbocation intermediate after the hydroxyl group is protonated and leaves.

Step 3: Determine Non-Carbocation Reaction

Identify which reaction does not involve a carbocation intermediate. In (a), the arenium ion is a type of carbocation. In (b), no carbocation is formed since the intermediate is a bromonium ion, not a classic carbocation. Reaction (c), the tertiary carbocation forms as an intermediate during the substitution process.

Step 4: Conclusion

Given the analyses above, option (b), which involves a bromonium ion rather than a carbocation, is the reaction that does not involve a carbocation intermediate.

Key Concepts

Friedel-Crafts Bromination

Friedel-Crafts Bromination is a type of electrophilic aromatic substitution reaction. This process involves the substitution of a hydrogen atom on an aromatic ring with a bromine atom. To facilitate this, a Lewis acid like aluminum bromide (AlBr₃) is necessary as a catalyst.
This catalyst helps in generating the bromonium ion, which is a powerful electrophile. In this reaction, the benzene ring acts as a nucleophile, attacking the electrophilic bromonium ion.
This results in the formation of a carbocation intermediate known as an arenium ion. Eventually, the hydrogen is replaced by the bromine, completing the substitution.

• The catalyst not only activates the bromine but also stabilizes the carbocation formed during the intermediate stage.
• Although instead of a direct carbocation, the arenium ion is a stabilized version of it.
• Friedel-Crafts reactions are highly useful in organic synthesis for modifying benzene rings.

Understanding this mechanism is crucial for comprehending how aromatic compounds undergo substitution reactions.

Electrophilic Addition

Electrophilic addition reactions are key reactions that occur in alkenes or alkynes. These reactions typically involve the addition of elements across a carbon–carbon double or triple bond.
In the context of the problem, an alkene reacts with bromine to form a dibromo compound. Unlike Friedel-Crafts Bromination, electrophilic addition to alkenes does not form carbocations but rather forms a temporary bromonium ion.

• During this reaction, the double bond of the alkene acts as a nucleophile.
• It attacks the electrophilic bromine to form the bromonium ion.
• This intermediate is a three-membered ring, quite distinct from typical carbocations.

As the reaction proceeds, the bromonium ion is opened by a bromide ion, resulting in the addition product where both bromine atoms attach across the initial double bond.

Substitution Reactions

Substitution reactions are fundamental in organic chemistry, involving the replacement of one group in a molecule with another.
One classic example is when an alcohol reacts with hydrogen bromide (HBr) to form an alkyl bromide.
This type of reaction might involve carbocations.

• Initially, the \(\mathrm{OH}\) group in the alcohol is protonated by \(\mathrm{HBr}\) to form \(\mathrm{H}_2\mathrm{O}\) as a leaving group.
• This departure creates a carbocation intermediate, which can be quite stable, especially within tertiary carbons.
• The bromide ion then acts as a nucleophile, attacking the carbocation forming the alkyl bromide.

Understanding substitution reactions and the role of carbocations is essential for chemists when predicting reaction outcomes, especially SN1 reactions that proceed through such intermediates.

Bromonium Ion

A bromonium ion is a crucial intermediate in certain electrophilic addition reactions involving alkenes and bromine.
Unlike most carbocations, a bromonium ion is a positively charged ion, where bromine has formed a three-membered ring with two carbons.
This structure is surprisingly stable due to its cyclic nature.

• The creation of the bromonium ion involves initial electrophilic attack of bromine by the alkene.
• This forms a charge-stabilized intermediate, rather than an unstable carbocation.
• Its stability prevents rearrangements common in typical carbocation intermediates.

The bromonium ion can then be attacked by a nucleophile, such as a bromide ion, which results in a dibromo compound. Understanding the dynamics of bromonium ions helps explain why certain reactions avoid carbocation intermediates altogether, as seen in electrophilic addition to alkenes.

Organic Reaction Mechanisms

Organic reaction mechanisms detail each step and transformation during chemical reactions involving organic compounds.
They provide a roadmap for understanding how substrates transform into products.
Insights into these steps can clarify reaction conditions and intermediates, including carbocations and bromonium ions.

• A deep grasp of mechanisms allows chemists to predict how changes in conditions can affect the outcome.
• They guide the synthesis of complex organic molecules.
• They also determine how and why specific intermediates form, affect reaction rates, and influence product distribution.

Comprehensive knowledge of organic reaction mechanisms is invaluable for students and professionals alike, offering deeper insight into the vast array of possible organic transformations.