Question

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

Short answer

Reaction (b) does not involve a carbocation intermediate.

Step-by-step solution

Understanding Reaction Intermediates

Carbocations are positively charged ions that often form as intermediates during certain types of organic reactions, particularly those involving substitution or electrophilic addition.

Analyzing Reaction (a)

In reaction (a), benzene reacts with bromine in the presence of aluminum bromide ( ext{AlBr}_3). This is an electrophilic aromatic substitution. Here, a bromonium ion forms that attacks the benzene, leading to ext{C}_6 ext{H}_5 ext{Br} after deprotonation. A carbocation intermediate does form temporarily on benzene.

Analyzing Reaction (b)

This reaction involves an alkene reacting with bromine to form a vicinal dibromide ( ext{BrCH}_2- ext{CH}_2 ext{Br} ). This is an anti-addition reaction via a bromonium ion, not a carbocation.

Analyzing Reaction (c)

In this reaction, ext{(CH}_3 ext{)}_3 ext{COH} reacts with ext{HBr} to form ext{(CH}_3 ext{)}_3 ext{CBr} . This reaction proceeds via the formation of a tertiary carbocation after protonation of the alcohol, followed by nucleophilic attack by bromide.

Determining the Correct Answer

Reaction (b) does not involve a carbocation intermediate, while reactions (a) and (c) do involve carbocations. Thus, option (d) claiming both (b) and (c) is incorrect. The correct choice, based on the absence of a carbocation intermediate, specifically applies to option (b).

Key concepts

Organic Reaction Mechanisms

Organic reaction mechanisms describe the step-by-step process by which reactions occur in organic chemistry. They serve as a map, showing how molecules transform from reactants to products. During these transformations, intermediate species are often formed. These intermediates include:

• Carbocations: Positively charged ions where a carbon atom has only six electrons in its outer shell, making it very reactive.
• Radicals: Atoms or molecules with an unpaired electron, which are highly reactive.
• Anions: Negatively charged ions, featuring carbon with a full octet.

Carbocations are most commonly encountered in substitution, addition, and rearrangement reactions. Knowing the intermediate helps predict the course of the reaction, and understanding these mechanisms is crucial for predicting reactant outcome and designing new synthetic pathways.
Intermediates form differently based on the structure of the reactants and the conditions under which the reaction occurs. This can influence the rate and direction of the reaction. By studying mechanisms, chemists gain deeper insight into how and why reactions take place.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a key reaction involving aromatic compounds, such as benzene. In EAS, an electrophile replaces one of the hydrogen atoms in the aromatic ring. Aromatic compounds are stable, and their reactions often involve temporary instability, making EAS mechanisms vital to understand.
The process typically involves:

• Formation of an electrophile: Often through the interaction of a halogen with a Lewis acid (e.g., \( ext{Br}_2 \) with \( ext{AlBr}_3 \)).
• Electrophile attacks the aromatic ring: Momentarily disrupting the aromaticity, creating a sigma complex or arenium ion, stabilizing through resonance.
• Deprotonation: Restores the aromaticity, resulting in the substituted aromatic product.

Despite its temporary loss of stability, the intermediate allows the molecule to return to an aromatic state, facilitating the substitution. The role of carbocations in the EAS mechanism is transient but crucial, making understanding these processes essential for manipulating benzene derivatives in organic synthesis.

Anti-Addition Reaction

Anti-addition reactions, particularly in alkenes, involve the addition of two substituents to opposite sides of a double bond. They occur in a stereospecific manner, meaning the spatial orientation of the reactants affects the outcome. Anti-addition is prominently observed in bromination reactions involving alkenes.
The general mechanism proceeds as follows:

• Formation of a bromonium ion: A halogen forms a cyclic ion with the existing double bond.
• Attack by a nucleophile (often anions or other halogen atoms) on the opposite side of the cyclic bromonium ion.

The resulting stereochemical outcome is the creation of a vicinal dibromide, where the two bromine atoms are added on opposite sides of the former double bond. This anti-addition approach ensures the molecular structure maintains maximum stability throughout the process, with no carbocations formed as intermediaries. Recognizing anti-addition is crucial as it dictates the stereochemistry necessary for chiral molecules in synthesis.

Alkene Reactions

Alkene reactions are an exciting area of organic chemistry due to the rich variety of transformations they can undergo. Alkenes are hydrocarbons containing a carbon-carbon double bond, which is reactive and serves as a site for many chemical transformations.
Common alkene reactions include:

• Addition Reactions: Generally, the double bond is broken as atoms or groups add to the carbons.
• Rearrangements: Where molecular structures are reformed, sometimes involving shifts in connectivity, especially in the presence of acid.
• Oxidation: Adding oxygen to the alkene, often converting it into an epoxide, diol, or other oxygen-containing product.

Addition reactions are further classified based on the type of addition:

• Syn-addition: Adding substituents to the same side of the double bond.
• Anti-addition: Adding substituents to opposite sides, as seen in bromination.

Alkene reactions do not always involve carbocation intermediates; rather, they depend on the specific mechanism at play. Mastery of these diverse reactions allows chemists to synthesize a wide array of organic compounds.