Which of the following reaction does not involve a...

Which of the following reaction does not involve a carbocation as intermediate? (a) $\mathrm{C}_6 \mathrm{H}_6+\mathrm{Br}_2 \xrightarrow{\mathrm{AlBr}_3} \mathrm{C}_6 \mathrm{H}_5 \mathrm{Br}$ (b) $\mathrm{CH}_2=\mathrm{CH}_2+\mathrm{Br}_2 \longrightarrow \mathrm{BrCH}_2-\mathrm{CH}_2 \mathrm{Br}$ (c) $\left(\mathrm{CH}_3\right)_3 \mathrm{COH}+\mathrm{HBr} \mathrm{H} \xrightarrow{+}\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{H}_2 \mathrm{O}$ (d) Both (b) and (c)

Which of the following reaction does not involve a carbocation as intermediate?
(a) $\mathrm{C}_6 \mathrm{H}_6+\mathrm{Br}_2 \xrightarrow{\mathrm{AlBr}_3} \mathrm{C}_6 \mathrm{H}_5 \mathrm{Br}$
(b) $\mathrm{CH}_2=\mathrm{CH}_2+\mathrm{Br}_2 \longrightarrow \mathrm{BrCH}_2-\mathrm{CH}_2 \mathrm{Br}$
(c) $\left(\mathrm{CH}_3\right)_3 \mathrm{COH}+\mathrm{HBr} \mathrm{H} \xrightarrow{+}\left(\mathrm{CH}_3\right)_3 \mathrm{CBr}+\mathrm{H}_2 \mathrm{O}$
(d) Both (b) and (c)

Key Concepts

Carbocation Intermediate

A carbocation is a positively charged carbon species that forms during many chemical reactions when a leaving group departs, leaving behind an electron-deficient carbon atom. These intermediates are key in mechanisms such as SN1 reactions and can be stabilized through hyperconjugation and resonance.

Bromonium Ion Intermediate

A bromonium ion is a cyclic, positively charged intermediate formed when a halogen adds to an alkene. Instead of generating a free carbocation, the halogen bridges the two carbon atoms of the double bond, creating a three-membered ring structure that avoids the formation of a discrete carbocation.

Electrophilic Aromatic Substitution

This reaction mechanism involves an aromatic ring interacting with an electrophile to form a sigma complex, which is a resonance-stabilized intermediate. Although this intermediate has carbocation-like character, its stabilization by the aromatic system means it is not considered a free carbocation in the same sense as those in SN1 reactions.

SN1 Reaction Mechanism

The SN1 reaction mechanism proceeds via the formation of a discrete carbocation intermediate after the loss of a leaving group. This unstable intermediate is then attacked by a nucleophile, making SN1 reactions common in systems where the carbocation formed is particularly stable, such as tertiary centers.

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