Reaction Intermediates in Organic Chemistry for JEE Main

Organic reaction intermediates are the transient species formed during the conversion of reactants to products. Understanding carbocations, carbanions, free radicals, and carbenes explains why reactions have the regiochemistry and stereochemistry they do. JEE Main tests their stability order, their role in determining products, and the reaction conditions that generate each type. This conceptual knowledge predicts outcomes without memorising individual reactions.

Carbocations: Stability and Rearrangements

A carbocation (carbonium ion) is a carbon bearing a positive charge and only three bonds. Stability order: tertiary > secondary > primary > methyl. Alkyl groups stabilise via hyperconjugation and inductive donation of electron density. Vinyl and phenyl cations are very unstable. Allyl and benzyl cations are stabilised by resonance: the positive charge delocalises over two carbons (allyl) or the ring (benzyl). Rearrangements (1,2-hydride or 1,2-methyl shifts) occur when a neighbouring bond migrates to the carbocation, generating a more stable cation. Recognising the rearrangement product requires asking: is there a more stable cation one shift away? See the reaction mechanisms context in our reaction mechanisms guide.

Carbanions: Stability Order

A carbanion bears a negative charge and an electron pair at carbon. Stability is the inverse of carbocations: primary > secondary > tertiary (electron-withdrawing inductive effect from neighbouring bonds destabilises the anion). Resonance-stabilised carbanions (allyl, benzyl, next to carbonyl) are more stable than simple alkyl carbanions. The pKa of terminal alkynes is ~25 (acetylide carbanion) vs alkanes ~50 — alkynes form isolable carbanions with strong bases like NaNH₂. This acidity is exploited in acetylide syntheses.

Free Radicals: Stability and Halogenation

Free radicals have an unpaired electron at carbon. Stability: tertiary > secondary > primary > methyl (hyperconjugation stabilises). Allyl and benzyl radicals are even more stable via resonance. Free radicals are formed by homolytic bond cleavage (UV light, peroxides, high temperature). Radical halogenation of alkanes: Cl₂ is less selective (attacks primary and secondary C–H roughly equally); Br₂ is highly selective (strongly prefers tertiary C–H). This selectivity difference is a frequently-tested conceptual point. UV light but no peroxide → radical mechanism; HX without UV or peroxide → ionic mechanism.

Carbenes and Nitrenes

Carbenes (:CR₂) have a carbon with two bonds and either a lone pair or two unpaired electrons (singlet vs triplet carbene). Generated from diazomethane (CH₂N₂) or CHCl₃/base. Carbenes insert into C–H bonds and add to double bonds: addition of :CH₂ to ethylene gives cyclopropane. Nitrenes (R–N:) are the nitrogen analogues and appear in Curtius and Lossen rearrangements. JEE occasionally tests carbene addition to alkenes as a route to cyclopropane that is distinct from the Simmons-Smith reaction.

Using Intermediate Stability to Predict Products

The key skill is using intermediate stability to predict the major product without knowing the full mechanism. For electrophilic addition to alkenes: the more stable carbocation forms → Markovnikov product. For radical halogenation: the more stable radical forms → selective product at most substituted position (Br₂) or roughly statistical (Cl₂). This stability-based reasoning is the tool JEE tests most in mechanism questions. After mastering intermediates, practise predicting products from reaction conditions, then take a free mock test on organic mechanisms.