Reactive Intermediates

CompleteFundamentals of Chemistry

Short-lived species formed during reactions — carbocations, carbanions, free radicals, carbenes, arynes and nitrenes.

Full notes

Definition: Reactive intermediates are short-lived, high-energy species occurring between reactants and products in a multistep reaction.

1. Carbocations (R₃C⁺)

sp², planar, 6 valence electrons — electron-deficient. Stability: 3° > 2° > 1° > CH₃⁺ (+I and hyperconjugation). Rearrange (1,2-hydride/alkyl shifts) to give more stable ions.

2. Carbanions (R₃C⁻)

8 valence electrons, sp³-like, electron-rich nucleophiles. Stability order is reverse: CH₃⁻ > 1° > 2° > 3°. Stabilized by −I / −M groups.

3. Free radicals (R₃C•)

Neutral, one unpaired electron, formed by homolytic cleavage. Stability: 3° > 2° > 1° > CH₃• (hyperconjugation). Planar or pyramidal.

4. Carbenes, arynes, nitrenes

Carbene (:CH₂): divalent carbon, 6 e⁻; singlet (paired) and triplet (unpaired) states; intermediate in Simmons-Smith and Reimer-Tiemann reactions.
Aryne (benzyne): highly strained triple bond in a benzene ring; intermediate in nucleophilic aromatic substitution via elimination-addition.
Nitrene (:NH): nitrogen analogue of carbene, 6 e⁻; intermediates in Curtius/Hofmann rearrangements.

Intermediate	Stability order	Hybridization
Carbocation	3°>2°>1°>CH₃⁺	sp²
Carbanion	CH₃⁻>1°>2°>3°	sp³
Free radical	3°>2°>1°>CH₃•	sp² (approx)

Note: Exam point: state the stability order AND justify with hyperconjugation/hydration (+I) — those ordering rules carry marks.

Exam notes

Carbocation sp² planar 6e⁻, stable 3°>2°>1°. Carbanion sp³ 8e⁻, reverse order. Radical like carbocation order. Carbene/aryne/nitrene are 6e⁻ intermediates. 1,2-shifts make more stable ions.

Important points

Stability rules come from +I / hyperconjugation acting on electron deficiency.

Common mistakes

Applying the carbocation order to carbanions.

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