Comparing Reactive Intermediates

Introduction

Three carbon-centred intermediates may look similar in abbreviated formulas yet differ by only one or two electrons: carbocation, radical and carbanion. Their charges, orbitals and reactions are not interchangeable. Comparing them side by side makes mechanism claims more precise and prevents using a stability rule for the wrong electron count.

Core explanation

At a simple methyl centre, CH₃⁺ has three C–H bonds and a positive charge, with a vacant p orbital in the elementary planar model. CH₃· has three bonds and one unpaired electron, often in a p-like orbital, and is neutral overall. CH₃⁻ has three bonds and a lone pair, with negative formal charge. These species have the same nuclei but different electrons. One cannot turn one into another by rotating a structure or moving a resonance arrow without changing electron inventory.

An ordinary carbocation is an electron-pair acceptor, making it electrophilic toward a suitable nucleophile. A carbanion can donate its lone pair and often acts as a base or nucleophile. A radical participates in single-electron chemistry, such as chain propagation or recombination, although radical reactions can form ordinary two-electron bonds when two unpaired electrons pair. The specific reaction role depends on partners and conditions, but the electronic starting point constrains what arrows and products make sense.

Alkyl substitution is often stabilising for simple carbocations through induction and hyperconjugation and for simple alkyl radicals through hyperconjugation. For a localised carbanion, electron-donating alkyl groups need not stabilise an already negative centre in the same way; electron-withdrawing groups can be helpful. Do not mechanically apply “tertiary always most stable” to carbanions. In all three classes, resonance may be important when the centre is allylic or benzylic, but the actual electron moved differs: empty orbital for cation, unpaired electron for radical, lone pair for anion.

An enolate illustrates why a category can be model-dependent in a delocalised ion. One resonance contributor places negative charge on α-carbon, a carbanion-like site; another places it on oxygen. The full species is not just a tiny isolated carbanion at one carbon. Similarly, an allyl cation's formal positive charge appears at different carbons in contributors, but the actual ion has delocalised charge. Resonance moves electron placement without shifting nuclei or changing net charge.

Intermediate and transition state must be separated. An intermediate corresponds to a species at a local minimum on a reaction energy profile, however short-lived. A transition state is a high-energy configuration at a barrier top, not a stable species that can be bottled by slowing time. A curved-arrow mechanism may show an intermediate between two arrows, but evidence such as kinetics, trapping or spectroscopy is needed to support its existence. A net reaction formula does not by itself establish whether an intermediate occurs.

Solvent and counterions can transform comparisons. A free carbocation is stabilised by a polar environment and may remain paired with its leaving anion. A carbanion may coordinate a metal cation or be strongly solvated. A radical may remain in a solvent cage near the fragment from which it formed. Therefore, isolated Lewis sketches are models of electron configuration, not complete pictures of the reaction mixture.

Step-by-step reasoning

1. Count the bonds, lone pair, unpaired electron and formal charge at the carbon. 2. Classify the species as cation, anion or radical. 3. Choose full-headed pair arrows or single-headed fishhooks appropriately. 4. Analyse induction, hyperconjugation and resonance for that electron type. 5. Distinguish proposed intermediates from transition states and observed products.

Visual explanation

Place CH₃⁺, CH₃· and CH₃⁻ in three boxes. Mark empty p, singly occupied orbital and lone pair respectively. Draw a lower row with a generic energy diagram showing two peaks around one valley; label the valley intermediate and peaks transition states.

Real-world analogy

Three accounts with a deficit, one unpaired token and a surplus require different transactions. Electron-deficient, odd-electron and electron-rich carbon centres likewise react differently despite sharing the same carbon skeleton.

Real-world example

An alkene addition may proceed through a carbocation in one mechanism, a radical in another and no isolated intermediate in a concerted pathway. The product alone may not prove which happened; solvent, initiator and stereochemical evidence help discriminate.

Why?

Why can the same allyl framework stabilise different intermediate classes? Its connected p orbitals can spread a positive charge, an unpaired electron or a negative lone pair in distinct resonance descriptions, lowering localisation in each case.

Common misconception

“A radical is halfway between a cation and anion in every property.” It has an intermediate electron count in the methyl example, but its one-electron reaction pathways are qualitatively different from simple average behaviour.

Worked example

Classify three drawings with carbon bonded to three H: one marked +, one marked · and one marked − with a lone pair. The + species is a methyl carbocation and electrophilic; the dot species is a methyl radical with an unpaired electron; the − species is a methyl carbanion and a strong electron-pair donor/base. The correct mechanism arrows are pair arrows for cation–nucleophile or carbanion reactions, but fishhooks for radical electron transfers.

Quick check

1. Which of CH₃⁺, CH₃· and CH₃⁻ has a nonbonding electron pair in its simplest Lewis drawing? Answer: CH₃⁻, the carbanion.

Exam focus

Start with electrons and charge, not the word “carbon intermediate.” Treat resonance stabilisation separately from the alkyl substitution trend, and do not label a transition-state peak as a discrete intermediate.

Advanced insight

Modern calculations can distribute formal charge and spin density over several atoms even when a Lewis formula labels one centre. Mechanistic labels remain useful, but experimental trapping and spectroscopy test whether a proposed intermediate has physical support.

Summary

Carbocations, carbon radicals and carbanions differ by vacant orbital, unpaired electron and lone pair, with positive, often neutral and negative charge respectively. Their stabilising effects and reaction arrows must match their electron structures.

Practice questions

1. Which methyl species is an electron-pair acceptor? Answer: CH₃⁺, the carbocation. 2. Which methyl species has an unpaired electron? Answer: CH₃·, the radical. 3. Is a transition state a local energy minimum? Answer: No. It lies at a barrier peak along a reaction path. 4. Can resonance stabilise cation, anion and radical allyl species? Answer: Yes, though their electron movements and charges differ.