Reaction Intermediates: Cations to Nitrenes

A reaction intermediate is a short-lived, high-energy species that forms during a reaction but does not appear in the overall equation. Carbocations, carbanions, free radicals, carbenes, and nitrenes are the five great intermediates of organic chemistry: too unstable to bottle, yet nearly every substitution, addition, and elimination passes through one of them.

The slowest step of a reaction almost always involves forming one. Know which intermediate forms and how stable it is, and you can predict the rate, the product, and even the stereochemistry.

Key Concepts

• Carbocation (C+): carbon with only 6 valence electrons (a sextet) and a positive charge; planar, sp2 hybridized, with an empty p orbital. Formed by heterolysis of a C−X bond.
• Carbanion (C-): carbon with 8 valence electrons including a lone pair, carrying a negative charge; pyramidal like ammonia.
• Free radical (C·): carbon with 7 valence electrons, one of them unpaired; nearly planar. Formed by homolysis, where a bond splits so each atom keeps one electron.
• Carbene (:C): neutral carbon with only 6 valence electrons — two bonds plus one lone pair. Singlet carbenes have the pair in one orbital; triplet carbenes have two unpaired electrons.
• Nitrene (:N): the nitrogen analogue of a carbene — neutral nitrogen with 6 valence electrons. Formed by decomposition of azides, and used in C−H amination reactions.

Formulas

• Electron count on carbon: a carbocation carbon owns 6 valence electrons (3 bonds × 2, all shared), a radical carbon owns 7 (3 bonds + 1 unpaired), a carbanion carbon owns 8 (3 bonds + 1 lone pair), and a carbene carbon owns 6 (2 bonds + 1 lone pair). Only the carbanion has a complete octet.
• Formal charge of methyl cation CH3+: carbon has 4 valence electrons, 0 nonbonding, 6 bonding → 4 − 0 − 3 = +1. Charge is in units of the elementary charge e.

Worked Example

Classify the intermediate formed when (CH3)3C−Br ionizes in water, and verify its charge and electron count.

1. Ionization is heterolysis: the C−Br bond's electron pair goes entirely to bromine, giving (CH3)3C+ and Br-. Unequal splitting means ions, not radicals.
2. The carbon fragment is a carbocation: central carbon bonded to three methyl groups, so it has 3 bonds = 6 shared electrons and no lone pair.
3. Formal charge on that carbon: 4 valence − 0 nonbonding − (6 ÷ 2) bonding share = 4 − 3 = +1. The species is the tert-butyl cation.
4. Electron count = 6, a sextet — the incomplete octet is why water immediately attacks it in the SN1 second step.

Common Misconceptions

• Intermediates can be isolated in a bottle. Almost never. Their lifetimes are fractions of a second; only exceptionally stabilized ones (like the triphenylmethyl cation) can be isolated as salts.
• Carbenes and nitrenes have full octets. Wrong. Both have only six valence electrons on the central atom, making them as electron-hungry as carbocations despite being neutral.

Real-World Applications

• Polymer manufacture: cationic polymerization of isobutene to make butyl rubber passes through carbocation intermediates; radical polymerization of ethene to polythene passes through carbon radicals.
• Petroleum refining: catalytic cracking of long alkanes into petrol-range molecules runs through carbocation intermediates on the catalyst surface.
• Drug synthesis: the Simmons–Smith reaction uses a carbene-like species to add a CH2 group across a double bond, building cyclopropane rings found in several medicines.

Carbocations, carbanions, radicals, carbenes, and nitrenes are the brief middlemen of organic reactions. Count their electrons and charges — sextet versus octet, +1 versus −1 — and you can tell how each one was born and what it will do next.