Carbenes and Nitrenes: Structure, Spin State and Reactions

Electron count and geometry

A carbene is a neutral divalent carbon species, R₂C:, carrying two bonds and one lone pair. That is six valence electrons, so it is electron deficient and highly reactive. A nitrene, R–N:, is the nitrogen analogue with one bond and two lone pairs, also six valence electrons.

The two non-bonding electrons can be arranged in two ways, and the arrangement changes the geometry as well as the chemistry.

How they are generated

• Alpha elimination. Chloroform with strong base loses a proton and then chloride to give dichlorocarbene. Both groups leave from the same carbon, which is what distinguishes alpha from the familiar beta elimination.
• Diazo decomposition. Diazomethane on photolysis loses nitrogen gas to give methylene. Photolysis tends to give the singlet initially; the singlet can then relax to the triplet on collision with an inert gas.
• Simmons–Smith reagent. Diiodomethane with a zinc–copper couple gives a carbenoid, a species that behaves like a carbene without ever being free. It cyclopropanates alkenes cleanly and stereospecifically.
• Nitrenes come from azides by loss of nitrogen, thermally or photochemically, and from the Curtius and Hofmann rearrangement families where the nitrene-like centre is generated but migration is concerted.

The Skell test: how spin state was proved

This is the classic experiment and it appears repeatedly in examinations because it links mechanism to observable stereochemistry.

A singlet carbene has both a filled orbital and an empty orbital, so it can form both new C–C bonds at once. Addition to cis-2-butene gives only the cis-dimethylcyclopropane. Nothing rotates because there is no intermediate.

A triplet carbene is a diradical. It forms one bond, giving a 1,3-diradical, and the second bond cannot form until one electron flips its spin, which is slow. During that delay the central C–C bond rotates freely, so both cis and trans products appear from a single alkene geometry.

Other reactions worth knowing

Insertion into C–H bonds

Free methylene is so reactive that it inserts almost indiscriminately into C–H bonds, which makes it useless synthetically. Selectivity improves markedly for stabilised carbenes such as dichlorocarbene and for metal-bound carbenoids, which is why synthesis uses those instead.

The Reimer–Tiemann reaction

Phenol with chloroform and aqueous hydroxide gives salicylaldehyde. The electrophile is dichlorocarbene generated in situ, the phenoxide attacks it at the ortho position, and hydrolysis of the resulting dichloromethyl group gives the aldehyde. Naming dichlorocarbene as the attacking species is the point of the question.

Wolff rearrangement

An alpha-diazoketone loses nitrogen to give a ketocarbene, which rearranges by migration of the adjacent group to give a ketene. This is the key step of the Arndt–Eistert homologation, which lengthens a carboxylic acid chain by one carbon.

Nitrene chemistry

Nitrenes add to alkenes to give aziridines and insert into C–H bonds to give amines, mirroring carbene behaviour throughout. Acyl nitrenes rearrange readily to isocyanates, which connects this topic directly to the Curtius and Hofmann rearrangements.

Frequently asked questions

How do I decide whether a carbene will be singlet or triplet?

Look at the substituents. Groups with lone pairs, such as halogens, oxygen or nitrogen, donate into the empty p orbital and stabilise the singlet. Simple alkyl or hydrogen substituents leave the triplet lower in energy. Dichlorocarbene is the standard singlet example and methylene the standard triplet.

Why is singlet addition stereospecific but triplet addition is not?

The singlet forms both bonds in one concerted step, so there is no intermediate in which rotation could occur. The triplet forms a diradical intermediate that must undergo spin inversion before closing, and rotation about the central bond is faster than that inversion.

What is a carbenoid, and how does it differ from a carbene?

A carbenoid is a species that transfers a carbene-like unit without a free carbene ever existing. The Simmons–Smith reagent is the standard example. Because no free carbene forms, the reaction is cleaner and reliably stereospecific.

What is alpha elimination?

Loss of two groups from the same carbon atom, as when chloroform loses a proton and then a chloride to give dichlorocarbene. It contrasts with beta elimination, where the two groups leave from adjacent carbons to give an alkene.

How are nitrenes related to the Curtius rearrangement?

Both involve an electron-deficient nitrogen generated by loss of nitrogen gas from an azide. In the Curtius the migration is concerted with that loss, so a free nitrene is not an intermediate, but the electronic situation is the same and the two topics are usually taught together.