Arrangement of electrons in singlet and triplet carbene.
The difference in the arrangement of electrons in singlet and triplet carbene.
The difference in the arrangement of electrons in singlet and triplet carbene.

Core Concepts

In this topic you will learn about properties of the difference between singlet and triplet carbene, their generation and properties.

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What are Carbenes?

Carbenes are divalent (covalently bonded to two other atoms or groups) neutral carbon atoms with six electrons in the outermost shell, out of which two are unshared valence electrons. They are the intermediate species formed by breaking groups attached to carbon atoms. The 6 electrons in the valence shell cause electron deficiency, thus making them short-lived and highly reactive. The simplest form of carbene is methylene: CH2

General Formula= R−:C−R’ or R=C:

:CH2 (Methylene)

Carbenes are classified as singlet or triplet depending on whether the two nonbonding electrons lie in the same or different orbitals.

Singlet Carbene

The carbon atom in carbene is usually sp2 hybridized with three hybrid orbitals. Two of the hybrid orbitals are covalently bonded to other atoms, leaving one unhybridized p orbital and one hybridized sp2 orbital. The two non-bonding electrons sit paired in the hybridized sp2 orbital and have opposite spins. Thus, it is paramagnetic.

Paired electrons in sp2 hybrid orbital of singlet carbene.
Paired electrons in sp2 hybrid orbital of singlet carbene.

Triplet Carbene

When the non-bonding electrons sit in two different orbitals (empty p-orbital and hybridized sp2 orbital) they acquire parallel spin and are known as triplet carbene. It has linear as well as bent geometry. The linear geometry arises when the central carbon atom forms two sp hybrid orbital. In this case, two vacant p-orbitals get filled by the non-bonding electron.

Bent and linear triplet carbene
Bent and linear triplet carbene


Generally, carbenes have an angular structure and exist in their triplet state. The triplet state is more stable than the singlet state because it has two unpaired electrons and has energy 8 Kcal/mol lower than the singlet According to Hund’s rule of multiplicity, the multiplicity state is given by (2S+1), where S is the number of unpaired electrons. A triplet state having two unpaired electrons has a high multiplicity state which results in lower energy and high stability.

Generation of Carbene

The diazo compound and hydrazones are common precursors for the generation of carbene.

Diazo compound

Formation of Carbene
Formation of Carbene

Diazoalkane when treated with heat or light generates carbene after the removal of nitrogen.


Formation of carbene from hydrazone
Formation of carbene from hydrazone

Ketone and aldehyde react with hydrazine to form hydrazone, which oxidizes to form a Diazo compound.

Reimer–Tiemann reaction

The Reimer-Tiemann reaction is a simple reaction where the deprotonation of chloroform leads to the formation of reactive dichlorocarbene. Here, the carbene is an electron deficient species due to the presence of two electron-withdrawing chlorine atoms, making it a good site for nucleophilic attack by the negatively changed phenoxide.

Carbenes Practice Problems

Problem 1

What is the role of Carbene is chemical reaction?

Problem 2

What is the difference between singlet and triplet carbene ?

Problem 3

Are carbenes electrophilic or nucleophilic?

Problem 4

What are the effects of substituents on Carbene?

Carbenes Practice Problem Solution

  1. Carbenes are highly reactive intermediates and assist the chemical reaction by forming new C-C bonds and adding to double bonds. Fischer carbene, a type of divalent organic ligand is a transition metal carbene complex.

3. Carbenes are neutral species that can be electrophilic or nucleophilic. The electrophilic nature is due to the absence of an octet state, whereas the presence of electron- donating substituents makes them nucleophilic, for example, Schrock carbenes.

4. If substituents attached to carbene are electron withdrawing it prefers singlet form whereas, if the substituents are electron donating triplet form is preferred.