A carbene is a highly reactive organic species in which a carbon atom has only six valence electrons (instead of the usual eight) and is bonded to two substituents.

General Features:

• General formula: R₂C:
• Structure: Carbon in carbene is divalent (forms 2 bonds).
• Electron deficiency: Only 6 valence electrons → makes carbenes very reactive.
• Bond angle: Usually about 104–150°, depending on type.

Types of Carbenes:

1. Singlet carbene
   • Both non-bonded electrons are paired in the same orbital.
   • Usually bent structure (~104° bond angle).
   • Electrophilic (electron-loving).
2. Triplet carbene
   • The two non-bonded electrons occupy different orbitals with parallel spins.
   • Linear or nearly linear structure (~130–150° bond angle).
   • More stable than singlet.

Examples:

• Methylene (:CH₂) → simplest carbene.
• Dichlorocarbene (:CCl₂) → formed in Reimer–Tiemann reaction.

Preparation:

• By photolysis or pyrolysis of diazo compounds (e.g., CH₂N₂ → :CH₂ + N₂).
• By decomposition of haloforms (CHCl₃ + base → :CCl₂).

Reactivity:

• Carbenes add to double bonds → form cyclopropanes.
• Insert into C–H bonds.
• Highly reactive intermediates in organic chemistr