Carbenes

Carbenes

source : Quora.com

 Carbene

These are highly reactive neutral species containing a divalent carbon. It acts as reaction intermediate.In carbenes  , carbon atom has four electrons in the valence shell of which two electrons are unshared.

Ex. –    :CH₂  (methylene carbene) ,   :CCl₂   (dichlorocarbene)

:CH₂  (methylene carbene) is parent carbene from which all other carbene compounds are derived.

Methods of preparation-

1) By thermal decomposition of diazo methane –

Methylene carbene is formed

CH₂N₂ ——— > :CH₂ + N₂

2) From halogen –

Carbene is formed by the  alpha elimination of  HX from haloform with base.

CHCl₃ + B^– ————> BH + :CCl₂ (dichloro Carbene) + Cl^–

3) By photochemical decomposition of ketene-

Carbene is formed.

CH₂CO  ———> :CH₂ (Carbene) + CO

Structure –

Carbenes are classified as either SINGLET CARBENE  & TRIPLET CARBENE.

SINGLET CARBENE-

In singlet state  carbon atom is in sp² hybridisation state. One of sp² hybrid orbital contains  unshared electron  pair with opposite spins & two sp² hybrid orbitals  form two covalent bonds . The geometry is planar.There is no magnetic moment in this state.

source : Quora

TRIPLET CARBENE –

In many triplet Carbene , carbon atom is in sp hybridization state. Two hybrid orbitals form sigma bond . Two remaining electrons with parallel spin occupy mutually perpendicular py & Pz orbitals. Geometry is linear . Such carbene would have magnetic moment.

Triplet carbenes are also known where carbon atom is in sp2 hybridisation state.Two nonbonding electrons occupy an sp2 atomic orbital & an unhybridized  p-atomic orbital with parallel spin.Their geometry is bent .

source : OChemPal

Applications-

Carbene acts as reaction intermediate in so many organic reactions .

1) Carbyl amine reaction –

        DichloroCarbene is reaction intermediate  in this reaction.

RNH₂ + CHCl₃  + KOH  (alc. ) ———–> RNC  +  3KCl  + 3 H₂O

2) Reimer  Tiemann reaction –

DichloroCarbene is reaction intermediate in this reaction .

3) Wittig reaction –

 4) In the preparation of allenes-

Stability of Carbenes –

Carbenes in which the carbon of carbene is attached  to two atoms , each bearing a lone pair of electron are more stable due to resonance.

Triplet Carbenes are more stable than singlet Carbene.