Cabenes - A Brief Introduction

A carbene is a neutral molecule which contains a carbon atom with an electron valency of 6 (two bonds and two nonbonding electrons). They are a very reactive type of compound, often too reactive to be observed.

Carbenes can be classified into two major forms:

1. Singlet Carbene
2. Triplet Carbene

 The two classes of carbenes are singlet and triplet carbenes. Singlet carbenes are spin-paired. In the language of valence bond theory, the molecule adopts an sp² hybrid structure. Triplet carbenes have two unpaired electrons.

Properties Of Singlet Carbenes

•  spin multiplicity (2s+1) = 1

• diamagnetic in nature.

• Both Non-Bonding electrons are paired.

• sp² hybridised   

• less stable

Properties of Triplet Carbene 

•  spin multiplicity (2s+1) =3

• paramagnetic in nature.

• Both Non-Bonding electrons are unpaired.

• sp² hybridised   

• more stable

 

Why is Triplet Carbene more stable than Singlet Carbene?

Fairly well asked question, One of the most basic explanation to this question is, In singlet carbene, there are two electrons which are in the same orbital and they repell each one another since they have both the same negative electric charge. in a triplet carbene the two electrons are in different orbitals and there is no repusive force between them

 

Methods Of Formation Of carbenes:

 1. From Haloform

[NOTE:as the Leaving Group ability of the leaving group increases, the rate of formation of carbene increases]

2. From Ketene

3. From Diazo Compounds

 

 4. From Acid Halide

5. From Tosylhydrazones

 6. From Active methylene Compounds

 7. Shapiro Reaction

In this Reaction, the tosyl hydrazones (p-Toluenesulfonyl hydrazones) of aliphatic aldehydes or ketones furnish more substituted alkenes when treated with strong bases like NaOMe, NaH, LiH, NaNH₂ etc.

mechanism:

In aprotic solvents, the diazo compound loses dinitrogen and gives a carbene, which undergoes a faster 1,2-hydrogen shift to furnish a Z-alkene predominantly. 

Chemical Properties of Carbenes

In this section we are going to discuss some Common Name Reactions in which Carbenes are obtained as reaction intermediate.

Wolf Rearrangement

Insertion Reactions of Carbene

1. Insertion with Alkene:

i)insertion of singlet carbene:

ii) insertion of triplet carbene:

2. Insertion with Benzene:

3. Insertion with Pyrrol (Riemann Tiemann Reaction)

Simon Smith Carbene

Arndt Ester Synthesis:

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