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:
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Carbenes can be classified into two major forms:
- Singlet Carbene
- Triplet Carbene
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)