1. Synthesis of Heterocyclic Rings 5 (carbenes and nitrenes)

2. Carbenes are uncharged, electron deficient molecular species that contain a divalent carbon atom surrounded by a sextet of electrons.
Generally there are two types of carbenes; singlet or triplet carbenes.
Singlet carbenes have a pair of electrons and an sp2 hybrid structure. They have triagonal planar geometry like a carbocation, and show electrophilic character.
Triplet carbenes have two unpaired electrons (diradicallike). They may be either sp2 hybrid or linear sp hybrid. Most carbenes have a nonlinear triplet ground state
Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media.
in general, triplet is the more stable state (the ground state) and singlet is the excited state species.

3. Nitrenes are uncharged, electron deficient molecular species that contain a monovalent nitrogen atom surrounded by a sextet of electrons. Nitrenes are very reactive and not isolated as such. They are formed as reactive intermediates in the reactions:

4. Formation Reactions of Carbenes

6. Reactions of Carbenes Addition to multiple bonds

8. Insertions of carbene (3 C-H > 2 C-H > 1 C-H

10. Formation of nitrene
from thermolysis or photolysis of azides

11. from isocyanates, with expulsion of carbon monoxide
R-N=C=O gives R-N

12. From N-amino heterocycles

13. From photolysis of Sulfilimines:

14. Curtius rearrangement