Answer to Question #32453 in Organic Chemistry for naeim
The term "carbene" may also refer to the specific compound H2C: , also called methylene, the parent hydride from which all other carbene compounds are formally derived.
Carbenes are classified as either singlets or triplets depending upon their electronic structure. Most carbenes are very short lived, although persistent carbenes are known.
One well studied carbene is Cl2C: , or dichlorocarbene, which can be generated in situ from chloroform and a strong base. There are three possible spin states for a carbene R2C:, namely, triplet R2C(↑)(↑) (state = number of unpaired spins plus one) and two singlet states R2C(↑)(↑) and R2C(↑↓)(0). So yes the last state can be less stable because of e- e- repulsion between the two electrons on the same MO (for your purposes an sp2 hybrid orbital). The reason why the triplet state is lower in energy than the other singlet state is due to the spin exchange interaction (Hund's rule; a quantum mechanical interaction that "spreads out" the electrons with parallel spins). As you know p2 is most stable as px(↑)py(↑). This is found for R = H, alkyl, phenyl carbenes. In carbenes with a heteroatom (eg, (MeO)2C: , (R2N)2C: ) there is interaction of the filled p AO on N or O with the formally empty 2pz AO on the C atom and this raises the energy of the empty pz AO on C and the two electrons pair up to give to give the now more stable singlet carbene: R2C(↑↓)(0)←N(O)2p
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