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The structure of carbenes depends on how they are made

المؤلف:  Jonathan Clayden , Nick Greeves , Stuart Warren

المصدر:  ORGANIC CHEMISTRY

الجزء والصفحة:  ص1013

2025-07-31

568

The structure of carbenes depends on how they are made

So far we have considered only the most stable possible structure, singlet or triplet, of a given carbene. In reality, a carbene will be formed in a chemical reaction and may well be formed as the less stable of the alternatives. If a reaction occurs by an ionic mechanism on a molecule with all electrons paired (as most molecules are!) then it must be formed as a singlet. Follow the α elimination mechanism, for example.

The starting material, a molecule of chloroform CHCl3, has all paired electrons. The C–H σ bond breaks and the two paired electrons from it form the lone pair of the carbanion. The carbanion also has all paired electrons. The two paired electrons of one of the C–Cl bonds leaves the carbanion and the carbene is formed. It has two paired electrons in each of the two remaining C–Cl bonds and the lone pair, also paired. It is formed as a singlet. As it happens, the singlet version of CCl₂ is also the more stable. But if the carbene were instead CH₂ and if it reacted rapidly, it might not have a chance to change into the more stable triplet state. Since carbenes are very reactive, this question can be important. In explaining their reactions in the next section we shall need to consider:

 • how the carbene was formed

• how rapidly it reacts

• whether it can change into the other state (singlet or triplet).