. Explain Briefly Structure and Reaction of Carbenes
Structure: A singlet carbene is thought to posses a bent sp2 hybrid structure in which the paired electrons occupy the vacant sp2 orbital. A triplet carbene can be either bent sp2 hybrid with an electron in each unoccupied orbital, or a linear sp hybrid with an electron in each to the unoccupied p-orbital. It has however, been shown that several carbenes are in a non-linear triplet ground state. However, the dihalogenocarbenes and carbenes with oxygen, nitrogen and sulphur atoms attached to the bivalent carbon, exist probably as singlets. The singlet and triplet state of a carbene display different chemical behaviour. Thus addition of singlet carbenes to olefinic double bond to form cyclopropane derivatives is much more stereoselective than addition of triplet carbenes.
Reactions: These add to carbon double bonds and also to aromatic systems and in the later case the initial product rearranges to give ring enlargement products (a car-benoids –organometallic or complexed intermediates which, while not free carbenes afford products expected from carbenes are usually called carbenoids). When a carbene is generated in a three membered ring allenes are formed by rearrangement. However, a similar formation at a cyclopropylmethyl carbon gives ring expansion. Carbenes are also involved in Reimer —Tiemann reaction.