Chemistry CARBENE & ITS STABILITY

Carbene :

It is a species having a neutral carbon with two bond pairs and two unshared electrons. It is of two types : Singlet and triplet.

(i) Singlet : Total spin `= + 1/2 - 1/2 = 0,` only one value.

`-underset(..)overset(|) C`

(ii) Triplet : It is more stable according to Hund's rule.

`+ 1/2 + 1/2 ,` total spin `= 1`

`-1/2 - 1/2 ,` total spin `= -1` Three different values of total spin.

`-1/2 +1/2 ,` total spin `= 0`

`-underset(.)overset(|)C*`

Both the unpaired electrons have parallel spins in ground state of triplet carbene. See fig.

Carbenes are neutral intermediates having bivalent carbon, in which a carbon atom is covalently bonded to two other groups and has two valency electrons distributed between two non bonding orbitals. When the two electrons have paired spin the carbene is a singlet, if the spins of the electrons are parallel it is a triplet.

Structure :

A singlet carbene is thought to posses a bent `sp^2` hybrid structure in which the paired electrons occupy the vacant `sp^2` orbital. A triplet carbene can be either bent `sp^2` 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.

Generation & Reactions :

`text(Generation :)` Carbenes are obtained by thermal or photochemical decomposition of diazoalkanes. These can also be obtained by `alpha`-elimination of a hydrogen halide from a haloform with base, or of a halogen from a gem dihalide with a metal.

`text(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. See fig.

Formation of Carbene :

`text(By dehydrohalogenation of Alkyl halide using a base :)` See fig.1.

`text(Note:)` Removal of `H^+` and `X^-` from same atom occurs only if their removal from adjacent atoms is not possible. See fig.2.

`text(By Dehalogenation of geminal dihalides using Mg or Zn :)` See fig.3.

`text(Note :)` Removal of two halogen groups from same atom occurs only if their removal from adjacent atoms is not possible. See fig.4.

`text(By photolysis or pyrolysis of Ketene :)` See fig.5.

`text(By photolysis or pyrolysis of Diazoalkane :)` See fig.6.

Behaviour of Carbene :

(i) Singlet carbene prefers to undergo shifting of a group or atom from adjacent position. See fig.1.

(ii) Singlet carbene can behave either like electrophile only (due to presence of vacant orbital) or like electrophile as well as like nucleophile in same step (due to presence of vacant orbital as well as lone pair). See fig.2.

(iii) Singlet carbene can slowly convert to more stable triplet carbene. This is significant only if singlet carbene is unable to react in singlet state itself.

(iv) Triplet carbene can react like a diradical. See fig.3.