Chemistry STABILITY OF CARBANIONS

Stability Of Carbanions :

See fig.1.

The stability order `3° < 2° < 1° <` methyl can be explained on the basis of `+I` effect of alkyl groups (electron-releasing) which increases `-ve` charge and decreases stability. The stability of allyl and benzyl carbanions is due to resonance. See fig.2.

Formation :

See fig.1.

Carbanion itself is very strong base hence this method is not generally useful. However, this method can be used in some cases where the carbanion is very stable (if that hydrogen is acidic). See fig.2.

From Alkyl Halide :

We know that `RX` give `R^(+)` so if we add two electrons, we may get alkyl carbanion. Hence we should add a metal which can give two electrons. Water cannot be used as solvent because metal can liberate `H_2` gas.

`R -X oversettext(Zn or Mg in dry )-> RMgX` {Grignard's reagent) or `RZnX`

`R-X oversettext(Li in dry ether)-> R-Li + LiX`

Behaviour of Carbanions :

Carbanions are very strong nucleophile and very strong bases. They prefer to abstract any acidic hydrogen whose acidic strength is comparable to ammonia or larger. They can also lose a leaving group from adjacent position.