Chemistry STABILITY OF CARBOCATIONS AND FREE RADICALS
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STABILITY OF CARBOCATIONS AND FREE RADICALS

Stability of Carbocation :

Stability of carbocation depends on factors affecting electron density. Any factor decreasing the positive charge will increase the stability. Effect of resonance is generally much more, direct inductive effect dominates over hyperconjugation but hyperconjugation generally dominates over indirect inductive effect. Aromatic carbocations will be highly stable while antiaromatic carbocations will be highly unstable. See fig.1.

The stability order `3^o > 2^o > 1^o >` methyl can be explained due to positive inductive effect of alkyl groups. Alternatively, it can also be explained on the basis of hyperconjugation effect with `9`, `5`, `3` and zero hyper conjugation in above `3^o , 2^o, 1^o` and methyl carbocations. See fig.2.

Formation of carbocation :

From alkyl halide `(overset(delta+)R - overset(delta-)X)`

`R - X underset(text(not) H^(+))oversettext(Lewis acid) -> R^(+) + [LA -X]^(-)`

`H^(+)` cannot be used to remove `X^(-)` because `HX -> H^(+) + X^(-)` has very high tendency to occur in forward direction and very low tendency to occur in backward direction.

`text(From alcohal :)` See fig.1.

`H^(+)` can be used to remove `OH^-` because it (`H_2O -> H^(+) + OH^(-)`) has very low tendency to occur in forward direction and very high tendency to occur in backward direction.

`R - OH underset(H_2SO_4)oversettext(Lewis acid)-> R^(+) +OH^(-)`

(`H_3PO_4` can also be used but not `HNO_3` because it is an oxidizing agent and can oxidize alcohols)

`text(From aldehyde and ketones :)` See fig.2.

`text(From alkene :)` `CH_3 - CH=CH_2 overset(H_2SO_4)-> CH_3-CH_2 -overset(oplus)CH_2 & CH_3-overset(oplus)CH -CH_3`

Behaviour of Carbocation :

(i) Carbocation prefers to undergo rearrangement to more stable carbocation by shifting an atom or group from adjacent atom along with electrons of the bond. See fig.1.

(ii) Carbocation can react with nucleophiles or can react like electrophiles. See fig.2.

(iii) Carbocations can lose `H^(+)`, even in presence of weak bases like `H_2O`, `ROH` etc; from adjacent atom to form a `pi`-bond. See fig.3.

Stability of Carbon Free Radicals :

Stability of free radicals mainly depends on resonance effect and hyperconjugation. The inductive effect has very little role in stability of free radicals. See fig.1.

The stability order of free radicals `3° > 2° > 1° >` methyl can be explained on the basis of inductive effect. It can, however, be explained on the basis of hyper conjugation effect with `9`, `6`, `3` and zero hyper conjugation in `3^o`, `2^o`, `1^o` and methyl free radicals. The stability of allyl and benzyl free radicals is due to resonance. See fig.2.

In carbon free radicals (neutral species), hyperconjugation effect dominates over direct inductive effect also. See fig.3.

Formation of Carbon Free Radical : By homolytic cleavage of a bond of carbon

See fig.1.

`text(From Alkyl Halides :)`

We know that `RX` give `R^(+)` so if we add one electron, we may get neutral alkyl free radical. Hence we should add a metal which can give one electron. See fig.2.

`text(By Homolytic cleavage of a pi)` `text((carbon-carbon pi))` `text(bond :)`

When any free radical reacts with alkene breaking the pi bond results in the formation of free radical. See fig.3.

`text(Behaviour of carbon free radical :)`

Carbon free radical will behave as a usual radical i.e. it can either combine with another radical or can do homolytic cleavage of a covalent bond.
 
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