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Addition of nucleophile to Aldehydes and Ketones

Nucleophile is the molecule with one ore more lone pairs or have negative charge.

Cyanide (CN) is potent nucleophile that can attake carbonyl group forming cyanohydrin

In cyanide carbon bears the negative charge which make it the HOMO suitable for attaking electrophile to revise nucleophile read this. About carbonyl which is electrophile E+ it has a π* orbital (LUMO) which can accept electrons from CN forming a new structure to under stand exactly wat happen click here.

The mechanism:

Cyanide (Nu) attacks carbonyl group (E+) at first froming alkoxide (O ) then alkoxide is protonated to from cyanohydrin

Note:

This mechanism takes place in acidic medium or water to protonate the alkoxide (O ) to yiled hydroxyl group.

Examples:

Stability:

Fomation of cyanohydrin is reversible reaction so to get yiled the equillibrium should favour products. Aldehyde favours product than ketones due to: The reaction converts carbon from sp2 to sp3 meaning from 120 degree to 109 degree meaning the substitunets is getting more closer. The smaller angle make compound less stable if there is large bulk groups repulsion. In aldehyde no problem in this as one of groups is hydrogen which is small ,while in ketone the large groups hinders the reaction and this is called hindrance effect.

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