`=>` A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of `color{red}(sp^2)` hybridised orbitals of carbonyl carbon (Fig. 12.2).

`=>` The hybridisation of carbon changes from `color{red}(sp^2)` to `color{red}(sp^3)` in this process, and a tetrahedral alkoxide intermediate is produced.

`=>` This intermediate captures a proton from the reaction medium to give the electrically neutral product.

`=>` The net result is addition of `color{red}(Nu^–)` and `color{red}(H^+)` across the carbon oxygen double bond as shown in Fig. 12.2.

`=>` Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons.

`color{green}(text(Sterically))`, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent.

`color{green}(text(Electronically))`, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl more effectively than in former.

(a) `color{green}(text(Addition of Hydrogen Cyanide))` `color{red}((HCN))` : Aldehydes and ketones react with hydrogen cyanide `color{red}((HCN))` to yield cyanohydrins. See fig.1.

● This reaction occurs very slowly with pure `color{red}(HCN)`.

● Therefore, it is catalysed by a base and the generated cyanide ion `color{red}((CN^-))` being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin.

● Cyanohydrins are useful synthetic intermediates.

(b) `color{green}(text(Addition of Sodium Hydrogensulphite ))` : Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products. See fig.2.

● The position of the equilibrium lies largely to the right hand side for most aldehydes and to the left for most ketones due to steric reasons.

● The hydrogensulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali.

● Therefore, these are useful for separation and purification of aldehydes.

(c) `color{green}(text(Addition of Grignard reagents ))` : Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones.

● The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields an alcohol. See fig.3.

● The overall reactions using different aldehydes and ketones are as shown in fig.4.

(d) `color{green}(text(Addition of alcohols ))` : Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as `text(hemiacetals)`, which further react with one more molecule of alcohol to give a gem-dialkoxy compound known as `color{green}(text(acetal))` as shown in fig.5.

● Ketones react with ethylene glycol under similar conditions to form cyclic products known as `color{green}(text(ethylene glycol ketals))`.

● Dry hydrogen chloride protonates the oxygen of the carbonyl compounds and therefore, increases the electrophilicity of the carbonyl carbon facilitating the nucleophilic attack of ethylene glycol.

● Acetals and ketals are hydrolysed with aqueous mineral acids to yield corresponding aldehydes and ketones respectively.

(e) `color{green}(text(Addition of Ammonia and its derivatives ))` : Nucleophiles, such as ammonia and its derivatives `color{red}(H_2N-Z)` add to the carbonyl group of aldehydes and ketones. See fig.6.

● The reaction is reversible and catalysed by acid.

● The equilibrium favours the product formation due to rapid dehydration of the intermediate to form `color{red}(> C = N -Z)`.