Chemistry Chemical Reactions of Carbonyl Compounds : Nucleophilic Addition Reactions

Topics Covered :

● Nucleophilic Addition Reactions
● Mechanism of Nucleophilic Addition Reactions
● Reactivity
● Examples of Nucleophilic Addition and Nucleophilic Addition-Elimination Reactions

Nucleophilic addition reactions :

Contrary to electrophilic addition reactions observed in alkenes, the aldehydes and ketones undergo nucleophilic addition reactions.

Mechanism of nucleophilic addition reactions :

`=>` 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.

Reactivity :

`=>` 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.
Q 3012167039

Would you expect benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Explain your answer.


The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance and hence it is less reactive than propanal.

Some important examples of nucleophilic addition and nucleophilic addition-elimination reactions :

(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)`.