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Nucleophilic Addition Reactions
in Aldehydes and Ketones

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Nucleophilic addition reactions are common in aldehydes and ketones as we already discussed in properties of aldehydes and ketones. Some addition reactions are given below :

Addition of Hydrogen Cyanide to Aldehydes and Ketones

Aldehydes and ketones react with hydrogen cyanides to give cyanohydrin. The reaction of ethanal with HCN is given below. The reaction of HCN with propanone is left for you to practice. Do not worry, just click on the image to check the answer (make sure your javascript is on).

Addition of hydrogen cyanide to aldehydes : formation of cyanohydrin

Mechanism

The C=O bond is polar due to the difference in electronegativities of carbon and oxygen atoms. The oxygen of C=O acquires a partial negative charge while carbon acquires a partial positive charge. The partial positive charge on carbon attracts the CN of HCN (H+CN). Because of this reason, the double bond breaks and a new C-CN bond is formed. As a result, oxygen becomes unstable due to excess electrons, and hence attracts H+ of H+CN.

mechanism of addition of HCN to aldehydes and ketones

Addition of Sodium Hydrogen Sulphite to Aldehydes and Ketones

Aldehydes react with sodium bisulphite (sodium hydrogen sulphite) to give addition products. Ketones, except for methyl ketones, do not react with sodium bisulphite due to steric hinderance (crowding).

addition of sodium bisuphite to aldehyde

Mechanism

mechanism of addition of sodium bisuphite to aldehydes and ketones

Addition of Grignard's Reagents to Aldehydes and Ketones

We have already studied that aldehydes and ketones react with Grignard's reagents to form alcohols. Refer to preparation of alcohols for detailed reactions.

Addition of Alcohols to Aldehydes and Ketones

Aldehydes react with alcohols in the presence of dry HCl to give hemiacetals. Hemiacetals being unstable react with another molecule of alcohol to give acetals. The general reaction is given below. Can you predict the products when methanol adds up to ethanal? Remember, you have to click on the image to check the answer.

Addition of alcohols to aldehydes | Formation of acetals

The reaction of aldehydes with dihydric alcohols such as ethane-1,2-diol yields cyclic acetals.

Formation of cyclic acetal

Similarly, cyclic ketals are formed when ketones react with dihydric alcohols.

Formation of cyclic ketal

Dry HCl gas is used to shift the equilibrium to the right.

Acetals and ketals on hydrolysis with aqueous mineral acids give back corresponding aldehydes and ketones.

Questions

Question : Arrange the following compounds in increasing order of their reactivity towards HCN:
acetaldehyde, acetone, di-tert-butyl ketone, Methyl tert-butyl ketone

Answer : The given compounds are:

Acetaldehyde

Acetaldehyde

Acetone

Acetone

Tert-butylmethyl ketone

Tert-butylmethyl ketone

Di-tert-butyl ketone

Di-tert-butyl ketone

We discussed in properties of aldehydes and ketones that inductive effect and steric hinderance affect the reactivity of aldehydes and ketones towards nucleophilic addition reactions.
Inductive effect: Alkyl groups being electron donating in nature, decrease the reactivity towards nucleophilic addition reaction when attached to the carbonyl group.
Steric effect: The more the number of alkyl groups and the bigger the alkyl group attached to the carbonyl carbon, the more difficult for a nucleophile to attack.

Hence, the order is:
Di-tert-butyl ketone < tert-Butyl methyl ketone < Acetone < Acetaldehyde

Question : Cyclohexanone forms cyanohydrin in good yield but 2,2,6-trimethylcyclohexanone does not. Explain.

Answer : The given compounds are:

Cyclohexanone
2,2,6-trimethylcyclohexanone

The three methyl groups in 2,2,6-trimethylcyclohexanone hinder the carbonyl carbon making the nucleophilic attack of CN ion difficult. In other words, 2,2,6-trimethylcyclohexanone does not react easily with CN ion to give cyanohydrin due to steric hindrance. There is no such hindrance in cyclohexanone; hence, it reacts with HCN easily to give cyanohydrin.

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