Notes On Aldehydes And Ketones: Chemical Reactions - I - CBSE Class 12 Chemistry
The carbonyl group governs the chemistry of aldehydes and ketones by providing a site for nucleophilic addition and by increasing the acidity of the hydrogen atoms to the alpha carbon. Both of these effects are due to the ability of oxygen to accommodate the negative charge. The typical reactions of aldehydes and ketones are nucleophilic addition reactions. The reactivity of the carbonyl group towards nucleophilic addition depends mainly upon the electronic factor and the stearic factor. Electronically, alkyl groups are electron-releasing groups that release electrons and stabilise the carbonyl carbon by reducing the magnitude of positive charge on it. Hence, the electrophilicity on the carbonyl Carbon, therefore the reactivity decreases with an increase in the number of alkyl groups. Stearically the presence of bulkier alkyl groups in ketones hinders the approach of the nucleophile towards the carbonyl carbon. Hence, the bulkier the alkyl group the lesser is the reactivity. When hydrogen cyanide is added to the carbonyl group of aldehydes and ketones, it yields compounds known as cyanohydrins. Aldehyde reacts with alcohol in the presence of anhydrous hydrogen chloride to form an unstable intermediate known as hemiacetal. Aldehydes and ketones react with primary amines to form substituted imines. Aldehydes and ketones react with hydroxylamine to form the corresponding oximes. Aldehydes and ketones react with hydrazine and its derivatives to form the corresponding hydrazones. Oximes and hydrazones are used to detect and purify aldehydes and ketones.

#### Summary

The carbonyl group governs the chemistry of aldehydes and ketones by providing a site for nucleophilic addition and by increasing the acidity of the hydrogen atoms to the alpha carbon. Both of these effects are due to the ability of oxygen to accommodate the negative charge. The typical reactions of aldehydes and ketones are nucleophilic addition reactions. The reactivity of the carbonyl group towards nucleophilic addition depends mainly upon the electronic factor and the stearic factor. Electronically, alkyl groups are electron-releasing groups that release electrons and stabilise the carbonyl carbon by reducing the magnitude of positive charge on it. Hence, the electrophilicity on the carbonyl Carbon, therefore the reactivity decreases with an increase in the number of alkyl groups. Stearically the presence of bulkier alkyl groups in ketones hinders the approach of the nucleophile towards the carbonyl carbon. Hence, the bulkier the alkyl group the lesser is the reactivity. When hydrogen cyanide is added to the carbonyl group of aldehydes and ketones, it yields compounds known as cyanohydrins. Aldehyde reacts with alcohol in the presence of anhydrous hydrogen chloride to form an unstable intermediate known as hemiacetal. Aldehydes and ketones react with primary amines to form substituted imines. Aldehydes and ketones react with hydroxylamine to form the corresponding oximes. Aldehydes and ketones react with hydrazine and its derivatives to form the corresponding hydrazones. Oximes and hydrazones are used to detect and purify aldehydes and ketones.

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