Aldehydes and ketones (A-level only)

Aldehydes and ketones are carbonyl compounds featuring a C=O group, which is central to their reactivity. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom (e.g., methanal, ethanal), while in ketones it is bonded to two carbon groups (e.g., propanone). This functional group makes them highly polar, enabling nucleophilic addition reactions—a key mechanism in A-level chemistry. Understanding these compounds is essential for grasping more advanced topics like carboxylic acids and their derivatives.

These compounds are widespread in nature and industry: aldehydes are used in perfumes and as preservatives (e.g., methanal in embalming), while ketones like propanone are common solvents. In the AQA A-Level syllabus, you will explore their preparation via oxidation of primary and secondary alcohols, their distinctive reactions with 2,4-dinitrophenylhydrazine (Brady's reagent) for identification, and the crucial nucleophilic addition mechanism involving reagents like HCN and NaBH₄. This topic also introduces the concept of chiral centres when unsymmetrical ketones react, linking to optical isomerism.

Mastering aldehydes and ketones builds on your knowledge of alcohols and bonding, and it lays the foundation for understanding carbonyl chemistry in biological molecules (e.g., sugars) and synthetic pathways. You'll need to recall reaction conditions, mechanisms, and tests—all of which are frequently examined. Pay close attention to the differences between aldehydes and ketones, especially in oxidation reactions, as this is a common source of exam questions.