Class 12 Named Reactions: Organised by What They Do

Why organise by transformation

In a conversion question you know the starting material and the target. What you need is a reaction that performs that change. Searching a mental list organised by chapter does not help, because the chapters are organised by functional group rather than by transformation.

Reactions that change the carbon count

Aldehyde and ketone reactions

The aldol and Cannizzaro pair is examined constantly because the requirement is opposite: aldol needs an alpha hydrogen, Cannizzaro needs none. Given a carbonyl compound and asked which reaction it undergoes, check for alpha hydrogens first — that single check answers the question.

Aromatic reactions

Reactions of amines and halides

• Carbylamine — detects a primary amine by a characteristic offensive smell.
• Hinsberg — distinguishes primary, secondary and tertiary amines by their behaviour with a sulphonyl chloride.
• Gabriel phthalimide — prepares a pure primary amine, avoiding the mixture that direct alkylation of ammonia gives.
• Finkelstein and Swarts — halogen exchange, useful for making iodides and fluorides that are hard to prepare directly.

The Gabriel synthesis is asked with a specific limitation attached: it does not work for aryl amines, because the aromatic halide will not undergo the required substitution. Knowing the limitation is usually worth as much as knowing the reaction.

How to revise them

Build one sheet organised by transformation rather than by chapter, and practise recalling from transformation to name and from name to transformation. Both directions are tested, and the second is harder because it requires recall rather than recognition.

Frequently asked questions

How do I decide between aldol and Cannizzaro?

Check for an alpha hydrogen. Present means aldol; absent means Cannizzaro. It is a single check that settles the question.

Why is acylation preferred over alkylation in Friedel–Crafts?

Because alkylation suffers carbocation rearrangement and gives multiple substitution, while acylation does neither. Reducing the ketone afterwards gives the straight-chain product cleanly.

Why use the Gabriel synthesis rather than alkylating ammonia?

Because direct alkylation gives a mixture of primary, secondary and tertiary amines together with the quaternary salt. Gabriel gives the primary amine alone.

Which named reactions appear most often?

Those with a clear diagnostic use — Cannizzaro, aldol, iodoform, Hofmann, Sandmeyer, Gabriel — because they can be asked as identification, as conversion, or as mechanism.