Benzaldehyde — Part 2: Named reactions, ring substitution and tests: visual lesson, ISC Class 12 Chemistry

Lesson

Benzaldehyde’s special reactions

Part 2 — reactions that depend on having no α-hydrogen, and substitution on the ring.

In this lesson you will

Write equations for the Cannizzaro, benzoin and Perkin reactions of benzaldehyde
Explain why the –CHO group is meta directing and write ring substitution reactions
Distinguish benzaldehyde from aliphatic aldehydes and state its uses

Concept

Cannizzaro reaction

With concentrated NaOH or KOH, benzaldehyde disproportionates: one molecule is reduced to benzyl alcohol, the other oxidised to sodium benzoate (acidify → benzoic acid). OH⁻ adds to one C=O; the intermediate transfers a hydride ion to a second molecule.

\ce{2C6H5CHO + NaOH -> C6H5CH2OH + C6H5COONa}

Concept

Benzoin condensation

Benzaldehyde heated with ethanolic KCN (CN⁻ catalyst) dimerises to benzoin, an α-hydroxy ketone: 2-hydroxy-1,2-diphenylethan-1-one.

\ce{2C6H5CHO ->[KCN][ethanol, \Delta] C6H5CH(OH)COC6H5}

Table of the benzoin condensation
Feature	Benzoin condensation
Reagent	Ethanolic KCN (catalytic CN⁻)
Product	Benzoin, C₆H₅CH(OH)COC₆H₅
Type	Self-condensation of aromatic aldehyde
Functional groups in product	Secondary alcohol and ketone

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