Benzaldehyde

Lesson

Part 1 — C₆H₅CHO: the simplest aromatic aldehyde, its preparation and its addition reactions.

In this lesson you will

Write the Etard reaction and other preparations of benzaldehyde
Describe the physical properties of benzaldehyde
Write oxidation, reduction and nucleophilic addition reactions of benzaldehyde
Write the products with hydroxylamine, hydrazine, phenylhydrazine and PCl₅

Table of uses and occurrence of benzaldehyde
Use or occurrence	Detail
Bitter almond oil	Natural source (as a glycoside, amygdalin)
Flavouring	Almond and cherry flavours
Perfumery	Soaps and cosmetics
Dyes	Malachite green
Synthesis	Cinnamic acid, mandelic acid, benzoin

Concept

Structure

In C₆H₅CHO the –CHO carbon is sp² and conjugated with the ring. Benzaldehyde has no α-hydrogen (the carbon next to C=O is a ring carbon with no H to lose as an enolate), so it cannot undergo aldol self-condensation. Conjugation slightly reduces the δ+ on the carbonyl carbon, so it is a little less reactive than aliphatic aldehydes.

Table comparing benzaldehyde and ethanal
Feature	Benzaldehyde	Ethanal
Carbonyl carbon	sp², conjugated with ring	sp²
α-Hydrogen	None	Three
Aldol with itself	No	Yes
Cannizzaro	Yes	No
Fehling’s test	Negative	Positive

Concept

Laboratory preparation: the Etard reaction

Toluene is oxidised by chromyl chloride (CrO₂Cl₂) in CS₂ or CCl₄; a brown chromium complex (Etard complex) separates, which is hydrolysed by water to benzaldehyde. Mild oxidation stops at the aldehyde. Other routes: side-chain chlorination of toluene to benzal chloride and hydrolysis; Gattermann–Koch (benzene + CO + HCl, AlCl₃/CuCl).

\ce{C6H5CH3 + 2Cl2 ->[h\nu] C6H5CHCl2 + 2HCl}

\ce{C6H5CHCl2 + H2O ->[373 K] C6H5CHO + 2HCl}

\ce{C6H6 + CO ->[HCl][AlCl3/CuCl] C6H5CHO}

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