Question

Write chemical equations of three methods of preparation and two chemical properties of benzaldehyde.
OR
Write short notes on the following: (i) Aldol condensation (ii) Cross aldol condensation (iii) Cannizzaro reaction (iv) Hell-Volhard-Zelinsky reaction (v) Kolbe electrolysis.

Hint:

Benzaldehyde has no alpha-hydrogen, so think Etard/Gattermann-Koch for preparation and oxidation/Cannizzaro for reactions. For the notes, sort the reactions by whether an alpha-hydrogen is present.

Answer 1

Option 1: Benzaldehyde (C₆H₅CHO)

Step 1: Three methods of preparation.
(a) Etard reaction (from toluene): Toluene is oxidised with chromyl chloride in CS₂; the chromium complex on hydrolysis gives benzaldehyde.
\( C_6H_5CH_3 + CrO_2Cl_2 \rightarrow \text{complex} \xrightarrow{H_2O} C_6H_5CHO \)
(b) Gattermann-Koch reaction: Benzene reacts with carbon monoxide and HCl in presence of anhydrous AlCl₃ and CuCl.
\( C_6H_6 + CO + HCl \xrightarrow{AlCl_3/CuCl} C_6H_5CHO + HCl \)
(c) Hydrolysis of benzal chloride: Benzal chloride on boiling with water (or aqueous alkali) gives benzaldehyde.
\( C_6H_5CHCl_2 + H_2O \rightarrow C_6H_5CHO + 2HCl \)

Step 2: Two chemical properties.
(a) Oxidation: Benzaldehyde is readily oxidised (even by air) to benzoic acid.
\( C_6H_5CHO + [O] \rightarrow C_6H_5COOH \)
(b) Cannizzaro reaction: Benzaldehyde has no alpha-hydrogen, so with concentrated NaOH it undergoes disproportionation (self oxidation-reduction) giving benzyl alcohol and sodium benzoate.
\( 2\,C_6H_5CHO + NaOH_{(conc)} \rightarrow C_6H_5CH_2OH + C_6H_5COONa \)

Option 2: Short notes.
(i) Aldol condensation: Aldehydes or ketones having an alpha-hydrogen, in the presence of dilute base, add to one another to give a beta-hydroxy aldehyde/ketone (an aldol); on heating this loses water to give an alpha,beta-unsaturated carbonyl compound.
\( 2\,CH_3CHO \xrightarrow{dil.\ NaOH} CH_3CH(OH)CH_2CHO \xrightarrow{\Delta} CH_3CH=CHCHO + H_2O \)
(ii) Cross aldol condensation: An aldol condensation between two different carbonyl compounds each carrying an alpha-hydrogen gives a mixture of four products. It is synthetically useful when one partner has no alpha-hydrogen (e.g. benzaldehyde with acetaldehyde gives cinnamaldehyde).
\( C_6H_5CHO + CH_3CHO \xrightarrow{dil.\ NaOH} C_6H_5CH=CHCHO + H_2O \)
(iii) Cannizzaro reaction: Aldehydes with no alpha-hydrogen, on treatment with concentrated alkali, undergo disproportionation - one molecule is oxidised to the carboxylate salt and another is reduced to the alcohol.
\( 2\,HCHO + NaOH \rightarrow CH_3OH + HCOONa \)
(iv) Hell-Volhard-Zelinsky (HVZ) reaction: Carboxylic acids having an alpha-hydrogen react with Cl₂/Br₂ in presence of a little red phosphorus to give alpha-halo acids.
\( CH_3COOH + Cl_2 \xrightarrow{red\ P} ClCH_2COOH + HCl \)
(v) Kolbe electrolysis: Electrolysis of an aqueous solution of the sodium or potassium salt of a carboxylic acid gives an alkane at the anode by decarboxylation and coupling of the two alkyl radicals.
\( 2\,CH_3COONa + 2H_2O \xrightarrow{electrolysis} C_2H_6 + 2CO_2 + 2NaOH + H_2 \)
\[\boxed{\text{Both alternatives complete}}\]