Question

Write short notes on the following. (i) Cross aldol condensation (ii) Stephen reaction (iii) Etard reaction. (2+1½+1½=5)
OR
How will you obtain (write chemical equations only): (i) Ethanoic anhydride from Ethanoic acid (ii) Benzoic acid from Ethyl benzoate (iii) Benzamide from benzoic acid. (2+1½+1½=5)

Hint:

Option 1: aldol between two different carbonyls (one lacking \(\alpha\)-H gives a single product); nitrile + SnCl\(_2\)/HCl then hydrolysis gives an aldehyde; toluene + chromyl chloride gives benzaldehyde. Option 2: dehydrate the acid with P\(_2\)O\(_5\); hydrolyse the ester then acidify; make the ammonium salt and heat.

Answer 1

Option 1

(i) Cross aldol condensation:
Step 1: An aldol condensation carried out between two different carbonyl compounds (two different aldehydes, two different ketones, or an aldehyde and a ketone) is called a cross aldol condensation. At least one of the reactants must possess an \(\alpha\)-hydrogen so that its enolate/carbanion can be generated by the base.
Step 2: A dilute base (NaOH) removes the acidic \(\alpha\)-hydrogen to give a carbanion, which adds to the carbonyl carbon of the second molecule to give a \(\beta\)-hydroxy carbonyl compound (aldol). On warming, this loses water to give an \(\alpha,\beta\)-unsaturated carbonyl compound.
Step 3: When one component has NO \(\alpha\)-hydrogen (e.g. benzaldehyde), a single clean product is obtained. Example (ethanal + benzaldehyde):
\[ C_6H_5CHO + CH_3CHO \xrightarrow{\text{dil. NaOH}} C_6H_5CH=CHCHO + H_2O \]
(3-phenylprop-2-enal, cinnamaldehyde). If both partners have \(\alpha\)-hydrogens, a mixture of four products results, so cross aldol is synthetically useful mainly when one partner lacks \(\alpha\)-H.

(ii) Stephen reaction:
Step 1: A nitrile (alkyl cyanide) is reduced to an imine using stannous chloride (SnCl\(_2\)) and hydrochloric acid.
Step 2: The intermediate imine salt is hydrolysed to the corresponding aldehyde.
\[ RC\equiv N \xrightarrow{SnCl_2/HCl} RCH=NH \xrightarrow{H_3O^+} RCHO + NH_3 \]
Thus a nitrile with n carbons gives an aldehyde with the same number of carbons. It is a method to prepare aldehydes from nitriles.

(iii) Etard reaction:
Step 1: Toluene (or a substituted toluene) is treated with chromyl chloride (CrO\(_2\)Cl\(_2\)) in a solvent such as CS\(_2\) or CCl\(_4\). The methyl group is oxidised and a brown chromium complex separates out.
Step 2: This complex is hydrolysed with water to give benzaldehyde, without over-oxidation to benzoic acid.
\[ C_6H_5CH_3 \xrightarrow{CrO_2Cl_2,\,CS_2} \text{chromium complex} \xrightarrow{H_3O^+} C_6H_5CHO \]

Option 2

(i) Ethanoic anhydride from ethanoic acid: Ethanoic acid is dehydrated by heating with a dehydrating agent such as P\(_2\)O\(_5\).
\[ 2\,CH_3COOH \xrightarrow{P_2O_5,\,\Delta} (CH_3CO)_2O + H_2O \]

(ii) Benzoic acid from ethyl benzoate: Alkaline hydrolysis of the ester followed by acidification.
\[ C_6H_5COOC_2H_5 + NaOH \rightarrow C_6H_5COONa + C_2H_5OH \]
\[ C_6H_5COONa + HCl \rightarrow C_6H_5COOH + NaCl \]

(iii) Benzamide from benzoic acid: Benzoic acid reacts with ammonia to form ammonium benzoate, which on heating loses water to give benzamide.
\[ C_6H_5COOH + NH_3 \rightarrow C_6H_5COONH_4 \xrightarrow{\Delta} C_6H_5CONH_2 + H_2O \]