Question

Aldehydes, Ketones and Carboxylic acids are Car bonyl compounds.

a) Aldehydes differ from Ketones in their oxidation reactions. Illustrate with one example.

b) How will you prepare benzaldehyde by Gatterman Koch reaction?

c) Write the reactions of carboxylic acid with the following reagents. (Write the chemical equations)

i) Thinoyl chloride (SOCl₂)

ii) Chlorine in presence of small amount of red phosphorous.

iii) Lithium Aluminium hydride (LiAlH₄) I ether.

a) Write a test to distinguish between aldehydes and ketones.

b) How will you prepare benzaldehyde by Etard’s rea dio n?

c) How willyou bring about the followng conversions? (Write the chemical equations)

i) Ethanol → Ethanoic acid

ii) Benzamide → benzoic acid

iii) Benzaldehyde → meta nitro benzaldehyde

Answer 1

a) Aldehydes are easily oxidised to carboxylic acids containing same number of carbon atoms on treatment with mild or strong oxidising agents. Ketones are oxidised under vigorous conditions i.e., with strong oxidising agents and at elevated temperatures to give carboxylic acids containing lesser number of carbon atoms.

OR

On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion.

Ketones will not give this reaction because Tollens’ reagent being a mild oxidising agent cannot oxidise ketones.

[Or, any other suitable example – Reaction with Fehling’s reagent, Reaction with Benedict’s reagent etc.]

b) When benzene is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde.

Or, the chemical equation:

a) Fehling’s Test – on heating an aliphatic aldehyde with Fehling’s reagent (aqueous copper sulphate + alkaline sodium-potassium tolerate), a reddish-brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion.

Aromatic aldehydes and ketones do not answer this test. This is because Fehling’s reagent being a mild oxidising agent cannot oxidise them.
[Or, any other suitable example – Fehling’s test, Benedict’s test etc.]

b) Toluene on treating with chromyl chloride (CrO₂Cl₂) in CS₂ forms a chromium complex WNCII on hydrolysis gives benzaldehyde.

Or, the chemical equation: