Question

a) Aldehydes and ketones are organic compounds containing carbonyl group.

i) Write a chemical reaction to distinguish between aldehydes and ketones.

ii) Aldehydes and ketones can be subjected to Clemmensen reduction and Wolf-Kishner reduction. Name the reagents used in both cases.

b) How will you make the following conversions?

i) Ethanoic acid to ethanol.

ii) Propanoic acid to 2-chloropropanoic acid.

iii) Toluene to benzoicacid.

Answer 1

a) i) Tollens’ Test: on warming an aldehyde with freshly prepared ammonical silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal.

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻→RCOO⁻ + 2Ag + 2H₂O + 4NH₃

Ketones, will not anwserTollens’ test.

ii) Clemensons reduction – Zinc amalgam and concentrated hydrochloric acid

Woif-Kishner reduction – Hydrazine, KOHI Ethylene glycol

b) i) By reduction – When ethanoic acid is treated with lithium aluminium hydride it is reduced to ethanol.

CH₃COOH \(\frac {LiAlH_4}{\longrightarrow}\) CH₃CH₂−OH

ii) By Hell-Volhartl-Zelinsky HVZ) reaction – When propanoic acid is treated with chlorine in presence of small amount of red phosphorus followed by hydrolysis 2-chloropropanoic acid is formed.

iii) When toluene is heated with alkaline solution of potassium permanganate the methyl side chain is oxidised to form benzoic acid.