Question

A mixture of two organic compounds is added to cold water. After filtration, water-insoluble compound (A) burns with a smoky flame and it does not respond to Lassaigne’* and Beilsteins test. When a small amount of this is added to NaHCO3 solution, a colourless gas is evolved with effervescence. When this compound is heated with CH3 OH in acidic medium, it gives the characteristic smell of the oil of wintergreen. Compound (B), which is water soluble, burns with a non-smoky flame and its sodium extract is prepared with cane sugar. It gives a Prussian blue colour with freshly prepared solution of FeSO4 + 2-3 drops NaOH and with few drops of H2SO4 . When a small amount of this compound is heated in a dry test tube, a colourless gas is evolved that turns moist red litmus paper blue and a white residue is left. This white residue is dissolved in water and a drop of CuSO4 is added in the basic medium-a violet colour is obtained. Identify the compounds (A) and (B) with the help of the reactions involved.

Answer 1

i. Compound (A) is insoluble in water and burns with a smoky flame; hence it should be an aromatic compound.

ii. It has no specific element such as N, * and halogens and gives a CO2 gas with NaHCO3 solution; hence it should contain (–COOH) group.

iii. It gives the oil of wintergreen (methyl salicylate) with CH3 OH in acidic medium; hence it is salicylic acid.

iv. Compound (B) is water soluble and burns with a non-smoky flame; hence it should be an aliphatic compound.

v. This compound has less carbon content because its sodium extract is prepared with sucrose and it gives a Prussian blue colour with the freshly prepared solution of FeSO4 + 2-3 drops of NaOH and a few drops of H2 SO4 ; hence it is a nitrogen-containing compound.

vi. On heating, it gives ammonia gas, which turns red litmus blue. Hence, it contains (-CONH2 ) group. vii. It also gives the biuret test.

Hence, this compound is urea (NH2 CONH2 ).