Practical Organic Chemistry: JEE Advanced PYQ, 2007–2026

Q1 · Chemistry · Practical Organic Chemistry

A mixture of naphthalene and sand is to be separated. Which method is most suitable?

• ASublimation: naphthalene passes directly from solid to vapour on gentle heating and deposits as pure crystals on a cold surface, leaving the sand behind.
• BFractional distillation: naphthalene is separated from sand by collecting the fractions at their different boiling points in a fractionating column.
• CFractional crystallisation from water: naphthalene dissolves in water on heating and separates as crystals on cooling, leaving sand undissolved.
• DMelting the mixture at 80 °C and decanting the liquid naphthalene, since sand is denser and settles to the bottom.

Q2 · Chemistry · Practical Organic Chemistry

A bottle of diethyl ether that has stood in air for a long time contains explosive peroxides as an impurity. Which treatment safely removes them before the ether is used?

• ADistilling the ether almost to dryness so that the less volatile peroxides stay behind
• BShaking with aqueous NaHCO3 and separating the layers
• CShaking with a freshly prepared aqueous solution of ferrous sulphate (Fe2+) and separating the layers
• DShaking with aqueous KMnO4

Q3 · Chemistry · Practical Organic Chemistry

A carbon-containing, sulphur-free organic compound is heated strongly with dry CuO, and the gas evolved is bubbled continuously and for a long time through a fixed volume of clear lime water. The clear solution finally obtained is then boiled. Which statement correctly describes the observations and their explanation?

• AThe lime water first turns milky because of CaCO₃. With excess CO₂ it becomes clear again because soluble Ca(HCO₃)₂ forms. On boiling, Ca(HCO₃)₂ decomposes and the solution turns milky again.
• BThe lime water turns milky and stays milky however much CO₂ is passed, because CaCO₃ is insoluble in water and cannot react further with CO₂.
• CThe lime water turns milky and then clear because of soluble Ca(HCO₃)₂. On boiling, the solution remains clear because Ca(HCO₃)₂ is thermally stable.
• DThe lime water turns milky and then clear because of soluble Ca(HCO₃)₂. On boiling, gas is evolved but the solution remains clear because CaCO₃ dissolves better in hot water.

Q4 · Chemistry · Practical Organic Chemistry

In Lassaigne's sodium fusion, the organic compound is heated with metallic sodium. What is the main role of the sodium?

• AIt acts as a reducing agent that converts covalently bonded N, S and halogens into ionic NaCN, Na2S and NaX, which dissolve in water.
• BIt acts as an oxidising agent that burns off carbon as CO2.
• CIt raises the melting point of the mixture so the compound does not vaporise.
• DIt forms an insoluble sodium salt of the element so that it precipitates out of the mixture.

Q5 · Chemistry · Practical Organic Chemistry

An alkaline aqueous solution contains sodium acetate and sodium 2-naphtholate. Carbon dioxide is bubbled through it to saturation and the solution is then shaken with diethyl ether. Take pKa values: acetic acid 4.8, carbonic acid (first dissociation) 6.4, 2-naphthol 9.5. Which species is mainly extracted into the ether layer?

• A2-Naphthol, while the acetate remains in the aqueous layer as sodium acetate
• BBoth 2-naphthol and acetic acid, because CO2 acidifies the whole solution
• CAcetic acid, while sodium 2-naphtholate remains in the aqueous layer
• DNeither, because carbonic acid is weaker than both acetic acid and 2-naphthol

Q6 · Chemistry · Practical Organic Chemistry

A hydrocarbon C4H6 decolourises bromine water and consumes 2 mol of H2 per mole on catalytic hydrogenation. It also gives a white precipitate with ammoniacal silver nitrate. The hydrocarbon is:

• ABut-1-yne
• BBut-2-yne
• CButa-1,3-diene
• DCyclobutene

Q7 · Chemistry · Practical Organic Chemistry

A mixture of salicylic acid and 4-methylphenol (p-cresol) is dissolved in diethyl ether. Shaking with which aqueous reagent transfers only the salicylic acid into the aqueous layer?

• AAqueous NaHCO3
• BAqueous NaOH
• CAqueous KCl
• DDilute HCl

Q8 · Chemistry · Practical Organic Chemistry

A racemic carboxylic acid, (±)-RCOOH, is treated with one equivalent of enantiomerically pure (R)-1-phenylethanamine and the mixture is crystallised from a suitable solvent. Which of the following statements about this resolution is correct?

• AThe two salts formed are enantiomers of each other, so they separate on crystallisation because their solubilities differ in the chiral solvent.
• BStarting from 10.0 g of the racemic acid, a single such resolution can give up to 10.0 g of the pure (R)-acid.
• CThe same separation could be done by using NaOH in place of the chiral amine, since the sodium salts of the (R)- and (S)-acids have different solubilities.
• DThe two salts are diastereomers with different solubilities. After fractional crystallisation, each salt is treated with a strong acid such as HCl to liberate the optically active carboxylic acid.

Q9 · Chemistry · Practical Organic Chemistry

Rectified spirit (about 95.6% ethanol by mass, rest water) forms an azeotrope, so fractional distillation alone cannot give absolute alcohol. A chemical drying step is therefore carried out before the final distillation. Which reagent and reason is correct?

• AQuicklime (CaO), because it combines with water to form non-volatile Ca(OH)₂ and does not react appreciably with ethanol
• BAnhydrous CaCl₂, because it is the usual drying agent and takes up water as hydrate
• CSodium metal, because it reacts vigorously with water and leaves pure ethanol
• DConcentrated H₂SO₄, because it is a powerful dehydrating agent

Q10 · Chemistry · Practical Organic Chemistry

After extraction, sodium propanoate is in the aqueous layer. Which reagent should be added to regenerate propanoic acid so that it can be recovered?

• ADilute HCl
• BAqueous NaOH
• CEthanol
• DSodium chloride solution