Question

An organic liquid A containing C, H and O with p$K_a$ value 15.9, used in polishing woods, on heating with concentrated sulphuric acid gives a gaseous product B with the empirical formula CH$_2$. B decolourises bromine water as well as dilute permanganate solution. The compound A and B are respectively

• A. methanol and methane
• B. ethanol and ethane
• C. phenol and ethanol
• D. ethanol and ethene
• E. ethanol and methane

Hint:

Chemistry Tip: If a gas decolourises bromine water and KMnO$_4$, first suspect an alkene or alkyne due to unsaturation.

Answer 1

The Correct Option is D

Concept:
We must identify compound A using all clues:

• Organic liquid containing C, H, O
• p$K_a = 15.9$
• Used in polishing woods
• On heating with conc. H$_2$SO$_4$ gives gaseous B
• B has empirical formula CH$_2$
• B decolourises bromine water and KMnO$_4$

These clues indicate alcohol dehydration producing an alkene.
Step 1: Identify compound A from p$K_a$.
A p$K_a$ around $15.9$ is characteristic of: $$\text{ethanol}$$ because alcohols are weak acids with p$K_a \approx 16$. Also ethanol is commonly used in:

• polishes
• varnishes
• solvents

So A is likely ethanol.
Step 2: Reaction with concentrated sulphuric acid.
When ethanol is heated with conc. H$_2$SO$_4$ at higher temperature, dehydration occurs: $$CH_3CH_2OH \xrightarrow[\Delta]{conc.\ H_2SO_4} CH_2=CH_2 + H_2O$$ So gaseous product B is ethene.
Step 3: Check empirical formula of B.
Ethene molecular formula: $$C_2H_4$$ Empirical formula: $$CH_2$$ Matches the question.
Step 4: Check test with bromine water and KMnO$_4$.
Ethene contains double bond. Therefore it:

• Decolourises bromine water
• Decolourises dilute alkaline KMnO$_4$ (Baeyer's test)

So this clue also confirms alkene.
Step 5: Reject other options.

• Methane / ethane do not decolourise bromine water easily.
• Phenol does not dehydrate to ethanol.
• Methanol gives different products.

Step 6: Final answer.
Thus: $$A=\text{ethanol}, \qquad B=\text{ethene}$$ Hence correct option is: $$\boxed{\text{(D)}}$$ :contentReference[oaicite:0]{index=0}