The confirmatory test for alkanoic acids in organic qualitative analysis is the?
turning of wet blue litmus paper red
reaction with alkanols to form esters
reaction with sodium hydroxide to form salt and water
reaction with aqueous Na2COsp3 to liberate a gas which turns lime water milky
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CH3COOH + C2H6O ---- CH3COOC2H5 + H20
Ethanoic Ethanol ester( ethyl ethanoate ) + water
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5
The confirmatory test for alkanoic acids in organic qualitative analysis is the?
A.
turning of wet blue litmus paper red
B.
reaction with alkanols to form esters
C.
reaction with sodium hydroxide to form salt and water
D.
reaction with aqueous Na2COsp3 to liberate a gas which turns lime water milky
The correct answer is:
D. reaction with aqueous Na₂CO₃ to liberate a gas which turns lime water milky
Explanation:
In organic qualitative analysis, the confirmatory test for alkanoic acids (carboxylic acids) is their reaction with a carbonate or hydrogen carbonate.
What happens?
Alkanoic acid + Sodium carbonate → Salt + Water + Carbon dioxide
Example:
2
𝐶
𝐻
3
𝐶
𝑂
𝑂
𝐻
+
𝑁
𝑎
2
𝐶
𝑂
3
→
2
𝐶
𝐻
3
𝐶
𝑂
𝑂
𝑁
𝑎
+
𝐻
2
𝑂
+
𝐶
𝑂
2
2CH
3
COOH+Na
2
CO
3
→2CH
3
COONa+H
2
O+CO
2
The CO₂ gas produced:
Causes effervescence (bubbling)
Turns limewater milky due to formation of CaCO₃
𝐶
𝑂
2
+
𝐶
𝑎
(
𝑂
𝐻
)
2
→
𝐶
𝑎
𝐶
𝑂
3
+
𝐻
2
𝑂
CO
2
+Ca(OH)
2
→CaCO
3
+H
2
O
Why the others are wrong:
A. Turning blue litmus red → Not confirmatory (many acids do this).
B. Reaction with alkanols → Requires catalyst (conc. H₂SO₄) and heating; not a simple confirmatory test.
C. Reaction with NaOH → Neutralization reaction; many acids do this.
The correct answer is:
D. reaction with aqueous Na₂CO₃ to liberate a gas which turns lime water milky
Why this is correct:
Alkanoic acids react with carbonates (like sodium carbonate, Na₂CO₃) to produce:
a salt
water
carbon dioxide (CO₂) gas
The CO₂ gas, when passed through limewater, turns it milky, which is a confirmatory test.
Why the others are not confirmatory:
A: Turning blue litmus red → shows acidity, but not specific to alkanoic acids
B: Ester formation → requires special conditions, not a simple test
C: Reaction with NaOH → neutralization, common to all acids
So, D is the specific confirmatory test.
