How many of the following compounds evolve carbon dioxide with an aqueous solution of sodium hydrogencarbonate?
Ethanoic acid, Phenol, Benzoic acid, Ethanol, Ethyl ethanoate, Methanoic acid, Benzaldehyde, Hexanedioic acid
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4
A weak base such as sodium hydrogencarbonate reacts only with a compound that is acidic enough, that is, one with a carboxyl group. The acid gives its salt with brisk effervescence of carbon dioxide; this is the test for the –COOH group.
\( \displaystyle \ce{RCOOH + NaHCO3 -> RCOONa + H2O + CO2 ^} \)
Evolve CO2 (carboxylic acids):
Ethanoic acid, \( \ce{CH3COOH} \)
Benzoic acid, \( \ce{C6H5COOH} \)
Methanoic acid, \( \ce{HCOOH} \)
Hexanedioic acid, \( \ce{HOOC(CH2)4COOH} \) (two –COOH groups)
No CO2:
Phenol — forms a salt with NaOH, but is too weak an acid to give CO2 with a hydrogencarbonate.
Ethanol — an even weaker acid than phenol.
Ethyl ethanoate — an ester; it has no acidic –COOH hydrogen.
Benzaldehyde — an aldehyde with no acidic hydrogen.
Compounds that are carboxylic acids: ethanoic, benzoic, methanoic and hexanedioic acid = 4.
NCERT (2026-27 edition):
8.9.1 Reactions Involving Cleavage of O–H Bond – “The carboxylic acids like alcohols evolve hydrogen with electropositive metals and form salts with alkalies similar to phenols. However, unlike phenols they react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide. This reaction is used to detect the presence of carboxyl group in an organic compound.”
“Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols (pKa is ~16 for ethanol and 10 for phenol).”