Formic acid does not respond to
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2,4-DNP test
Formic acid, \( \ce{H-COOH} \), is the one carboxylic acid whose carboxyl carbon carries a hydrogen atom. It therefore has an aldehyde-like H–C=O unit and acts as a reducing agent, besides being an acid. But its C=O group is part of a carboxyl group: the lone pair of the –OH oxygen is delocalised onto the carbonyl carbon, which makes it much less electrophilic. So formic acid, like other carboxylic acids, gives no 2,4-dinitrophenylhydrazone, which aldehydes and ketones form by addition–elimination at the C=O group.
Option (1) Fehling's test — Given by formic acid. Formic acid reduces the \( \ce{Cu^2+} \) of Fehling’s solution to red \( \ce{Cu2O} \) and is itself oxidised to \( \ce{CO2} \):
\( \displaystyle \ce{HCOOH + 2Cu^2+ + 4OH- -> CO2 + Cu2O v + 3H2O} \)
Option (2) Tollen's test — Given by formic acid. Formic acid reduces Tollens’ reagent to a silver mirror:
\( \displaystyle \ce{HCOOH + 2[Ag(NH3)2]+ + 2OH- -> 2Ag v + CO2 + 4NH3 + 2H2O} \)
Option (3) sodium bicarbonate test — Given by formic acid. Formic acid is a stronger acid than carbonic acid, so it liberates carbon dioxide with brisk effervescence:
\( \displaystyle \ce{HCOOH + NaHCO3 -> HCOONa + H2O + CO2 ^} \)
Option (4) 2,4-DNP test — Not given by formic acid. Its C=O group is part of the carboxyl group and is not electrophilic enough to add 2,4-dinitrophenylhydrazine.