Q."Formic acid behaves both as an aldehyde and an acid." Explain.

Formic acid's structure H-C(=O)-OH contains both -COOH (acid) and -CHO (aldehyde) features, so it behaves as both an acid and an aldehyde.

Structure of formic acid: HCOOH, i.e. H-C(=O)-O-H.

If we look at this structure, the same carbon carries:

• a carboxyl group -COOH (which makes it a carboxylic acid), and

• an aldehydic -CHO group, because the H-C(=O)- part is exactly an aldehyde (-CHO) group. Formic acid is the only carboxylic acid whose carboxyl carbon also bears an H directly, giving it the -CHO grouping.

1. Acidic character (behaves as an acid) — due to the -COOH group:

• It turns blue litmus red.

• It reacts with active metals and bases to form formate salts, e.g. with NaOH:

  HCOOH + NaOH --> HCOONa + H2O

• It reacts with alcohols to form esters (esterification):

  HCOOH + C2H5OH --> HCOOC2H5 + H2O

• It liberates CO2 from carbonates/bicarbonates.

2. Aldehydic character (behaves as an aldehyde) — due to the -CHO group. It is a good reducing agent:

• Tollens' test: it reduces ammoniacal silver nitrate, giving a silver mirror. HCOOH + 2 [Ag(NH3)2]+ + 2 OH- --> 2 Ag (mirror) + CO2 + H2O + 4 NH3 (formic acid is oxidised to CO2 and water)
• Fehling's test: it reduces Fehling's solution, giving a red precipitate of cuprous oxide (Cu2O). …