Formic acid occurs in the sting of bees, wasps, red ants and in stinging nettles. In traces it is also present in perspiration, urine, and blood. It was first obtained in 1670 by the distillation of red ants and hence the name (Latin formica-ant).
Preparation
1. By the oxidation of formaldehyde
\(HCO \xrightarrow {K_2Cr_2O_7 - H_2SO_4} HCOOH\)
2. By the hydrolysis of hydrogen cyanide, HCN.
3. In laboratory, it is prepared by heating glycerol with oxalic acid at 100-110°C. Since formic acid boils at 100-5°C, anhydrous formic acid can't be obtained by fractional distillation. Anhydrous formic acid can be obtained by distilling a mixture of sodium formate and sodium hydrogen sulphate.
\(HCOONa +NaHSO_4 \longrightarrow HCOOH +Na_2SO_4\)
Manufacture : By heating NaOH and CO at 200°C under 6-10 atmospheric pressure.
\(CO + NaOH \rightarrow HCOONa \xrightarrow {HCl} HCOOH\)
Properties
In addition to properties discussed earlier, formic acid gives the following reactions.
1. Action of conc. H2SO4 (Dehydration).
\(HCOOH \xrightarrow {H_2SO_4,heat}H_2O +CO\)
2. Action of heat.
\(HCOOH \xrightarrow {160° C}H_2+CO_2\)
3. Reducing agent. (Reactions due to -CHO group). Like aldehydes but unlike other acids, formic acid reduces Tollen's reagent, Fehling solution, mercuric chloride and potassium permanganate.
2KMnO4 + 3H2SO4 + 5HCOOH- K2SO4 + 2MnSO4 + 5CO2 + 8H2O However, remember that formic acid does not react with NH2OH, semicarbazide, phenylhydrazine, etc.
