Contents
Laboratory preparation of anhydrous formic acid
Formic acid is prepared in the laboratory by the decarboxylation of oxalic acid with glycerol at 1100C.
This reaction occurs in following steps:
Procedure: About 50ml of anhydrous glycerol and 40gm of oxalic acid crystals are placed in a flask and all the apparatus are fitted as shown in figure. The flask is heated at 1100C till the evolution of carbon dioxide (Marked by effervescence) ceases. The reaction flask is then cooled and a fresh lot of oxalic acid (40gm) is added. The mixture is again heated at 1100C and the distilled aqueous solution of formic acid is collected in the receiver.
For anhydrous formic acid:
The aqueous formic acid is neutralized with lead carbonate and the lead formate solution is then evaporated to get crystals of lead formate. The dry lead formate is packed in the inner tube of condenser and treated with dry H2S gas. As a result anhydrous formic acid is formed which is collected in the receiver.
Anhydrous formic acid so obtained contains traces of H2S. it is mixed with some lead formate and is distilled to obtain pure formic acid.
Note: Anhydrous HCOOH can’t be obtained by distillation of aqueous HCOOH as b.pt. of HCOOH (b.pt. 100.50C) is almost similar to that of water.
Q) Aqueous formic acid can not be dried by conc. H2SO4, P2O5 and solid KOH, why?
Aqueous formic acid can not be dried by conc. H2SO4, anhy.P2O5 and solid KOH because formic acid itself reacts with these compounds.
Abnormal behaviour of formic acid
In formic acid molecule, the carboxylic acid group is attached to hydrogen atom (not to an alkyl group as in case of other higher members). As a result, it possesses dual functional groups i.e. carboxylic group as well as aldehyde group.
Hence, it behaves as an acid and an aldehyde.
1. Action with Tollen’s reagent:
When formic acid is boiled with Tollen’s reagent, a silver mirror is deposited.
Acetic acid does not give this test.
2. Action with Fehling’s solution:
When formic acid is warmed with Fehling’s solution, a brick red ppt. of cuprous oxide is obtained.
Acetic acid does not give this test.
3. Action with acidified KMnO4 :
When formic acid is treated with acidified KMnO4 solution, the pink colour of KMnO4 is discharged.
Acetic acid does not give this test.
4. Action of conc.H2SO4:
Formic acid when heated with conc. H2SO4, is dehydrated to form CO and H2O.
No action of conc.H2SO4 on acetic acid.
5. Action with PCl5/PCl3/SOCl2:
Formic acid reacts with PCl5 to form formyl chloride (methanoyl chloride), which subsequently decomposes into CO and HCl.
Acetic acid reacts with PCl5 to give acetyl chloride.
Q) Write the chemical reaction to distinguish methanoic acid and ethanoic acid.
Q) Why does formic acid gives silver mirror with Tollen’s reagent?
Uses of formic acid
Formic acid is used:
- In the preservation of fruits.
- As an antiseptic.
- As a coagulating agent for rubber latex.
- In leather tanning.
- In the laboratory preparation of carbon monoxide.
- In the treatment of gout.
References
- https://genuinechemistry.com/2019/02/formic-acid-preparation-properties-uses.html
- https://www.britannica.com/science/formic-acid
- Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
- Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
- Morrison, R.T. , Boyd, R.N., Organic Chemistry, Sixth edition, Prentice-Hall of India Pvt. Ltd., 2008.
- March, j., Advanced Organic Chemistry, Fourth edition, Wiley Eastern Ltd. India, 2005.
