Science and Technology for UPSC Examinations [Chemistry] Part 18- Carbonyl Compounds – Aldehydes

Aldehydes and ketones are called carbonyl compounds as they contain the carbonyl group, >C=O. The functional group of an Aldehydes is -CHO and that of the ketones is >C=O. Both Aldehydes and ketones have the same general formula, CnH2nO. The general formula for Aldehydes is R-CHO, where R is an alkyl group and the general formula for ketones is R-CO-R¢, where R and R’ are alkyl group which may or may not be the same.


The common names assigned to Aldehydes are based on the names of acids produced by their oxidation. For example, formaldehyde and acetaldehyde are so called because on oxidation they produce formic acid and acetic acid respectively. The suffix of the names of acid ‘ic’ is replaced by ‘Aldehydes’. The formulae, common names and IUPAC names of first four members of the series are given below:


Formaldehyde or Methanal is the first member of the aldehyde series. The chemical formula of methanal is HCHO. It is prepared by the controlled oxidation of methanol (CH3OH) at 873 – 923K using silver, iron oxide or molybdenum oxide as catalyst

Properties of Formaldehyde

Formaldehyde is a colorless, pungent smelling gas. It is highly soluble in water. It can be easily condensed into liquid. The liquid HCHO boils at 252K. It causes irritation of skin, eyes, nose and throat. Its solution acts as an antiseptic and a disinfectant.

Applications of Formaldehyde
  • Formaldehyde is oxidized to methanoic (Formic) acid in the presence of oxidizing agents like alkaline potassium permanganate or Ammoniacal silver nitrate. Formic Acid occurs naturally, most notably in the venom of bee and ant stings.
  • A major use of formic acid is as a preservative and antibacterial agent in livestock feed
  • An aqueous solution of formaldehyde is called Formalin which contains about 40% HCHO. It is a powerful disinfectant and antiseptic. It is used for preserving dead bodies, biological specimens and sterilising surgical instruments.
  • It is used in the manufacture of paints and dyes.
  • Formaldehyde is condensed with phenol in the manufacture of bakelite, a plastic which is used for making electrical switches.
  • Formaldehyde is condensed with ammonia to produce urotropine, (CH2)6N4 which is an important medicine in urinary ailments.


Acetaldehyde or ethanal occurs widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part of their normal metabolism. It is also produced by metabolism of Alcohol in our body and is popularly believed to be a cause of hangovers from alcohol consumption. Pathways of exposure include air, water, land or groundwater as well as drink and smoke. Acetaldehyde derived from the consumption of ethanol binds to proteins to form DNA adducts. This DNA adduct is what causes Cancer linked to alcohol overuse. Acetaldehyde is a probable or possible carcinogen in humans. Many microbes produce acetaldehyde from ethanol in our body which lead to stomach and colon cancer.

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