Aldehydes

CARBONYL COMPOUNDS
The compounds containing carbonyl group () are called carbonyl compounds. The majority of biologically important compounds contain carbonyl group. For example,

Name	Structural Formula		Structural Formula
Aldehydes		or	RCHO
Ketones		or	RCOR’
Carboxylic acids		or	RCOOH
Esters		or	RCOOR

Where, R represents the alkyl group.

ALDEHYDES

Aldehydes are the compounds which contain carbonyl group. That is why; these compounds are also called carbonyl compounds. The structure of Aldehyde group is:

 

 

 

 

 

Where R represents the alkyl group and carbonyl group is attached to one hydrogen atom. The aldehyde (  CHO ) functional group is also called as aldehydic group. General formula of aldehydes is C_nH_2n+1CHO.

Nomenclature of Aldehydes

According to IUPAC nomenclature aldehydes are named by replacing ‘e’ in the name of parent alkane by ‘al’. For example,

Molecular Formula	Common Name	IUPAC Name
HCHO	Formaldehyde	Methanal
CH3CHO	Acetaldehyde	Ethanal
C2H5CHO	Propionaldehyde	Propanal
C3H7CHO	n – Butryaldehyde	Butanal

Formaldehyde is the simplest aldehyde and it is produced on commercial scale from methane gas which is main component of natural gas.

 

 

 

 

Preparation of Methanal

Molecular formula of methanal is HCHO. It can be prepared by following two methods.

1. From Methanol

Methanal can be prepared by dehydrogenation process by passing methanol over copper or silver at temperature 573 K.

Methanal can also be prepared by oxidation of methanol with potassium dichromate and sulphuric acid.

2. From Distillation of Calcium Salts of Acids

Methanal can be prepared by distilling the calcium salts of the acids. For example,

 

 

 

 

 

1. Properties of Methanal

   Physical Properties
   Methanal is a colourless gas with pungent odour.

2. Its boiling point is 252 K.
3. Formaldehyde is soluble in water. Its 40% solution in water is called formalin. Formalin is very important preservative in biology because it is used for preserving biological specimens.

Chemical Properties

Following are the important chemical reactions of methanal:

1) Oxidation Reaction

Methanal can be easily oxidized into methanoic acid on treatment with common oxidizing agents like KMnO₄.

 

 

 

 

2) Reduction Reaction

Methanal gives methanol on catalytic hydrogenation in the presence of catalysts such as Ni, Pt or Pd.

 

3) Reaction with Hydrogen Cyanide

Methanal reacts with hydrogen cyanide to form methanal cyanohydrin.

 

 

4) Reaction with Tollen’s Reagent

Preparation of Tollen’s Reagent

Tollen’s reagent is ammoniacal solution of silver nitrate. Its chemical formula is [Ag(NH₃)₂]OH. Tollen’s reagent can be prepared by adding ammonium hydroxide (NH₄OH) to silver nitrate solution (AgNO₃).

 

 

 

The black precipitates of AgOH are dissolved in excess of NH₄OH by adding more NH₄OH in it, to produce [Ag(NH₃)₂]OH molecule which is Tollen’s reagent.

 

 

Reaction of Methanal with Tollen’s Reagent (Silver Mirror Test)

When methanal is heated with Tollen’s reagent then a bright silver mirror is produced on the inner side of the test tube.

 

5) Reaction with Fehling’s Solution

Preparation of Fehling’s Solution

Fehling’s solution is an alkaline solution of copper sulphate (also called Fehling solution A) and sodium potassium tartarate (also called Fehling solution B). The chemical formula of Fehling’s solution is [Cu(OH)₂ + NaOH]. It is prepared by mixing Fehling solution A and Fehling solution B in equal amount.

Reaction of Methanal with Fehling’s Solution

When methanal reacts with blue coloured Fehling’s solution, red precipitates of cuprous oxide (Cu₂O) are formed and the colour of Fehling’s solution changes from blue to red.

 

Uses of Methanal

1. Formaldehyde is used in the manufacture of synthetic polymers like bakelite and synthetic dyes.
2. Formaldehyde is used to preserve biological specimens in the form of formalin.
3. Formaldehyde is also used as an insecticide.
4. Formaldehyde is used in leather industry.
5. Formaldehyde is used for silver polishing of mirrors.

Test your understanding and answer these questions:

1. What are aldehydes? give examples.