Formaldehyde : Preparation, Reaction, Uses

Formaldehyde

Formaldehyde is also known as methanal. Formaldehyde is a colorless, pungent-smelling gas, b.p. -21^oC, extremely soluble in water. It is powerful disinfectant and antiseptic.

Preparation of formaldehyde

Formaldehyde is prepared industrially:

1.  BY the dehydrogenation or air oxidation of methanol in presence of a silver catalyst at 400^oC:

CH₂O   +  H₂    (Ag)⟵    CH₃OH    (air+Ag) ⟶     CH₂O   +   H₂O

In the oxidation method, the condensate obtained is a mixture of formaldehyde, methanol and water. It is freed from excess of methanol by distillation, and the resulting mixture is known as formalin (40 per cent formaldehyde, 8 per cent methanol, 52 per cent water).

2.  By the air oxidation of methane or propane-butane mixtures (from natural gas) in the presence of various metallic oxides.

Reactions of Formaldehyde

Formaldehyde undergoes many of the general reactions of aldehydes, but differs in certain respects. When treated with ammonia, it does not form an aldehyde-ammonia, but gives instead hexa-methylenetetramine (m.p. 260^oC)

6HCHO   +   4NH₃    ⟶   (CH₂)₆N₄   +   6H₂O     (80%)

The structure of hexamethylenetetramine consists of three fused chair conformations.

Formaldehyde, since it has no a-hydrogen atoms, readily undergoes the Cannizzaro reaction.

Primary and secondary amines are methylated by heating with formaldehyde (formalin solution) and an excess of formic acid at 100^oC, e.g., ethylmethylamine from ethylamine:

C₂H₅NH₂   +   CH₂O   +   HCO₂H    ⟶    C₂H₅NHCH₃   +   CO₂   +   H₂O

This reaction is known as the Eschweiler-Clarke methylation

Polymers of formaldehyde

1.  In dilute aqueous solution, formaldehyde is almost 100 per cent hydrated to form methylene glycol (Bieber et al., 1947). This is believed to be the reason for the stability of dilute formaldehyde solutions.

CH₂O   +   H₂O    ⇌    CH₂(OH)₂

2.  When a formaldehyde solution is evaporated to dryness, a white crystalline solid, m.p. 121-123^oC, is obtained. This is known as paraformaldehyde, (CH₂O)_n.H₂O, and it appears to be mixture of polymers, n having values between 6 and 50. Paraformaldehyde reforms formaldehyde when heated.

Formaldehyde cannot be separated from methanol (in formalin) by fractionation; pure aqueous formaldehyde may be obtained by refluxing paraformaldehyde with water until solution is complete.

3.  When a formaldehyde solution is treated with concentrated sulphuric acid, polyoxymethylenes, (CH₂O)_n.H₂O –n is greater than 100 – are formed. Polyxymethylenes are white solids, insoluble in water, and reform formaldehyde when heated.

4.  When allowed to stand at room temperature, formaldehyde gas slowly polymerises to a white solid, trioxymethylene (metaformaldehyde, trioxan) (CH₂O)₃, m.p. 61-62^oC. Trioxan is prepared by distilling formaldehyde solution (60 per cent) containing a little sulphuric acid. This trimer is soluble in water, and does not show any reducing properties.  Hence it is believed to have a cyclic structure (in which there is no free aldehyde group).

Trioxan is very useful for generating formaldehyde since (a) it is an anhydrous form of formaldehyde; (b) the rate of depolymerisation can be controlled; and (c) it is soluble in organic solvents.

5.  Formaldehyde polymerises in the presence of weak alkalis, e.g., calcium hydroxide, to a mixture of sugars of formula C₆H₁₂O₆, which is known as formose or α-acrose.

Condensation reactions of formaldehyde

Formaldehyde can participate in the ‘crossed’ Cannizzaro reaction, and the nature of the final product depends on the structure of the other aldehyde. Aldehydes with no α-hydrogen atoms readily undergo the crossed Cannizzaro reaction; e.g., benzaldehyde forms benzyl alcohol:

C₆H₅CHO   +   HCHO   +   NaOH    ⟶    C₆H₅CH₂OH   +   HCO₂Na

Aldehydes with one α-hydrogen atom react as follows:

This β-hydrxyaldehyde in the presence of excess of formaldehyde forms a substituted trimethylene glycol:

Thus the first step in the above reaction is the replacement of the α-hydrogen atom by a hydroxymethyl group, CH₂OH, and the second step is the crossed Cannizzaro reaction.

In a similar manner, aldehydes with two α-hydrogen atoms are converted first into the hydroxymethyl, then into the bishydroxymethyl, and finally into the trishydroxymethyl compounds:

A special case is acetaldehyde, which has three α-hydrogen atoms. This reaction is best carried out by adding powdered calcium oxide to a suspension of paraformaldehyde in water containing acetaldehyde. Tetrakishydroxymethylmethane (tetramethylolmethane) or pentaerythriol is formed.

CH₃CHO    +   4HCHO   [Ca(OH)₂ ] ⟶      C(CH₂OH)₄   +   HCO₂Ca/2   (55-57%)

Pentaerythritol is important industrially since its tetranitrate is a powerful explosive.

Use of formaldehyde

Formaldehyde is a powerful disinfectant and antiseptic, and so is used for preserving anatomical specimens. Its main uses are in the manufacture of dyes, the hardening of casein and gelatin, and for making plastics.