Condensation reactions of formaldehyde

Condensation reactions of formaldehyde

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

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.