Question

How is formaldehyde prepared from (a) Methylene dichloride (b) Calcium formate.
Explain Cannizaro reaction.

Answer 1

When methylene dichloride is hydrolysed by boiling with aqueous KOH, formaldehyde(methanal) is obtained. Cannizzaro reaction Aldehydes which do not have an $\alpha$-hydrogen atom undergo self-oxidation and reduction(disproportionation) reaction on heating with concentrated alkali ($50\%$ aqueous or ethanolic solution of NaOH or KOH). In this reaction, one molecule of the aldehyde is reduced to alcohol while another aldehyde molecule is oxidised to carboxylic acid salt. This self redox reaction or disproportionation reaction is called Cannizzaro reaction.