Carbonyl compounds. Aldehydes: preparation and properties
Compounds with a carbonyl group >C=O are oxo compounds; those in which the carbonyl carbon is bonded to hydrogen are aldehydes (–CHO group), general formula CₙH₂ₙO. Members: methanal HCHO (formic aldehyde, formaldehyde), ethanal CH₃CHO (acetaldehyde), propanal; the name is the alkane name plus “-al”. Preparation: oxidizing a primary alcohol with copper(II) oxide or air oxygen (CH₃CH₂OH + CuO → CH₃CHO + Cu + H₂O), hydrating acetylene (Kucherov: HC≡CH + H₂O → CH₃CHO, mercury salts), hydrolyzing a 1,1-dihaloalkane. Aldehydes are reduced by hydrogen (Ni) to primary alcohols: CH₃CHO + H₂ → CH₃CH₂OH. They oxidize easily, which gives their qualitative tests: “silver mirror” — with an ammoniacal solution of Ag₂O, CH₃CHO + Ag₂O → CH₃COOH + 2Ag (a silver film on the glass); with copper(II) hydroxide on heating the blue precipitate turns first yellow CuOH, then red Cu₂O: CH₃CHO + 2Cu(OH)₂ → CH₃COOH + Cu₂O + 2H₂O. With an excess of oxidant methanal is oxidized up to CO₂ and gives 4 Ag per molecule. With phenol it undergoes polycondensation to phenol–formaldehyde resin. A 40 % solution of methanal is formalin; methanal is toxic with a choking smell.
Compare on the board the aldehyde group (–CH=O) and the carboxyl group (–COOH): the C–O bonds and the position of H. The teacher alone demonstrates the silver mirror and Cu(OH)₂ tests; watch and record the observations.