Acetaldehyde

         Acetaldehyde is the second member of the aldehyde sequence. It happens in certain fruits. It was first prepared by Scheele in 1774 by oxidation of ethyl alcohol.

        Preparation of Acetaldehyde : It may be prepared by any of the general procedures. The summary of the procedures is shown below

         (i) By oxidation reaction of ethyl alcohol with acidified potassium dichromate or with air in presence of a catalyst like silver at 300°C.

         (ii) By dehydrogenation of ethyl alcohol. The vapours of ethyl alcohol are gone through copper at 300°C.

         (iii) By heating the mixture of calcium formate and calcium acetate.

         (iv) By temperature ethylidene chloride with caustic soda or caustic potash solution.

         (v) By the reaction of acetyl chloride with hydrogen in presence of a catalyst palladium suspended in barium sulphate (Rosenmund's reaction).

         (vi) By the reduction of CH₃CN with stannous chloride and HCl in ether and hydrolysis (Stephen's method).

         (vii) By hydration of acetylene with dil. H₂SO₄ and HgSO₄ at 60°C.

         (viii) By ozonolysis of subsequent breaking of ozonide and butene-2.

         (ix) Laboratory preparation : Acetaldehyde is prepared in the laboratory by oxidation of ethyl alcohol with acidified potassium dichromate or acidified sodium dichromate.

         To recover acetaldehyde, the distillate is treated with dry ammonia when crystallised product, acetaldehyde ammonia, is created. It is washed and filtered with dry ether. The dried crystals are then redefined with dilute sulphuric acid when pure acetaldehyde is collected.

         (x) Manufacture : Acetaldehyde can be manufactured by one of the following methods:

         (a) By air oxidation of ethyl alcohol

         (b) By dehydrogenation of alcohol

         (c) By hydration of acetylene

         (d) From ethylene (Wacker process)

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