Oxidation Of Alcohol

Speaking broadly, oxidising agents convert ethyl alcohol into aldehyde and acetic acid: –

C2H5.OH+O=CH3.CHO + H1O;
C2H5.OH+O2=CH3.COOH + H1O.

The character and quantity of the products vary, however, according to the conditions of the oxidation. When this is carried out with potassium dichromate or permanganate in aqueous-acid solution, the foregoing are the products; with permanganate in neutral solution, acetic acid, or acetic acid and a little carbonic acid, are given; and with alkaline permanganate solution, oxalic, acetic, and carbonic acids are produced.

A nearly quantitative conversion of ethyl alcohol into acetic acid can be obtained by treating an aqueous solution of the alcohol with potassium dichromate and sulphuric acid. Various proportions of the reagents have been used for this purpose. In oxidising a mixture of ethyl and methyl alcohols, Thorpe and Holmes2 employ 7 5 grams of dichromate and 30 grams of sulphuric acid per gram of alcohol, in a total volume of about 57 c.c. The dichromate and acid are added in two stages, and the mixture is finally boiled (see p. 188). A small quantity of carbonic acid is also produced from the ethyl alcohol at the same time – about 0.01 gram for each gram of alcohol.

Dox and Lamb,3 for oxidising small quantities of alcohol ranging from 0 2 to 2 0 grams, use an oxidising mixture composed of potassium dichromate, sulphuric acid, and water in the proportions of 1:2:7 by weight. About 150 c.c. of this mixture are used for the quantity of alcohol mentioned.

1 Compt. rend,, 1913, 156, 68-71; 1910, 151, 478.

2 Trans. Chem. Soc, 1904, 85, 1.

3 J. Amer. Chem. Soc, 1916, 38, 2563.

Evans and Day1 have studied the action of neutral and alkaline solutions of potassium permanganate on ethyl alcohol. Using about 30 grams of this salt in a litre of water at 50°, and 3 to 6 grams of alcohol, they found that with no potassium hydroxide added the product was almost wholly acetic acid, with a very small proportion of carbonic acid. When, however, the solution was made alkaline with potassium hydroxide, oxalic acid was also one of the products, and the proportion of carbonic acid was much increased. The concentration of the alkali ranged from 5 3 to 340 8 grams per litre; and at the higher strengths (85 grams and upwards) the amount of oxalic acid produced was almost as much as that of the acetic acid. One experiment may be quoted to illustrate the whole: – Permanganate used, 30 grams; alkali, 170.4; alcohol, 3.34; oxalic acid obtained, 2 0 grams; acetic acid, 2 11; carbonic acid, 1.18. To reduce completely 30 grams of permanganate in a litre of water at 50°, 6 grams of alcohol were required when no alkali was added; but 3 5 grams sufficed for the reduction when potassium hydroxide was present in the proportion of 105 grams per litre.

Ethyl alcohol (and also methyl, propyl, butyl, and iso-amyl alcohols) are oxidised by potassium permanganate or hydrogen peroxide in the presence of ferrous salts, which act as catalysts.2 Ferric and manganous salts are without catalytic effect on the oxidation. When ferrous sulphate is present, permanganate oxidises ethyl alcohol, in dilute solution, almost exclusively to the aldehyde; but in presence of ferrous oxalate the oxidation proceeds further, part of the aldehyde being transformed into acetic acid. These oxidations proceed so regularly that the course of the reaction may be investigated quantitatively. Hydrogen peroxide acts still more energetically on ethyl alcohol, which, in presence of ferrous sulphate or oxalate, is oxidised to a mixture of acetaldehyde and acetic acid, the aldehyde predominating with the former, and the acid with the latter, catalyst. These oxidations serve as striking lecture experiments, decolorised magenta solution being used as indicator.

Charcoal acts as an oxidiser of alcohol by reason of its adsorbed, oxygen, and ferrous salts increase the oxidising power. The addition of a ferrous salt to a solution of alcohol containing charcoal results in considerable increase of the amount of aldehyde formed. Ferric salts produce no such increase.

1 J. Amer. Chem. Soc, 1916, 38, 375. 2 Doroschewsky and Bardt, J. Buss. Phys. Chem. Soc, 1914, 46, 754-85; J. Chem. Soc, 1915, 108 (Abst.), ii, 331.

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