The first catalysed reaction
The conversion of ethanol to ethanoic acid, commonly known as acetic acid, was the first reaction to be attempted in the presence of a catalyst. This introduction was done in the late 1700 by Johann Wolfgang D’bereiner, even before the term catalyst was introduced. Ethanoic acid had been first identified by German chemist George Stahl earlier that century, produced by air oxidation of ethanol present in alcoholic drinks. Although this process was quickly applied to the production of a weak solution of acetic acid called “vin aigre” which literally means “sour wine”, it wasn’t until 1949 that American chemist Frank Westheimer finally unveiled the complicated mechanism behind this reaction.
Although more sophisticated synthetic methods have been developed to synthesise ethanoic acid, the production of wine vinegar still maintains the tradition of relying on this oxidation reaction to convert ethanol into ethanoic acid.
Ethanol is converted to ethanoic acid through an oxidation reaction
The early process merely involved leaving an alcoholic drink (containing ethanol) exposed to air to obtain vinegar. This basic procedure has been significantly improved, and the reaction is now conducted using a catalyst at 150 degree Celsius under reflux and high pressure. Under these conditions, a 90% yield can be obtained, representing a simple and eco-friendly way to generate ethanoic acid. The reaction can be subdivided in two stages:
- First stage: Partial oxidation generating ethanal
- Second stage: Ethanal oxidation to ethanoic acid (full ethanol oxidation)
The reaction can be catalysed with potassium dichromate (K2Cr2O7) or sodium dichromate (Na2Cr2O7) in combination with sulphuric acid, to generate the reactive oxygen. Conventionally, this oxygen is represented as [O].
K2Cr2O7 + H2SO4 à Na2SO4 + Cr2(SO4)3 + H2O + [O]
Partial oxidation to ethanal
The first stage involves the production of ethanal (which is an aldehyde). During the initial stages, excess ethanol (CH3CH2OH ) means not enough oxidising agent is able to continue the reaction to the second stage, and there is a temporary accumulation of ethanal (CH3CHO).
CH3CH2OH + Cr2O72- + H+ à CH3CHO + Cr3+ + H2O
Full oxidation to ethanoic acid
As the ethanol is consumed in the partial oxidation, the oxidising agent (now present in excess) is free to convert ethanal to ethanoic acid and the full oxidation can be completed. Once all reagents are consumed, the solution is allowed to cool and ethanoic acid is removed by distillation.
CH3CHO + Cr2O72- à CH3COOH + Cr3+ + H2O
In reality, this reaction has very little importance in an industrial setting, as acetic acid is produced more efficiently via the Monsanto and Cativa processes using methanol. There are some applications which use ethanal as a raw product, but in that case, this product is generated by oxidation of ethylene. This reaction remains however, a favourite amongst chemistry teachers to explain the oxidation of a primary alcohol partially to aldehyde and completely to carboxylic acid.