Ethanol (CH3CH2OH) is the primary alcohol that is used in alcoholic drinks and denatured alcohol. Among its many applications, it is used to form ethanoic acid which is most popularly found in a kitchen essential: vinegar.
What is Ethanoic Acid?
Ethanoic acid (CH3COOH), also known as acetic acid, is the most typical example of a carboxylic acid. A versatile chemical recognised by its acidic smell and taste, ethanoic acid is the active ingredient in vinegar. It is also used as a preservative, and many foods are stored in it because bacteria is unable to withstand the acidic environment.
Ethanoic acid is formed when ethanol is oxidised. This reaction actually happens every day when bottles of wine are left open for a while. As a ‘typical’ acid, it:
- Reacts with metals to produce hydrogen
- Reacts with bases or carbonates to form salts
- Turns litmus paper and universal indicator red
Making Ethanoic Acid
Ethanoic acid is the most common example of a carboxylic acid. These acids are a series of organic compounds that contain a carboxyl group (COOH), and there are a variety of methods that are used to produce them.
The most common method is oxidising primary alcohols. This is done in two simplified stages. First, the primary alcohol is oxidised to an aldehyde. Next, the aldehyde is converted to an acid.
The alcohol is oxidised by using potassium dichromate solution in the presence of dilute sulphuric acid. It is the dichromate ion that is the oxidising agent, and dilute sulphuric acid is used to acidify the solution.
Oxidation of Ethanol
Potassium dichromate solution is acidified with dilute sulphuric acid to oxidise ethanol into ethanoic acid. There are few steps that comprise this process:
- Ethanol is heated under reflux with an excess mixture of potassium dichromate solution and dilute sulphuric acid. Heating under reflux is achieved by heating the mixture in a flask with a condenser placed vertically inside so that any vapour is liquefied and returned. It prevents any aldehyde that is formed from escaping before full oxidation has occurred
- Excess oxidising agent is used to ensure that there is enough for the ethanol to convert to a carboxylic acid. If there was too little oxidising agent, ethanol would only partially oxidise into an aldehyde
- During the reaction, potassium dichromate turns from orange to green. It also eventually reduces to chromium (III) sulphate
- The ethanol will first oxidise into acetaldehyde, which can be identified by its sweet smell. This reaction will also produce water. The mixture is then left to oxidise further
- When oxidation is complete, acetaldehyde will convert into ethanoic acid which has a comparatively sharper smell – like vinegar. The solution is then distilled in order to eliminate any water. The result is an aqueous solution of ethanoic acid
During the early stages of this reaction, it is important to thoroughly mix the solution before heating. If this is not done, the reaction can be vigorous and can suddenly boil and explode out of the condenser. Therefore, always make sure you are being careful when attempting this reaction.
In the Body
Ethanol is also converted into ethanoic acid in the body. In our post about the science of alcohol, we looked into the effects alcohol has on our body and why these occur. Since alcohol contains ethanol, a similar process as above happens in our bodies in order to break down the ethanol and get it out of our system.
Ethanol is metabolised in the liver by an enzyme called alcohol dehydrogenase. This reaction produces the same half-way product that forms in the oxidation of ethanol: acetaldehyde. When there is an accumulation of this toxic compound in the body, we feel nausea and headaches. In other words, acetaldehyde is why we get hangovers. Therefore, it is important for the body to break this down quickly.
It does this by metabolising acetaldehyde with another enzyme, aldehyde dehydrogenase. This breaks down the toxic compound into a non-toxic compound: ethanoic, or acetic acid. Because it is more soluble in water, ethanoic acid can then be easily carried by the bloodstream to the kidneys, where it exits the body via urination.
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Ethanoic Acid 50% v/v