In a recent blog post, we looked at how the oxidation of primary alcohols into carboxylic acids is the process used when converting ethanol into ethanoic acid. But what is oxidation exactly?
Oxidation is one of the key types of chemical reactions. It is seen everywhere, from the roar of your fire to the rust on your bike. Ostensibly, oxidation is when a substance gains an oxygen. For example, you can say that magnesium is oxidised when it reacts with oxygen to form magnesium oxide.
However, this is an outdated definition that first came about when oxygen was the only recognised oxidising agent. In modern-day chemistry, this is not the case. In fact, oxidation doesn’t necessarily have anything to do with oxygen.
How Do We Define Oxidation?
Put simply, it is the loss of electrons from a molecule, atom, or compound during a chemical reaction. This electron loss correspondingly increases the oxidation state, and this process can happen with or without oxygen. For example, when iron is expressed as Fe3+, it tells us that it has an increased oxidation state and has lost 3 electrons.
Therefore, a compound that readily loses its electrons is easily oxidised. An increased oxidation state is expressed with a “+” sign, and appears next to the number of electrons that have been lost.
Oxidation is closely related to reduction, which is another important chemical reaction. Rather than losing electrons, reduction is characterised by a molecule, atom, or compound gaining electrons. This decreases the oxidation state as the electrons are held onto tightly, and this is alternatively expressed with a “–“ next to the number of electrons that have been gained. When oxidation and reduction occur together, it is known as a redox reaction.
Examples of Oxidation
Oxidation surrounds us every day and is an important part of many chemical reactions. Here are 5 examples of oxidation that all undergo different processes.
When a compound is oxidised, the loss of electrons causes its properties to change. The most common example of this is iron, which is a structurally sound metal when unoxidised. However, when iron reacts with oxygen, it becomes rust, also known as iron oxide.
Unlike unoxidised iron, rust is a brittle powder-like substance. The process of rusting requires three elements: water, oxygen, and iron. When oxygen combines with metal, electrons are released. Water behaves as an effective electrolyte, which the newly liberated electrons travel through towards the cathode (a piece of metal that accepts electrons). Meanwhile, the metal from the anode (a piece of metal that gives up electrons easily) begins to disappear into the electrical flow. As it is swept away, it is converted into metal cations. This is when rust is formed.
We all know that when a piece of sliced apple is left out in the air it turns brown. This happens because when the skin of a fruit is ruptured, the walls and membranes of the cells within the fruit are exposed to oxygen.
When this happens, the compounds in fruit incorporate oxygen into their molecular structure and react with it. The resulting brown colour of the oxidised organic compounds occurs because an enzyme called polyphenol oxidase is able to oxidise with phenolic compounds found in fruit tissue. This causes the phenolic compounds to condense into brown spots.
Oxidation plays an important role in the chemistry of fire, and is one of the faces on the fire tetrahedron. During a fire, an oxidising agent is required to support burning. It does this by causing the compounds released by the combustible fuel source to break apart and lose electrons. These then recombine with oxygen to produce water vapour, carbon dioxide, various combustion products, and heat. This is recognised as burning.
There are many substances that can be used as oxidising agents in a fire. These agents are defined as materials that readily produce oxygen or other oxidising substances like bromine and chlorine. While oxygen in the air is the most common oxidising agent used for fires, hydrogen peroxide and halogens are also very common.
One of the common processes where alcohol is oxidised in the laboratory is when ethanol is being converted into ethanoic acid. Potassium dichromate with dilute sulphuric acid is often used to oxidise ethanol in this reaction.
We already know that oxidation doesn’t exclusively refer to the gain of oxygen, and rather encompasses the loss of electrons. In this process, the oxidation of ethanol is characterised by the loss of hydrogen.
Ethanol is oxidised into acetaldehyde by losing two hydrogen atoms to the oxidising agent. This is physically expressed in the colour change of the dichromate solution, which changes from orange to green as the oxidising agent reduces to chromium (III) sulphate. The acetaldehyde is then oxidised into ethanoic acid by gaining an oxygen atom. You can see the reaction for this below:
Biological Oxidation of Alcohol
The same process as above also occurs in the human body. The biological oxidation of alcohol happens when we drink alcohol beverages. Our bodies work to break down the alcohol in our system by converting the ethanol into ethanoic acid. In order to do this, it must first oxidise the ethanol into acetaldehyde.
The oxidation process in the body is catalysed by enzymes and coenzymes. Ethanol is oxidised into acetaldehyde by NAD+ (nicotinamide adenine dinucleotide), a coenzyme that behaves as the oxidising agent. Meanwhile, the enzyme alcohol dehydrogenase catalyses the reaction. NAD+ incurs oxidation by removing the hydrogens and electrons.
Acetaldehyde is very toxic, and is also the culprit behind hangovers. This why it is important that our bodies convert it into ethanoic acid quickly so that it doesn’t build up. To do this, another enzyme known as aldehyde dehydrogenase oxidises acetaldehyde until it has fully converted.
Sometimes oxidation is helpful. Sometimes it’s destructive. Whichever way you look at it, though, it is a vital reaction that takes place in many chemical processes. At ReAgent, we sell a variety of products that can be used as oxidising agents, including hydrogen peroxide. We have hundreds of high quality products available in our online shop, so why not take a look? For any queries, please get in touch today.