What Are Ketones in Chemistry?

Ketones are a class of organic compounds identifiable by their carbonyl group, i.e. the group that has a carbon-oxygen double bond. 

A by-product of metabolism, ketones are naturally present in humans, plants and animals. They’re also used as solvents in a wide range of industries. 

Continue reading to learn more about ketones, the different types, and some common applications.

What are ketones?

Ketones are a large group of organic compounds characterised by the carbonyl functional group. The functional group is connected to two R groups that can represent a wide variety of carbon-containing substituents. The generic formula for ketones can therefore be written as R−C(=O)−R’. The simplest form of ketone is acetone (CH₃)₂CO, a clear, volatile liquid with a sweet smell.

Ketones exist naturally as a by-product of metabolism. In humans, they are formed in the liver through the breakdown of fats. They act as an alternative source of energy when blood glucose is low, for example, when someone is fasting or during long periods of exercise. 

However, if you eat a normal diet and your blood and urine contain high levels of ketones, it could indicate diabetes. Essentially it’s a sign that the cells cannot effectively metabolise the glucose in food. 

In terms of industrial applications, ketones are mass-produced primarily as solvents. These chemicals are also used as polymer precursors and pharmaceutical reagents. 

The most important ketones that have biological significance, industrial applications, and commercial value are acetone, methyl ethyl ketone, and cyclohexanone.

Types of ketones

Ketones are usually classified according to their respective substituents, which means they can either be symmetrical or asymmetrical (more on this later). Symmetrical ketones have the same type of substituents on either side, whereas asymmetrical ketones have different types.

Ketones can also be categorised into diketones, unsaturated ketones, and cyclic ketones. 

Each class is identifiable by their substituents, the number of functional groups, and the molecules. While the ketones in a particular class may have similar chemical and physical properties, there are also some significant differences.

• Diketones – As the name suggests, diketones have two ketone (carbonyl) functional groups. The dual carbonyl (C=O) groups provide unusual properties such as smell and flavour. Hence, some diketones are used as artificial flavourings in processed food.
• Unsaturated ketones – These ketones contain alkene and alkyne substituents. They are unsaturated because the substituents contain double or triple bonds. 
• Cyclic ketones – These ketones have cyclic molecular structures, which means some of their atoms are connected in such a way that they form rings. Cyclic ketones have the simplest generic chemical formula of (CH₂)nCO.

Examples of ketones

Aside from acetone, here are some examples of ketones and their respective chemical formulas:

Diacetyl

Diacetyl is a type of diketone that has the chemical formula CH₃C(O)C(O)CH₃. It’s the simplest example of a diketone and is commonly used as an artificial butter flavouring for popcorn.

Methyl vinyl ketone

An example of an unsaturated ketone, methyl vinyl ketone has the chemical formula CH₃C(O)CH=CH₂. This type of ketone is useful in Robinson annulation reactions – a method developed in 1935 to form six-membered ring compounds through the formation of three new carbon–carbon bonds.

Cyclohexanone 

Cyclohexanone has the chemical formula (CH₂)₅CO. It’s a symmetrical cyclic ketone that’s commonly used as a monomer in nylon manufacturing.

Methyl ethyl ketone

Methyl ethyl ketone, otherwise known as 2-butanone, is a colourless liquid with the chemical formula C₄H₈O or CH₃COCH₂CH₃. Chronic inhalation of this ketone’s vapour can result in minor neurological problems, kidney disease, liver disease, and respiratory irritation.

Applications of ketones

Ketones are a metabolic by-product of fat breakdown and serve as an alternative source of calories for biological systems. As we explain below, they also have several commercial and industrial uses.

Ketones in biological systems

Ketones act as an energy source for extrahepatic tissues, including those of the brain, heart, and skeletal muscles. But that’s not their only function. Ketones also serve as signalling mediators and drivers of protein post-translational modification (PTM). They play an important role in controlling and modulating inflammation, too.

Industrial and commercial applications of ketones

Ketones are used as solvents and reagents for many manufacturing and chemical processes. 

Take acetone, for example. In addition to its many household uses (the most famous being nail polish remover), acetone is involved in the production of plastics, pharmaceuticals, and cosmetics, to name a few. It’s also used to manufacture other chemicals such as methyl methacrylate and bisphenol A.

Methyl ethyl ketone – another commercially important ketone – also has a wide range of industrial applications, mainly as a highly effective solvent. It’s commonly used in surface coatings such as automotive paints, as well as glues, cleaners, and textiles. 

Symmetrical vs. asymmetrical ketones

As previously mentioned, ketones can be broadly classified into two categories – namely symmetrical and asymmetrical ketones. 

Symmetrical ketones have the same type of substituents, while asymmetrical ketones are structurally imbalanced because they contain different types of substituents.

Safety and handling of ketones

Ketones are highly volatile, often flammable substances that are capable of dissolving many organic compounds, including non-polar compounds. They’re also poisonous when ingested. Depending on the type, they can even irritate the skin and mucous membranes. Inhaling ketone vapours can cause respiratory problems, too. 

Their hazardous nature means ketones should be handled very carefully. Always wear the appropriate PPE and ensure any chemicals are stored out of reach of children and animals and per the manufacturer’s instructions.

Conclusion

Ketones are organic compounds with a carbonyl group that contains a carbon atom covalently bonded to an oxygen atom. They have important roles in biological systems and industrial applications. Ketones can be classified into different groups based on their structure and substituent compositions. Some of the most common types of ketones include acetone, cyclohexanone, and methyl ethyl ketone.