What Are Ketones in Chemistry?

Lucy Bell-Young

by Lucy Bell-Young

25th August 2021

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Ketones word concept on cubes

Ketones are a large group of organic compounds that have biological and industrial importance. Ketones are identifiable by their carbonyl group, i.e. the group that has a carbon-oxygen double bond. In medical diagnosis, high levels of ketones in the blood and in the urine is indicative of a diabetic complication known as diabetic ketoacidosis, or DKA.

Ketones play the central role in the controversial “keto diet” promoted by dieting companies and health gurus. While this diet might be effective for a few select people, it does pose a number of health risks, often leading to kidney stones and liver problems.

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 importance, industrial applications, and commercial value are acetone, methyl ethyl ketone, and cyclohexanone.

What Are Some Ketone Examples?

In standard IUPAC (International Union of Pure & Applied Chemistry) nomenclature, ketones are named based on the alkanes attached to the carbonyl functional group. The suffix -ane of the alkane group is changed into -anone, while the position of the carbonyl group is indicated by a number. Some important ketones, however, retain their common names. For example, the simplest ketone, which is acetone (CH3−CO−CH3), has the alternate name 2-propanone, though it isn’t widely known as this. 

Aside from acetone, here are some examples of ketones and their respective chemical formula and molecular structure:

  • 2-butanone (C4H8O): More commonly known as methyl ethyl ketone, this is used as a solvent for paints and glues. It’s also used as a cleaning agent. Chronic inhalation of this ketone’s vapour can result in minor neurological problems, kidney disease, liver disease, and respiratory irritation:
  • 2-pentanone (C5H10O): This is also known as methyl propyl ketone. It’s a colourless liquid with a smell similar to nail polish. It has a role as a plant metabolite, and its molecular structure looks like this:
  • 2-hexanone (C6H12O): Otherwise known as methyl butyl ketone or MBK, this is used as a general solvent and as an ingredient in paint. It can easily dissolve nitrate, vinyl polymers, and copolymers. It can also dissolve both natural and synthetic resins:
  • 3-octanone (C8H16O): This is also known as ethyl amyl ketone. It’s produced by the human body as a metabolite in urine. It’s also a metabolite of some fungi and plants. It can be used to attract insects for entomological studies, and can also be used as a biomarker for diagnostic purposes:
  • 3-nonanone (C9H18O): Otherwise known as ethyl hexyl ketone, this compound has a sweet, fresh, and fruity taste. It can be found in fruits and milk:
  • 3-decanone (C10H20O): The other name for this compound is ethyl heptyl ketone. It’s useful as a food additive and it has a role as a metabolite. This is its chemical structure:
  • 1-Phenylethan-1-one (C8H8O): This is also known by other names, such as acetophenone and phenylethanone. It’s the simplest aromatic ketone and is mainly used as a precursor to synthetic resins and fragrances:

The high polarity of the carbonyl group of ketones make these organic compounds highly reactive, and they can readily react in various ways. The typical reactions that ketones undergo include oxidation-reduction reactions and nucleophilic addition. The polarity of the carbonyl group also has a direct effect on the physical properties of ketones.

The Structure of Ketones

Ketones are organic compounds that have a carbonyl group with two alkane chains attached to it. Ketones are named after these substituents. The structural formula of ketones can generalised as follows:

On the basis of how the carboxyl group is attached to the R and R’ groups, ketones can be broadly classified either as symmetrical or asymmetrical derivatives of hydrocarbon substituents. The hydrocarbons can either be aliphatic (alkanes, alkenes, and alkynes) or aromatic.

If the chemical formula and structure suggest equal numbers of hydrocarbons on either side of the carbonyl group, the ketone is classified as symmetrical. One good example of a symmetrical ketone is benzophenone, the chemical formula for which can be written as C6H5C(O)C6H5. An example of an asymmetrical ketone is acetophenone, which has the chemical formula C6H5C(O)CH3.

Ketones can be simple or complex with many aromatic rings. For instance, tetracycline is a type of ketone that’s used as an antibiotic. It’s typically prescribed to treat a range of infections, such as acne, cholera, brucellosis, plague, malaria, and syphilis. The molecular structure of this substance is illustrated below:

The ketone carbon is classified as having sp2 hybridised orbitals. This translates to trigonal planar bond angles around the ketone carbon. This makes the carbonyl group of a ketone molecule highly polar. At the oxygen part of the carbonyl group, ketones are nucleophilic, meaning they have the tendency to donate electrons. Meanwhile, at the carbon part, ketones are electrophilic, meaning they have the tendency to accept electrons.

A Look at Some Popular Ketone Groups

Aside from the two generalised classes of ketones (symmetrical or asymmetrical), these compounds can also be classified into three main groups:

  • Diketones: These are some of the most well known examples. They have two carbonyl groups, and some of them have very unusual properties, such as diacetyl, which was once used as butter flavouring in popcorn.
  • Unsaturated ketones: These contain alkene and alkyne units.
  • Cyclic ketones: These are very useful compounds and contain at least one cyclic or aromatic hydrocarbon. They are commonly used as precursors to polymers.

Are Aldehydes Similar to Ketones?

Structurally and chemically, ketones and aldehydes are very similar. Both have at least one carbonyl functional group, as well as hydrocarbon substituents. However, the main difference is that aldehyde has one hydrogen attached to the carbonyl group. The molecular structure of aldehydes can be generalised as follows:

Many species of aldehydes have industrial applications, but formaldehyde is produced the most on an industrial scale. Aside from preserving organic tissues, such as in the case of embalming, formaldehyde is mainly used in the production of synthetic resins, and it’s also a precursor to polyurethanes.

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