A Level Chemistry Revision: Organic Chemistry – Aldehydes & Ketones

by Kate Onissiphorou

If you’re preparing for your A level organic chemistry exam, you’ll need to have a good understanding of aldehydes and ketones. This includes learning their names, functional groups, and properties.

To help with your revision, we’ve put together an overview of the key points you need to know.

Aldehydes and ketones: an introduction

Aldehydes and ketones are types of organic compounds that have a carbonyl functional group.

Put simply, a carbonyl group contains a carbon atom that is double-bonded to an oxygen atom. Consequently, aldehydes and ketones have a few similar chemical and physical properties. 

For example, both are polar compounds due to the slightly stronger pull of the oxygen atom on the electron pairs. During a chemical reaction, the carbonyl group undergoes an addition-elimination reaction that breaks the carbon-oxygen double bond.

The main difference between aldehydes and ketones is the presence of hydrogen attached to the carboxyl functional group:

  • A ketone has two R groups with carbon-containing substituents attached to the carboxyl functional group
  • An aldehyde has one R group with carbon-containing substituents and one hydrogen attached to the carbonyl functional group

The presence of hydrogen attached to the carboxyl functional group means aldehydes are easy to oxidise and therefore strong reducing agents. Ketones, however, are resistant to oxidation and can only be oxidised by strong agents like potassium permanganate solution.

Naming aldehydes and ketones

Many aldehydes and ketones have common names and alternative chemical names However, these are less precise and non-standardised, which can lead to confusion and disagreements. To prevent this, the IUPAC nomenclature system is used to standardise naming.

Naming aldehydes and ketones

Aldehydes

Aldehyde names are based on the longest carbon chain that contains the aldehyde group. The parent alkane suffix -e is changed into -al. For example, a derivative of butane CH3CH2CH2CHO is named butanal.

In cases where the aldehyde group is attached to a ring, -carbaldehyde is sometimes used as a suffix. For instance, the aldehyde C6H11CHO is named cyclohexanecarbaldehyde.

However, if the compound is derived from carboxylic acid, the prefix oxo- may be added. This also indicates the position of the aldehyde carbon.

Justus Von Liebig coined the name aldehyde in 1835 in a paper describing the preparation of acetaldehyde or ethanal. It was a shortened version of the Latin dehydrogenatus. He derived an aldehyde from the dehydrogenation of alcohol. Historically, aldehydes were named based on their corresponding alcohols.

Ketones

The word ketone originates from the old German word for acetone, aketon. Many common names of ketones, such as acetone and benzophenone, are still used today. The IUPAC naming convention drops the suffix -ane of the parent alkane and replaces it with -anone. The position of the carbonyl group is indicated by a number, such as 2-propanone.

If there are multiple and different types of alkyl groups that branch out of the carbonyl group, the names are written in alphabetical order and the word ketone is added at the end. Methyl ethyl ketone is one such example. The prefix di- is added if two alkyl groups are the same.

Properties and structures

Aldehydes

Aldehydes have a wide range of properties depending on the attached R group. They tend to behave like strong reducing agents due to the presence of hydrogen. Aldehydes with shorter R groups are generally soluble in water, while volatile aldehydes have strong pungent odours.

When it comes to structure, aldehydes are distinguished from ketones by the hydrogen attached to the carbonyl group. This makes the aldehyde group polar. The aldehyde carbon can be described as sp2 hybridised, which is the mixing of one s orbital with two p orbitals.

Aldehydes occur naturally as byproducts of metabolic processes. For example, food additives such as cinnamaldehyde, cilantro, and vanillin are found in the essential oils of plants.

Ketones

The ketone carbon is also sp2 hybridised. However, unlike aldehydes, ketones are resistant to oxidation reactions. They can only be oxidised by strong oxidising agents because of the two R substituents. Ketone molecules form trigonal planar around the ketonic carbon, with bond angles of around 120°.

Ketones are polar because of the slight pull of the oxygen on the electrons. Consequently, the oxygen part is nucleophilic while the carbon part is electrophilic. They tend to be more soluble in water but they’re not good donors of hydrogen bonds.

Ketones are divided into three categories: diketones; unsaturated ketones; and cyclic ketones. They all occur in nature but some are synthesised.

  • Diketones as the name implies, diketones contain two ketone groups. Some have unusual chemical and physical properties. For example, diacetyl a once common butter flavouring of popcorn is a type of ketone with the formula CH3C(O)C(O)CH3.
  • Unsaturated ketones these are ketones that contain alkene and alkyne substituents.
  • Cyclic ketones many ketones have a cyclic structure. They have biological roles, for example, in the production of pheromones, along with a variety of industrial applications. Cyclic ketones are used to manufacture nylon, for instance.

Identification and analysis

You can use a range of quantitative and qualitative analytical methods to identify aldehydes and ketones. Blood sample tube for a ketone test

Spectroscopic techniques, for example, can identify the compositions of ketones and aldehydes. You can also adopt qualitative methods such as Brady’s test, which detects the presence of ketones and aldehydes.

Chemical reactions

Both aldehydes and ketones can undergo various chemical reactions. They may behave similarly in some reactions and different in others. The types of chemical reactions they can undergo include:

  • Hydration reactions
  • Acid-base reactions
  • Oxidation-reduction reactions
  • Nucleophilic addition reactions

In a nucleophilic reaction, the carbonyl carbon of an aldehyde becomes sp3-hybridised while the oxygen is protonated. The generalised reactions are as follows:

RCHO + Nu → RCH(Nu)O

RCH(Nu)O + H+ → RCH(Nu)OH

Oxidation reactions of aldehydes vary depending on whether the solution is acidic or basic. The half reaction equations are set out below. The first equation shows a reaction performed under acidic conditions while the second is conducted under basic or alkaline conditions.

Half reaction equations

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