Aldehydes vs Ketones: What’s The Difference?

Lucy Bell-Young

by Lucy Bell-Young

8th September 2021

Blog

Various organic compound molecules written on white paper

Aldehydes and ketones are similar in chemical composition and molecular structure. They both have a carbonyl functional group, but the main difference is in the atoms bonded to the central carbon. While aldehydes have an R group chain of hydrocarbon substituents and a hydrogen attached to the central carbon, ketones have R and R’ substituents attached to the central carbon.

Both aldehydes and ketones occur naturally and have important biological roles in metabolism, as well as other physiological functions. They also both have a wide range of industrial and commercial uses. They’re mass produced on an industrial scale and used as solvents, reagents, and precursors to many products, such as various types of polymers. They’re also used as disinfectants.

The Differences Between Aldehyde and Ketone

The key difference between aldehydes and ketones is the type of groups or atoms attached to the central carbon of the carbonyl functional group. This functional group is a carbon that’s double bonded to an oxygen atom.

Aldehydes have an R group hydrocarbon attached to one side, and a hydrogen atom attached to the other side. Ketones, on the other hand, have two alkyl groups (R and R’) attached to both sides of the functional group. These could either be the same substituents, making it a symmetrical molecule, or they could be different types of hydrocarbon substituents, making the molecule asymmetrical. In contrast, aldehydes are always asymmetrical.

Here are five differences between aldehydes and ketones:

  1. Generic forms: Aldehydes have the R-CHO generic form, while ketones have the R-CO-R’ generic form. This difference in form determines how they react with other compounds. It also determines their physical properties, such as boiling point and miscibility with water.
  1. Chemical reactivity: Aldehydes are generally more chemically reactive than ketones. They can easily undergo oxidation, which forms carboxylic acids. Meanwhile, ketones cannot be easily oxidised without breaking the carbon chain.
  1. Carbonyl location: Due to the hydrogen attachment in aldehydes, which gives the molecule its asymmetry, the aldehyde functional group is always located at the end of the carbon chain. This is also responsible for aldehydes’ higher reactivity. The ketone functional group is always located in the middle of the carbon chain, even if the molecule is asymmetrical.
  1. Typical occurrence: Aldehydes are usually found with volatile compounds, such as fragrances. Meanwhile, ketones are usually found in sugars. Most organs of the human body can use ketones as an alternative energy source when the blood sugar level is low. The brain, in particular, mainly uses ketones as an energy source when glucose is not readily available.
  1. IUPAC nomenclature: The standard nomenclature of aldehydes uses the suffix -al while the standard nomenclature of ketones uses the suffix -one. For example, formaldehyde is systematically named as methanal because it’s derived from the oxidation of methanol alcohol. Meanwhile, the simplest ketone is called acetone, but the more formal IUPAC name of this ketone is Propan-2-one.
Functional groups of aldehyde and ketone

Are Aldehydes or Ketones Stronger? 

Ketones are stronger. The strength of the molecular bonds of a substance is directly related to its melting point and boiling point. Put simply, stronger intermolecular bonds are harder to break and, as a result, require more energy. The dipole-dipole bonds between the molecules of aldehydes are relatively weak compared to the bonds between the molecules of ketones.

Given an aldehyde and ketone with the same molecular weight, the boiling point of the aldehyde will always be lower than that of the ketone. This is due to the higher polarity of the ketone, which is also related to the higher dipole moment. Therefore, in terms of intermolecular bonds, ketones are stronger than aldehydes. 

Why Are Aldehydes More Acidic Than Ketones?

Many textbooks on organic chemistry state that aldehydes are comparably more acidic than ketones. This is indicated by the lower pKa of aldehydes, and is mainly because of the lower tendency of aldehydes to donate electrons compared to the alkyl group of ketones. Aldehydes also react faster to nucleophilic attacks compared to ketones.

According to the Lewis definitions of acids and bases, an acid is an electron acceptor, while a base is an electron donor. The alkyl groups in ketones are electron donors, albeit in a relatively weak manner compared to other substances. The anions tend to be destabilised by this, with a tendency to push electrons towards an electrostatically unfavourable system. This means that the anion of a ketone that has extra alkyl groups is a bit unstable compared to an aldehyde. Therefore, a ketone is less acidic.

Why Are Aldehydes and Ketones Classified Differently?

Both compounds are classified as carbonyl compounds because they both contain a carbonyl functional group. The physical and chemical properties of carbonyl compounds are largely influenced by the structure and positioning of their functional group.

Aldehydes and ketones are classified differently despite their similarities mainly because of the lone hydrogen that is bonded to the carbon of the functional group. This seemingly minor difference gives rise to the different chemical and physical properties of aldehydes as compared to ketones.

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