Aldehydes and Ketones

Aldehydes and Ketones

Shiken premium Upgrade Banner

Have you ever left a bottle of wine open for too long and found that it's become sour and acidic? This is because the ethanol in the wine has turned into ethanoic acid - a process known as oxidising an alcohol. But before it becomes a carboxylic acid, the alcohol first turns into something else entirely: an aldehyde. Aldehydes and ketones are organic molecules that contain a special group called the carbonyl functional group. This group consists of a carbon atom joined to an oxygen atom by a double bond, which we represent as .

So what's the difference between aldehydes and ketones? Well, in aldehydes, the carbon atom in the  bond is attached to at least one hydrogen atom, giving them the general formula . On the other hand, the carbon atom in the  bond in ketones is attached to two organic R groups, which may be the same or different. We represent ketones using the formula. In this article, we'll introduce you to the world of aldehydes and ketones in organic chemistry. We'll start by learning their general formulas before diving into their nomenclature and properties. We'll also explore how they compare to other organic molecules and take a brief look at some common reactions involving aldehydes and ketones. So get ready to expand your knowledge on these important organic compounds! And remember, the next time you open that bottle of wine, make sure to finish it off before it turns into an aldehyde.

bonded to two organic R groups
Bonded to two organic R groups

Aldehydes and ketones are structural isomers of each other. These are molecules with the same molecular formulas but different structural formulas. More specifically, they are functional group isomers - isomers with different functional groups. For example, the molecule  could either be propanal, with the structural formula , or propanone, with the structural formula. The following table should help summarise the differences between aldehydes and ketones:

The differences between aldehydes and ketones
The differences between aldehydes and ketones

Naming aldehydes and ketones is actually quite simple once you understand the nomenclature laws. Let's start with aldehydes. To name an aldehyde, you simply replace the -e at the end of the parent alkane name with -al. The carbon atom in the carbonyl group is always given the number 1, and any other substituents are named using the appropriate prefixes.

For example, if we have a molecule with a butyl group attached to the second carbon atom and an aldehyde group on the first carbon atom, we would name it 2-butanal.

Naming ketones follows a similar pattern. You replace the -e at the end of the parent alkane name with -one, and the carbon atoms in the carbonyl group are given the lowest possible numbers. If both R groups are the same, you use the prefix di-. For example, a molecule with a methyl group on one side of the carbonyl group and an ethyl group on the other side would be named 2-propanone, or simply acetone if the R groups are both methyl. By understanding these simple naming rules, you can easily name any aldehyde or ketone molecule you come across in organic chemistry.

Naming aldehydes

Aldehydes use the suffix - al . We include the carbon atom in thebond when finding their root name, which you'll remember indicates the length of their longest carbon chain. However, we don't need to use a number to show where to find the  group, as in aldehydes it is always located at one end of the molecule. Instead, we always assume the carbonyl group is located on carbon 1. For example, this aldehyde has three carbon atoms in its longest chain and no other functional groups. We call it propanal.

An example of an aldehyde
An example of an aldehyde

The following aldehydes are a bit more tricky. It also has a chain three carbon atoms long, but contains two side groups: a methyl group shown in green and a chlorine atom shown in blue. If we count the carbon atom of the carbonyl group as carbon 1, then the methyl group is attached to carbon 2 and the chlorine atom is attached to carbon 3. Remember that we list other side groups in alphabetical order. This molecule is therefore called 3-chloro-2-methylpropanal.


Naming ketones

When it comes to naming ketones, we use the suffix -one. It's important to note that we must include the carbonyl group when counting the number of carbon atoms in the molecule's longest carbon chain. In longer ketones, we need to show the position of the carbonyl group using a number. This follows the 'lowest numbers' rule, where we count from the end of the chain that gives the carbonyl group the lowest number possible. For example, let's consider a ketone with five carbon atoms in its backbone and a carbonyl group on either the second or fourth carbon atom. Since 2 is lower than 4, we correctly name it pentan-2-one. This naming convention makes it easy to identify and understand the structure of different ketones in organic chemistry.


The next ketone has a carbon chain four atoms long. This gives it a root name of -but-. Taking the carbonyl group as carbon 2, we can see that it also has a bromine atom attached to carbon 3, and so it is therefore called 3-bromobutan-2-one.



What are the properties of aldehydes and ketones?

Take a look at the following table. It shows the electronegativities of several elements.

Electronegativity of select elements
Electronegativity of select elements

You'll notice that oxygen has a much higher electronegativity than carbon. In the carbonyl bond, oxygen attracts the shared bonding pair of electrons towards itself, becoming partially negatively charged and leaving the carbon atom partially positively charged. This makes the bond polar and creates a dipole moment , influencing the properties of aldehydes and ketones.

The polar bond in an aldehyde, shown here in methanal
The polar bond in an aldehyde, shown here in methanal

If you aren't sure about polar bonds, take a look at Polarity. Let's explore some of these properties now.

Boiling point

Aldehydes and ketones have higher melting and boiling points compared to alkanes with similar molecular masses due to the presence of polar double bonds, which results in permanent dipole-dipole forces between molecules. However, their boiling points are not as high as similar alcohols because alcohols can form hydrogen bonds between molecules, which are much stronger than permanent dipole-dipole forces.

As with alkanes, the boiling point of aldehydes and ketones increases with increasing chain length. This is because larger molecules have more electrons and form stronger temporary dipoles, resulting in stronger van der Waals attraction between molecules. To learn more about van der Waals attraction, hydrogen bonding, and permanent dipole-dipole forces, you can check out the topic of intermolecular forces. Understanding these concepts is crucial in chemistry, as they help us understand the physical properties of different molecules and how they interact with each other.


Generally, aldehydes and ketones are soluble in water.

Although aldehydes and ketones molecules can't form hydrogen bonds with each other, they can form hydrogen bonds with water. This makes them soluble in water. The oxygen atom in the carbonyl group has two lone pairs of electrons that are attracted to the densely charged hydrogen atoms in water molecules, releasing lots of energy. However, longer-chain aldehydes and ketones are less soluble in water than shorter-chain ones. Their long, non-polar hydrocarbon chains get in the way of the hydrogen bonds and interfere with the bonding.

Hydrogen bonding between propanone and water
Hydrogen bonding between propanone and water

Aldehydes and ketones are highly reactive molecules due to the polarity of the carbonyl group. The partially positively charged carbon atom in the carbonyl group is easily attacked by nucleophiles, which are electron pair donors. Examples of nucleophiles include the cyanide ion, which is negatively charged and contains a lone pair of electrons.

Aldehydes and ketones are also unsaturated molecules as they contain a double bond. Therefore, they readily participate in addition reactions, and most of their reactions are nucleophilic additions. For instance, the reaction between an aldehyde and cyanide ion produces a hydroxynitrile, which contains both nitrile and alcohol functional groups. It's important to note that aldehydes and ketones are named using different suffixes. Aldehydes are named using the suffix -al, while ketones are named using the suffix -one. In addition, they have higher melting and boiling points compared to similar alkanes due to the polarity of the carbonyl group. They are also soluble in water due to the formation of hydrogen bonds. In summary, aldehydes and ketones are highly reactive molecules, and their reactions mainly involve nucleophilic additions. Understanding their reactivity is crucial in organic chemistry as it helps us predict and understand the formation of different compounds. 

Aldehydes and Ketones

What is the difference between aldehydes and ketones?

Both aldehydes and ketones contain the carbonyl functional group, C=O. However, in aldehydes this function group is bonded to at least one hydrogen atom and either zero or one R groups, whereas in ketones this functional group is bonded to two R groups.

How are ketones and aldehydes related?

Aldehydes and ketones both contain the carbonyl functional group, C=O. They are also structural isomers of each other.

What are aldehydes and ketones used for? 

Methanal is a common fungicide, germicide, and tanning agent, but other aldehydes are used as solvents and flavouring agents. For example, the molecule cinnamaldehyde, responsible for the main flavour in cinnamon, is an aldehyde. Ketones are often used as solvents and are found in some steroids.

What are common examples of aldehydes and ketones? 

The most widely known aldehyde is methanal, also known as formaldehyde. It is used as a fungicide, herbicide, and tanning agent. The simplest ketone is propanone, a molecule with a carbon chain that is just three carbon atoms long.

What are aldehydes and ketones?

Aldehydes and ketones are organic molecules containing the carbonyl group, C=O.

Join Shiken For FREE

Gumbo Study Buddy

Explore More Subject Explanations

Try Shiken Premium
for Free

14-day free trial. Cancel anytime.
Get Started
Join 20,000+ learners worldwide.
The first 14 days are on us
96% of learners report x2 faster learning
Free hands-on onboarding & support
Cancel Anytime