Aldehydes and Ketones

If you leave a bottle of wine open for long enough, it eventually becomes sour and acidic. The pH drops and you have turned ethanol into ethanoic acid. This is an example of oxidising an alcohol. But before the alcohol turns into a carboxylic acid, it first oxidises into something else entirely: an aldehyde.

• This article is an introduction to aldehydes and ketones in organic chemistry.
• We will start by learning the general formulas of aldehydes and ketones before looking at their nomenclature.
• We'll then explore some of their properties and see how they compare to other organic molecules.
• Finally, we'll briefly look at some of the common reactions involving aldehydes and ketones.

Difference between aldehydes and ketones

As we mentioned above, aldehyde and ketones are organic molecules Containing the carbonyl group, $\mathrm{C}=\mathrm{O}$. In aldehydes, the carbon atom in the $\mathrm{C}=\mathrm{O}$ bond is attached to at least one hydrogen atom. This gives aldehydes the general formula $\mathrm{RCHO}$. In contrast, the carbon atom in the $\mathrm{C}=\mathrm{O}$ bond in ketones is attached to two organic R groups. These may be the same or different. We represent ketones using the formula $\mathrm{RCOR}\text{'}$.

The general formulas for an aldehyde (left) and a ketone (right). Note that the carbonyl group in a ketone is never attached to a hydrogen atom. Instead, it is always bonded to two organic R groups. Anna Brewer StudySmarter Originals

The following table should help summarise the differences between aldehydes and ketones:

The differences between aldehydes and ketones, with examples. Note the different numbers of hydrogen atoms bonded to the carbonyl group in each case. Anna Brewer StudySmarter Originals

How do we name aldehydes and ketones?

Naming aldehydes and ketones is pretty straightforward. If you aren't familiar with classifying compounds we'd recommend first checking out Organic Compounds, but if you feel confident with nomenclature laws, you shouldn't find this too much of a challenge.

Naming aldehydes

Aldehydes use the suffix - al . We include the carbon atom in the$\mathrm{C}=\mathrm{O}$bond 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 $\mathrm{C}=\mathrm{O}$ group, as in aldehydes it is always located at one end of the molecule. Instead, we always assume the carbonyl group $\mathrm{C}=\mathrm{O}$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. Anna Brewer, StudySmarters Originals

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.

3-chloro-2-methylpropanal. Anna Brewer, StudySmarter Originals.

Naming ketones

To name ketones, we use the suffix - one . Remember to include the carbonyl group when counting the number of carbon atoms in the molecule's longest carbon chain. You should also note that in longer ketones, we need to show the position of the carbonyl group using a number, making sure to give it the lowest number possible. This is an example of the 'lowest numbers' rule and could mean counting from the left side of the chain or from the right.

For example, this ketone has five carbons atoms in its backbone and a carbonyl group in either position 2 or 4, depending on which side of the molecule you count from. As 2 is lower than 4, we correctly classify it as pentan-2-one:

Pentan-2-one, shown with the correct numbering of the carbon atoms in its backbone. Anna Brewer StudySmarter Originals

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.

Anna Brewer, StudySmarter Originals

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. Anna Brewer, StudySmarter Originals

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 $\mathrm{C}=\mathrm{O}$bond polar and creates a dipole moment , influencing the properties of aldehydes and ketones.

The polar bond in an aldehyde, shown here in methanal. Anna Brewer, StudySmarter Originals

Let's explore some of these properties now.

Boiling point

Aldehydes and ketones have high melting and boiling points compared to alkanes with a similar molecular mass. This is because aldehydes and ketones experience permanent dipole-dipole forces between molecules due to their polar $\mathrm{C}=\mathrm{O}$double bond, as explored above. However, their boiling points are not as high as similar alcohols. You may remember that alcohols can form hydrogen bonds between molecules as they have an oxygen atom bonded to a hydrogen atom. These hydrogen bonds are much stronger than permanent dipole-dipole forces and require more energy to overcome.

Like with alkanes, the boiling point of carbonyls increases as chain length increases. This is because larger molecules have more electrons and form stronger temporary dipoles. This increases the strength of van der Waals attraction between molecules.

Solubility

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. As a result, propanone dissolves well in aqueous solution. Anna Brewer, StudySmarter Originals

How do aldehydes and ketones react?

As we explored above, the $\mathrm{C}=\mathrm{O}$bond in aldehydes and ketones is strongly polar. As a result, the partially positively charged carbon atom is easily attacked by nucleophiles.

Examples include the cyanide ion, ${\mathrm{CN}}^{-}$. Nucleophiles are always negatively or partially negatively charged and contain a lone pair of electrons.

You might also have noticed that aldehydes and ketones are unsaturated - they contain a double bond. They therefore readily take part in addition reactions. In fact, most of their reactions are nucleophilic additions. For example, reacting cyanide with an aldehyde produces a hydroxynitrile, which is a molecule containing both nitrile and alcohol functional groups, $\mathrm{C}\equiv \mathrm{N}$ and $-\mathrm{OH}$ respectively.