Aldehydes and Ketones (HSC SSCE Chemistry): Revision Notes

Aldehydes and Ketones

Introduction to carbonyl compounds

Carbonyl compounds are organic molecules that feature a carbon atom connected to an oxygen atom through a double bond. This special arrangement, known as the carbonyl group ($\text{C}=\text{O}$), gives these compounds unique properties and behaviours.

What are aldehydes?

Aldehydes are organic molecules characterized by having their carbon-oxygen double bond positioned at the end of the carbon chain. This terminal placement is what distinguishes aldehydes from other carbonyl compounds.

The general molecular formula for aldehydes can be written as:

$\text{R}\text{CHO} \text{ or } \text{R}-\text{CH}=\text{O}$

where $\text{R}$ represents any alkyl group (a chain of carbon and hydrogen atoms).

Common examples of aldehydes

• Methanal ($\text{HCHO}$): Also called formaldehyde, this is the simplest aldehyde. It is manufactured in large quantities and used extensively in making plastics. The first synthetic plastic, bakelite, was created using phenol and formaldehyde.
• Ethanal ($\text{CH}_3\text{CHO}$): A two-carbon aldehyde commonly used in industrial processes.
• 2-methylbutanal: A branched aldehyde with a methyl side group, demonstrating how aldehydes can have more complex structures.

What are ketones?

Ketones differ from aldehydes in a key way: the carbonyl group ($\text{C}=\text{O}$) can be located on any carbon atom except those at the ends of the hydrocarbon chain. This means the carbonyl group sits within the carbon chain, rather than at its terminus.

The general molecular formula for ketones is:

$\text{RCOR}' \text{ or } \text{R}-\text{C}(=\text{O})-\text{R}'$

where $\text{R}$ and $\text{R}'$ represent alkyl groups that can be the same or different.

Common examples of ketones

• Propanone ($\text{CH}_3\text{COCH}_3$): Commonly known as acetone, this is the simplest ketone. It is widely used as a solvent in laboratories and industry, including in nail polish remover.
• Butanone ($\text{CH}_3\text{COCH}_2\text{CH}_3$): A four-carbon ketone used as an industrial solvent.
• 3-pentanone ($\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3$): A five-carbon ketone where the carbonyl group is positioned at carbon 3.

Naming aldehydes and ketones

Understanding how to name these compounds correctly is essential for communicating clearly in chemistry. The naming system follows specific rules that help identify the structure from the name alone.

How to name aldehydes

The suffix '-al' is used for aldehydes, replacing the '-e' from the parent alkane name.

Step-by-step process:

1. Identify the longest carbon chain that includes the $\text{CHO}$ group
2. Remove the 'e' from the parent alkane name and add 'al' (for example, ethane → ethanal)
3. Number the carbon atoms starting from the aldehyde group. Since the aldehyde functional group is always at position 1, you don't need to include this number in the name
4. Name any side branches (alkyl groups) using the same rules as for branched alkanes, with their position numbers

How to name ketones

The suffix '-one' is used for ketones, replacing the '-e' from the parent alkane name.

Step-by-step process:

1. Identify the longest carbon chain that includes the $\text{C}=\text{O}$ group
2. Remove the 'e' from the parent alkane name and add 'one' (for example, propane → propanone)
3. Number the carbon atoms from the end that gives the smallest number to the $\text{C}=\text{O}$ group
4. Add a number before the name to show the position of the carbonyl group (for example, 2-pentanone)
5. Name any side branches (alkyl groups) using the same rules as for branched alkanes, with their position numbers

Functional group isomers

Aldehydes and ketones with the same number of carbon and hydrogen atoms are called functional group isomers. They share the same molecular formula but have different functional groups, which means they have different structural formulae and different properties.

Worked example: identifying and naming carbonyl compounds

Let's practice identifying whether molecules are aldehydes or ketones and naming them correctly.

When approaching these structures, identify the longest carbon chain, locate the carbonyl group, number from the appropriate end, and then name any branches systematically.

Properties of aldehydes and ketones

The carbonyl group gives aldehydes and ketones distinctive physical and chemical properties that set them apart from other organic compounds.

Polarity of the carbonyl group

Oxygen is more electronegative than carbon, meaning it has a stronger pull on shared electrons in the carbon-oxygen bond. This creates a highly polar bond, with oxygen carrying a partial negative charge ($\delta^-$) and carbon carrying a partial positive charge ($\delta^+$).

Boiling points

The strength of intermolecular forces directly affects boiling points:

Aldehydes and ketones vs hydrocarbons: Aldehydes and ketones have higher boiling points than hydrocarbons of similar molecular mass. Hydrocarbons only experience weak dispersion forces, which require less energy to overcome than the dipole-dipole forces between carbonyl compound molecules.

Aldehydes and ketones vs alcohols: However, aldehydes and ketones have lower boiling points than alcohols of similar size. Alcohols form hydrogen bonds between molecules due to their hydroxyl group ($\text{OH}$). Hydrogen bonds are stronger than dipole-dipole forces and require more energy to break.

Solubility in water

The highly polar carbonyl group forms attractions with polar water molecules, making aldehydes and ketones more soluble than hydrocarbons but less soluble than alcohols.

Effect of chain length: Small aldehydes and ketones are soluble in water because the polar carbonyl group dominates the molecule's behaviour. However, as the carbon chain length increases, solubility in water decreases. This happens because the longer non-polar hydrocarbon portion of the molecule begins to outweigh the effect of the polar carbonyl group.

Remember!