How to Understand Aldehydes and Ketones in A-Level Chemistry

If carbonyls - aldehydes and ketones - feel confusing at first, it may be because you’re expected to link bond polarity, mechanisms and functional group conversions in one sitting.

The good news is that most of the topic comes back to one key idea: the reactivity of carbonyl compounds is linked to the idea that the C=O bond is polar. Once you understand what that means, the reactions become much easier to predict.

Some key terms

Before looking at carbonyl reactions, make sure you fully understand these core concepts:

• A nucleophile is an electron pair donor. It’s electron-rich.

• An electrophile is an electron pair acceptor. It’s electron-poor.

• A curly arrow shows the movement of a pair of electrons from an electron-rich region to an electron-poor one.

A curly arrow should always begin where the electrons are coming from, such as a lone pair, a negative charge associated with an electron pair, or a bond.

How do I know when to use dipoles?

You would use dipoles to show uneven electron pair sharing in a covalent bond. In other words, when you have a polar bond. A bond is polar when the two bonded atoms differ in electronegativity and therefore attract the shared pair of electrons by different unequally.

The atom that attracts the electrons more strongly (more electronegative) becomes δ−, while the other becomes δ+.

In a carbonyl bond, oxygen is more electronegative than carbon, so the C=O bond is polar:

Cδ+ = Oδ−

This matters because the δ+ carbon is electron-poor, so it can be attacked by a nucleophile. That is the starting point for nucleophilic addition reactions of aldehydes and ketones.

How to understand the nucleophilic addition mechanism?

It is important to remember the golden rule for ALL curly arrow mechanisms: curly arrows move a pair of electrons from an electron-rich place to an electron-poor place.

So following this logic, in a nucleophilic addition mechanism, the general pattern is:

1. A nucleophile attacks the δ+ carbon in the C=O group. Draw a curly arrow from nucleophile to Cδ+ e.g :CN- to Cδ+

2. In the same step, the electrons in the C=O bond move onto Oδ-. Draw a curly arrow from double bond to Oδ-

3. The pi-bond breaks. An intermediate containing C—O⁻ forms. Draw intermediate with new bond between C and nucleophile.

4. The O⁻ is then protonated to give an OH group. Draw a curly arrow from O⁻ to H+.

When drawing the mechanism, remember that the curly arrow travels from the electron-rich nucleophile towards the electron-poor carbonyl carbon. You also need a second curly arrow from the C=O bond towards oxygen.

How does NaBH₄ reduce aldehydes and ketones?

Sodium borohydride, NaBH₄, is a reducing agent for aldehydes and ketones. The nucleophile involved is the hydride ion, H⁻. Note this is not a proton H+. It attacks the δ+ carbon of the carbonyl group by nucleophilic addition.

The products are:

Aldehyde → primary alcohol

For example: CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH

Ketone → secondary alcohol

For example: CH₃COCH₂CH₃ + 2[H] → CH₃CH(OH)CH₂CH₃

In balanced equations, use [H] to represent the reducing agent rather than writing NaBH₄ directly. Treat [H] as a regular H atom when balancing.

How are aldehydes connected to alcohols and carboxylic acids?

Carbonyl compounds sit naturally between alcohols and carboxylic acids in organic reaction pathways. Carbonyl compounds are also an important part of OCR A-Level Chemistry Paper 2, where they can appear alongside alcohols, organic synthesis and analysis questions.

A primary alcohol can be oxidised to an aldehyde, and the aldehyde can then be oxidised further to a carboxylic acid.

For example:

1. CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O

2. CH₃CH₂CHO + [O] → CH₃CH₂COOH

Ketones behave differently. They are formed by oxidation of secondary alcohols, but they are not oxidised by acidified potassium dichromate under these conditions.

That makes aldehydes and ketones useful linking functional groups when revising organic synthesis.

Reactions of aldehydes and ketones summarised

It helps to see carbonyl chemistry as a small reaction map rather than a collection of separate facts. The main reactions in this topic are oxidation, reduction and nucleophilic addition.

Reaction	Reagents and conditions	Balanced equation / example	Final organic product
Oxidation of an aldehyde	Acidified potassium dichromate, Cr₂O₇²⁻/H⁺	CH₃CH₂CHO + [O] → CH₃CH₂COOH	Carboxylic acid
Oxidation of a ketone	Acidified potassium dichromate, Cr₂O₇²⁻/H⁺	No reaction under these conditions	No oxidation product
Reduction of an aldehyde	NaBH₄	CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH	Primary alcohol
Reduction of a ketone	NaBH₄	CH₃COCH₂CH₃ + 2[H] → CH₃CH(OH)CH₂CH₃	Secondary alcohol
Nucleophilic addition with HCN	HCN, or NaCN + H₂SO₄	RCHO + HCN → RCH(OH)CN	Hydroxynitrile
Nucleophilic addition with HCN to a ketone	HCN, or NaCN + H₂SO₄	R₂CO + HCN → R₂C(OH)CN	Hydroxynitrile

What do OCR examiners expect in a carbonyl mechanism?

For a nucleophilic addition mechanism, check that you have shown:

1. δ+ on carbon and δ− on oxygen

2. The nucleophile with the correct negative charge

3. A curly arrow from the lone pair or negative charge on the nucleophile to the carbonyl carbon

4. A curly arrow from the C=O bond to oxygen

5. The correct intermediate showing an O⁻

6. Protonation to give the final product

A common student mistake when doing the reduction mechanism with NaBH₄ is using H⁺ instead of H⁻. Remember, the attacking species is hydride. It is a nucleophile and must be electron rich, not poor.

The simplest way to understand aldehydes and ketones

Don’t try to memorise every reaction separately. Start with the carbonyl bond:

Cδ+ = Oδ−

Then ask:

Where are the electrons coming from?
From the nucleophile.

Where are they going?
Towards the electrophilic δ+ carbon.

If you can follow that electron movement, nucleophilic addition and NaBH₄ reduction become much easier to understand. You can use the nucleophile —> electrophile curly arrow rule for any mechanism.

Want to get more confident with organic mechanisms?

If curly arrows, nucleophiles and electrophiles still feel like something you have to memorise, my Organic Mechanisms Guide will help you see the patterns behind the reactions.

It breaks down the main A-Level organic mechanisms step by step, so you can practise where the arrows start, where they go, and what examiners are looking for. It’s designed for OCR and AQA exam boards.

Download the Organic Mechanisms Guide and use it alongside your carbonyl revision to make mechanisms feel much less intimidating.