Last updated: 24 August 2026
Ketones sit at an important crossroads in Organic Chemistry. They can be formed by oxidising secondary alcohols, converted into alcohols again by reduction, and attacked by nucleophiles because of the polarity of their carbonyl group. Once you understand what the C=O bond does to electron distribution, the reactions of ketones become much easier to predict.
In this chapter, we will use propanone, CH₃COCH₃, and phenylethanone, C₆H₅COCH₃, as our main examples. The aim is not simply to memorise a collection of reagents. Instead, we will keep returning to one central question: what does the carbonyl group make possible?
Try these five questions before reading the chapter. Use the structures and reaction schemes carefully — Organic Chemistry often becomes much easier once you learn to read what is happening to the molecule.
- 5 diagnostic questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
1. What makes a compound a ketone?
A ketone contains a carbonyl group, C=O, in which the carbonyl carbon is bonded to two carbon-containing groups. We can represent its general structure as R–CO–R′, where R and R′ may be alkyl or aryl groups.
This distinguishes a ketone from an aldehyde. In an aldehyde, the carbonyl carbon has at least one hydrogen attached to it. In a ketone, both of the remaining bonds from the carbonyl carbon lead to carbon atoms.
A carbonyl compound is a ketone when the carbonyl carbon is bonded to two carbon groups.
Propanone and phenylethanone
Two important examples are propanone and phenylethanone. Propanone has two methyl groups attached to the carbonyl carbon, whereas phenylethanone has a methyl group on one side and a phenyl group on the other.
Aldehyde: the carbonyl carbon is bonded to at least one H atom, giving a group such as –CHO.
Ketone: the carbonyl carbon is bonded to two carbon atoms, giving the general arrangement R–CO–R′.
Why is the C=O bond so important?
Oxygen is more electronegative than carbon, so it attracts the bonding electrons in the C=O bond more strongly. The carbonyl bond is therefore polar:
The oxygen becomes partially negative, while the carbonyl carbon becomes partially positive. The carbon is therefore an electrophilic centre: it attracts electron-rich species such as nucleophiles.
When you see a ketone reacting with a nucleophile, do not begin by memorising an arrow. First identify the δ⁺ carbonyl carbon. That tells you where an electron-pair donor is most likely to attack.
2. Preparing ketones by oxidation of secondary alcohols
One of the most important ways of preparing a ketone is to oxidise a secondary alcohol. A secondary alcohol has the –OH-bearing carbon bonded to two other carbon atoms.
A common oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇/H⁺. The mixture is heated under reflux so that the alcohol is oxidised efficiently without losing volatile organic material.
Oxidation of propan-2-ol
When acidified potassium dichromate(VI) is used, the dichromate(VI) solution changes from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺.
Phenylethanone can be prepared in the same way
Oxidation is not limited to simple aliphatic alcohols. The secondary alcohol 1-phenylethanol is oxidised to phenylethanone.
Why are ketones not readily oxidised further?
An aldehyde has a hydrogen atom attached directly to its carbonyl carbon and can normally be oxidised further to a carboxylic acid. A ketone does not have this carbonyl hydrogen.
Further oxidation of a ketone generally requires breaking a carbon–carbon bond. This does not occur readily under the mild oxidation conditions normally used to distinguish aldehydes from ketones.
Secondary alcohol → oxidation → ketone. Under ordinary laboratory oxidation conditions, the ketone is not readily oxidised further.
3. Nucleophilic addition of hydrogen cyanide
The characteristic reaction of a ketone is nucleophilic addition. Hydrogen cyanide provides a useful example because the reaction shows clearly how the polarity of C=O controls the mechanism.
The reaction adds CN and OH across the carbonyl group, forming a compound containing both an –OH group and a nitrile group.
Why is KCN added if HCN is the reagent?
HCN is a weak acid and therefore produces only a relatively small concentration of CN⁻ ions by ionisation. However, the nucleophile required for the first step is CN⁻, not a neutral HCN molecule.
A small amount of KCN, or another soluble cyanide such as NaCN, supplies enough CN⁻ ions to initiate the reaction. The CN⁻ ion is later regenerated, so only a catalytic amount is required.
Step 1: CN⁻ attacks the carbonyl carbon
The CN⁻ ion is electron-rich and acts as a nucleophile. It donates an electron pair to the δ⁺ carbonyl carbon. At the same time, the π electrons of the C=O bond move onto oxygen.
This produces an intermediate containing an O⁻ group.
In nucleophilic addition to C=O, the nucleophile attacks the δ⁺ carbonyl carbon and the C=O π bond is broken.
Step 2: the O⁻ intermediate is protonated
The negatively charged oxygen then accepts H⁺ from an HCN molecule. The –OH group is formed and CN⁻ is regenerated.
This regeneration explains why CN⁻ can act catalytically. However, HCN itself is consumed overall: one H atom and one CN group from HCN ultimately become incorporated into the organic product.
HCN: the overall reagent. It ultimately supplies the H and CN incorporated into the cyanohydrin and is therefore consumed overall.
KCN or NaCN: provides the initial CN⁻ nucleophile. Because CN⁻ is regenerated during the mechanism, only a small amount is required.
Why is this an addition reaction?
Before the reaction, carbon and oxygen are joined by a double bond. After the reaction, the C=O π bond has been replaced by two new single bonds: one between the carbonyl carbon and CN, and one O–H bond.
No small molecule is eliminated from the carbon skeleton, so the process is classified as nucleophilic addition.
Phenylethanone produces a chiral product
The carbonyl carbon in phenylethanone is trigonal planar. A CN⁻ nucleophile can therefore attack from either side of the planar carbonyl group.
After addition, the former carbonyl carbon is bonded to four different groups: –OH, –CN, –CH₃ and –C₆H₅. It has become a chiral centre.
With no other chiral influence present, attack from the two faces is equally likely. Equal amounts of the two enantiomers are therefore formed, giving a racemic mixture.
Propanone behaves differently. Its product has two identical CH₃ groups attached to the carbon bearing –OH and –CN, so that carbon is not chiral.
4. Chemical tests for ketones
Chemical tests involving ketones are easiest to understand as a sequence. First ask whether a carbonyl group is present. Then, if necessary, decide whether that carbonyl compound is an aldehyde or a ketone. Finally, a more specific test such as the iodoform test may reveal additional structural information.
2,4-DNPH tests for aldehydes and ketones
2,4-Dinitrophenylhydrazine, commonly abbreviated 2,4-DNPH, reacts with aldehydes and ketones.
A positive result is the formation of an orange or yellow precipitate.
A positive 2,4-DNPH test tells you that a suitable carbonyl compound is present. It does not by itself tell you whether the compound is an aldehyde or a ketone.
Distinguishing a ketone from an aldehyde
Aldehydes are more readily oxidised than ketones. This difference allows oxidation-based tests to distinguish them.
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollens’ reagent | Silver mirror or grey deposit of silver | No visible change |
| Fehling’s solution | Brick-red Cu₂O precipitate for suitable aldehydes | No visible change |
| 2,4-DNPH | Orange or yellow precipitate | Orange or yellow precipitate |
For a straightforward comparison such as propanal against propanone, Tollens’ reagent is particularly useful. Propanal gives a positive result, whereas propanone does not.
Why do aldehydes react with Tollens’ reagent but ketones usually do not?
The structural difference is important. An aldehyde has an H atom bonded directly to the carbonyl carbon. It can therefore be oxidised to a carboxylic acid without breaking the carbon skeleton.
A ketone such as propanone has two carbon groups bonded to the carbonyl carbon and no carbonyl H. Oxidising the ketone further would generally require a more extensive structural change, such as breaking a C–C bond. This does not occur under the mild conditions of the Tollens’ test.
Aldehyde + Tollens’ reagent → oxidation occurs → Ag⁺ is reduced to Ag. Ketone + Tollens’ reagent → no oxidation under these mild conditions → no silver mirror.
The iodoform test identifies the CH₃CO– arrangement
The iodoform test provides more specific structural information. A methyl ketone contains the group:
Such ketones give a positive iodoform test when warmed with iodine in alkaline solution.
Both propanone and phenylethanone contain the CH₃CO– group, so both give a positive iodoform test.
But not every ketone gives the iodoform reaction. For example, pentan-3-one has no CH₃ group directly attached to the carbonyl carbon, so it gives a negative result.
5. Reduction of ketones to secondary alcohols
Reduction reverses the change produced when a secondary alcohol is oxidised. The carbonyl group of a ketone is converted into a C–OH group.
Reduction of propanone
Common reducing agents
| Reducing agent | Typical conditions | Key idea |
|---|---|---|
| NaBH₄ | Aqueous or alcoholic medium | Convenient reducing agent for aldehydes and ketones |
| LiAlH₄ | Dry ether, followed by hydrolysis | Powerful reducing agent that must be kept away from water initially |
| H₂ / Ni | Hydrogen with a nickel catalyst, usually with heating | Catalytic hydrogenation can also reduce the carbonyl group |
Why is the product always a secondary alcohol?
Before reduction, the carbonyl carbon of a ketone is already attached to two carbon groups. Reduction effectively adds hydrogen across the C=O group: hydrogen is added to the carbonyl carbon and the oxygen becomes part of an –OH group.
The carbon bearing –OH therefore ends up attached to two carbon groups and one hydrogen atom. That is the definition of a secondary alcohol.
6. Bringing the reactions together
Ketone chemistry becomes much easier when you see oxidation, reduction and nucleophilic addition as transformations of the same carbonyl carbon rather than as unrelated reactions.
Oxidation: secondary alcohol → ketone. The carbon bearing –OH becomes a carbonyl carbon.
Reduction: ketone → secondary alcohol. The C=O group is converted back into C–OH.
Nucleophilic addition: the C=O π bond is broken as a nucleophile forms a new bond to the carbonyl carbon.
A useful synthesis sequence is: propan-2-ol → propanone → cyanohydrin. The first step requires oxidation with acidified K₂Cr₂O₇ under reflux. The second requires HCN with catalytic KCN. Reading the functional-group change at each stage tells you which reagent to choose.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying that 2,4-DNPH specifically identifies a ketone | Both aldehydes and ketones normally give a positive 2,4-DNPH test. | Use 2,4-DNPH to detect a carbonyl compound, then use another test such as Tollens’ reagent if you need to distinguish an aldehyde from a ketone. |
| Saying that HCN directly attacks the carbonyl carbon | The attacking nucleophile in the usual mechanism is CN⁻. | State that KCN or NaCN provides CN⁻, which attacks the δ⁺ carbonyl carbon. |
| Thinking that HCN is regenerated completely and is therefore not consumed | CN⁻ is regenerated, but HCN supplies the H and CN incorporated into the product. | Distinguish the catalytic CN⁻ ion from HCN, which is an overall reactant. |
| Assuming every ketone gives a positive iodoform test | The test requires the CH₃CO– structural arrangement. | Look specifically for a methyl group directly bonded to the carbonyl carbon. |
| Drawing a primary alcohol as the reduction product of a ketone | The carbonyl carbon of a ketone is already attached to two carbon groups. | Replace C=O by CH–OH while keeping both carbon groups attached. The product is therefore a secondary alcohol. |
What you should be able to do
Recognise a ketone
Identify R–CO–R′ in displayed, condensed and skeletal structures and distinguish a ketone from an aldehyde.
Prepare ketones by oxidation
Predict the ketone formed when a secondary alcohol is oxidised and state suitable reagents and conditions.
Explain nucleophilic addition
Use the polarity of C=O to explain attack by CN⁻ and describe the two stages of HCN addition.
Interpret carbonyl tests
Use 2,4-DNPH and oxidation-based tests appropriately and state the correct observations.
Recognise methyl ketones
Identify the CH₃CO– group and predict whether a ketone gives a positive iodoform test.
Reduce ketones
Predict the secondary alcohol formed using reagents such as NaBH₄ or LiAlH₄ and use the transformation in synthesis questions.
These ten questions test whether you can apply the chapter to structures, reactions and unfamiliar examples.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Questions students often ask about ketones
Once the main reactions are connected to the polarity and structure of C=O, several details that initially seem like separate facts start to make sense.
What is the simplest way to distinguish a ketone from an aldehyde structurally?
Look at the carbonyl carbon. In a ketone, it is bonded to two carbon atoms. In an aldehyde, at least one of the groups attached to the carbonyl carbon is H. For example, CH₃COCH₃ is a ketone, whereas CH₃CHO is an aldehyde.
Why can a secondary alcohol be oxidised to a ketone but the ketone does not normally oxidise further?
Oxidation of a secondary alcohol creates the C=O group without breaking the carbon skeleton. Once the ketone has formed, the carbonyl carbon has carbon groups on both sides and no carbonyl hydrogen. Further oxidation under ordinary laboratory conditions would generally require more extensive changes such as breaking a C–C bond, so ketones are much less readily oxidised than aldehydes.
If CN⁻ is regenerated during HCN addition, does that mean HCN is not used up?
No. It is the CN⁻ ion that is regenerated and can therefore act catalytically. HCN is still an overall reactant. One H and one CN group ultimately become incorporated into the organic product, so HCN is consumed as the reaction proceeds.
Why does phenylethanone form a racemic mixture with HCN but propanone does not?
CN⁻ can attack either face of the planar C=O group. In phenylethanone, the product carbon becomes bonded to four different groups — OH, CN, CH₃ and C₆H₅ — so it is chiral and the two possible attacks produce enantiomers. In the propanone product, however, that carbon is bonded to two identical CH₃ groups and is therefore not a chiral centre.
Do all ketones give a positive iodoform test?
No. For a ketone, the key arrangement is CH₃CO–. Propanone and phenylethanone both contain this methyl-ketone group and therefore give a pale yellow precipitate of CHI₃. A ketone such as pentan-3-one lacks CH₃CO– and gives a negative result.
Ketone chemistry is largely the chemistry of the polar C=O bond. Secondary alcohols can be oxidised to create it, nucleophiles attack its δ⁺ carbon, and reducing agents convert it back into a secondary alcohol. Once that structure–reactivity connection is clear, reactions of propanone, phenylethanone and unfamiliar ketones can be reasoned through rather than memorised one reaction at a time.