Ketone: Structure, Properties & Naming

A ketone is an organic compound in which the carbonyl carbon is bonded to two other carbon atoms. Because both neighbors are carbon groups rather than hydrogen, the carbonyl is locked inside the chain rather than sitting at the end. That one structural difference from aldehydes shapes how ketones are named, how reactive they are, and why they fail the classic silver-mirror test.

The simplest ketone is propanone (acetone), and it is a good mental anchor for the whole family.

The Carbonyl Group in a Ketone

Like all carbonyl compounds, a ketone is built around a carbon double-bonded to oxygen, $\text{C=O}$. The difference is the company that carbon keeps: in a ketone both of the remaining bonds go to carbon groups, giving the general formula $\text{R-CO-R}'$.

The carbonyl carbon is $sp^2$ hybridized and planar, with bond angles near $120^\circ$. As in aldehydes, oxygen pulls electron density toward itself, so the carbon is partially positive:

$$\overset{\delta+}{\text{C}}=\overset{\delta-}{\text{O}}$$

This polarity makes the carbonyl carbon an electrophile, but two effects make ketones less reactive than aldehydes. First, the two alkyl groups push electron density onto the carbon and reduce its positive character. Second, those same groups crowd the carbon and block incoming nucleophiles. The result is a carbonyl that is real but harder to attack.

Naming Ketones with IUPAC Rules

The systematic name of a ketone replaces the final -e of the parent alkane with -one.

• Choose the longest chain that contains the carbonyl carbon.
• Number the chain so the carbonyl carbon receives the lowest possible locant.
• Place that number just before the -one suffix when the position is ambiguous.

Formula	IUPAC name	Common name
$\text{CH}_3\text{COCH}_3$	propanone	acetone
$\text{CH}_3\text{COCH}_2\text{CH}_3$	butanone	methyl ethyl ketone
$\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3$	pentan-2-one	methyl propyl ketone

In propanone the carbonyl can only sit at carbon $2$, so no locant is required. In longer chains the locant is essential: pentan-2-one and pentan-3-one are different compounds.

Physical Properties

Ketones share the polar carbonyl, so they behave much like aldehydes physically.

• Boiling point: the dipole raises boiling points above those of similar alkanes, but without an $\text{O-H}$ group ketones cannot hydrogen-bond to one another, so they boil below comparable alcohols.
• Solubility: small ketones such as acetone are fully miscible with water because the carbonyl oxygen accepts hydrogen bonds. Solubility decreases as the carbon chain grows.
• Volatility and use as solvents: acetone is volatile and dissolves a wide range of organic substances, which is why it is a common laboratory and industrial solvent.

Key Reactions of Ketones

Nucleophilic Addition

Ketones undergo the same nucleophilic addition as aldehydes, just more slowly. A nucleophile adds to the carbonyl carbon and oxygen takes the negative charge. Hydrogen cyanide, for instance, gives a cyanohydrin:

$$\text{R}_2\text{CO} + \text{HCN} \rightarrow \text{R}_2\text{C(OH)CN}$$

The reaction is slower than for aldehydes because of the steric crowding and the electron donation from the two alkyl groups.

Reduction

Reducing agents such as $\text{NaBH}_4$ or $\text{LiAlH}_4$ add hydrogen across the carbonyl. Because the carbon already carries two alkyl groups, the product is a secondary alcohol:

$$\text{R}_2\text{CO} + 2[\text{H}] \rightarrow \text{R}_2\text{CHOH}$$

This is a reliable way to remember the difference: aldehydes reduce to primary alcohols, ketones to secondary alcohols.

Resistance to Mild Oxidation

Unlike aldehydes, a ketone carbonyl carbon has no hydrogen to remove, so mild oxidizing agents cannot convert it to a carboxylic acid. Ketones are essentially inert to Tollens', Fehling's, and Benedict's reagents. Only harsh conditions break carbon-carbon bonds and cleave the molecule, which is rarely a useful test.

Telling Ketones from Aldehydes

The lab distinction follows directly from oxidation behavior.

Test	Reagent	Ketone	Aldehyde
Tollens'	ammoniacal $\text{Ag(NH}_3)_2^+$	no change	silver mirror
Fehling's	blue $\text{Cu}^{2+}$ complex	no change	brick-red ppt
Benedict's	blue $\text{Cu}^{2+}$ complex	no change	brick-red ppt

A ketone leaves all three reagents unchanged. To confirm a carbonyl is present at all, both aldehydes and ketones give a yellow-orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNP); once that confirms a carbonyl, a negative Tollens' result points specifically to a ketone.

Worked Example 1: Naming a Ketone

Name $\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3$.

The longest chain has five carbons, and the carbonyl carbon falls in the middle. Numbering from either end places the carbonyl at carbon $3$, so the locant is $3$. Replacing the final -e of pentane with -one gives pentan-3-one. The locant is required here because pentan-2-one is a different molecule.

Worked Example 2: Identifying an Unknown

An unknown liquid gives an orange precipitate with 2,4-DNP but no reaction with Tollens' reagent. Reduction with $\text{NaBH}_4$ produces a secondary alcohol. Identify the functional group.

The 2,4-DNP result confirms a carbonyl compound, so it is either an aldehyde or a ketone. The negative Tollens' test rules out an aldehyde. Reduction to a secondary alcohol confirms the carbonyl carbon was flanked by two carbon groups. The compound is a ketone.

Ketones Around You

Acetone is the most familiar ketone, found in nail-polish remover and used industrially as a solvent for paints and plastics. The body makes its own ketones, called ketone bodies, when it burns fat for fuel during fasting or low-carbohydrate diets; acetone is one of them and can give breath a faintly sweet smell. Many fragrances and natural flavors are also ketones, so this family reaches from the lab bench into metabolism, cosmetics, and the kitchen.