Carbonyl Compounds: Aldehydes and Ketones

The carbonyl group — a carbon double-bonded to an oxygen — is one of the most important functional groups in organic chemistry. It appears in aldehydes, ketones, carboxylic acids, esters, acyl chlorides, amides and many other families. This lesson looks at the two simplest carbonyl compounds — aldehydes and ketones — their structure, bonding, naming, and physical properties.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.1.2 (a)–(b): the structure of carbonyl compounds and their nomenclature.

1. The Carbonyl Group

The carbonyl group is the C=O double bond. In aldehydes and ketones the carbonyl carbon is bonded only to H and/or carbon, never directly to a heteroatom such as O, N or Cl (those would be carboxylic acids, amides or acyl chlorides).

Key Definition — Carbonyl group: The functional group >C=O, where a carbon atom is double-bonded to an oxygen atom.

The C=O double bond consists of one sigma (σ) bond and one pi (π) bond. The carbonyl carbon is sp² hybridised and the three atoms attached to it lie in a trigonal planar arrangement with bond angles close to 120°.

Because oxygen is much more electronegative than carbon (3.44 vs 2.55), the C=O bond is strongly polarised:

$\delta^+C = O\delta^-$

This polarisation is the single most important thing to remember about carbonyls. It makes the carbonyl carbon an electrophile — a site of attack by nucleophiles — and underlies nearly every reaction you will meet in the next few lessons.

graph LR
  A[Carbonyl C=O] --> B[Sigma + pi bond]
  A --> C[sp2 C, trigonal planar 120 deg]
  A --> D[Polarised: C delta + / O delta -]
  D --> E[Carbon is electrophilic]
  E --> F[Attacked by nucleophiles]

2. Aldehydes vs Ketones: Spot the Difference

The distinction between an aldehyde and a ketone rests on what is attached to the carbonyl carbon.

Feature	Aldehyde	Ketone
Carbonyl C is bonded to…	At least one H	Two carbon atoms (no H on C=O carbon)
General formula	R–CHO	R–CO–R'
Position of carbonyl	End of chain	Middle of chain
Example	Ethanal CH₃CHO	Propanone CH₃COCH₃

The presence of that C–H bond on the carbonyl carbon of aldehydes makes aldehydes more reactive and more easily oxidised than ketones. This difference underpins every chemical test for aldehydes vs ketones that you will meet in Lesson 2.

graph TD
  A[Carbonyl R-CO-X] --> B{What is X?}
  B -->|H| C[Aldehyde R-CHO<br/>Carbonyl at end of chain<br/>Easily oxidised]
  B -->|Carbon R'| D[Ketone R-CO-R'<br/>Carbonyl in middle of chain<br/>Resists oxidation]

Common examples

Compound	Structure	Class
Methanal (formaldehyde)	HCHO	Aldehyde
Ethanal (acetaldehyde)	CH₃CHO	Aldehyde
Propanal	CH₃CH₂CHO	Aldehyde
Benzaldehyde	C₆H₅CHO	Aromatic aldehyde
Propanone (acetone)	CH₃COCH₃	Ketone
Butanone	CH₃COCH₂CH₃	Ketone
Pentan-2-one	CH₃COCH₂CH₂CH₃	Ketone
Pentan-3-one	CH₃CH₂COCH₂CH₃	Ketone

3. Nomenclature: Naming Aldehydes and Ketones

OCR expects you to name and draw aldehydes and ketones up to about 6 carbons, including branched chains.

3.1 Aldehydes

Aldehydes take the suffix -al. The carbonyl is always at carbon 1 — you do not need a locant for it because no other position is possible.

Rules:

Find the longest carbon chain containing the –CHO group.
Number from the CHO carbon as C1.
Replace the final e of the parent alkane with al.
Add substituents as prefixes with locants.

Examples:

CH₃CHO → ethanal
CH₃CH₂CHO → propanal
(CH₃)₂CHCHO → 2-methylpropanal
CH₃CH(CH₃)CH₂CHO → 3-methylbutanal

3.2 Ketones

Ketones take the suffix -one. Because the C=O can be anywhere in the middle of the chain, you usually need a locant to say where.

Rules:

Find the longest carbon chain containing the C=O.
Number to give the C=O carbon the lowest possible locant.
Replace the final e of the parent alkane with one.
Add the locant before the one.

Examples:

CH₃COCH₃ → propanone (no locant needed — position 2 is the only possibility)
CH₃COCH₂CH₃ → butanone (position 2 is the only possibility for a 4-carbon ketone)
CH₃CH₂COCH₂CH₃ → pentan-3-one
CH₃COCH₂CH₂CH₃ → pentan-2-one
CH₃COCH(CH₃)CH₃ → 3-methylbutan-2-one

Exam Tip: For 4-carbon ketones (butanone) and 3-carbon ketones (propanone) you do not write a locant. For 5 carbons and longer, you must — OCR will penalise you for writing "pentanone" because it is ambiguous.

4. Physical Properties

4.1 Boiling Points

Carbonyl compounds are polar because of the δ+C=Oδ- dipole. They cannot form hydrogen bonds with themselves (they lack an O–H or N–H), so their intermolecular forces are:

London forces (all molecules)
Permanent dipole-dipole interactions (due to C=O polarity)

Compare some similar-sized molecules:

Compound	Mᵣ	Boiling point (°C)	Strongest IMF
Butane C₄H₁₀	58	–1	London
Propanal C₂H₅CHO	58	48	Permanent dipole
Propanone CH₃COCH₃	58	56	Permanent dipole
Propan-1-ol C₃H₇OH	60	97	Hydrogen bonding

Propanal and propanone boil ~50 °C higher than butane because of their dipole-dipole forces. They boil ~50 °C lower than propan-1-ol because they cannot H-bond with themselves (propan-1-ol can).

4.2 Solubility in Water

Small carbonyls (C1–C4) are miscible with water because the lone pairs on the carbonyl oxygen can accept hydrogen bonds from water's O–H. As the carbon chain grows, the hydrophobic tail dominates and solubility drops. By C7 or so, most carbonyls are insoluble in water but soluble in non-polar solvents.

Key Insight: Carbonyl compounds can accept hydrogen bonds from water but cannot donate them to each other. This explains both their relatively low boiling points and their small-molecule water solubility.

5. Why the Carbonyl is So Reactive

The polarisation of C=O means two reactive sites coexist in a single functional group:

The carbonyl carbon is electrophilic and is attacked by nucleophiles (CN⁻, H⁻ from NaBH₄, water, amines, etc.).
The carbonyl oxygen has lone pairs and is weakly basic — it can accept a proton from acids, making the carbon even more electrophilic.

This combination makes carbonyls the central "hub" of organic synthesis. Every major transformation you will study in the next few lessons — reductions, additions, condensations, esterifications, nucleophilic substitutions — goes through (or begins at) a C=O group.

graph TD
  A[Carbonyl C=O] --> B[Reductions: NaBH4 -> alcohol]
  A --> C[Additions: HCN -> hydroxynitrile]
  A --> D[Oxidation of aldehydes -> carboxylic acid]
  A --> E[Tests: Tollens, Fehlings, 2,4-DNP]
  A --> F[Combines with alcohol + acid -> ester]

Exam Tips

Always draw the carbonyl C=O with a clear double-bond line — examiners routinely reject skeletal drawings where the C=O looks like a single bond.
When naming ketones of 5+ carbons, include the locant (pentan-2-one, not "pentanone").
Remember the difference: aldehydes have at least one H on the C=O carbon; ketones have no H on the C=O carbon.
When asked to explain boiling points, always mention both London forces and permanent dipole-dipole for carbonyls, and explain why hydrogen bonds are absent.
The carbonyl carbon is δ⁺ and is attacked by nucleophiles, not electrophiles. This is the opposite of alkenes — do not confuse the two.

Common Exam Mistakes

Writing "ethanone" for CH₃CHO. Ethanone does not exist — the two-carbon carbonyl with a C=O on C1 is ethanal (an aldehyde).
Saying carbonyls can hydrogen-bond with themselves — they cannot, because they lack O–H or N–H.
Forgetting to number ketones from the end that gives the C=O the lowest locant.
Drawing the C=O as polar but then treating the carbon as nucleophilic. The carbon is δ⁺ and so is the electrophile.
Confusing "carbonyl" with "carboxyl". Carbonyl = C=O; carboxyl = –COOH (which contains a C=O but also an OH).

Reference: OCR A-Level Chemistry A (H432), Module 6 — Organic Chemistry and Analysis, section 6.1.2 (a)–(b): the carbonyl group in aldehydes and ketones; systematic nomenclature and general physical properties of carbonyl compounds.

Lesson 1: Carbonyl Compounds — Aldehydes and Ketones