Acetone: the structure (a carbonyl group between two methyl groups, trigonal planar at 120°), the formula C₃H₆O and the name propan-2-one, the properties table (boiling point 56 °C, miscible with water, highly flammable), why it dissolves nail polish, how it is made and what it is used for, acetone in the body, the safety facts, and a printable sheet with practice

"Acetone structure" is searched about 9,700 times a month in the United States, by GCSE, A-level and Grade 10 to 12 chemistry students meeting their first ketone, and by nursing students who meet acetone as a ketone body. This page draws the molecule three ways and explains the shape, gives the properties table from the NIST Chemistry WebBook and PubChem, explains why acetone is such a good solvent and why it evaporates so fast, covers how it is made and used, what it does in the human body, and the safety facts, with a printable sheet and practice set. The companion page for the UK syllabus is ethanoic acid.

Download the acetone sheet with practice (PDF) (A4, the structure, the properties table and the uses on page 1, the ketone family and the practice set with answers on page 2, free).

The structure

Acetone is the simplest ketone: a carbonyl group (a carbon double-bonded to an oxygen, C=O) with a carbon on each side. In acetone both of those carbons are methyl groups, CH₃.

• Displayed formula: every atom and bond drawn. Three carbons in a row; the middle one has a double bond up to O; the outer two each carry three hydrogens.
• Structural formula: CH₃COCH₃, or (CH₃)₂CO, or CH₃-CO-CH₃.
• Skeletal formula: a V shape (two lines meeting at a point) with =O rising from the point. Each end of the V is a CH₃; the point is the carbonyl carbon.
• Molecular formula: C₃H₆O. Molar mass 58.08 g/mol (PubChem CID 180).
• IUPAC name: propan-2-one. Propan- for the three-carbon chain, -one for the ketone, 2 for the carbon that carries the C=O. (The "2" is often dropped, since in a three-carbon chain the ketone can only be in the middle; "propanone" is also correct.)

Shape. The carbonyl carbon has three regions of electron density (two single bonds and one double bond) and no lone pairs, so VSEPR gives trigonal planar, with bond angles close to 120°. The two methyl carbons are tetrahedral (109.5°). The C=O bond is short and strongly polar: oxygen pulls electron density toward itself, leaving the carbonyl carbon partly positive. That polarity drives both the solvent behaviour and the reactions.

The properties table

Two numbers explain most of what acetone does. The boiling point of 56 °C is low for a polar molecule of this size because acetone cannot hydrogen-bond with itself (no O–H or N–H), so only dipole-dipole forces hold the liquid together; it evaporates quickly and feels cold on skin. The miscibility with water comes from the carbonyl oxygen accepting hydrogen bonds from water molecules, while the two methyl groups let it mix with oils and non-polar solvents as well. A molecule that mixes with both water and oil is a useful solvent.

Why it is such a good solvent

Acetone is polar aprotic: polar (it dissolves salts and polar molecules reasonably well) but with no acidic hydrogen to donate. Its small size and low viscosity let it penetrate films and residues, and its fast evaporation means it leaves no trace. That is why it is:

• the active ingredient of most nail polish remover (it dissolves the nitrocellulose or acrylic film);
• a paint, lacquer and resin thinner and the cleaner for brushes and fibreglass tools;
• the laboratory rinse for glassware (it dissolves organic residue, mixes with the water left behind and dries in seconds);
• a degreaser in workshops and for electronics (where it is kept away from the plastics it would attack);
• the solvent that dissolves some plastics: polystyrene, ABS, acetate and some acrylics soften or dissolve in it, which is both a use (smoothing 3D prints, cementing plastic) and a hazard (never store it in a polystyrene cup).

How it is made

Almost all acetone is made as the co-product of phenol in the cumene process: benzene and propene form cumene, which is oxidised to cumene hydroperoxide and then split by acid into phenol and acetone. World production is several million tonnes a year. Smaller amounts come from the dehydrogenation of propan-2-ol (isopropyl alcohol), and in the laboratory a secondary alcohol oxidised with acidified potassium dichromate gives a ketone: propan-2-ol → propanone, the reaction A-level students write.

What it is used for

Acetone in the body

The liver makes three ketone bodies when it burns fat for fuel: acetoacetate, β-hydroxybutyrate and, from the breakdown of acetoacetate, acetone. Acetone leaves mostly through the breath, which is the fruity or nail-polish smell on someone who is fasting, on a ketogenic diet, or in the dangerous state of diabetic ketoacidosis. Breath acetone is measured in some medical and diet-monitoring devices. The normal blood level is tiny; the body clears it within hours.

Safety

• Highly flammable. Flash point about −20 °C, so at any room temperature the vapour above the liquid is ignitable; vapour is heavier than air and can travel to a flame. GHS: flammable liquid category 2. Keep away from hobs, candles, pilot lights and sparks; store in a closed metal or glass container.
• Irritant. Vapour irritates eyes, nose and throat; the liquid dries and cracks skin by removing its oils. Splashes in the eye need rinsing for several minutes.
• Drowsiness. Breathing a lot of vapour in a closed room causes headache and dizziness; use it with a window open.
• Low toxicity. Acetone is one of the least toxic industrial solvents; small amounts are made by the body and it is not classed as a carcinogen. Swallowing it is still a poison-control call.
• Plastics. It dissolves polystyrene, ABS and some acrylics, so never decant it into a plastic cup or use it near a plastic screen.

The ketone family

Ketones are the homologous series R–CO–R′ with the general formula CₙH₂ₙO (the same as aldehydes). Propanone is the first; butanone (methyl ethyl ketone, MEK, another industrial solvent) is next; pentan-2-one and pentan-3-one show why the position number matters from five carbons. Ketones resist oxidation (no hydrogen on the carbonyl carbon), which is the test that separates them from aldehydes: aldehydes turn Tollens' reagent to a silver mirror and Fehling's solution brick-red; ketones do neither. Ketones are reduced by sodium borohydride or lithium aluminium hydride to secondary alcohols (propanone → propan-2-ol), and they undergo nucleophilic addition at the carbonyl carbon, the A-level mechanism, with HCN giving a hydroxynitrile.

Where it sits in each curriculum

Practice (answers in the PDF)

1. Draw the displayed formula of acetone and state the bond angle at the carbonyl carbon.
2. Give the IUPAC name of acetone and explain each part of it.
3. Acetone (58 g/mol) boils at 56 °C; propan-1-ol (60 g/mol) boils at 97 °C. Explain the difference.
4. Why is acetone miscible with water but hexane is not?
5. Name the secondary alcohol that oxidises to acetone and the reagent used.
6. A student stores acetone in a polystyrene cup. What happens, and what else is wrong with the storage?

Download the sheet and the practice set (PDF)

Where to get help

A 1-on-1 online chemistry class with a tutor who teaches GCSE and A-level chemistry, AP Chemistry, CBSE Class 11 and 12 and the Australian Years 11 and 12 every week: organic chemistry taught from the shape and the polarity of the molecule outward, so properties and reactions follow from the structure instead of being memorised. The chemistry page lists the courses; the free 30-minute first class is a real lesson on this week's topic. Related pages: ethanoic acid and the 100 common compounds chart.

Questions parents ask

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