n-Propyl acetate: CAS, isomers, selection

Technical article · Eapearl Chemical · 15 May 2026

n-Propyl acetate (CAS 109-60-4) is the ester specified when ethyl acetate flashes off too quickly for the film being formed or is restricted in a given market, and when n-butyl acetate would hold the film open too long. It is bought for its position on the evaporation ladder rather than for any unusual solvent power. The single largest commercial risk attached to it is the word “propyl” standing alone on a document, because that word describes two different products with two different CAS numbers.

“Propyl acetate” names two products, not one

Two esters answer to the name propyl acetate. n-Propyl acetate is CAS 109-60-4. Isopropyl acetate is CAS 108-21-4. Both are C₅H₁₀O₂ with a molar mass of 102.13, so neither the molecular formula nor the mass printed on a specification sheet can separate them. A purchase order, packing list or customs declaration that says only “propyl acetate” has not specified a product; the ambiguity is resolved downstream by whoever picks the drum.

This is not a theoretical failure mode. It produces deliveries that pass goods-in on name and appearance, dissolve the same resins, carry an almost identical safety data sheet, and then behave differently in the tank. The remedy is procedural rather than technical: treat the CAS number as the primary key on the enquiry, the order, the label, the packing list and the certificate of analysis, and treat the trade name as secondary text.

Same mass, different molecule

The two esters are structural isomers. In n-propyl acetate the acetate group sits on a straight three-carbon chain; in isopropyl acetate it sits on the middle carbon of a branched chain. Branching changes how closely the molecules pack and therefore changes volatility, while leaving elemental composition untouched. A combustion analysis, a molar mass figure or a purity claim expressed only as “ester content” will not distinguish them. Gas chromatography against an authentic standard will, and so will refractive index; both are routine and both belong on the incoming-goods check when a site stocks more than one propyl ester.

Two isomers, two behaviours

Branching lowers the boiling point, so isopropyl acetate boils below n-propyl acetate and leaves a wet film sooner. Substituting one for the other therefore shifts the whole flash-off profile of a coating forwards: solvent balance in a nitrocellulose or acrylic lacquer changes, the window in which the film can still flow out narrows, and the tendency to blush or to trap solvent under a skinned surface changes with it. Because both isomers carry the same harmonised classification, the safety data sheets look close enough to pass a paper comparison, which is exactly why the substitution slips through.

Where n-propyl acetate sits on the ladder

Ranked by how fast they leave a film, the common acetate solvents run from methyl acetate as the fastest, through ethyl acetate, then isopropyl acetate, then n-propyl acetate, to n-butyl acetate as the slowest of the group. n-Propyl acetate occupies the step between ethyl and butyl, and that is the whole commercial case for it. It buys a longer open time than ethyl acetate without committing the formulation to the slow tail of butyl acetate, and it does so with a solvency profile that is not dramatically different from either neighbour. A wider view of the family is set out in our note on common ester solvents and their uses, and the full range sits under esters.

Buying a position, not a solvency

These acetates dissolve broadly similar resin classes, so arguments about which one to buy are rarely won on solvent power. They are won on drying window, flash-point band, odour profile, market restrictions and price against the rest of the blend. When a specification moves away from ethyl acetate, the move is usually driven either by a restriction on that material in the end market or by a film defect caused by drying too fast; in both cases the replacement has to be slower but not much weaker, which is the description of n-propyl acetate.

Checks before making the substitution

1. Re-balance the fast and slow fractions of the blend rather than swapping one ester for another one-for-one.
2. Confirm the flammability category has not changed in a way that affects the storage licence or the transport classification.
3. Re-test application at the extremes of workshop temperature and humidity, not at mid-range only.
4. Confirm the receiving specification names the isomer by CAS, so the second delivery matches the qualified one.

Classification: what is law and what is only evidence

All five esters discussed here carry harmonised entries in Annex VI to the CLP Regulation. A harmonised entry is law. It is the minimum classification and labelling that must be applied for the endpoints it covers, and a supplier or downstream user may not soften it on the strength of their own data. n-Propyl acetate, isopropyl acetate, ethyl acetate and methyl acetate all take Danger with H225, H336 and H319 plus the supplemental statement EUH066. n-Butyl acetate takes Warning with H226 and H336 plus EUH066, a different flammability category and no harmonised eye irritation entry, which is a real difference in labelling and in storage classification, not a formatting difference.

Self-classification data is a different kind of object. A consensus among notifiers in a classification inventory is evidence about what registrants concluded, useful for endpoints that Annex VI does not cover, and frequently inconsistent between notifiers. It is not law. Where a self-classification disagrees with the harmonised entry on an endpoint the harmonised entry covers, the harmonised entry governs. Where an endpoint is simply absent from Annex VI, the supplier must classify on their own data, and that classification, not a database tally, is what appears on the safety data sheet. Our quality and compliance page sets out how these obligations are documented for each consignment.

Handling, storage and transport consequences

H225 means highly flammable liquid and vapour, and the practical consequences follow from the vapour rather than the liquid: vapour is denser than air, collects in pits, sumps and the bottom of vessels, and travels to remote ignition sources. Bonding and earthing during transfer, ventilation at low level, and control of static during drum filling are the controls that matter. H336 drives ventilation and exposure control in enclosed application areas. EUH066 is a supplemental statement about repeated exposure causing skin dryness or cracking, which is a glove-selection and hand-care matter rather than a classification for acute toxicity.

In storage, keep the material cool, ventilated and away from oxidising agents and from strong acids and bases, which promote hydrolysis of the ester back to acetic acid and propan-1-ol. Free water and heat push the same reaction, which is why acidity appears as a routine specification parameter and why drums that have been opened repeatedly in humid conditions should be re-tested before use in a sensitive coating. Packaging, container options and consignment arrangements are covered under logistics.

What to put on the order and what to check on arrival

The order should carry the CAS number first, then the name, then the grade and the pack. The certificate of analysis should carry assay by gas chromatography, water by Karl Fischer, acidity expressed as acetic acid, colour, non-volatile matter and density or refractive index at a stated reference temperature. Every numeric limit for the grade actually supplied belongs to that certificate and to the current safety data sheet; a knowledge-base page can give direction and ranking, and should not be quoted as a specification. If your site holds more than one propyl ester, add an isomer identity check to goods-in and record the CAS number on the internal batch label, not just the trade name.

For current grades, pack sizes and documentation for any of the esters named here, contact us with the CAS number and the intended application.