Methyl octanoate, also called methyl caprylate (CAS 111-11-5), is the methyl ester of octanoic acid, the eight-carbon fatty acid also known as caprylic acid. At room temperature it is a colourless to light yellow oily liquid with a winy, fruity and sweet orange aroma. The molecule combines a short C8 chain with an ester group, so it works both as an aroma chemical and as a reactive intermediate for downstream synthesis.
Physical and Chemical Properties
Methyl octanoate is a combustible organic liquid that is practically insoluble in water but readily soluble in alcohol, ether and oils. The table below lists the properties most often requested by formulators and buyers.
| Property | Typical value |
|---|---|
| Chemical name | Methyl octanoate (methyl caprylate) |
| CAS number | 111-11-5 |
| Molecular formula | C9H18O2 |
| Molecular weight | 158.24 |
| Appearance | Colourless to light yellow oily liquid |
| Odour | Winy, fruity, sweet orange |
| Boiling point | 194-195 °C |
| Melting point | -37.3 °C |
| Relative density (20 °C) | 0.875-0.890 |
| Refractive index (20 °C) | 1.413-1.422 |
| Solubility in water | Practically insoluble |
| Solubility in organics | Soluble in ethanol, diethyl ether and fixed oils |
Main Application Areas
Food flavourings. The ester is mainly used to build pineapple, berry and other fruit flavour accords. Under the Chinese national food additive standard GB 2760 it is permitted for use as a food flavouring, and it is dosed at the low levels typical for fruity top notes.
Cosmetic and pharmaceutical intermediates. It serves as a raw material and synthetic intermediate in cosmetic and pharmaceutical manufacture, where its mild odour and solubility in alcohol and oils simplify blending.
Plant growth regulator. Methyl octanoate is also applied as a plant growth regulator in agricultural programmes.
Fuel and biodiesel research. Because of its straight-chain C8 ester structure, it is studied as a potential biodiesel component and as a diesel-blend reference ester for ignition and density studies.
Flavour and fragrance compounding. Perfumers and flavourists use it as a fruity modifier and as a diluent in concentrated flavour blends.
Manufacturing Routes and Quality Control
Two commercial routes dominate. The first is direct esterification of octanoic acid with methanol in the presence of an acid catalyst, followed by neutralisation, washing and vacuum distillation. The second is alcoholysis (transesterification) of coconut-oil fatty acids in the presence of acidic gas, which delivers the ester together with other methyl esters of the C8-C18 series that are then separated by distillation.
Because methyl octanoate is judged largely on odour and purity, every batch should be released against a gas chromatography assay, an acid value limit for residual free fatty acid, a water content limit, a colour reading and an odour panel result. Keeping residual methanol and free octanoic acid low is what protects the fruity note from turning sour or solvent-like in storage.
Handling, Storage and Packaging
Wear gloves and eye protection, and avoid direct contact with skin and eyes; the ester is a mild irritant and a defatting agent on repeated contact.
Work with local exhaust ventilation, because the vapour is combustible and the odour becomes noticeable well below any hazardous concentration.
Store in sealed containers in a cool, dry, well-ventilated place away from oxidising agents, strong acids and ignition sources.
Prefer stainless steel, lined drums or HDPE containers; keep drums closed when not in use to limit moisture pickup and odour loss.
For transport, classify on the basis of the flash point determined for the actual batch rather than on general data for the ester family.
Frequently Asked Questions
Q: What is methyl octanoate used for?
Its largest use is as a food flavouring for pineapple, berry and fruit accords; it is also used as a cosmetic and pharmaceutical intermediate, as a plant growth regulator and as an ester of interest in biodiesel research.
Q: Is methyl octanoate the same as methyl caprylate?
Yes. Methyl octanoate and methyl caprylate are two names for the same C8 methyl ester, because octanoic acid is also called caprylic acid.
Q: What is the CAS number of methyl octanoate?
The CAS registry number is 111-11-5, with the molecular formula C9H18O2 and a molecular weight of 158.24.
Q: Is methyl octanoate soluble in water?
No. It is practically insoluble in water, but it dissolves readily in ethanol, diethyl ether, other organic solvents and fixed oils.
Q: How is methyl octanoate produced?
It is made either by acid-catalysed esterification of octanoic acid with methanol, or by alcoholysis of coconut-oil fatty acids followed by distillation of the methyl ester fraction.
Q: How should methyl octanoate be stored?
Keep it in sealed containers in a cool, dry, ventilated area away from oxidisers, acids and ignition sources, and handle it with gloves and eye protection.
Q: What purity is available?
Commercial material is normally supplied at 98 percent or higher by gas chromatography, with residual free fatty acid and water controlled to protect the fruity odour profile.