Look up “isooctanoic acid” and the sources contradict each other. PubChem files it as a separate compound from 2-ethylhexanoic acid, with a different structure and a different InChIKey. One supplier catalog ties the name to CAS 149-57-5, another to CAS 25103-52-0, and a granted patent flatly calls the two names interchangeable.
That disagreement matters the moment a spec sheet, SDS, or synonym list puts both names in front of you and you have to order against one of them. The two names denote two distinct branched-C8 acids that happen to share a molecular formula, and the CAS number, not the trade name, is what tells them apart.
Treat the name as marketing and the CAS number plus the isomer assay as the spec, and the confusion resolves quickly.
How 2-EHA and Isooctanoic Acid Differ by CAS Number
Isooctanoic acid and 2-ethylhexanoic acid are not automatically the same substance. Under their authoritative CAS numbers they resolve to two different branched-C8 carboxylic acids.
2-Ethylhexanoic acid (2-EHA) is CAS 149-57-5, a single defined isomer with the branch on the carbon next to the acid group. “Isooctanoic acid” most precisely points to CAS 25103-52-0, a mixture of branched-C8 isomers whose representative structure is a different molecule, 6-methylheptanoic acid.
| Attribute | 2-Ethylhexanoic acid | “Isooctanoic acid” (commercial) |
|---|---|---|
| CAS number | 149-57-5 | 25103-52-0 |
| IUPAC name | 2-ethylhexanoic acid | 6-methylheptanoic acid (representative) |
| InChIKey | OBETXYAYXDNJHR-UHFFFAOYSA-N | OEOIWYCWCDBOPA-UHFFFAOYSA-N |
| Formula / MW | C8H16O2 / 144.21 | C8H16O2 / 144.21 |
| Composition | Single defined isomer | Branched-C8 isomer mixture |
| Common synonyms | 2-ethylcaproic acid | isocaprylic acid, Cekanoic C8 |
The InChIKey, the hashed structure identifier a database assigns to a molecule, is the tell. Two different InChIKeys mean PubChem has resolved the two names to two different structures, and neither record lists the other name as a synonym.
The branch position is the physical difference. 2-EHA carries an ethyl group on the alpha carbon, while the isooctanoic representative structure carries a methyl branch five carbons down the chain. The settled identity here belongs to 2-ethylhexanoic acid: one structure, one InChIKey, one CAS.
Two acids can share a formula and a molecular weight and still behave like different materials once they hit your reactor.
Why Isooctanoic Acid and 2-EHA Get Confused
The two names get confused because both acids share the formula C8H16O2 and a molecular weight of 144.21, and both trace back to the same oxo-synthesis family. A property card cannot separate them, so the trade name drifted into service as a loose label for either one.
The confusion has a production root. The classic route to 2-EHA runs butanal through 2-ethylhexenal and 2-ethylhexanal to the acid, and that oxo product has been sold under the name “isooctanoic acid” for decades.
A granted patent on isooctanoic-acid preparation states outright that 2-ethylhexanoic acid is “also known as isooctanoic acid,” and it cites no CAS number anywhere. That name-only habit is exactly how the ambiguity spreads.
The cross-wiring runs in every direction. One supplier catalog pairs the name “2-ethylhexanoic acid” with CAS 25103-52-0; another labels isooctanoic acid a nine-carbon pelargonic acid, which is simply wrong.
A name that resolves three different ways is not a specification. It is the start of a question.
How to Tell Which Acid a Spec Actually Means
The CAS number and the isomer assay on the COA identify which acid a document means; the trade name does not. Read the CAS first, 149-57-5 for the single isomer and 25103-52-0 for the mixture, then confirm it against the isomer distribution, because bulk properties will not save you.
The property card is a dead end here. 2-EHA boils at about 228 °C with a density near 0.90 g/mL, and the mixed grade’s reported boiling point sits at roughly the same value.
Those figures carry “(estimate)” flags in the reference databases. Both describe isomers of one C8 acid, so a boiling point or density number cannot tell you which material is in the drum.
Confirm the CAS and the assay together, not either one alone. Some supplier pages print the wrong CAS beside the right name, so a lone CAS field can mislead as easily as the trade name can.
For CAS 25103-52-0, the assay should state the isomer distribution, because the commercial grade is a UVCB, a substance of unknown or variable composition rather than a single defined compound.
A CAS number printed next to a name is a claim, not a guarantee. Check both fields before the material ships.
When to Specify 2-EHA vs Isooctanoic Acid
Applications built on a single defined isomer call for CAS 149-57-5. Metal octoates and 2-ethylhexanoate salts, ester synthesis, and alkyd paint driers all depend on a consistent molecular structure, and the mixed-isomer grade (25103-52-0) fits only where a variable composition is acceptable.
Cobalt, zirconium, and calcium octoate driers are the clearest case. These salts are built directly on the 2-ethylhexanoate ligand, so a batch-to-batch shift in isomer distribution changes the salt you are making.
Industrial octoate, ester, and drier specifications call for the defined single isomer. Bastone Petrochem supplies it as 2-ethylhexanoic acid, CAS 149-57-5 with full COA documentation for exactly these uses.
The mixed grade earns its place where the process tolerates a spread of isomers, and some heat-stabilizer and general metal-salt uses treat the branched-C8 acid as a class rather than one molecule. If your process was qualified on a defined isomer, substituting the mixture is a re-qualification, not a drop-in.
Match the isomer definition your application was qualified on, not the name on the invoice.
The Bottom Line
When a document says “isooctanoic acid,” do not assume it means 2-EHA. Read the CAS number, then confirm the isomer assay, before the material is ordered or accepted.
CAS 149-57-5 and CAS 25103-52-0 are two different branched-C8 acids that share a formula but not a structure. Only one of them, the single defined isomer 149-57-5, is the settled feedstock for octoates, esters, and driers.
The fix costs nothing: one glance at the CAS field plus one line on the COA closes a gap that a shared formula and a swapped trade name would otherwise leave open in your order.