What Is Isononanoic Acid? Structure, Isomers & Nomenclature Explained
A plain-English look at the branched C9 acid behind ester lubricants, metal-salt driers and non-phthalate plasticisers - 3,5,5-trimethylhexanoic acid, CAS 26896-18-4.
💡 Quick answer: Isononanoic acid (INA) is a colourless, branched-chain C9 carboxylic acid. Its dominant molecule is 3,5,5-trimethylhexanoic acid - a six-carbon acid chain carrying three methyl branches. That branching is the whole story: it lowers the freezing point, boosts solubility in oils, and gives good hydrolytic stability, which is exactly why INA is prized for making synthetic esters and metal carboxylate salts.
🧪 Isononanoic Acid at a Glance
Before the chemistry, here is the compound's core identity. (For the full certified specification, purity grades and COA, see the isononanoic acid product page - this article focuses on what the molecule is.)
| Common name | Isononanoic acid (INA) · iso-C9 acid |
| IUPAC name (dominant isomer) | 3,5,5-Trimethylhexanoic acid |
| CAS number | 26896-18-4 (commercial mixed isomer) · 3302-10-1 (pure dominant isomer) |
| EC number | 248-191-6 |
| Molecular formula / weight | C₉H₁₈O₂ / 158.24 g/mol |
| Physical character | Colourless, water-white liquid; faint fatty-acid odour; density ≈ 0.90 g/cm³; boils ≈ 215–220 °C; weak acid (pKa ≈ 4.9) |
🔬 The Molecular Structure: A Branched C9 Acid
Every carboxylic acid shares the same reactive handle - the carboxyl group (–COOH). What sets acids apart is the carbon chain attached to it. In isononanoic acid, that chain is nine carbons in total but heavily branched, and it is the branching pattern that defines the molecule's behaviour.
The dominant isomer, 3,5,5-trimethylhexanoic acid, has this skeleton:
Read it from right to left: a carboxyl group, then a carbon bearing a methyl branch (the 3-position), a spacer carbon, and finally a tert-butyl end group - a quaternary carbon carrying three methyls (the 5,5,5 cluster). Compare that to a straight-chain C9 acid (nonanoic/pelargonic acid), where all nine carbons line up in a single row. Same carbon count, very different molecule.
✅ Why branching matters (in three effects):
🔹 Lower melting/freezing point. Branches stop molecules from packing neatly into a solid, so INA stays liquid at low temperatures - useful for cold-weather formulations and low-pour-point ester lubricants.
🔹 Better oil solubility. The bulky, lipophilic branched tail dissolves readily in hydrocarbon oils, which is why INA-based metal salts make more concentrated, cold-stable drier solutions than their straight-chain cousins.
🔹 Good hydrolytic stability. The branch near the acid group shields it sterically, slowing ester breakdown - a key reason INA esters survive demanding lubricant service.
These effects are explored in depth across the cluster - see how the same branched chain builds high-performance TMP and pentaerythritol ester lubricants, and how INA's chain length compares with C8 and C10 acids in the INA vs neodecanoic acid selection guide.
📋 Nomenclature: Why It's "3,5,5-Trimethylhexanoic Acid" - Not "Trimethylheptanoic"
Isononanoic acid is one of those compounds whose name is regularly written incorrectly in supplier documents. Getting it right matters for REACH registrations, safety data sheets and procurement contracts.
⚠️ The common error: Some sources call INA "trimethylheptanoic acid," implying a seven-carbon principal chain. That is a systematic naming mistake.
✅ The correct name: The IUPAC rule is to name the acid after the longest continuous carbon chain that contains the carboxyl group. In 3,5,5-trimethylhexanoic acid that chain is six carbons (hexanoic acid), with methyl substituents at positions 3, 5 and 5.
The molecule still contains nine carbons overall (hence "iso-nonanoic"), but three of them sit on branches, not on the main chain - so the parent is hexanoic, not heptanoic or nonanoic.
For regulated filings, use "3,5,5-trimethylhexanoic acid" with CAS 3302-10-1 when referring to the pure dominant isomer, or "isononanoic acid" with CAS 26896-18-4 for the commercial Oxo-process mixture. The naming conventions behind this are set by IUPAC ↗.
⚗️ One CAS Number, a Family of Isomers
Here is a subtlety worth understanding: commercial CAS 26896-18-4 does not describe a single pure compound. It designates isononanoic acid as an Oxo-derived mixture of branched C9 carboxylic acid isomers, in which 3,5,5-trimethylhexanoic acid is overwhelmingly dominant - typically ≥ 99% by gas chromatography in high-purity grades.
This mirrors how many branched industrial acids and alcohols are supplied: the manufacturing route produces a family of close isomers, and the CAS number names the family. When a single, defined isomer is required, chemists reference the pure-isomer CAS 3302-10-1 instead. Both point to essentially the same molecule for practical purposes - the distinction is one of regulatory precision, not different chemistry. The molecule is catalogued in detail on PubChem (CID 17759) ↗ and ECHA ↗.
Where does INA come from in the first place? It is produced by the Oxo process - hydroformylation of C8 olefins to a C9 aldehyde, followed by oxidation to the acid. That production route is covered in full in how isononanoic acid is made via the Oxo process.
🧭 Where INA Sits in the Branched-Acid Family
Isononanoic acid occupies the C9 slot in a small, commercially important family of branched carboxylic acids used to build esters and metal soaps:
| Acid | Chain | Character |
|---|---|---|
| 2-Ethylhexanoic acid (2-EHA) | C8, branched | Lowest cost, widest use - but carries Repr. 1B / SVHC flags |
| Isononanoic acid (INA) | C9, branched (Oxo) | Water-white colour, good oil solubility, no Repr. 1B |
| Neodecanoic acid (NDA) | C10, neo (Koch) | Best hydrolysis resistance (quaternary α-carbon) |
A big part of INA's modern appeal is regulatory: it delivers branched-C-chain performance close to 2-EHA without 2-EHA's reproductive-toxicant classification. That comparison is a decision in its own right - see isononanoic acid vs 2-ethylhexanoic acid (2-EHA).
🏭 What Is Isononanoic Acid Used For?
Because INA is a reactive acid with a helpful branched tail, it is almost always used as a building block rather than as an end product. Its main destinations:
Synthetic ester lubricants
TMP and pentaerythritol isononanoate esters for turbine, compressor, aviation and refrigeration oils.
Metal-salt driers & catalysts
Cobalt, zirconium, cerium and bismuth isononanoates for coatings and non-tin PU catalysis.
Plasticisers & surfactants
Non-phthalate isononanoate ester plasticisers, PVC co-stabilisers and branched-chain surfactants.
Each application is a full article in this series. If you formulate esters, you may also be interested in Sinolook's finished ester plasticisers such as TOPM (Tetraoctyl Pyromellitate) and TM810 (Tri(octyl,decyl) Trimellitate).
💡 Don't confuse the acid with the alcohol. Isononanoic acid (this compound, CAS 26896-18-4) is different from isononanol - the C9 branched alcohol. The plasticiser DINP is made from the alcohol, not this acid. It's the single most common procurement mix-up: isononanoic acid vs isononanol, explained.
🇪🇺 🇺🇸 Regulatory Snapshot
At a high level, INA carries only a mild irritant classification under EU CLP (Skin Irrit. 2 / Eye Irrit. 2) - no carcinogen, mutagen or reproductive-toxicant (CMR) classification. It is not on the REACH SVHC Candidate List, and in the US it is TSCA-listed and confirmed non-HAP. This clean profile is a core reason formulators choose INA over 2-EHA. Regional compliance details, including downstream metal-salt derivative status, are covered in the dedicated isononanoic acid regulatory guide, and authoritative data is available from US EPA CompTox ↗.
❓ Frequently Asked Questions
🔹 Is isononanoic acid the same as nonanoic (pelargonic) acid?
No. Both are C9 acids, but nonanoic acid is straight-chain and isononanoic acid is branched (3,5,5-trimethylhexanoic acid). The branching gives INA a lower freezing point and better oil solubility.
🔹 What is the molecular formula and weight of isononanoic acid?
C₉H₁₈O₂, molecular weight 158.24 g/mol. Its theoretical acid value (pure dominant isomer) is about 354.5 mg KOH/g.
🔹 Why does INA have two CAS numbers?
CAS 26896-18-4 designates the commercial Oxo-process mixture of branched C9 isomers; CAS 3302-10-1 is the pure dominant isomer, 3,5,5-trimethylhexanoic acid. High-purity grades are ≥ 99% that dominant isomer.
🔹 Is isononanoic acid hazardous?
It is a mild skin and eye irritant (EU CLP H315 / H319) and a combustible liquid, but carries no CMR classification. Standard nitrile gloves and eye protection are adequate for routine handling; always follow the current SDS.
📚 Authoritative References
🔗 Related Articles
How Isononanoic Acid Is Made: The Oxo Process
Hydroformylation → C9 aldehyde → oxidation, and how it shapes purity. →
The Repr. 1B-free substitution guide for metal salts and esters. →
✅ Source High-Purity Isononanoic Acid from Sinolook
≥ 99.5% C9 acid, water-white (Pt-Co ≤ 15), no Repr. 1B - with EU CLP SDS, REACH and TSCA documentation. Qualification samples and full specifications available on request.
