2-Ethylhexanoic Acid

2-Ethylhexanoic acid is a branched-chain C8 carboxylic acid used as a corrosion inhibitor in organic acid technology (OAT) and hybrid OAT antifreeze/coolant formulations, typically as its sodium or potassium salt. It forms a protective film on aluminum and ferrous metal surfaces in the cooling system, replacing the silicate and phosphate inhibitor packages used in older IAT (inorganic additive technology) coolants. It is a common component of VW/Audi G12-family and G13 (Si-OAT / TL 774-J) coolant formulations from multiple manufacturers.

At the pure-substance level, 2-ethylhexanoic acid carries a GHS Reproductive Toxicity Category 2 classification (H361d, suspected of damaging the unborn child) based on animal developmental-toxicity studies. This drives the WARNING signal word on finished coolant products where it is disclosed above the SDS reporting threshold. The chemical was added to California's Proposition 65 list for developmental toxicity in 2009 but was formally delisted in December 2013 following an OEHHA reconsideration of the listing basis; it does not currently trigger a Prop 65 warning on its own, though the ethylene glycol base of most coolants containing it independently carries an active Prop 65 listing.

Aquatic ecotoxicity data (fish, Daphnia, and algae endpoints all in the 100-180 mg/L range) sit above the acute Category 3 threshold, so this ingredient does not independently drive an aquatic-toxicity classification. It is a small, biodegradable organic molecule with no bioaccumulation, persistence, or PFAS concerns.