Acidichloride was a gas that was toxic to most humanoids.

While under the effects of hyper-evolution in 2372, the alveoli in Tom Paris's lungs changed so that he could only breathe a mixture of 80% nitrogen and 20% acidichloride. (VOY: "Threshold")

There is a similar sounding real-world compound, acetyl chloride, but it is synthetic (not appearing in nature) and is a liquid at standard temperature and pressure.
Acid dichloride is not a rigorous chemical name, but taken most literally, corresponds to phosgene, a compound notorious for its use as a poison gas in World War I.

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