Acetyl chloride - CH3C(O)Cl
Acetyl chloride is a colorless liquid with a very pungent smell, which fumes in contact with humid air. It is volatile and its vapor reacts with water vapor in the air, giving acetic acid and hydrogen chloride. The hydrogen chloride reacts with more water, giving rise to formation of small droplets of hydrochloric acid and it is this which causes the strong fuming in humid air.
This compound is used as a strong acetylating agent in organic chemistry. It is capable of replacing OH-groups by CH3C(O)O- groups and HCl is formed as a byproduct. Per example, ethanol reacts with acetyl chloride, giving ethyl acetate and hydrogen chloride.