Propionic acid (systematically named propanoic acid) is a naturally-occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor. The anion CH3CH2COO- as well as the salts and esters of propionic acid are known as propionates (or propanoates). Propionic acid has physical properties intermediate between those of the smaller carboxylic acids, formic and acetic acid, and the larger fatty acids. It is miscible with water, but can be removed from water by adding salt. As with acetic and formic acids, it consists of hydrogen bonded pairs of molecules both as the liquid and vapor. Propionic acid displays the general properties of carboxylic acids: it can form amide, ester, anhydride, and chloride derivatives. It can undergo alpha-halogenation with bromine in the presence of PBr3 as catalyst (the HVZ reaction) to form CH3CHBrCOOH.