The propionate or propanoate ion is C 2 H 5 C O O − (the conjugate base of propionic acid).A propionic or propanoic compound is a small salt or ester of propionic acid. In these compounds, propionate is often written in shorthand, as CH 3 CH 2 CO 2 or simply EtCO 2. propionic or propanoic compound is a small salt or ester of propionic acid. In these compounds, propionate is often written in shorthand, as CH 3 CH 2 CO 2 or simply EtCO 2.