Acetic anhydride

\( \newcommand\) • • • • • • • • This page explains what acid anhydrides are and looks at their simple physical properties such as boiling points. It introduces their chemical reactivity in a general way. A If you took two ethanoic acid molecules and removed a molecule of water between them you would get the acid anhydride, ethanoic anhydride (old name: acetic anhydride). You can actually make ethanoic anhydride by dehydrating ethanoic acid, but it is normally made in a more efficient, round-about way. Appearance Ethanoic anhydride is a colorless liquid, smelling strongly of vinegar (ethanoic acid). The smell is because ethanoic anhydride reacts with water vapor in the air (and moisture in your nose) to produce ethanoic acid again. This reaction with water is given in detail on another page. (Find it from the acid anhydrides menu - link at the bottom of this page.) Boiling point Ethanoic anhydride boils at 140°C. This is because it is a fairly big polar molecule and so has both van der Waals dispersion forces and dipole-dipole attractions. It does not, however, form hydrogen bonds. That means that its boiling point isn't as high as a carboxylic acid of similar size. For example, pentanoic acid (the most similarly sized acid) boils at 186°C. Comparing acid anhydrides with acyl chlorides (acid chlorides) You have almost certainly come across acid anhydrides for the first time just after looking at acyl chlorides, or you may be studying them at the same time as acyl chlorides...