(o-). A prefix meaning “straight ahead.” Compare with meta- (m-) meaning “beyond,” para- (p-) meaning “opposite.” These prefixes are used in organic chemistry in naming disubstitution products derived from benzene in which the substituent atoms or radicals are located in certain definite positions on the benzene ring. This is illustrated in the diagram, where A and B represent the substituent atoms or groups. When attached to adjoining carbon atoms, B is in the o- position in respect to A (also called the 1,2-position). If B is located on the third carbon atom in respect to A, it is in the m- position (also called 1,3-); when B is attached to the opposite carbon atom, it is in the p- position (1,4).
In organic compounds, these prefixes usually appear in italics (often abbreviated o-, m-, and p-) and are ignored in alphabetizing.In inorganic chemistry, the prefix ortho designates the most highly hydrated acid, or its salt, to contrast with the meta, or less hydrated acid or salt. For example, H3PO4 is orthophosphoric acid and HPO3 is metaphosphoric acid. These are considered in alphabetizing.
