Science, Tech, Math › Science The Definition of Ortho, Meta, and Para in Organic Chemistry Share Flipboard Email Print jxfzsy / Getty Images Science Chemistry Molecules Basics Chemical Laws Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated October 02, 2019 The terms ortho, meta, and para are prefixes used in organic chemistry to indicate the position of non-hydrogen substituents on a hydrocarbon ring (benzene derivative). The prefixes derive from Greek words meaning correct/straight, following/after, and similar, respectively. Ortho, meta, and para historically carried different meanings, but in 1879 the American Chemical Society settled upon the following definitions, which remain in use today. Ortho Ortho describes a molecule with substituents at the 1 and 2 positions on an aromatic compound. In other words, the substituent is adjacent or next to the primary carbon on the ring. The symbol for ortho is o- or 1,2- Meta Meta is used to describe a molecule with substituents are at the 1 and 3 positions on an aromatic compound.The symbol for meta is m- or 1,3 Para Para describes a molecule with substituents at the 1 and 4 positions on an aromatic compound. In other words, the substituent is directly opposite the primary carbon of the ring.The symbol for para is p- or 1,4- For more organic chemistry definitions, see the organic chemistry glossary.