Ortho Meta Para in Organic Chemistry

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Ortho Meta Para in Organic Chemistry

These chemical structures show the difference between ortho-, meta- and para-xylene.
These chemical structures show the difference between ortho-, meta- and para-xylene. Todd Helmenstine

The terms ortho-, meta-, and para- are 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. Although ortho-, meta-, and para- historically carried different meanings, in 1879 the 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 describing 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-

Ipso, Meso, and Peri Substitutions