Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.

Ortho, meta, and para substitution

The toluidines serve as an example for these three types of substitution.

Synthesis

Electron donating groups, for example amino, hydroxyl, alkyl, and phenyl groups tend to be ortho/para-directors, and electron withdrawing groups such as nitro, nitrile, and ketone groups, tend to be meta-directors.

Properties

Although the specifics vary depending on the compound, in simple disubstituted arenes, the three isomers tend to have rather similar boiling points. However, the para isomer usually has the highest melting point, and the lowest solubility in a given solvent, of the three isomers.

Separation of ortho and para isomers

Because electron donating groups are both ortho and para directors, separation of these isomers is a common problem in synthetic chemistry. Several methods exist in order to separate these isomers:

Ipso, meso, and peri substitution

Cine and tele substitution

Origins

The prefixes ortho, meta, and para are all derived from Greek, meaning correct, following, and beside, respectively. The relationship to the current meaning is perhaps not obvious. The ortho description was historically used to designate the original compound, and an isomer was often called the meta compound. For instance, the trivial names orthophosphoric acid and trimetaphosphoric acid have nothing to do with aromatics at all. Likewise, the description para was reserved for just closely related compounds. Thus Jöns Jakob Berzelius originally called the racemic form of tartaric acid "paratartaric acid" (another obsolete term: racemic acid) in 1830. The use of the prefixes ortho, meta and para to distinguish isomers of disubstituted aromatic rings starts with Wilhelm Körner in 1867, although he applied the ortho prefix to a 1,4-isomer and the meta prefix to a 1,2-isomer. It was the German chemist Karl Gräbe who, in 1869, first used the prefixes ortho-, meta-, para- to denote specific relative locations of the substituents on a disubstituted aromatic ring (namely naphthalene). In 1870, the German chemist Viktor Meyer first applied Gräbe's nomenclature to benzene. The current nomenclature was introduced by the Chemical Society in 1879.

Examples

Examples of the use of this nomenclature are given for isomers of cresol, C6H4(OH)(CH3):