Arene substitution patterns
In ortho-substitution, two substituents occupy positions next to each other, which may be numbered 1 and 2. In the diagram, these positions are marked G (for Group) and ortho.
In meta-substitution the substituents occupy positions 1 and 3 (corresponding to G and meta in the diagram). In para-substitution, the substituents occupy the opposite ends (positions 1 and 4). The toluidines serve as an example for these three types of substitution.
In special cases, ipso-substitution describes two substituents sharing the same ring position in an intermediate compound in an electrophilic aromatic substitution.
In naphthalenes, substitution specifically in the 1 and 8 positions is called peri-substitution.
Two additional terms related to substitution processes are cine-substitution in which the entering group takes up a position adjacent to that occupied by the leaving group for example observed in aryne chemistry and tele-substitution when the new position is more than one atom away on the ring
The meanings of the prefixes ortho, meta and para are all derived from Greek: respectively meaning straight or correct, following or after and akin to or similar. 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, see for instance the trivial names orthophosphoric acid and trimetaphosphoric acid having nothing to do with aromats at all. Likewise the description para was reserved for just closely related compounds. Thus Berzelius originally called the racemic form of aspartic acid paraaspartic acid (another obsolete term: racemic acid) in 1830. The use of the descriptions ortho, meta and para for multiple substituted aromatic rings starts with Wilhelm Körner in the period 1866–1874 although he chose to reserve the ortho prefix for the 1,4 isomer and the meta prefix for the 1,2-isomer. The current nomenclature (different again from that of Körner) was introduced by the Chemical Society in 1879 .