Para- (chemistry)

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

* 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 R and ''ortho''.
* In ''meta''-substitution the substituents occupy positions 1 and 3 (corresponding to R and ''meta'' in the diagram).
* In ''para''-substitution, the substituents occupy the opposite ends (positions 1 and 4, corresponding to R and ''para'' in the diagram).
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:
*Column chromatography will often separate these isomers, as the ''ortho'' is more polar than the ''para'' in general.
* Fractional crystallisation can be used to obtain pure ''para'' product, relying on the principle that it is less soluble than the ''ortho'' and thus will crystallise first. Care must be taken to avoid cocrystallisation of the ''ortho'' isomer.
*Many nitro compounds' ''ortho'' and ''para'' isomers have quite different boiling points. These isomers can often be separated by distillation. These separated isomers can be converted to diazonium salts and used to prepare other pure ''ortho'' or ''para'' compounds.

''Ipso'', ''meso'', and ''peri'' substitution

Image:Ipso Substitution V.3.svg, ''ipso''- substitution.
Image:Meso Substitution V.1.svg, ''meso''- substitution.
Image:Peri Substitution V.1.svg, ''peri''- substitution.

* ''Ipso''-substitution describes two substituents sharing the same ring position in an intermediate compound in an electrophilic aromatic substitution. Trimethylsilyl, ''tert''-butyl, and isopropyl groups can form stable carbocations, hence are ''ipso'' directing groups.
* ''Meso''-substitution refers to the substituents occupying a benzylic position. It is observed in compounds such as calixarenes and acridines.
* ''Peri''-substitution occurs in naphthalenes for substituents at the 1 and 8 positions.

''Cine'' and ''tele'' substitution

* In ''cine''-substitution, the entering group takes up a position adjacent to that occupied by the leaving group. For example, ''cine''-substitution is observed in aryne chemistry.
* ''Tele''-substitution occurs when the new position is more than one atom away on the ring.

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):

File:Ortho-cresol-2D-skeletal.png, ''o''-cresol
File:Meta-cresol-2D-skeletal.png, ''m''-cresol
File:Para-cresol-vertical-2D-skeletal.png, ''p''-cresol

There are three arene substitution isomers of dihydroxybenzene (C₆H₄(OH)₂) – the ''ortho'' isomer catechol, the ''meta'' isomer resorcinol, and the ''para'' isomer hydroquinone:

File:Pyrocatechol.svg, catechol
File:Resorcin.svg, resorcinol
File:Hydrochinon.svg, hydroquinone

There are three arene substitution isomers of benzenedicarboxylic acid (C₆H₄(COOH)₂) – the ''ortho'' isomer phthalic acid, the ''meta'' isomer isophthalic acid, and the ''para'' isomer terephthalic acid:

File:Phthalsäure.svg, phthalic acid
File:Isophthalsäure.svg, isophthalic acid
File:Terephthalsäure.svg, terephthalic acid

These terms can also be used in six-membered heterocyclic aromatic systems such as pyridine, where the nitrogen atom is considered one of the substituents. For example, nicotinamide and niacin, shown ''meta'' substitutions on a pyridine ring, while the cation of pralidoxime is an ''ortho'' isomer.

File:Niacin structure.svg, niacin
File:Nicotinamid.svg, nicotinamide
File:Pralidoxime-2D-skeletal.png, pralidoxime

See also

* Descriptor (chemistry)
* Isomer
* Structural isomerism

References

{{Navbox stereochemistry

Aromatic compounds
Chemical nomenclature