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Arene substitution pattern

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Arene substitution pattern

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.

Contents

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.
  • Ipso-substitution describes two substituents sharing the same ring position in an intermediate compound in an electrophilic aromatic substitution. Me3Si, t-Bu, and iPr groups can form stable carbocation, 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 Berzelius originally called the racemic form of aspartic acid paraaspartic acid (another obsolete term: racemic acid) in 1830. The use of the prefixes ortho, meta and para to distinguish isomers of di-substituted aromatic rings starts with Wilhelm Körner in 1867, although he applied 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 di-substituted aromatic ring (viz, 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):

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

    References

    Arene substitution pattern Wikipedia
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