meta

(noun)

A prefix used to name an aromatic ring with two substituents separated by one carbon on the ring.

Related Terms

  • para
  • ortho

Examples of meta in the following topics:

  • Elimination

    • For example, treatment of para-chlorotoluene with sodium hydroxide solution at temperatures above 350 ºC gave an equimolar mixture of meta- and para-cresols (hydroxytoluenes).
    • However, ortho-chloroanisole gave exclusively meta-methoxyaniline under the same conditions.
    • In the absence of steric hindrance (top example) equal amounts of meta- and para-cresols are obtained.
  • Substitution Reactions of Benzene Derivatives

    • If reaction occurs equally well at all available sites, the expected statistical mixture of isomeric products would be 40% ortho, 40% meta and 20% para.
    • Bromination of nitrobenzene requires strong heating and produces the meta-bromo isomer as the chief product.
    • With some exceptions, such as the halogens, deactivating substituents direct substitution to the meta location.
    • Toluene gives 58.5% ortho-nitrotoluene, 37% para-nitrotoluene and only 4.5% of the meta isomer.
    • Consequently, meta-products predominate when electrophilic substitution is forced to occur.
  • Benzen Derivatives

    • In the case of disubstituted benzenes, the prefixes ortho, meta & para are commonly used to indicate a 1,2- or 1,3- or 1,4- relationship respectively.
    • Some disubstituted toluenes have singular names (e.g. xylene, cresol & toluidine) and their isomers are normally designated by the ortho, meta or para prefix.
  • Directed ortho-Metalation

    • Electrophilic substitution of aromatic rings generally gives a mixture of ortho and para substitution products when an existing substituent activates the ring or meta products when the substituent is deactivating.
    • Direct electrophilic substitution would normally occur at the meta position, so the action of the amide DMG is particularly noteworthy.
  • Characteristics of Specific Substitution Reactions

    • We find, for example, that nitration of nitrobenzene occurs smoothly at 95 ºC, giving meta-dinitrobenzene, whereas bromination of nitrobenzene (ferric catalyst) requires a temperature of 140 ºC.
    • The bulky tert-butyl group ends up attached to the reactive meta-xylene ring at the least hindered site.
    • This may not be the site of initial bonding, since polyalkylbenzenes rearrange under Friedel-Crafts conditions (para-dipropylbenzene rearranges to meta-dipropylbenzene on heating with AlCl3).
    • Since meta-substitution favors a single product, separation of trace isomers is normally not a problem.
  • Background & Introduction

    • Discrete Kekule formulas demonstrate that this benzyl-like delocalization places the electron on ortho and para carbons, but not on meta carbons.
    • If the para-locations are themselves hindered by large meta substituents, then an unstable hexaarylethane may actually be formed.
  • Electrophilic Substitution of Disubstituted Benzene Rings

    • The first three examples have two similar directing groups in a meta-relationship to each other.
    • Example 6 is interesting in that it demonstrates the conversion of an activating ortho/para-directing group into a deactivating meta-directing "onium" cation [–NH(CH3)2(+)] in a strong acid environment.
  • Properties of Aromatic Compounds

    • This nomenclature tutorial video takes you through the IUPAC rules for naming disubstituted benzene compounds using ortho-, meta-, and para- prefixes.
  • Oxidation of Phenols

    • Note that meta-quinones having similar structures do not exist.
  • Naming Aromatic Compounds

    • Disubstituted benzene rings can be named based on the relative positions of the substituents: the prefix ortho- is used if the substituents occupy adjacent positions on the ring (1,2), meta- is used if the substituents are separated by one ring position (1,3), and para- if they are found on opposite sides of the ring (1,4).
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