meta

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).