para

Examples of para in the following topics:

• Acidity of Phenols

  • This phenolic acidity is further enhanced by electron-withdrawing substituents ortho and para to the hydroxyl group, as displayed in the following diagram.
  • It is noteworthy that the influence of a nitro substituent is over ten times stronger in the para-location than it is meta, despite the fact that the latter position is closer to the hydroxyl group.
  • Furthermore additional nitro groups have an additive influence if they are positioned in ortho or para locations.
  • As noted in our earlier treatment of electrophilic aromatic substitution reactions, an oxygen substituent enhances the reactivity of the ring and favors electrophile attack at ortho and para sites.
  • The additional resonance stabilization provided by ortho and para nitro substituents will be displayed in the third diagram above.

• Substitution Reactions of Benzene Derivatives

  • In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent.
  • Toluene gives 58.5% ortho-nitrotoluene, 37% para-nitrotoluene and only 4.5% of the meta isomer.
  • Although chlorobenzene is much less reactive than benzene, the rate of ortho and para-substitution greatly exceeds that of meta-substitution, giving a product mixture of 30% ortho and 70% para-nitrochlorobenzene.
  • This happens only for ortho and para electrophilic attack, so such substituents favor formation of those products.
  • Consequently, all these substituents direct substitution to ortho and para sites.

• 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).
  • In the absence of steric hindrance (top example) equal amounts of meta- and para-cresols are obtained.

• Background & Introduction

  • Discrete Kekule formulas demonstrate that this benzyl-like delocalization places the electron on ortho and para carbons, but not on meta carbons.
  • More than fifty years later, the reactive dimer of triphenylmethyl radical was shown to be the para-coupled compound drawn above and not hexaphenylethane.
  • Since the electron delocalization noted above places radical character at the para carbons of the phenyl groups, bonding to this relatively unhindered location is preferred, although at the cost of one benzene ring's aromaticity.
  • If the para-locations are themselves hindered by large meta substituents, then an unstable hexaarylethane may actually be formed.

• Oxidation of Phenols

  • Among the colored products from the oxidation of phenol by chromic acid is the dicarbonyl compound para-benzoquinone (also known as 1,4-benzoquinone or simply quinone); an ortho isomer is also known.
  • Although chromic acid oxidation of phenols having an unsubstituted para-position gives some p-quinone product, the reaction is complex and is not synthetically useful.

• Characteristics of Specific Substitution Reactions

  • When the molar ratio of benzene to alkyl halide falls below 1:1, para-ditert-butylbenzene becomes the major product.
  • 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).
  • This is particularly true for cases of ortho-para substitution, which often produce significant amounts of the minor isomer.
  • As a rule, para-isomers predominate except for some reactions of toluene and related alkyl benzenes.
  • Separation of these mixtures is aided by the fact that para-isomers have significantly higher melting points than their ortho counterparts; consequently, fractional crystallization is often an effective isolation technique.

• Reactions of Substituent Groups

  • The strongest activating and ortho/para-directing substituents are the amino (-NH2) and hydroxyl (-OH) groups.
  • For example, acetylation of aniline gives acetanilide (first step in the following equation), which undergoes nitration at low temperature, yielding the para-nitro product in high yield.
  • The modifying acetyl group can then be removed by acid-catalyzed hydrolysis (last step), to yield para-nitroaniline.
  • Although the activating influence of the amino group has been reduced by this procedure, the acetyl derivative remains an ortho/para-directing and activating substituent.
  • However, the overall influence of the modified substituent is still activating and ortho/para-directing.

• Organometallic Reagents from Geminal Dihalides

  • For example, 1,4-dibromobutane and para-dibromobenzene react with excess magnesium in ether to generate di-Grignard reagents that may be used in the same manner as any simple Grignard reagent.
  • In the case of para-chlorobromobenzene, the greater reactivity of bromine permits the preparation of para-chlorophenylmagnesium bromide in good yield.

• Reactions of Aromatic Compounds

  • It states that an electron-donating substituent generally accelerates substitution and directs reactivity toward the positions that are ortho and para to it on the ring, while an electron-withdrawing substituent will slow reaction progress and favor the meta position on the ring.
  • EAS occurs ortho or para to electron donating groups, such as amines, due to the stabilization of the intermediate positive charge.
  • Due to the electrons provided by the NH2 group, this intermediate is stabilized, and the para-substitution is favored.

• 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.