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Electrophilic Aromatic Substitution

  1. Nitration
    Description: 45960.png
    i.e., Description: 45968.png
    Description: 45979.png
    Description: 46027.png
  2. Sulphonation
    Sulphonation is done by heating the substrate with conc. sulphuric acid or fuming sulphuric acid containing varying proportions of sulphur trioxide.
    Description: 46024.png
    Description: 45997.png
    Sulphonation, like iodination, is reversible and is believed to take place in concentrated sulphuric acid via the pathway,
    Description: 46009.png
    In oleum, the s-complex is believed to undergo protonation of the Description: 46017.png before undergoing C – H fission to yield the SO3H analogue. Like iodination, sulphonation exhibits a kinetic isotope effect, indicating that C – H bond breaking is involved in the rate-limiting step of the reaction, i.e., k–1 > k2.
  3. Halogenation
    Description: 46040.png
    Description: 46084.png
    Description: 46076.png
    Kinetic isotope effects have not been observed for chlorination and only rarely for bromination, i.e., the reactions normally follow pathway like nitration. In iodination, which only takes place with iodine itself on activated species, kinetic isotope effects are the rule. Iodination is often assisted by the presence of bases or of oxidizing agents, which remove HI and thus displace the above equilibrium to the right. Oxidizing agents also tend to produce I, or a complex containing positively polarized iodine, from I2, thus providing a more effective electrophile.
    Halogenation may also be carried out by use of interhalogen compounds Description: 46098.png etc., attack occurring through the less electronegative halogen, as this will constitute the electrophilic end of the molecule.
  4. Friedel crafts reaction: Alkylation or acylation of aromatic ring with alkyl halides or acyl halides in the presence of a Lewis acid, generally anhydrous AlCl3 (Friedel crafts catalyst) is called Friedel crafts reaction.
    Description: 46107.png
    where X = Cl, Br, I, but the most effective catalyst is anhydrous AlCl3.
  • Alkylation
    Description: 46117.png
    1. Nature of alkyl groups: If the alkyl group is simple CH3– or CH3CH2–, then a complex between alkyl halide and Lewis acid is the electrophile as shown in the second mechanism. But because of the relative stability of s- and t-carbonium ions, adducts with s- and t-alkyl halides ionize. It is now the carbonium ion that is predominantly the active species. e.g.,
      Description: 46130.png
    2. Temperature: Not only nature of the alkyl group, but also temperature determines the nature of electrophile. For example, n-alkyl group can be introduced to a fair extent without rearrangement at low temperatures, because ionization of the adduct is retarded. But at higher temperature, carbonium ion is formed which rearranges and the product is rearranged alkyl benzene. Thus n-propylchloride gives isopropyl benzene.
      Description: 46139.png
      In the same way, isobutyl chloride gives t-butyl benzene.
    3. Nature of Lewis acid as catalyst: The order of effectiveness of Lewis acid catalysts has been shown to be
      AlCl3 > FeCl3 > BF3 > TiCl3 > ZnCl2 > SnCl4
      The action of Me3CCH2Cl/AlCl3 on benzene is found to yield almost completely the rearranged product, PhCMe2CH2Me, which can be explained on the basis of the initial electrophilic complex being polarized enough to allow the rearrangement of [Me3CCH2]d+ --------- Cl --------- AlCl3d. By contrast Me3CCH2Cl/FeCl3 on benzene is found to yield almost completely the unrearranged product, Me3CCH2Ph. This is due to the fact that the complex with the weaker Lewis acid, FeCl3, is not now polarized enough to allow of rearrangement.
  • Acylation
    Mechanism may be represented as follows:
    Description: 46173.png

Directive influence of substituents in benzene

The first substituent may occupy any position in benzene ring, i.e., one and only one monosubstituted benzene is obtained. The next group may go to ortho, meta, or para position. It is the group already present in the benzene nucleus that determines how readily the attack occurs and at what position of the ring it occurs. In other words, the group attached to the ring not only affects the reactivity but also determines the orientation of substitution. This is called directive influence of substituents in benzene nucleus. The substituent group is able to activate or deactivate the ring due to a number of factors such as inductive effect, electromeric effect, resonance effect, and hyperconjugative effect. Depending on the directive influence, the substituent groups, except halogens, are divided into two different classes:
  • Class I: R, OH, OR, NH2, NHR, NHCOCH3, OCOCH3, Cl, Br, I, F, CH2Cl, SH, Ph, etc. These groups direct the incoming electrophile mainly to the o/p-positions.
  • Class II: NO2, CHO, CO2H, COCl, CONH2, CO2R, SO3H, SO2Cl, COCH3, CN, CCl3, Description: 46201.png, etc. These groups direct the incoming electrophile mainly to the m-position.
ortho, para
–OH, –NH2,
–NHR, –NR2
–SO3H, –CN, –CHO
–NO2, –CF3,
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Orientation in benzene ring with two substituents

When the two groups direct differently, i.e., one is of activating type and the other is of deactivating type, then activating group decides the orientation.
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When both the groups are activating, then more activating group decides the orientation.
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When both the groups are deactivating, then more deactivating group decides the orientation.
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Substitution in other aromatic systems

  1. Naphthalene and anthracene rings
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    Description: 46359.png
    In each of the two transition states, the positive charge is more extensively delocalized than in reaction with benzene, leading to lower activation energies. Further, the three-starred structures are of benzenoid type and therefore of lower energy content than the remainder in which the benzenoid nature of the second ring has been destroyed. Since there are two such low-energy contributors for 1-substitution as compared with one for 2-substitution, it is understandable that the 1-position should be the more reactive than the 2-position.
    In anthracene, the electrophile attacks preferentially at the 9 or 10-positions, since the arenium ion formed by the electrophilic attack at any of these positions can have two intact benzene rings in its canonical forms, while attack of electrophile at any other positions (1 or 2) would give arenium ion having a naphthalene ring in its canonical forms. The resonance energy of two benzene rings is more than the resonance energy of a naphthalene ring.
    Description: 46369.png
  2. Heterocyclic rings
    Pyrrole is highly reactive at both 2- and 3-positions. The reason is that the transition state for substitution at each position is strongly stabilized by the accommodation of the positive charge by nitrogen, in just the way that aniline owes its reactivity to the exocyclic nitrogen. 2-substitution predominates because the positive charge in the transition state is delocalized over a total of three atoms, compared with two for 3-substitution.
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    Description: 46387.png
    The reactivity order for a 5-membered heterocyclics towards electrophilic substitution would be pyrrole > furan > thiophene.
    Description: 46398.png
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