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organic reaction mechanismsorganic reaction mechanismsRevising Advanced Organic Chemistry  Doc Brown's GCE Chemistry

Revision Notes PART 10 Summary of organic reaction mechanisms - A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

Part 10.8 Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions

Part 10.8 AROMATIC HYDROCARBONS (ARENES) - introduction to arene electrophilic substitutions. Nitration to give nitro-aromatics like nitrobenzene. The orientation of products in aromatic substitution (1,2-; 1,3-; and 1,4- positions for two substituents in the benzene ring, old names - ortho/meta/para substitution products). The revision notes include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained for the reaction mechanisms of aromatic hydrocarbons like benzene and methylbenzene.


Part 10.8 AROMATIC HYDROCARBONS (Arenes)

10.8.1 Introduction to the reactivity of aromatic compounds

e.g. the arenes benzene and methyl benzene

Why do aromatic hydrocarbon molecules primarily react via electrophilic substitution reaction?

The five reactions described are electrophilic substitution reactions involving the generation of a powerful electrophile (electron pair acceptor) which subsequently attacks the electron rich Π (pi) electron system of the double bond. Arenes tend to undergo substitution, rather than addition, because substitutions allows the very stable benzene ring to remain intact.


10.8.2 The electrophilic substitution of an arene - nitration mechanism

Organic synthesis of nitro aromatic compounds by reaction of conc. sulfuric acid & nitric acid with benzene/methylbenzene

  • Examples of aromatic nitration substitution reactions

    • (i) (c) doc b + HNO3 ==> (c) doc b + H2O

      • benzene + nitric acid ==> nitrobenzene + water

      • -

    • (ii) (c) doc b + HNO3 ==> (c) doc b + H2O

      • methylbenzene + nitric acid ==> methyl-2/3/4-nitrobenzene + water

      • Three structural-positional isomers C7H7NO2 formed in different proportions.

      • -

    • (iii) (c) doc b HNO3 ==> (c) doc b + H2O

      • benzoic acid + nitric acid ==> 3-nitrobenzoic acid + water

        • the 3-nitrobenzoic acid is the majority product, BUT, you will also get some 2-nitrobenzoic acid and 4-nitrobenzoic acid.

        • -

    • (iv) (c) doc b+ HNO3 ==> (c) doc b + H2O

      • nitrobenzene + nitric acid ==> 1,3-dinitrobenzene

      • 1,3-dinitrobenzene is the majority product, BUT, you will still get some 1,2-dinitrobenzene and 1,4-dinitrobenzene.

      • -

    • (v) (c) doc b + HNO3 ==> (c) doc b (c) doc b (c) doc b + H2O

      • chlorobenzene + nitric acid ==> chloronitrobenzenes

      • 3 structural-positional isomers of C6H4NO2Cl, 1-chloro-2-nitrobenzene, 1-chloro-3-nitrobenzene, 1-chloro-4-nitrobenzene,  formed in different proportions.

      • -

  • What is the mechanism for nitrating benzene? or methyl benzene?

  • for benzene : C6H6 + HNO3 ==> C6H5NO2 + H2O    [see mechanism 19 below]

  • for methyl benzene: C6H5CH3 + HNO3 ==> O2NC6H4CH3 + H2O

  • The nitrating mixture consists of concentrated nitric acid (source of the nitro group -NO2) and concentrated sulphuric acid which acts as a catalyst and as a strong acid.

organic reaction mechanisms

mechanism 19 - electrophilic substitution in the nitration of the benzene ring

  • [mechanism 19 above] Benzene is converted into nitrobenzene, when R = H.

  • When R = CH3, methylbenzene will form a mixture of the three possible substitution products methyl-2/3/4-nitrobenzene,

    • and methyl-3-nitrobenzene is the minority product, the mechanism above would show the formation of one of the major products methyl-2-nitrobenzene.

  • Step (1) The sulphuric acid protonates the nitric acid (strong acid, but weaker than H2SO4)

  • Step (2) The protonated nitric acid loses a water molecule via a sulphuric acid molecule, to generate the electrophile, the nitronium ion, NO2+. This is a much more powerful electrophile, i.e. electron pair acceptor, than the original nitric acid, and is needed to attack the very stable aromatic ring of benzene.

    • Steps (1) and (2) can be written as: 2H2SO4 + HNO3 ==> NO2+ + H3O+ + 2HSO4- 

  • Step (3) An electron pair from the delocalised pi electrons of the benzene ring forms a C-N bond with the electron pair accepting nitronium ion forming a highly unstable carbocation. It is very unstable because the stable electron arrangement of the benzene ring is partially broken to give a 'saturated' C (top right of ring).

  • Step (4) The hydrogensulphate ion (HSO4-, formed in step (1), abstracts a proton from the highly unstable intermediate carbocation to give the nitro-aromatic product and reform the sulphuric acid catalyst as well as the stable benzene ring.

    • Note that the hydrogensulphate ion, HSO4- has been shown in the style of -:O-SO2-OH, to emphasize the importance a lone pair on the oxygen abstracting the proton from the aromatic carbocation.

  • GENERAL COMMENT to compare aromatic electrophilic substitution with the electrophilic addition with alkenes:

    • Like alkenes, arenes are susceptible to electrophilic attack because of the high electron density of the pi electrons involved in the carbon-carbon bonding and both show little reactivity towards nucleophilic reagents.

    • However two points should be considered because of the particular stability of the aromatic (benzene) ring.

      1. This makes aromatic compounds less reactive than alkenes, which readily undergo addition rather than substitution.

      2. Aromatics do not usually undergo addition because this will remove the stability conferred on the molecule by the benzene ring. By under going substitution rather than addition, the stable aromatic ring is preserved.

  • FURTHER COMMENTS

    • The overall nitration reaction is the substitution of -H by -NO2 


ALL OF THESE ARE NOW ON SEPARATE PAGES

10.8.3 The electrophilic substitution of an arene - chlorination mechanism (example of aromatic halogenation)

10.8.4 The electrophilic substitution of an arene - alkylation mechanism (Friedel-Crafts reaction)

10.8.5 The electrophilic substitution of an arene - acylation mechanism (Friedel-Crafts reaction)

10.8.6 The electrophilic substitution of an arene - sulphonation mechanism


10.8.7 The orientation of products in aromatic electrophilic substitution reactions

  • Certain groups, already present, can increase the electron density of the benzene ring and make the aromatic compound more reactive towards electrophiles such as those described above. However the effect seems to enhance the reactivity at the 2 and 4 substitution positions more than the 3 substitution position.

    • Groups that increase reactivity are e.g. -CH3, -Cl, -OH, -NH2, -NHCOCH3, and favour substitution at the 2 and 4 positions (typically 90-100% combined).

    • They all, by some means, have a small, but significant, electron donating (+I inductive effect) on the ring of pi electrons.

    • For example, methyl benzene is significantly more reactive than benzene and when nitrated, over 90% of the products are either methyl-2-nitrobenzene or methyl-4-nitrobenzene.

  • Certain groups, already present, can decrease the electron density of the benzene ring and make the aromatic compound less reactive towards electrophiles such as described above. However the effect seems to decrease the reactivity at the 2 and 4 substitution positions more than the 3 substitution position.

    • Groups that decrease reactivity, by some means, are e.g. -NO2, COOH, -CHO, -SO2OH, and favour substitution at the 3 position (typically 70-90%) and their effect does fit in with them all being strongly electronegative groupings giving a -I inductive effect.

    • For example, nitrobenzene is much less reactive than benzene and on nitration, 93% of the product is 1,3-dinitrobenzene.

  • -


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms electrophilic substitution nitration methylbenzene benzene C6H6 + HNO3 ==> C6H5NO2 + H2O nitration of methylbenzene C7H8 C6H5CH3 + HNO3 ==> O2NC6H4CH3 + H2O 2H2SO4 + HNO3 ==> NO2+ + H3O+ + 2HSO4-


APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)


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