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 - SULFONATION

Part 10.8 AROMATIC HYDROCARBONS (ARENES) - introduction to arene electrophilic substitutions. Sulphonation/sulfonation to give a sulphonic/sulfonic acid like benzenesulphonic acid/benzenesulfonic acid, 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.


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.


10.8.6 The electrophilic substitution of an arene - sulphonation mechanism

The organic synthesis of sulfonic/sulphonic acids by reaction of benzene/methylbenzene with conc. sulfuric/sulphuric acid

  • Examples of aromatic sulfonation substitution reactions

  • (i) (c) doc b + H2SO4 ==> (c) doc b + H2O

    • benzene + sulfuric acid ==> benzenesulfonic acid + water

    • (c) doc b shows the full structure of the sulfonic acid group

  • (ii) (c) doc b + H2SO4 ==> (c) doc b  (c) doc b (c) doc b + H2O

    • methylbenzene + sulfuric acid ==> 2/3/4-methylbenzenesulfonic acid + water

    • Three structural-positional isomers C7H8SO3 formed in different proportions

  • What is the mechanism for sulphonating/sulfonating benzene? or methyl benzene?

  • for R = H, benzene: C6H6 + SO3 ==> C6H5SO2OH  [see mechanism 44 below]

  • or C6H6 + H2SO4 ==> C6H5SO2OH + H2O

  • Benzene is heated with concentrated sulphuric acid or even better, 'fuming' sulphuric acid, which has a higher sulphur trioxide content and more efficient at introducing the sulphonic acid group into the benzene ring.

organic reaction mechanisms

mechanism 25 - electrophilic substitution by an acyl group in the benzene ring

  • [mechanism 44 above]  

    • Step (1) Sulphur trioxide is formed (or already present). It is a powerful electrophile, i.e. electron pair acceptor because of the effect of the three very electronegative oxygen atoms bonded to the central sulphur atom.

    • Step (2) An electron pair from the delocalised pi electrons of the benzene ring forms a C-S bond with the electron pair accepting sulphur trioxide forming a second 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 (3) A hydrogensulphate ion removes a proton and the complete benzene ring is reformed giving the anion of the aromatic sulphonic acid.

    • Step (4) is a proton transfer to give the sulphonic acid.

  • for R = CH3, methylbenzene: C6H5CH3 + H2SO4 ==> CH3C6H4SO2OH + H2O

    • and again there is the potential to form three position isomers by substituting in the 2, 3 or 4 position on the ring, the mechanism diagram shows the formation of methyl-2-benzenesulphonic acid.


    • The overall sulfonation reaction is the substitution of -H by -SO2OH




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 benzene methylbenzene C6H6 + SO3 ==> C6H5SO2OH or C6H6 + H2SO4 ==> C6H5SO2OH + H2O R = CH3, benzene: C6H5CH3 + H2SO4 ==> CH3C6H4SO2OH + H2O sulfonation reaction is the substitution of -H by -SO2OH sulphonation

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

GCSE grade 9-1 & IGCSE CHEMISTRY Doc Brown's Travel Pictures & Notes
All website content Dr Phil Brown 2000 onwards. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial. Email doc b:

 Doc Brown's Chemistry 


For latest updates see