Organic chemistry Part 10: Reactivity and reaction mechanisms

Part 10.8 Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions - SULFONATION yielding sulfonic acids

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Part 10.8 Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions - SULFONATION

Part 10.8 AROMATIC HYDROCARBONS (ARENES) - introduction to arene electrophilic substitutions.

 Sulfonation to give a sulfonic/sulfonic acid like benzenesulfonic 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.

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

The five reactions described Part 10.8 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 benzene ring.

Arenes tend to undergo substitution, rather than addition, because substitutions allows the very stable benzene ring to remain intact.

See also note on orientation of substituted products from electrophilic substitution in the benzene ring.

• Examples of aromatic sulfonation substitution reactions

• (i) + H₂SO₄ ==> + H₂O

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

  • shows the full structure of the sulfonic acid group

• (ii) + H₂SO₄ ==>    + H₂O

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

  • Three structural-positional isomers C₇H₈SO₃ formed in different proportions

• What is the mechanism for 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 sulfuric acid or even better, 'fuming' sulfuric acid, which has a higher sulfur trioxide content and more efficient at introducing the sulfonic acid group into the benzene ring.

mechanism 25 - electrophilic substitution by a sulfonate group in the benzene ring

• [mechanism 44 above]  

  • Step (1) Sulfur 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 sulfur 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 sulfur trioxide forming a second highly unstable intermediate. It is very unstable because the stable electron arrangement of the benzene ring pi electrons is partially broken to give a 'saturated' C (top right of ring).

  • Step (3) A hydrogensulfate ion removes a proton and the complete benzene ring of pi electrons is reformed giving the anion of the aromatic sulfonic acid.

  • Step (4) is a proton transfer to give the sulfonic 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-benzenesulfonic acid.

• FURTHER COMMENTS

  • 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 Sulfonation, Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to A level chemistry course specifications are unofficial. Website content © Dr Phil Brown 2000+. All copyrights reserved on these organic chemistry exam revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, these A level chemistry revision notes are suitable for use of pre-university students studying AQA advanced level organic chemistry revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, Edexcel advanced level organic chemistry revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, OCR advanced level organic chemistry revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, IB advanced level organic chemistry revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, WJEC (Eduqas) advanced level organic chemistry revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, CIE Cambridge advanced level organic chemistry revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, CCEA advanced level organic chemistry revision notes on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, and useful for US grade 11 grade 12 AP honors organic chemistry courses involving electrophilic substitution mechanism of sulfonation of benzene & methylbenzene Explaining the importance of electrophilic substitution mechanism of sulfonation of benzene & methylbenzene in organic chemistry, What you need to know about electrophilic substitution mechanism of sulfonation of benzene & methylbenzene for organic chemistry, Explaining the use of electrophilic substitution mechanism of sulfonation of benzene & methylbenzene knowledge in organic chemistry, Examples of electrophilic substitution mechanism of sulfonation of benzene & methylbenzene explained when studying organic chemistry, What is the significance of electrophilic substitution mechanism of sulfonation of benzene & methylbenzene in organic chemistry, What is the use of electrophilic substitution mechanism of sulfonation of benzene & methylbenzene in organic chemistry  Describing and explaining the theory of electrophilic substitution mechanism of sulfonation of benzene & methylbenzene when studying organic chemistry, exam revision notes for electrophilic substitution mechanism of sulfonation of benzene & methylbenzene in exams, online help for electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, revision notes for electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, what do I need to learn for electrophilic substitution mechanism of sulfonation of benzene & methylbenzene in exams? revision summary for electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, help in teaching electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, learning notes for electrophilic substitution mechanism of sulfonation of benzene & methylbenzene, help to pass the electrophilic substitution mechanism of sulfonation of benzene & methylbenzene exam, how to prepare for examination questions on electrophilic substitution mechanism of sulfonation of benzene & methylbenzene? Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry GCE A Level Revision Notes PART 10 Summary of organic reaction mechanisms - A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

APPENDIX

  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)