HOME PAGE * KS3 SCIENCES * GCSE BIOLOGY  CHEMISTRY  PHYSICS * ADVANCED LEVEL CHEMISTRY

A Level Organic Chemistry: Reaction mechanisms - aromatic electrophilic sulfonation

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. Sulfonation/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.

[SEARCH BOX]

See also INDEX of all AROMATIC COMPOUND chemistry revision notes



 

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 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.


 

10.8.6 The electrophilic substitution of an arene - sulfonation mechanism

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

organic reaction mechanisms

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


TOP OF PAGE

See also INDEX of all AROMATIC COMPOUND chemistry revision notes

[SEARCH BOX]

Doc Brown's Advanced Level Chemistry Revision Notes

APPENDIX

See also INDEX of all AROMATIC COMPOUND chemistry revision notes

  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

See also INDEX of all AROMATIC COMPOUND chemistry revision notes

Doc Brown's Advanced Level Chemistry Revision Notes

TOP OF PAGE

See also INDEX of all AROMATIC COMPOUND chemistry revision notes

[SEARCH BOX]

KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
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 science course specifications are unofficial.

 Doc Brown's Chemistry 

*

TOP OF PAGE

best Christmas gift deals latest video game release, best Christmas gift deals best bargains in new year January shop sales latest pop music releases, best New Year sales deals download free music, latest film releases, best Christmas gifts for teenagers latest high street fashion in clothes, fashionable trending in clothing, best Christmas gift deals best bargains in new year January shop salesgirls buy clothes, spend a lot of money on clothes, best New Year sales deals shoes, sweets and chocolates, magazines and make-up best Christmas gifts for teenagers  Boys buy food and drink, computer games best Christmas gift deals best bargains in new year January shop sales DVDs and CDs, girls and boys spend a lot of money on credit for mobile phones best New Year sales deals best bargains in new year January shop sales buses and trains to transport them going out best Christmas gifts for teenagers best bargains in new year January shop sales Girls spend a lot of money on clothes best Christmas gift deals  color colour preferences in clothes, cool sunglasses best New Year sales deals boys buy expensive things like best Christmas gifts for teenagers designer sports clothes and trainers, teenagers save for holidays, best New Year sales deals clothes, mobile phones, birthday presents and electronic goods, teenage accessories, Favourite style of jeans. best Christmas gifts for teenagers A typical girl’s shopping list includes mobile phone credit deals best Christmas gift deals fashionable quality cool airpods, air pods, fashionable clothes best New Year sales deals the most popular favourite sneakers best Christmas gifts for teenagers fancy shoes, sweets, chocolates, magazines, best Christmas gift deals best bargains in new year January shop sales lip moisturizer best bargains in new year January shop sale slots on make-up, well being, teenage decor decorating their room best New Year sales deals teenagers like LED string lantern lights, best Christmas gifts for teenagers beauty products for teenagers, denim jackets, scrunchies coolness, fashionable back packs, typical boy’s shopping list includes mobile credit deals, eating out takeaway food and drinks, what teenagers like to buy in clothes best Christmas gifts for teenagers computer games, DVDs, CDs, what teenagers talk about best Christmas gift deals what teenagers worry about, what teenagers like to do for fun sports best New Year sales deals  what cool things do teenagers buy, resale websites like eBay Teenager