DOC BROWN'S Science-CHEMISTRY HOMEPAGE KS3 SCIENCE QUIZZES and WORKSHEETS (~US grades 6-8)
GCSE SCIENCE help links GCSE ADDITIONAL SCIENCE help links
KS3 BIOLOGY Quizzes KS3 CHEMISTRY Quizzes & Worksheets KS3 PHYSICS Quizzes
KS4 Science GCSE/IGCSE CHEMISTRY NOTES (~US grades 8-10) KS4 Science GCSE/IGCSE CHEMISTRY QUIZZES and WORKSHEETS (~US grades 8-10) ADVANCED LEVEL CHEMISTRY QUIZZES and WORKSHEETS (~US grades 11-12)
Custom Search

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 - introduction to arene electrophilic substitutions. Nitration to give nitro-aromatics like nitrobenzene, Chlorination to chloro-aromatics like chlorobenzene, Alkylation to give alkyl-aromatics like methylbenzene [Friedel-Crafts reaction], Acylation to give aromatic ketones [Friedel-Crafts reaction], Sulphonation/sulfonation to give a sulphonic/sulfonic acid like benzenesulphonic acid/benzenesulfonic acid, The orientation of products in aromatic substitution (1,5/3,5/4 positions, 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

  • 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 

 

 


 

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

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

  • C6H6 + Cl2 ==> C6H5Cl + HCl    [see mechanism 21 below]

  • Chlorine is bubbled into a mixture of the arene and anhydrous aluminium chloride catalyst. Other catalysts like anhydrous iron(III) chloride can be used, and they are collectively known as halogen carriers.

organic reaction mechanisms

mechanism 21 - electrophilic substitution by halogen in a benzene ring

  • [mechanism 21 above] When R = H, benzene forms chlorobenzene.

    • Step (1) The non-polar and uncharged chlorine molecule is not a strong enough an electrophile to disrupt the pi electron system of the benzene ring. The aluminium chloride reacts with a chlorine molecule to form a positive chlorine ion Cl+ which is a much stronger electron pair accepting electrophile and a tetrachloroaluminate(III) ion (either this or an AlCl3-Cl2 complex - details not needed for A level).

    • Step (2) An electron pair from the delocalised pi electrons of the benzene ring forms a C-Cl bond with the electron pair accepting positive chlorine 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 (3) The tetrachloroaluminate(III) ion, formed in step (1), abstracts a proton from the highly unstable intermediate carbocation to give the chloro-aromatic product, hydrogen chloride gas and reform the aluminium chloride catalyst.

  • Also consider C6H5CH3 + Cl2 ==> ClC6H4CH3 + HCl

  • when R = CH3, methylbenzene forms a mixture of chloro-2/3/4-methylbenzene.

    • chloro-3-methylbenzene is the minority product and the mechanism above would show the formation of chloro-2-methylbenzene.

  • FURTHER COMMENTS

    • The overall halogenation reaction is the substitution of -H by -Cl 

    • Bromination can be carried in the same way by mixing bromine, the aromatic hydrocarbon (arene) with a halogen carrier catalyst such as anhydrous AlBr3 or FeBr3.

    • Why do aromatic compounds tend to react by electrophilic substitution BUT alkenes tend to react by electrophilic addition?

      • They both interact with electrophiles because they both have 'electron rich' electron pair donating bonding systems i.e. the >C=C< double bond in alkenes and the delocalised electrons of the benzene ring, but the benzene ring has a particularly high stability which is preserved on substitution. For the same reason alkenes are generally more reactive than arenes.

    • If methyl benzene is reacted with chlorine in the presence of uv light, substitution takes place in the alkyl side chain. In other words it behaves like an alkane and undergoes a free radical substitution reaction. The initial product is chloromethylbenzene, C6H5CH2Cl, and further substitution products can be formed C6H5CHCl2 and C6H5CCl3. This illustrates the significance of changing reaction conditions which function via a different mechanism to give a different product.

      • initiation:

        • Cl-Cl ==> Cl. + Cl. 

      • chain propagations:

        • Cl. + C6H5CH3 ==> HCl + C6H5CH2.

          • then C6H5CH2. + Cl-Cl ==> C6H5CH2Cl + Cl. 

      • chain terminations:

        • 2C6H5CH2. ==> C6H5CH2CH2C6H5 or 2Cl. ==> Cl2 or C6H5CH2. + Cl. ==> C6H5CH2Cl

 

 


 

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

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

  • C6H6 + R3C-Cl ==> C6H5-CR3 + HCl (R = H, alkyl, aryl)   [see mechanism 23 below]

  • The arene is refluxed with a chloroalkane and anhydrous aluminium chloride catalyst.

organic reaction mechanisms

mechanism 23 - electrophilic substitution by an alkyl group in the benzene ring

  • [mechanism 23 above] If R' = H, benzene would form methylbenzene if chloromethane was used.

    • Step (1) The weakly polar and uncharged halogenoalkane molecule is not a strong enough an electrophile to disrupt the pi electron system of the benzene ring. The aluminium chloride reacts with the halogenoalkane molecule to form a carbocation which is a much stronger electron pair accepting electrophile than the original acid chloride (either this or an AlCl3-R3Cl complex - details not needed for A level).

    • Step (2) An electron pair from the delocalised electrons of the benzene ring forms a C-C bond with the electron pair accepting carbocation 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) is a proton transfer, as the tetrachloroaluminate(III) ion [formed in step (1)], abstracts a proton from the highly unstable intermediate carbocation to give the alkyl-aromatic product, hydrogen chloride gas and reform the aluminium chloride catalyst.

  • If R' = CH3 methylbenzene: C6H5CH3 + R3C-Cl ==> R3C-C6H4CH3 + HCl

  • A mixture of polysubstituted alkyl aromatic compounds are formed.

    • e.g. using chloromethane, 1,2- or 1,3- or 1,4-dimethylbenzene will be formed,

      • so if R=H, the mechanism above would show the formation of 1,2-dimethylbenzene.

  • FURTHER COMMENTS

    • The overall alkylation reaction is the substitution of -H by -CR3 

    • Bromoalkanes can also be used for alkylation, but more expensive. Similar catalysts can be used e.g. anhydrous AlBr3 or FeBr3.

 

 


 

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

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

  • for R = H, benzene: C6H6 + R'COCl ==> C6H5COR' + HCl   [see mechanism 25 below]

  • Benzene is refluxed with an acid chloride and anhydrous aluminium chloride catalyst and a ketone is formed.

organic reaction mechanisms

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

  • [mechanism 25 above] If ethanoyl chloride, CH3COCl, was used (R=CH3-), benzene forms phenylethanone, C6H5-CO-CH3.

    • Step (1) Although the acid chloride molecule is polar, it is still not a strong enough electrophile to disrupt the pi electron system of the benzene ring. The aluminium chloride reacts with an acid chloride molecule to form a carbocation (acylonium ion, RCO+) which is a much stronger electron pair accepting electrophile than the original acid chloride (either this or an AlCl3-RCOCl complex - details not needed for A level).

    • Step (2) An electron pair from the delocalised pi electrons of the benzene ring forms a C-C bond with the electron pair accepting carbocation 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) is a proton transfer, as the tetrachloroaluminate(III) ion [formed in step (1)], abstracts a proton from the second highly unstable intermediate carbocation to give the ketone product, hydrogen chloride gas and reforming the aluminium chloride catalyst.

  • for R = CH3, methylbenzene: C6H5CH3 + R'COCl ==> R'COC6H4CH3 + HCl

    • and again there is the potential to form three position isomers by substituting in the 2, 3 or 4 position on the ring.

  • FURTHER COMMENTS

    • The overall acylation reaction is the substitution of -H by RCO

 

 


 

10.8.6 The electrophilic substitution of an arene - sulphonation mechanism

  • 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.

  • 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.

  • -


LINKS TO ASSOCIATED ADVANCED ORGANIC CHEMISTRY PAGES

All Advanced Organic Chemistry Notes

Summary of Organic Functional Groups

Quiz on Organic Structure Recognition

Summary of organic chemistry functional group tests

The shapes and bond angles of simple organic molecules

click me! An ALIPHATIC ORGANIC STRUCTURE and NOMENCLATURE BUMPER multiple choice QUIZ

click me! Type in name (short answer) BUMPER ALIPHATIC ORGANIC NOMENCLATURE QUIZ


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms electrophilic substitution nitration methylbenzene nitration benzene C6H6 + HNO3 ==> C6H5NO2 + H2O nitration of methylbenzene C7H8 C6H5CH3 + HNO3 ==> O2NC6H4CH3 + H2O 2H2SO4 + HNO3 ==> NO2+ + H3O+ + 2HSO4- C6H6 + Cl2 ==> C6H5Cl + HCl initiation: free radical substitution chlorination chain propagations: Cl. + C6H5CH3 ==> HCl + C6H5CH2. then C6H5CH2. + Cl-Cl ==> C6H5CH2Cl + Cl. Cl-Cl ==> Cl. chain terminations: 2C6H5CH2. ==> C6H5CH2CH2C6H5 or 2Cl. ==> Cl2 or C6H5CH2. + Cl. ==> C6H5CH2Cl + Cl. C6H6 + R3C-Cl ==> C6H5-CR3 + HCl R' = CH3 electrophilic substitution alkylation  methylbenzene C7H8: C6H5CH3 + R3C-Cl ==> R3C-C6H4CH3 + HCl benzene: C6H6 + R'COCl ==> C6H5COR' + HCl If ethanoyl chloride, CH3COCl, was used (R=CH3-), benzene forms phenylethanone, C6H5-CO-CH3 RCO+ R = CH3, acylation of benzene methylbenzene : C6H5CH3 + R'COCl ==> R'COC6H4CH3 + HCl 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


Revision notes for GCE Advanced Subsidiary Level AS Advanced A Level A2 IB Revise AQA GCE Chemistry OCR GCE A Level Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 A Level Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses) for revising A level chemistry courses revision guides

Website content copyright © Dr W P Brown 2000-2013 All rights reserved on revision notes, puzzles, quizzes, worksheets, x-words etc. * Copying of website material is not permitted chemhelp@tiscali.co.uk

Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

SITE HELP SEARCH - ENTER SPECIFIC WORDS/FORMULA etc.