Revising organic chemistry  Doc Brown's  Summary of organic reaction mechanisms

Part IV Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions

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.

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

• MECHANISM Introduction and terms used in organic chemistry

•  MECHANISM INDEX

• Part Ia ALKANES - introduction

  • Free radical chlorination/bromination to give halogenoalkanes (haloalkanes, alkyl halides)

  • Free radical thermal cracking to give shorter alkanes and alkenes

  • Ionic catalytic cracking to give shorter alkanes and alkenes

• Part Ib ALKENES - introduction

  • Electrophilic addition of hydrogen bromide [HBr_{(conc. aq)} and HBr_{(g/non-polar solvent)}] to form halogenoalkanes

  • Electrophilic addition of bromine ...

    • with pure bromine or in non-polar solvent (non-aqueous Br_2(l/solvent)) to give dibromoalkanes

    • or using bromine water [aqueous Br_2(aq)] to give bromo-alcohols

  • Electrophilic addition of sulphuric acid

  • Electrophilic addition of water [acid catalyst] to form alcohols

  • Free radical polymerisation to give poly(alkene) polymers

  • Hydrogenation to give saturated alkanes (in kinetics notes)

• Part II HALOGENOALKANES - introduction (haloalkanes, alkyl halides)

  • Nucleophilic substitution by water/hydroxide ion [S_N1 or S_N2, hydrolysis to give alcohols]

    • with extra notes on kinetics, rds, molecularity, activated complex etc.

  • Nucleophilic substitution by cyanide ion to give a nitrile [S_N1 or S_N2]

  • Nucleophilic substitution by ammonia/primary amine to give primary/secondary amines etc. [S_N1 or S_N2]

  • Elimination of hydrogen bromide to form alkenes [E1 and E2]

• Part III ALCOHOLS - introduction

  • Conversion of an alcohol to a halogenoalkane

  • Elimination of water from an alcohol to give an alkene [acid catalysed, E1 and E2]

• and Carbonyl compounds - ALDEHYDES and KETONES

  • Nucleophilic addition of hydrogen cyanide to form a hydroxy-nitrile

  • Addition of hydrogen - reduction with LiAlH₄ or NaBH₄ to give alcohols

  • The iodination of ketones e.g. a 2-one like propanone (a methyl ketone) to give iodo-ketones

• and ACID CHLORIDES - introduction - all nucleophilic addition-elimination reactions

  • Hydrolysis with water to give a carboxylic acid

  • Esterification with alcohols to give an ester

  • Amide formation from reaction with ammonia or primary amines

• Part IV AROMATIC HYDROCARBONS - introduction to arene electrophilic substitutions [this page]

  • 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)

• EMAIL query?comments

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.

• 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.

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 

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

• 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.

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

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

• 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.

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.

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

• 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.

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, benzene: 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

• 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.

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, benzene: 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

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.

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