Revising Advanced Organic Chemistry  Doc Brown's GCE 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

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Part 10.8 Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions - CHLORINATION & BROMINATION

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

Chlorination/bromination to aromatics like chlorobenzene or bromobenzene.

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.

Part 10.8 AROMATIC HYDROCARBONS (Arenes)

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.

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

Organic synthesis of chloro/bromo aromatic compounds by reaction of chlorine/bromine with benzene/methylbenzene

• Examples of aromatic chlorination/bromination substitution reactions

  • (i) + Cl₂ ==> + HCl

    • benzene + chlorine ==> chlorobenzene + hydrogen chloride

  • (ii) + Cl₂ ==> + HCl

    • methylbenzene + chlorine ==> chloro-2/3/4-methylbenzene + hydrogen chloride

    • Three positional structural isomers of C₇H₇Cl formed in different proportions.

    • -

  • (iii) + Cl₂ ==> + HCl

    • chlorobenzene + chlorine ==> 1,2- or 1,3- or 1,4-dichlorobenzene + hydrogen chloride

    • Three positional structural isomers of  C₆H₄Cl₂ formed in different proportions.

  • (iv) To add bromine examples, BUT they are just the same as above with Br instead of Cl

• What is the mechanism for chlorinating/bromination benzene? or chlorinating/brominating methyl benzene? via ring substitution reactions.

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

TO ADD BROMINATION

• 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.

    • AND you can write out the mechanism in exactly the same way, but putting in Br instead of Cl.

  • 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 π 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

10.8.7 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 methylbenzene benzene C6H6 + Cl2 ==> C6H5Cl + HCl initiation: free radical substitution chlorination chain propagations: Cl. + C6H5CH3 ==> HCl + C6H5CH2

APPENDIX

See also INDEX of all AROMATIC COMPOUND chemistry revision notes

 COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

• PART 10 An Introduction to organic chemical reaction mechanisms and technical terms explained

• Organic Synthesis - MECHANISMS INDEX

• Part 10.2 ALKANES - introduction

to their chemistry *

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

• Free radical mechanism for cracking hydrocarbons to give shorter alkanes and alkenes

• Ionic mechanism for cracking hydrocarbons

• Part 10.3 ALKENES - introduction to their chemistry

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

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

  • Free radical polymerisation to give poly(alkene) polymers e.g. ethene ==> poly(ethene)

  • Hydrogenation to give saturated alkanes

• Part 10.4 HALOGENOALKANES - introduction to the chemistry of 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, rate expression, 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 10.5 ALCOHOLS - introduction to their chemistry

  • Conversion of an alcohol to a halogenoalkane

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

• Part 10.6 Carbonyl compounds - ALDEHYDES and KETONES - introduction to their chemistry

  • 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

• Part 10.7 Carboxylic Acids and ACID CHLORIDES - introduction - all nucleophilic addition-elimination reactions

  • Hydrolysis of acid chlorides with water to give a carboxylic acid

  • Esterification of acid chlorides with alcohols to give an ester

  • Amide formation from reaction of acid chlorides with ammonia or primary amines

• 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 (2,4,6 or 3,5 positions, ortho, para & meta substitution products)

• Other important associated links

  • Isomerism - chain isomerism, positional isomerism, functional group isomerism & Tautomerism
  • Stereoisomerism introduction, E/Z isomerism ('ex' Geometric/Geometrical cis/trans Isomerism)
  • R/S Optical Isomerism and chiral auxiliary synthesis
  • 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
  • Aliphatic structure & nomenclature m/c BUMPER QUIZ
  • Type in name aliphatic structure and nomenclature BUMPER QUIZ

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