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 - CHLORINATION & BROMINATION

Part 10.8 AROMATIC HYDROCARBONS (ARENES) - introduction to arene electrophilic substitutions. Chlorination/bromination to aromatics like chlorobenzene, bromobenzene etc. 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)

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.

 


 

10.8.3 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) (c) doc b + Cl2 ==> (c) doc b + HCl

      • benzene + chlorine ==> chlorobenzene + hydrogen chloride

    • (ii) (c) doc b + Cl2 ==>(c) doc b (c) doc b (c) doc b + HCl

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

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

      • -

    • (iii) (c) doc b + Cl2 ==> (c) doc b (c) doc b (c) doc b + 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.

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.

      • 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 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

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