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A Level Organic Chemistry: Reaction mechanisms - aromatic electrophilic alkylation

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 - ALKYLATION

Part 10.8 AROMATIC HYDROCARBONS (ARENES) - introduction to arene electrophilic substitutions. Alkylation to give alkyl-aromatics like methylbenzene [Friedel-Crafts reaction]. 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.

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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.


 

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

Organic synthesis of alkyl substituted aromatic compounds by reaction of halogenoalkanes (haloalkanes) with benzene/methylbenzene

  • Examples of aromatic Friedel Crafts alkylation substitution reactions

    • (i) (c) doc b + CH3Cl ==> (c) doc b + HCl

      • benzene + chloromethane ==> methylbenzene + hydrogen chloride

    • (ii) (c) doc b + CH3Cl ==> (c) doc b (c) doc b (c) doc b + HCl

      • methylbenzene + chloromethane ==> 1,2- or 1,2- or 1,4dimethylbenzene + hydrogen chloride

    • Three positional structural isomers of  C8H10 formed in different proportions.

    • (c) doc b + CH3CH2Br ==> (c) doc b (c) doc b (c) doc b + HBr

    • methylbenzene + bromoethane ==> 1-ethyl-2/3/4-methylbenzene

    • Three positional structural isomers of molecular formula C9H12 formed in different proportions

  • 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 liquid chloroalkane or gaseous chloroalkane bubbled in plus 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 haloalkane (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.


10.8.7 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 + R3C-Cl ==> C6H5-CR3 + HCl R' = CH3 electrophilic substitution alkylation  methylbenzene C7H8: C6H5CH3 + R3C-Cl ==> R3C-C6H4CH3 + HCl benzene

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