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 - ACYLATION

Part 10.8 AROMATIC HYDROCARBONS (ARENES) - introduction to arene electrophilic substitutions. Acylation to give aromatic ketones [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.


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.


 

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

Organic synthesis of aromatic ketones by reaction of acid/acyl chlorides with benzene/methylbenzene

  • Examples of aromatic Friedel Crafts acylation substitution reactions

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

      • benzene + ethanoyl chloride ==> 1-phenylethanone + hydrogen chloride

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

      • benzene + benzoyl chloride ==> diphenylmethanone + hydrogen chloride

  • 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


 

10.8.7 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 benzene methylbenzene 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


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