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.

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.

See also note on orientation of substituted products from electrophilic substitution in the benzene ring.

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) + CH₃Cl ==> + HCl

    • benzene + chloromethane ==> methylbenzene + hydrogen chloride

  • (ii) + CH₃Cl ==> + HCl

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

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

  • + CH₃CH₂Br ==> + 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.

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, Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to A level chemistry course specifications are unofficial. Website content © Dr Phil Brown 2000+. All copyrights reserved on these organic chemistry exam revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, these A level chemistry revision notes are suitable for use of pre-university students studying AQA advanced level organic chemistry revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, Edexcel advanced level organic chemistry revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, OCR advanced level organic chemistry revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, IB advanced level organic chemistry revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, WJEC (Eduqas) advanced level organic chemistry revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, CIE Cambridge advanced level organic chemistry revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, CCEA advanced level organic chemistry revision notes on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, and useful for US grade 11 grade 12 AP honors organic chemistry courses involving electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction Explaining the importance of electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction in organic chemistry, What you need to know about electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction for organic chemistry, Explaining the use of electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction knowledge in organic chemistry, Examples of electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction explained when studying organic chemistry, What is the significance of electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction in organic chemistry, What is the use of electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction in organic chemistry  Describing and explaining the theory of electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction when studying organic chemistry, exam revision notes for electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction in exams, online help for electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, revision notes for electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, what do I need to learn for electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction in exams? revision summary for electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, help in teaching electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, learning notes for electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction, help to pass the electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction exam, how to prepare for examination questions on electrophilic substitution mechanism of alkylation of benzene & methylbenzene by the Friedel- Crafts reaction? Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic 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

APPENDIX

 COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)