Friedel–Crafts reaction

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Friedel-Crafts reaction
Named after Charles Friedel
James Crafts
Reaction type Coupling reaction
RSC ontology ID RXNO:0000369

The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring.[1] There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. Both proceed by electrophilic aromatic substitution. The general reaction scheme is shown below..

Rightward reaction arrow with minor substrate(s) from top left

AlCl3 catalyst, reflux
anhydrous conditions

Benzene monosubstituted with R group

Several reviews have been written.[2][3][4][5]

Friedel–Crafts alkylation[edit]

Friedel-Crafts alkylation
Named after Charles Friedel
James Crafts
Reaction type Coupling reaction
Organic Chemistry Portal friedel-crafts-alkylation
RSC ontology ID RXNO:0000046

Friedel–Crafts alkylation involves the alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid catalyst.[6] With anhydrous ferric chloride as a catalyst, the alkyl group attaches at the former site of the chloride ion. The general mechanism is shown below.[7]

Mechanism for the Friedel Crafts alkylation

This reaction has one big disadvantage, namely that the product is more nucleophilic than the reactant due to the electron donating alkyl-chain. Therefore, another hydrogen atom is substituted with an alkyl-chain, which leads to overalkylation of the molecule. Also, if the chloride ion is not attached to a tertiary carbon atom or secondary carbon atom, then the carbocation formed (R+) will undergo a carbocation rearrangement reaction. This reactivity is due to the relative stability of the tertiary and secondary carbocation over the primary carbocations.[7]

Steric hindrance can be exploited to limit the number of alkylations, as in the t-butylation of 1,4-dimethoxybenzene.[citation needed]

t-butylation of 1,4-dimethoxybenzene

Alkylations are not limited to alkyl halides: Friedel–Crafts reactions are possible with any carbocationic intermediate such as those derived from alkenes and a protic acid, Lewis acid, enones, and epoxides. An example is the synthesis of neophyl chloride from benzene and methallyl chloride:[8]

H2C=C(CH3)CH2Cl + C6H6 → C6H5C(CH3)2CH2Cl

In one study the electrophile is a bromonium ion derived from an alkene and NBS:[9]

Friedel–Crafts alkylation by an alkene

In this reaction samarium(III) triflate is believed to activate the NBS halogen donor in halonium ion formation.

Friedel–Crafts dealkylation[edit]

Friedel–Crafts alkylation has been hypothesized to be a reversible reaction. In a reversed Friedel–Crafts reaction or Friedel–Crafts dealkylation, alkyl groups are theoretically removed in the presence of protons and a Lewis acid.[citation needed]

For example, in a multiple addition of ethyl bromide to benzene, ortho and para substitution is expected after the first monosubstitution step because an alkyl group is an activating group. However, the actual reaction product is 1,3,5-triethylbenzene with all alkyl groups as a meta substituent.[10] Thermodynamic reaction control makes sure that thermodynamically favored meta substitution with steric hindrance minimized takes prevalence over less favorable ortho and para substitution by chemical equilibration. The ultimate reaction product is thus the result of a series of alkylations and dealkylations.[citation needed]

synthesis of 2,4,6-triethylbenzene

Friedel–Crafts acylation[edit]

Friedel-Crafts acylation
Named after Charles Friedel
James Crafts
Reaction type Coupling reaction
Organic Chemistry Portal friedel-crafts-acylation
RSC ontology ID RXNO:0000045

Friedel–Crafts acylation is the acylation of aromatic rings with an acyl chloride using a strong Lewis acid catalyst. Friedel–Crafts acylation is also possible with acid anhydrides.[11] Reaction conditions are similar to the Friedel–Crafts alkylation mentioned above. This reaction has several advantages over the alkylation reaction. Due to the electron-withdrawing effect of the carbonyl group, the ketone product is always less reactive than the original molecule, so multiple acylations do not occur. Also, there are no carbocation rearrangements, as the carbonium ion is stabilized by a resonance structure in which the positive charge is on the oxygen.[citation needed]

Friedel–Crafts acylation overview

The viability of the Friedel–Crafts acylation depends on the stability of the acyl chloride reagent. Formyl chloride, for example, is too unstable to be isolated. Thus, synthesis of benzaldehyde via the Friedel–Crafts pathway requires that formyl chloride be synthesized in situ. This is accomplished via the Gattermann-Koch reaction, accomplished by treating benzene with carbon monoxide and hydrogen chloride under high pressure, catalyzed by a mixture of aluminium chloride and cuprous chloride.

Reaction mechanism[edit]

In a simple mechanistic view, the first step consists of dissociation of a chloride ion to form an acyl cation (acylium ion)[citation needed]:

FC acylation step 1

In some cases, the Lewis acid binds to the oxygen of the acyl chloride to form an adduct.[7] Regardless, the resulting acylium ion or a related adduct is subject to nucleophilic attack by the arene:

Finally, chloride anion (or AlCl4) deprotonates the ring (an arenium ion) to form HCl, and the AlCl3 catalyst is regenerated:

FC acylation step III

If desired, the resulting ketone can be subsequently reduced to the corresponding alkane substituent by either Wolff–Kishner reduction or Clemmensen reduction. The net result is the same as the Friedel–Crafts alkylation except that rearrangement is not possible.[12]

Friedel–Crafts hydroxyalkylation[edit]

Arenes react with certain aldehydes and ketones to form the hydroxyalkylated product for example in the reaction of the mesityl derivative of glyoxal with benzene[13] to form a benzoin with an alcohol rather than a carbonyl group:

Friedel–Crafts hydroxyalkylation

Friedel–Crafts sulfonylation[edit]

Under Friedel–Crafts reaction conditions, arenes react with sulfonyl halides and sulfonic acid anhydrides affording sulfones. Commonly used catalysts include AlCl3, FeCl3, GaCl3, BF3, SbCl5, BiCl3 and Bi(OTf)3, among others.[14][15] Intramolecular Friedel–Crafts cyclization occurs with 2-phenyl-1-ethanesulfonyl chloride, 3-phenyl-1-propanesulfonyl chloride and 4-phenyl-1-butanesulfonyl chloride on heating in nitrobenzene with AlCl3.[16] Sulfenyl and sulfinyl chlorides also undergo Friedel–Crafts–type reactions, affording sulfides and sulfoxides, respectively.[17] Both aryl sulfinyl chlorides and diaryl sulfoxides can be prepared from arenes through reaction with thionyl chloride in the presence of catalysts such as BiCl3, Bi(OTf)3, LiClO4 or NaClO4.[18][19]

Scope and variations[edit]

This reaction is related to several classic named reactions:

Bogert-Cook Synthesis.png

  • The Darzens–Nenitzescu synthesis of ketones (1910, 1936) involves the acylation of cyclohexene with acetyl chloride to methylcyclohexenylketone.
  • In the related Nenitzescu reductive acylation (1936) a saturated hydrocarbon is added making it a reductive acylation to methylcyclohexylketone
  • The Nencki reaction (1881) is the ring acetylation of phenols with acids in the presence of zinc chloride.[33]
  • In a green chemistry variation aluminium chloride is replaced by graphite in an alkylation of p-xylene with 2-bromobutane. This variation will not work with primary halides from which less carbocation involvement is inferred.[34]


Friedel–Crafts reactions have been used in the synthesis of several triarylmethane and xanthene dyes.[35] Examples are the synthesis of thymolphthalein (a pH indicator) from two equivalents of thymol and phthalic anhydride:

Thymolphthalein Synthesis

A reaction of phthalic anhydride with resorcinol in the presence of zinc chloride gives the fluorophore Fluorescein. Replacing resorcinol by N,N-diethylaminophenol in this reaction gives rhodamine B:

Rhodamine B synthesis

Haworth reactions[edit]

The Haworth reaction is a classic method for the synthesis of 1-tetralone.[36] In it benzene is reacted with succinic anhydride, the intermediate product is reduced and a second FC acylation takes place with addition of acid.[37]

Haworth reaction

In a related reaction, phenanthrene is synthesized from naphthalene and succinic anhydride in a series of steps.

Haworth Phenanthrene synthesis

Friedel–Crafts test for aromatic hydrocarbons[edit]

Reaction of chloroform with aromatic compounds using an aluminium chloride catalyst gives triarylmethanes, which are often brightly colored, as is the case in triarylmethane dyes. This is a bench test for aromatic compounds.[38]

See also[edit]


  1. ^ Friedel, C.; Crafts, J. M. (1877) "Sur une nouvelle méthode générale de synthèse d’hydrocarbures, d’acétones, etc.," Compt. Rend., 84: 1392 & 1450.
  2. ^ Price, C. C. (1946). "The Alkylation of Aromatic Compounds by the Friedel-Crafts Method". Org. React. 3: 1. doi:10.1002/0471264180.or003.01. ISBN 0471264180. 
  3. ^ Groves, J. K. (1972). "The Friedel–Crafts acylation of alkenes". Chem. Soc. Rev. 1: 73. doi:10.1039/cs9720100073. 
  4. ^ Eyley, S. C. (1991). "The Aliphatic Friedel–Crafts Reaction". Comp. Org. Syn. 2: 707–731. doi:10.1016/B978-0-08-052349-1.00045-7. ISBN 978-0-08-052349-1. 
  5. ^ Heaney, H. (1991). "The Bimolecular Aromatic Friedel–Crafts Reaction". Comp. Org. Syn. 2: 733–752. doi:10.1016/B978-0-08-052349-1.00046-9. ISBN 978-0-08-052349-1. 
  6. ^ Rueping, M.; Nachtsheim, B. J. (2010). "A review of new developments in the Friedel–Crafts alkylation – From green chemistry to asymmetric catalysis". Beilstein J. Org. Chem. 6 (6). doi:10.3762/bjoc.6.6. 
  7. ^ a b c Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7 
  8. ^ Smith, W. T. Jr. and Sellas, J. T. (1963). "Neophyl chloride". Organic Syntheses. 
  9. ^ Hajra, S.; Maji, B.; Bar, S. (2007). "Samarium Triflate-Catalyzed Halogen-Promoted Friedel–Crafts Alkylation with Alkenes". Org. Lett. 9 (15): 2783–2786. doi:10.1021/ol070813t. 
  10. ^ Anslyn, E.; Wallace, K. J.; Hanes, R.; Morey, J.; Kilway, K. V.; Siegel, J. (2005). "Preparation of 1,3,5-Tris(aminomethyl)-2,4,6-triethylbenzene from Two Versatile 1,3,5-Tri(halosubstituted) 2,4,6-Triethylbenzene Derivatives". Synthesis. 2005 (12): 2080–2083. doi:10.1055/s-2005-869963. 
  11. ^ Somerville, L. F.; Allen, C. F. H. (1933). "β-Benzoylpropionic acid". Organic Syntheses. 13: 12. doi:10.15227/orgsyn.013.0012. 
  12. ^ Friedel-Crafts Acylation. Retrieved on 2014-01-11.
  13. ^ Fuson, R. C.; Weinstock, H. H.; Ullyot, G. E. (1935). "A New Synthesis of Benzoins. 2′,4′,6′-Trimethylbenzoin". J. Am. Chem. Soc. 57 (10): 1803–1804. doi:10.1021/ja01313a015. 
  14. ^ Truce, W. E.; Vriesen; C. W. (1953). "Friedel—Crafts Reactions of Methanesulfonyl Chloride with Benzene and Certain Substituted Benzenes". J. Am. Chem. Soc. 75 (20): 5032–5036. doi:10.1021/ja01116a043. 
  15. ^ Répichet, S.; Le Roux, C.; Hernandez, P.; Dubac, J.; Desmurs, J. R. (1999). "Bismuth(III) Trifluoromethanesulfonate: An Efficient Catalyst for the Sulfonylation of Arenes". The Journal of Organic Chemistry. 64 (17): 6479–6482. doi:10.1021/jo9902603. 
  16. ^ Truce, W. E.; Milionis, J. P. (1952). "Friedel-Crafts Cyclization of ω-Phenylalkanesulfonyl Chlorides". J. Am. Chem. Soc. 74 (4): 974–977. doi:10.1021/ja01124a031. 
  17. ^ Fujisawa, T.; Kakutani, M.; Kobayashi, N. (1973). "On the Reaction of p-Toluenesulfinyl Chloride with Anisole". Bull. Chem. Soc. Jpn. 46 (11): 3615–3617. doi:10.1246/bcsj.46.3615. 
  18. ^ Le Roux, C.; Mazières, S. P.; Peyronneau, M.; Roques, N. (2003). "Catalytic Lewis Acid Activationof Thionyl Chloride: Application to the Synthesis of ArylSulfinyl Chlorides Catalyzed by Bismuth(III) Salts". Synlett (5): 0631–0634. doi:10.1055/s-2003-38358. 
  19. ^ Bandgar, B. P.; Makone, S. S. (2004). "Lithium/Sodium Perchlorate Catalyzed Synthesis of Symmetrical Diaryl Sulfoxides". Syn. Commun. 34 (4): 743–750. doi:10.1081/SCC-120027723. 
  20. ^ Clemmensen, E. (1913). "Reduktion von Ketonen und Aldehyden zu den entsprechenden Kohlenwasserstoffen unter Anwendung von amalgamiertem Zink und Salzsäure". Chemische Berichte. 46: 1837–1843. doi:10.1002/cber.19130460292. 
  21. ^ Clemmensen, E. (1914). "Über eine allgemeine Methode zur Reduktion der Carbonylgruppe in Aldehyden und Ketonen zur Methylengruppe". Chemische Berichte. 47: 51–63. doi:10.1002/cber.19140470108. 
  22. ^ Clemmensen, E. (1914). "Über eine allgemeine Methode zur Reduktion der Carbonylgruppe in Aldehyden und Ketonen zur Methylengruppe. (III. Mitteilung.)". Chemische Berichte. 47: 681–687. doi:10.1002/cber.191404701107. 
  23. ^ Gattermann, L.; Koch, J. A. (1897). "Eine Synthese aromatischer Aldehyde". Ber. 30: 1622–1624. doi:10.1002/cber.18970300288. 
  24. ^ L. Gattermann; W. Berchelmann (1898). "Synthese aromatischer Oxyaldehyde". Berichte der deutschen chemischen Gesellschaft. 31 (2): 1765–1769. doi:10.1002/cber.18980310281. 
  25. ^ Hoesch, Kurt (1915). "Eine neue Synthese aromatischer Ketone. I. Darstellung einiger Phenol-ketone". Berichte der deutschen chemischen Gesellschaft. 48 (1): 1122–1133. doi:10.1002/cber.191504801156. 
  26. ^ J. Houben (1926). Über die Kern-Kondensation von Phenolen und Phenol-äthern mit Nitrilen zu Phenol- und Phenol-äther-Ketimiden und -Ketonen (I.). Berichte der deutschen chemischen Gesellschaft (A and B Series) 59 (11) 2878–2891. doi:10.1002/cber.19260591135
  27. ^ M B Smith, J March. March's Advanced Organic Chemistry (Wiley, 2001) (ISBN 0-471-58589-0)
  28. ^ Grzybowski, M.; Skonieczny, K.; Butenschön, H.; Gryko, D. T. (2013). "Comparison of Oxidative Aromatic Coupling and the Scholl Reaction". Angew. Chem. Int. Ed. 52: 9900–9930. doi:10.1002/anie.201210238. 
  29. ^ Zincke, Th.; Suhl. R. (1906). "Ueber die Einwirkung von Tetrachlorkohlenstoff und Aluminiumchlorid auf p-Kresol und p-Kresolderivate". Chemische Berichte. 39 (4): 4148–4153. doi:10.1002/cber.190603904115. 
  30. ^ Blanc, Gustave Louis (1923). Bulletin de la Société chimique de France [4]. 33: 313–319.  Missing or empty |title= (help)
  31. ^ G. Grassi and C. Maselli (1898) "Su alcuni derivati clorurati de trossimetilene" (On some chlorinated derivatives of 1,3,5-trioxane), Gazzetta Chimica Italiana, 28 (part 2) : 477-500 ; see especially p. 495.
  32. ^ This reaction with phosphorus pentoxide: Kamp, J. V. D.; Mosettig, E. (1936). "Trans- and Cis-As-Octahydrophenanthrene". Journal of the American Chemical Society. 58 (6): 1062–1063. doi:10.1021/ja01297a514. 
  33. ^ Nencki, M.; Sieber, N. (1881). "Ueber die Verbindungen der ein- und zweibasischen Fettsäuren mit Phenolen". J. Prakt. Chem. (in German). 23: 147–156. doi:10.1002/prac.18810230111. 
  34. ^ Sereda, Grigoriy A.; Rajpara, Vikul B. (2007). "A Green Alternative to Aluminum Chloride Alkylation of Xylene". J. Chem. Educ. 2007 (84): 692. Bibcode:2007JChEd..84..692S. doi:10.1021/ed084p692. 
  35. ^ McCullagh, James V.; Daggett, Kelly A. (2007). "Synthesis of Triarylmethane and Xanthene Dyes Using Electrophilic Aromatic Substitution Reactions". J. Chem. Educ. 84: 1799. doi:10.1021/ed084p1799. 
  36. ^ Haworth, Robert Downs (1932). "Syntheses of alkylphenanthrenes. Part I. 1-, 2-, 3-, and 4-Methylphenanthrenes". J. Chem. Soc.: 1125. doi:10.1039/JR9320001125. 
  37. ^ Li, Jie Jack (2003) Name Reactions: A Collection of Detailed Reaction Mechanisms, Springer, ISBN 3-540-40203-9, p. 175.
  38. ^ John C. Gilbert., Stephen F. Martin. Experimental Organic Chemistry: A Miniscale and Microscale Approach[permanent dead link]. Brooks/Cole CENGAGE Learning, 2011. pp 872. 25.10 Aromatic Hydrocarbons and Aryl Halides - Classification test. ISBN 978-1-4390-4914-3

Friedel–Crafts reactions published on Organic Syntheses[edit]