The Friedel-Crafts alkylation reaction uses a Lewis acid catalyst to attach an alkyl group to an electron-rich benzene ring or derivative. First, a carbocation electrophile is generated through Lewis acid-assisted ionization of an alkyl halide. The aromatic ring then attacks the carbocation, forming a carbon-carbon bond and a new carbocation intermediate. A final deprotonation reestablishes the aromaticity of the ring and regenerates the Lewis acid.

• Comparative studies using a series of benzyl halides have qualitatively grouped a variety of Lewis acids:(i) Very active: AlCl₃, AlBr₃, GaCl₃, GaCl₂, SbF₅, MoCl₅ (ii) Moderately active: InCl₃, LnBr₃, SbCl₅, FeCl₃, AlCl₃-CH₃NO₂, SbF₅-CH₃NO₂ (iii) Mild: BCl₃, SnCl₄, TiCl₄, TiBr₄, FeCl₂. ^[1]
• Other efficient catalysts include acidic oxides and sulfides, Brӧnsted and Lewis superacids, modified zeolites, acidic cation-exchange resins, supported metal halides, etc. ^[2]
• Proper catalyst choice can minimize possible product equilibrations that arise from reversibly formed carbocations. ^[3] Note also that harsh catalysts and alkylating agents may degrade the substrate. ^[2]
• Alkyl halide reactivity is based on both the size of the halide and the degree of branching of the alkyl group. The smaller the halide and the greater the degree of branching, the more reactive. ^[2]
• Aryl halides and vinylic halides do not react. ^[4]
• This reaction may be difficult to control as 1° and 2 °alkyl groups tend to rearrange to give a mixture of products. ^[2] Moreover, polyalkylation after the first alkyl group is introduced may be a problem ^[2] that can be minimized by using the aromatic reactant in excess. ^[3]
• Aromatic rings with electron-withdrawing substituents or amino groups will not react. ^[2,4]
  
• When the alkylating agent is an alkene or an alkyne, a cocatalyst that releases protons (such as water, an alcohol, or a protic acid) is also required for the reaction to occur. ^[2]

• Reagents: Lewis Acid
• Reactant: Aromatic or Aliphatic Substrate, Alkylating Agent (Alkyl Halide, Alkene, Alkyne, Alcohol, etc.)
• Product: Substituted Aromatic Compound
• Type of Reaction: Electrophilic Aromatic Substitution

Mechanism

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Related Compounds