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08
Feb
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.
- Reagents: Lewis Acid, Phenol alkylation
- Reactant: Aromatic or Aliphatic Substrate, Alkylating Agent (Alkyl Halide, Alkene, Alkyne, Alcohol, etc.)
- Product: Substituted Aromatic Compound
- Type of Reaction: Electrophilic Aromatic Substitution
Lab Tips
- 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][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][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]
2. Kürti, L., Czakó, B. (2005). Strategic Applications of Named Reactions in Organic Synthesis; Background and Detailed Mechanisms. Burlington, MA: Elsevier Academic Press.
3. Carey, F.A., Sundberg, R.J. (2001). Advanced Organic Chemistry Part B: Reactions and Synthesis (4th ed.). New York: Kluwer Academic/Plenum Publishers.
4. McMurry, J. (2000). Organic Chemistry (5th ed.). Pacific Grove, CA: Brooks/Cole.
Mechanism
Original Paper
Top Citations
Related Reactions
- Friedel-Crafts Acylation
Related Compounds
- Aluminum Chloride (CAS7446-70-0)
- Lewis Acid
- Alkyl halide
