Friedel-Crafts Alkylation_default

Introduction to Electrophilic Aromatic Substitution

• Electrophilic Aromatic Substitution (EAS): A chemical reaction where an electrophile reacts with an aromatic compound, substituting one of the hydrogen atoms on the aromatic ring.

• Focus on Alkylation: A specific type of EAS which involves the attachment of an alkyl group to an aromatic ring.

• Previous examples of EAS included sulfonation (addition of sulfonyl groups) and nitration (addition of nitro groups).

Friedel Crafts Alkylation

• General method for attaching alkyl groups to aromatic rings, primarily using benzene.

• Common reagents used in this reaction include:

  • Lewis Acids: Such as AlCl3, FeCl3, or FeBr3 which act to activate the alkyl halide, making it more reactive.

  • Alkyl Halides: These are compounds with the general formula R-X, where R represents the alkyl group and X represents the halogen.

  • Alternative Method: Alcohol combined with a strong acid can generate an alkyl group suitable for the reaction.

Mechanism of Alkylation

• Electrophile Generation: The process involves the formation of a carbocation, a positively charged species that is often unstable in solution but useful for visualization of alkylation steps.

• In this reaction, electrophiles are the species that accept electron pairs, while the aromatic pi bonds act as the source of electrons, functioning as nucleophiles.

Example Reactions

• Stability of tertiary carbocations makes them more likely to exist in solution for the reaction.

• Rearrangement Fear: Carbocations, especially the primary types, are prone to rearrangement into more stable forms, resulting in different reaction products.

• Example illustrates that adding an additional carbon atom can enhance the stability of the carbocation formed.

Carbocation Stability and Rearrangements

• Stability Order: The stability of carbocations follows the hierarchy: Tertiary > Secondary > Primary. More stable carbocations are less likely to undergo further rearrangements.

• Rearrangement Mechanism: An example includes a 1,2-hydride shift, where a hydrogen atom moves from one carbon to a neighboring one, transforming a primary carbocation into a more stable secondary one, leading to outcomes with both major and minor products.

Limitations of Alkylation Reactions

• Carbocation Rearrangements: Essential to consider during alkylation since rearrangements can lead to less stable intermediates or undesired products.

• Alkyl Halide Structure: The halide must be attached to an sp³ hybridized carbon to ensure the reaction proceeds effectively.

• Polyalkylation: The possibility of multiple alkyl groups attaching presents complexity and increases the potential for a mixture of products.

• Substituted Benzene Limitations: The presence of substituents on aromatic rings can deactivate them, changing their reactivity, which differs case by case; a detailed discussion on deactivation will take place in future learning materials.