18.2 Friedel Crafts Alkylation and Acylation | Organic Chemistry

The video begins with an introduction to Friedel-Crafts alkylation and acylation, including a brief mention of the Gatterman-Koch synthesis. The speaker outlines the weekly release schedule of the organic chemistry lessons and encourages viewers to subscribe for updates. The process of Friedel-Crafts alkylation is explained, starting with the formation of an electrophile, which is a carbocation in this case, using a Lewis acid catalyst and chlorine. The reaction mechanism involves benzene acting as a nucleophile, attacking the carbocation to form a sigma complex, followed by the restoration of aromaticity with the help of AlCl4. The video also highlights three main problems associated with Friedel-Crafts alkylation: polyalkylation, carbocation rearrangements, and the inability of strongly deactivated benzene rings to undergo the reaction.

The second paragraph delves into the problems associated with Friedel-Crafts alkylation in more detail. It discusses how using an excess of benzene can help mitigate polyalkylation issues, though it requires additional purification steps. The paragraph also addresses the issue of carbocation rearrangements, explaining that not all alkyl halides can yield good products due to these rearrangements. Furthermore, it emphasizes that strongly deactivated benzene rings, such as those with nitro groups, will not participate in Friedel-Crafts alkylation. However, the paragraph transitions into how Friedel-Crafts acylation can solve two of these problems: it prevents multiple reactions due to the deactivating nature of the acyl group and avoids rearrangements because the carbocation intermediate is resonance-stabilized.

The third paragraph focuses on the mechanism of Friedel-Crafts acylation, contrasting it with alkylation. It describes the use of an acid halide or acyl halide instead of an alkyl halide and the formation of a carbocation. The carbocation formed is a resonance-stabilized cation, known as the acylium ion, which is the electrophilic species in the reaction. The acylium ion reacts with the benzene ring, and the mechanism follows the typical Friedel-Crafts acylation pattern, resulting in the formation of a product that is less reactive than the original reactant, thus avoiding polyacylation. The paragraph also reiterates that strongly deactivated benzene rings will not undergo Friedel-Crafts acylation.

The final paragraph introduces the Gatterman-Koch synthesis, which is a special case of Friedel-Crafts acylation used to form an aldehyde group. The synthesis involves the reaction of carbon monoxide and hydrochloric acid to form a reactive species that can then react with a Lewis acid catalyst. The mechanism of this reaction mirrors that of Friedel-Crafts acylation, but it uses carbon monoxide and HCl as reagents instead of an acyl chloride. The paragraph concludes with an overview of the mechanisms of major EAS reactions and a teaser for the next lesson, which will cover activating and deactivating groups, as well as ortho, para, and meta directors. The speaker also encourages viewers to like, share, and check out additional resources on their website.