11.2 Common Patterns in Organic Synthesis Involving Alkenes | Retrosynthesis | Organic Chemistry

The video begins by introducing common patterns in organic synthesis, which is the main topic of the lesson. It references reactions such as SN1, SN2, E1, E2, alkene, alkyne, free radical halogenation, and others that the viewer is expected to be familiar with. The focus is on functional group conversions and the patterns that emerge in synthesis questions. The lesson is part of a series released during the 2020-21 school year, and the presenter encourages viewers to subscribe to the channel for updates. The process of retrosynthesis is introduced, where reactions are often worked backward from the final product to determine the steps needed to synthesize it. The first functional group conversion discussed is the transformation of an alkane into an alkyl halide, typically through bromination.

The paragraph delves into the possibilities available once an alkyl halide is synthesized. It outlines two main categories of reactions that can be performed with alkyl halides: substitution (SN2) and elimination (E2). The video explains the conditions required for each reaction, such as the need for a strong nucleophile for SN2 and a strong, bulky base for E2 to ensure the elimination pathway over substitution. The use of NBS is also mentioned for allylic bromination, which can lead to further synthetic possibilities. The paragraph concludes with a discussion on the various reactions that can be performed on alkenes, such as converting them into alcohols, alkyl halides, ethers, and more, and introduces the concept of retrosynthesis by working backward from an alkyl halide to a complex nitrile and alkene compound.

This section of the script focuses on the strategic aspects of synthesis, particularly the decision-making process when choosing between Zaitsev and Hoffman (anti-Zaitsev) products. It discusses the use of standard versus bulky bases in E2 elimination reactions and the impact on the substitution pattern of the resulting alkene. The paragraph also emphasizes the importance of selecting the shortest and most efficient synthesis pathway to maximize yield, as each step in a synthesis involves purification and potential product loss. An example synthesis is presented, starting from an alkyl halide and aiming to produce a compound that includes a nitrile and an alkene. The presenter illustrates the thought process of retrosynthesis, considering the functional group requirements and the most plausible reaction sequences to achieve the target molecule.

The paragraph discusses the starting point of many synthetic pathways: the alkane. It notes that the only feasible reaction with an alkane is free radical halogenation, which is typically carried out with Br2 and light or NBS. The focus is on the formation of an alkyl halide from a tertiary carbon, which is a key intermediate in synthesis. The video then explores the potential reactions available from an alkyl halide, including E2 elimination to form an alkene. It also touches on the Zaitsev's rule and the use of non-bulky bases to form more substituted, stable alkenes. The presenter contrasts this with the use of a bulky base like potassium t-butoxide to form the less substituted, or Hoffman, alkene. The paragraph concludes with a brief mention of the various alkene reactions that can be performed once an alkene is formed.

The final paragraph of the script emphasizes the concept of retrosynthesis, where the synthesis process is worked out backwards from the target molecule to identify the necessary precursors and reactions. It discusses the challenges of introducing a halogen to a primary carbon and the most viable methods to achieve this, such as anti-Markovnikov addition to an alkene using HBr and a peroxide. The paragraph also highlights the importance of considering the final product's functional groups when planning the synthesis route. An example synthesis is outlined, starting from an alkane and progressing through a series of logical steps to form the desired alkyl halide. The video concludes with a prompt for viewers to like, share, and explore additional study materials on the presenter's premium course.