Chem 51A 12/02/09 Ch. 8. Regiochemical and Stereochemical Course of E2 Reactions

This paragraph delves into the stereochemical aspects of elimination reactions, specifically focusing on the E2 mechanism. It uses the example of butane reacting with sodium ethoxide to form different stereoisomers of butene. The explanation covers how strong bases favor elimination over substitution, leading to major products such as trans-2-butene and minor products like cis-2-butene. The paragraph emphasizes the importance of understanding the reaction mechanism and the role of beta carbon in the formation of these isomers.

The second paragraph discusses the concept of transition states in elimination reactions and introduces Zaitsev's Rule, which states that more substituted alkene products are formed preferentially due to their thermodynamic stability. The explanation includes the impact of the transition state's energy on the reaction's outcome, illustrating how the stability of the product influences the reaction pathway and the ease of overcoming the energy barrier.

This paragraph explores energy diagrams to represent the progression from reactants to products in elimination reactions. It introduces the Hammond Postulate, which relates the energy of the transition state to the stability of the product. The discussion uses diagrams to show how more stable products have lower energy barriers, leading to a greater preference for their formation in reactions.

The focus of this paragraph is on the stereoisomers of alkenes, their nomenclature, and the rules for determining priority substituents. It explains the difference between cis (Z) and trans (E) isomers, using the example of 2-butene. The explanation includes how the arrangement of substituents on either side of the double bond influences the isomer's identity and how to denote this using the cis-trans or Z-E system.

This paragraph discusses the rigid nature of alkenes due to the presence of a carbon-carbon double bond. It contrasts the restricted rotation in double bonds with the free rotation around single bonds in alkanes. The explanation highlights the significant energy difference required to break the pi bond, thus 'locking' the conformation of alkenes.

The paragraph explains the stereochemical requirements for E2 elimination reactions, emphasizing the anti-periplanar alignment of the hydrogen and leaving group for the reaction to proceed efficiently. It uses the analogy of backside attack in SN2 reactions to illustrate the concept and discusses the implications of this requirement on the reaction's stereochemistry.

This paragraph examines the stereochemistry of E2 elimination in cyclic compounds, using the example of stereoisomers of 1-bromo-4-tert-butylcyclohexane. It illustrates how the conformation of the molecule affects the ease of elimination, with the more stable equatorial tert-butyl group influencing the reaction rate.

The paragraph explores the stereochemistry of E2 elimination in acyclic compounds, focusing on the stereoisomers of 1-bromo-1,2-diphenylmethane. It demonstrates how the stereochemistry of the starting material leads to different products, with the E2 elimination yielding either the E or Z alkene, depending on the initial stereoisomer.

The final paragraph summarizes the stereochemical outcomes of E2 elimination reactions, showing how the requirement for anti-periplanar elimination leads to the formation of either E or Z stereoisomers. It uses Newman projections to visualize the stereochemistry of the products and emphasizes the dynamic nature of acyclic compounds, which can rotate about single bonds to achieve the necessary anti-periplanar alignment.