16.6 Cycloaddition Reactions | Organic Chemistry

The script begins by introducing the topic of cycloaddtion reactions, following a previous lesson on Diels-Alder reactions. It sets the stage for a deeper exploration of these reactions, mentioning thermal versus photochemical conditions, concepts of suprafacial and antarafacial interactions, as well as the significance of symmetry in reactions. The lecturer also introduces the concept of frontier molecular orbital theory, also known as the Woodward-Hoffmann rules, and warns students of the complexity of the topic. The importance of understanding the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in predicting reaction outcomes is emphasized, along with the concept of symmetry-allowed and symmetry-forbidden reactions.

This paragraph delves into the specifics of how reactions proceed under thermal and photochemical conditions. It contrasts the Diels-Alder reaction, which occurs under thermal conditions, with photochemical reactions that require light energy. The concept of symmetry is revisited, with a focus on how electron promotion under photochemical conditions alters the symmetry of the molecular orbitals, leading to different allowed transition states. The paragraph explains how these changes affect the reaction mechanism and the resulting stereochemistry, highlighting the differences between suprafacial-suprafacial and suprafacial-antarafacial transition states.

The focus shifts to the stereochemical outcomes of cycloaddtion reactions, particularly in the context of the Diels-Alder reaction and its implications for product formation. The paragraph explains how stereochemistry can predict the cis or trans relationships of substituents in the product, based on whether the reaction is symmetry-allowed or forbidden. It also discusses how photochemical conditions can lead to different stereoisomers, with a detailed look at how the stereochemistry changes when there is a mismatch in symmetry between the reactants' molecular orbitals.

The script touches on the broader concept of frontier molecular orbital theory, which encompasses the Woodward Hoffmann rules. It explains how the number of pi electrons (4n + 2 or 4n) influences the symmetry of the HOMO and LUMO, and consequently, the allowedness of reactions under different conditions. The paragraph also discusses the generalization that reactions with an odd number of pi electron pairs (4n + 2) tend to have matching symmetries, leading to symmetry-allowed suprafacial-suprafacial reactions under thermal conditions. Conversely, reactions with an even number of pi electron pairs (4n) have mismatched symmetries, leading to symmetry-allowed suprafacial-antarafacial reactions under thermal conditions.

The script provides an example of a six plus two cycloaddition, involving 1,3,5-hexatriene and ethylene, which has six pi electrons. It explains that in this case, the symmetry of the HOMO and LUMO does not match, leading to a symmetry-allowed suprafacial-antarafacial transition state. The paragraph also discusses the implications of photochemical conditions, where electron promotion results in both orbitals being anti-symmetric, thus allowing a suprafacial-suprafacial transition state. The limitations of geometry in smaller molecules, such as ethylene, which prevents antarafacial interactions, are also covered.

The final paragraph explores the two plus two cycloaddition, where two ethylene molecules react to form cyclobutane. It explains that under thermal conditions, the reaction is geometry forbidden due to the impossibility of an antarafacial interaction with the small ethylene molecule. However, under photochemical conditions, an electron promotion allows for a symmetry-allowed suprafacial-suprafacial transition state, leading to the formation of cyclobutane. The paragraph concludes by emphasizing the importance of understanding these concepts to predict products and transition states, and the potential for these concepts to be used in reverse to deduce the conditions under which a given product was formed.

The script concludes with a summary of the key points covered in the lesson and encourages students to practice applying these concepts. It also provides information on where to find additional study resources, such as a study guide, practice problems, and rapid review for organic chemistry final exams, directing students to the instructor's premium course on Chatsprep.com.