2.4 Resonance | Organic Chemistry

The video begins by introducing the topic of resonance, a concept that deals with the delocalization of electrons, typically π electrons, within molecules. It emphasizes the importance of resonance in organic chemistry and mentions that it is a concept carried over from general chemistry. The instructor discusses the nitrate ion as an example, demonstrating how resonance can be represented by multiple equivalent structures that contribute to the overall molecular structure, known as the resonance hybrid. The summary also clarifies that resonance structures are not real but are conceptual tools to help understand the true structure of a molecule.

This paragraph focuses on the process of drawing resonance structures for complex organic molecules. It outlines the rules for moving electrons to create resonant structures: non-bonding electrons and π electrons can be moved, with specific restrictions on their movement. The paragraph also explains the use of arrows to depict the movement of electrons between structures. It further discusses the concept of resonance stabilization and its impact on the stability of molecules, noting that the more resonance structures a molecule has, the more stable it tends to be.

The video introduces the terms 'allylic' and 'benzylic,' which describe the position of a carbon atom in relation to a double bond or a benzene ring, respectively. It explains that when a carbocation, lone pair, or radical is in an allylic or benzylic position, resonance can occur with the π electrons of the adjacent double bond or benzene ring. The paragraph also generalizes that any non-bonding electrons one bond away from π electrons will undergo resonance stabilization.

The paragraph delves into the specifics of drawing resonance structures for different classes of compounds, including carbocations, lone pairs (carbanions), and radicals. It highlights that the approach for drawing resonance structures and the movement of electrons (arrow pushing) varies among these classes. The video provides a detailed example of stabilizing a carbocation through resonance by moving π electrons to form additional bonds, thereby reducing the positive charge and increasing stability.

This section discusses the stability trends for carbocations, carbanions, and radicals within the context of resonance structures. It explains that carbocations are more stable when they are tertiary rather than secondary or primary, due to the electron-donating effect of carbon atoms. Conversely, carbanions are more stable as primary, secondary, or tertiary in the reverse order, as additional carbon atoms increase electron density, which destabilizes carbanions. Radicals follow a similar stability trend to carbocations, with tertiary radicals being more stable than secondary or primary ones. The paragraph also explains how these stability trends affect the contribution of different resonance structures to the overall resonance hybrid.

The video continues to explore the concept of resonance by illustrating the sequential nature of drawing resonance structures. It emphasizes the importance of considering all possible resonance structures to understand the delocalization of electrons and the resulting stabilization. The paragraph also touches on the idea that resonance structures can involve the conversion between lone pairs and π bonds, and the movement of electrons must follow specific rules. It concludes with an example of stabilizing a lone pair through resonance by transforming it into a π bond, and vice versa.

The final paragraph addresses the connection between resonance and hybridization, highlighting that determining the hybridization of an atom based on a single resonance structure can be misleading. It explains that the true structure of a molecule is an average of all its resonance structures, known as the resonance hybrid. The video clarifies that an atom involved in pi bonding must be sp2 hybridized, as sp3 hybridized atoms cannot form pi bonds due to the lack of unhybridized p orbitals. It concludes by stressing the importance of considering the resonance hybrid when determining hybridization, especially in the presence of possible resonance.