Orbital Diagram:3-atoms- Allylic Ions: Videos & Practice Problems

In understanding molecular orbital diagrams for three-atom conjugated systems, it is essential to recognize the significance of the allylic position. The allylic position refers to the site adjacent to a double bond, which plays a crucial role in resonance. In a three-atom system, the presence of a double bond typically leads to an allylic position that can resonate, allowing electrons to shift between different atoms. This resonance can be effectively explained through molecular orbital theory.

When analyzing a propanyl ion, for instance, we consider three atomic orbitals, labeled A, B, and C. The double bond contributes two pi electrons, which occupy the most stable bonding molecular orbital, denoted as ψ₁. Any additional electrons, whether they are from an empty orbital, a radical, or a lone pair, will occupy the next available molecular orbital, ψ₂. Notably, ψ₂ contains a node at atom B, indicating that no electrons can pass through this atom. Consequently, this means that the allylic position cannot react at atom B, but can only react at atoms A or C.

This molecular orbital framework provides a deeper understanding of why resonance structures behave as they do. For example, while an allylic ion can resonate to the other side of the double bond, it cannot transition to the middle atom (B) because there are no electrons present in that orbital. Thus, the molecular orbital theory not only clarifies the behavior of allylic positions but also reinforces the principles of resonance that have been previously established.

Understanding the reactivity of radicals can be effectively approached through both resonance theory and molecular orbital (MO) theory. Radicals, which are species with unpaired electrons, can exhibit resonance, allowing them to stabilize and delocalize their unpaired electrons across multiple atoms. In resonance theory, we utilize fish hook arrows to illustrate the movement of these unpaired electrons. For a given radical, we can draw resonance structures that show how the radical can shift its position, typically resulting in new double bonds and new radical sites. This process indicates that the reactive sites of the radical are located at specific positions within the conjugated system, often denoted as positions A and C.

In resonance theory, we conclude that the radical character is predicted at these positions, meaning they are likely to participate in chemical reactions. When we analyze the radical using MO theory, we begin by identifying the number of atomic orbitals involved. For a conjugated system with three atoms, we have three energy states, which we label as Ψ₁, Ψ₂, and Ψ₃. The total number of electrons in this system includes one radical electron and two pi electrons from the double bonds, leading to a total of three electrons.

Constructing the MO diagram involves determining the phases of the orbitals and their nodes. The first orbital has no nodes, the second has one node, and the third has two nodes. According to the Aufbau principle, we fill the orbitals starting from the lowest energy level, adhering to the Pauli exclusion principle, which states that no two electrons can occupy the same quantum state. This filling results in a situation where the radical can only react at positions A and C, as position B is a node and cannot accommodate any electrons.

Thus, MO theory reinforces the findings from resonance theory, confirming that the radical's reactivity is limited to positions A and C, while position B is non-reactive due to the absence of electrons. This dual approach highlights the importance of both resonance and molecular orbital theory in predicting the behavior of radicals in chemical reactions, with MO theory providing a more comprehensive understanding of the underlying principles governing reactivity.