Organic Chemistry MCAT Practice Exam

What is the effect of unhybridized p orbitals on resonance?

1. They cannot participate in resonance

2. They allow electrons to interact with adjacent atoms

3. They lower the acidity of compounds

4. They increase molecular weight

The correct answer is: They allow electrons to interact with adjacent atoms

Unhybridized p orbitals play a crucial role in resonance stabilization of molecular structures. They are involved in the delocalization of electrons, which is essential for resonance to occur. When a molecule has unhybridized p orbitals, these orbitals can overlap with p orbitals of adjacent atoms or groups. This overlap allows electrons to be shared across multiple atoms, ultimately leading to a more stable arrangement because the electron density is distributed over a larger area. This delocalization lowers the overall energy of the molecule, enhancing its stability. For example, in the case of conjugated systems or aromatic compounds, the presence of unhybridized p orbitals allows for π-bonding interactions that contribute to the resonance significance of the compound. The ability of these p orbitals to interact facilitates resonance structures, where the actual structure of the molecule is a hybrid of all possible resonance forms. Thus, they allow for increased electron mobility, which is a foundational element in understanding the behavior of many organic compounds.