Organic Chemistry MCAT Practice Exam

Which of the following is a characteristic of good leaving groups?

1. Strong bases

2. Resonance stabilized

3. Negatively charged species

4. Unstable molecules

The correct answer is: Resonance stabilized

Good leaving groups are typically characterized by their ability to stabilize the negative charge that they carry when departing from a substrate. Resonance stabilization plays a critical role in this process. When a leaving group is resonance-stabilized, it can delocalize the negative charge across multiple atoms, reducing the energy and increasing the stability of the leaving group after it has departed. For instance, leaving groups like acetate (from acetic acid) are resonance-stabilized because the negative charge can be spread over the oxygen atoms. This makes it more favorable for these groups to leave compared to those that do not have such stabilizing factors. The other characteristics often found in potential leaving groups can lead to less favorable conditions. Strong bases, for example, are typically poor leaving groups because they hold onto their electrons tightly and are less likely to depart. Compared to resonance-stabilized groups, negatively charged species without resonance stabilization can also be less effective as leaving groups since localized charge can lead to instability upon departure. Unstable molecules do not typically serve well as leaving groups because their inherent instability would lead to a higher likelihood of rapid reactivity or decomposition rather than leaving cleanly. Thus, the resonance stabilization of leaving groups significantly enhances their ability to depart from a molecule, making