Bad leaving groups are typically strong bases that are unstable as independent ions, making their departure from a molecule energetically unfavorable.
Understanding Bad Leaving Groups
The effectiveness of a leaving group in a chemical reaction, such as substitution or elimination, is primarily determined by its ability to stabilize the negative charge it acquires upon departure. Good leaving groups are weak bases and can disperse this charge effectively, often due to resonance, electronegativity, or size. Conversely, bad leaving groups are strong bases, meaning they are eager to share their electrons and are highly reactive as free ions, thus making it difficult for them to leave a molecule.
Here's a breakdown of common leaving groups by their quality:
| Leaving Group Quality | Examples |
|---|---|
| Excellent | TsO⁻, NH₃ |
| Good | Br⁻ |
| Fair | Cl⁻ |
| Poor | F⁻ |
| Very Poor | HO⁻, NH₂⁻, RO⁻ |
Characteristics of Poor and Very Poor Leaving Groups
Poor Leaving Groups:
- Fluoride (F⁻): Among the halogens, fluoride is the smallest and most electronegative. While electronegative, its small size concentrates the negative charge, making it a stronger base than other halides like chloride (Cl⁻) or bromide (Br⁻). This renders F⁻ a poor leaving group compared to its heavier counterparts.
Very Poor Leaving Groups:
These groups are characterized by being exceptionally strong bases, making them highly reluctant to depart from a molecule. Their departure would lead to the formation of a highly unstable and reactive anion.
- Hydroxide (HO⁻): This is a very strong base. It is commonly found in alcohols and water, and its tendency to remain bonded makes it a poor candidate for leaving a carbon atom.
- Amide (NH₂⁻): Amide is an extremely strong base, much stronger than hydroxide. This is why ammonia (NH₃) is often protonated to form NH₄⁺ to be a better leaving group (as NH₃).
- Alkoxide (RO⁻): Similar to hydroxide, alkoxide ions (e.g., methoxide CH₃O⁻, ethoxide C₂H₅O⁻) are derived from alcohols and are very strong bases. Their instability as free anions means they are very poor leaving groups.
In summary, for a group to be a good leaving group, it must be able to exist as a stable, relatively unreactive species once it departs. Strong bases fail this criterion, making them bad leaving groups.
