What are considered as “good” leaving groups?

Good leaving groups are those that can stabilize negative charge effectively. In organic chemistry, leaving groups are typically groups or atoms that leave a molecule as a result of a chemical reaction, often taking with them a pair of electrons. Some characteristics of good leaving groups include:

Electronegativity: Leaving groups should be electronegative atoms or groups that can accommodate a negative charge well. Common leaving groups include halides (like Cl⁻, Br⁻, and I⁻), tosylates (OTs⁻), mesylates (OMs⁻), and sulfonates (OSO3R⁻).

Size: Smaller leaving groups are generally better because they can disperse negative charge more effectively. For example, iodide (I⁻) is a better leaving group than bromide (Br⁻), which is better than chloride (Cl⁻), due to the larger size of iodide.

Polarizability: Leaving groups should be polarizable, meaning they can easily accommodate charge. This is related to size; larger atoms or groups can more easily disperse negative charge.

Stability of the Leaving Group: Leaving groups should form stable ions or molecules after departure. For instance, stable anions like chloride (Cl⁻) are good leaving groups.

Solubility: Leaving groups should be able to leave the molecule in a manner that does not interfere with the solubility of the reaction components. This is especially important in polar solvents commonly used in many organic reactions.

In summary, good leaving groups are those that can stabilize negative charge effectively, typically by being electronegative, small, polarizable, stable, and compatible with the reaction conditions.