Leaving Group Ability of Halogens in Organic Chemistry

When discussing leaving group ability for halogens in chemical reactions, it is important to understand the relationship between leaving group propensity and basicity. Halogens serve as leaving groups in various organic reactions, with their ability influenced by factors such as bond length, electronegativity, and bond dissociation energy.

The trend in basicity for the halogens is fluoride (F-) > chloride (Cl-) > bromide (Br-) > iodide (I-), indicating that iodide is the weakest base and fluoride is the strongest base among them.

As a weaker base is typically a better leaving group, the correct order for leaving group ability, from fastest to slowest, for the halogens is iodide (-I) > bromide (-Br) > chloride (-Cl) > fluoride (-F). Fluoride is generally considered a poor leaving group due to its high basicity and strong bond strength, making it less likely to dissociate from the carbon atom in a reaction.

Furthermore, the bond dissociation energy decreases from F-C to I-C, making a carbon-iodine bond the weakest and therefore more reactive. Therefore, the fastest to the slowest halogen leaving groups are in the order: iodine (-I), bromine (-Br), chlorine (-Cl), and fluorine (-F) is typically not considered a good leaving group at all.