1. Polarizability: Bromine is more polarizable than chlorine. This means that the electron cloud of bromine is more easily distorted when it is bonded to a carbon atom. This distortion weakens the carbon-bromine bond, making it more likely to break during a substitution reaction.
2. Electronegativity: Bromine is less electronegative than chlorine. This means that bromine has a lower affinity for electrons than chlorine. When a bromine atom is bonded to a carbon atom, there is less of a competition for electrons between the carbon and bromine atoms. This weakening of the bond between carbon and bromine facilitates the departure of the bromine atom during a substitution reaction.
3. Bond Dissociation Energy: The carbon-bromine bond dissociation energy (BDE) is lower than the carbon-chlorine BDE. This means that it requires less energy to break the carbon-bromine bond than the carbon-chlorine bond. The lower BDE of the carbon-bromine bond contributes to the greater ease of bromine as a leaving group.
4. Nucleophilicity of the Leaving Group: The bromide ion (Br-) is a better nucleophile than the chloride ion (Cl-). This means that Br- is more reactive and more likely to participate in reactions with other molecules. The higher nucleophilicity of the bromide ion facilitates its departure from the molecule during a substitution reaction.
Overall, the combination of these factors - polarizability, electronegativity, bond dissociation energy, and nucleophilicity of the leaving group - makes bromine a better leaving group than chlorine in substitution reactions.
