When iodine replaces bromine in 2-bromobutane, does this lead to an overall lower energy level (electrons closer to nucleus) as well?

It seems to me that iodine is a nucleophile that replaces bromine because it has greater attraction to the carbon nucleus due to its electron properties. However, I thought that bonds are formed for there to be a lower overall energy level, meaning that electrons move closer to the nucleus. Since bromine has less electrons, it seems that the bond between carbon and bromine would have a lower overall energy level than the iodine carbon bond? I guess im asking if bond energy plays a role here, or if the nucleophilic attack occurs indepedent of this?




In hydroxylic solvents nucleophilicity increases as you go from F>Cl>Br>I. Fluoride is the most basic of the four and highly solvated. Iodide is the least basic, most polarizable and most nucleophilic. So bromide would rather be solvated than iodide and iodide would rather be a nucleophile. Moreover, in such reactions, an excess of nucleophile is generally employed. In polar, aprotic solvents (dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO)), the order of nucleophilicity is reversed. You cannot just consider bond strengths or heats of formation of the alkyl halides without considering the status of the halides (solvated, unsolvated). One must consider all of the reaction components.


 0