SN2 Reaction: Predicting Reactivity of Five Compounds

[leopard]

Which of these five compounds will react easier after an S_N2 mechanism?

Since Cl is more electronegative than I, it is a better leaving group, so I would expect the answer to be B or D.

In B the halide is tertiary while in D it is primary, so I expect D to be the answer.

The correct answer is E. Why?

 

[chemisttree]

Because iodide is a better leaving group than chloride. You should have a ranking of rates of reactions for homologous halides in the chapter somewhere.