In a reaction that proceeds by an E2 mechanism, what effect does changing the substrate from an alkyl iodide to an alkyl bromide have on the rate of the reaction?

Question

Answer Choices:

A. Increases the rate

B. Decreases the rate

C. No effect on the rate

Answer 1

Hello Tierra

B. Decreases the rate.

In an E2 reaction, halides (Cl, Br, I) are good leaving groups. The order of their reactivity is I > Br > Cl.

As you go from I to Cl in the periodic table, the electronegativity of the halogen increases. This makes the alkyl-halide bond stronger. More energy will be required to break the alkyl-Br bond than the alkyl-I bond. Thus, it will take longer for the alkyl-Br substrate to reach the transition state resulting in a decreased rate for the overall reaction.