Question 3.48: Consider the activation energies for the following nucleophi......

Organic Chemistry Study Guide: Key Concepts, Problems, and Solutions [1424398]

Consider the activation energies for the following nucleophilic substitution reactions. Which reaction occurs at the faster rate?
$CH_3−I + Cl^{−} → CH_3−Cl + I^{−}$ $E_a$ = 104 kJ mole $^{−1}$
CH $_3−I + Br^{−}$ → $CH_3−Br + I^{−}$ $E_a$ = 96 kJ mole $^{−1}$

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The reaction of bromide ion with iodomethane has the lower activation energy ( $E_a$ ), so it proceeds at the faster rate.

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