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SOLUTION 21.1b

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The first reaction is faster. The substrate substitution and base are the same, but iodide is the better leaving group over chloride. I- is larger and the weaker base (more stable anion). In both E1 and E2 reactions, the type of leaving group affects the activation energy.
Debatable that they are approximately the same rate. In both cases, the protic solvent is the base (both weak bases). The identity of the base is the only difference between the two reactions. Since the strength and concentration of base do not affect the rate of E1 reactions, they should proceed at the same rate. A slight difference in rate may come from the solvent effects. Water is the more polar of the two solvents, which could stabilize the carbocation enough MORE than NH3 to increase reaction rate in water.
 

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