Predicting order of nucleophilic substitution reactivity

Question

1. What order of reactivity do you predict will be observed when each alkyl halide is mixed with sodium iodide in acetone?

   • 1-Chlorobutane
   • 1-Bromobutane
   • 2-Chloro-2-methylpropane
   • Bromobenzene
   • 2-Chlorobutane
   • 1-Chloro-2-butene

I know that $\ce{I-}$ is the best leaving group, followed by $\ce{Br-}$ and $\ce{Cl-}$.
For the $\ce{NaI}$ reaction, tertiary halides should react fastest and primary halides should react slowest.

2. What order of reactivity do you predict will b observed when each alkyl halide is mixed with silver nitrate in ethanol?

   • 1-Chlorobutane
   • 1-Bromobutane
   • 2-Chloro-2-methylpropane
   • Bromobenzene
   • 2-Chlorobutane
   • 1-Chloro-2-butene

For the $\ce{AgNO3}$ reaction, primary halides should react fastest and tertiary halides should react slowest.

Which takes preference, leaving group or rank? Also, how do I rank 1-chlorobutane vs. 1-chloro-2-butene? They are both primary and the halide is $\ce{Cl-}$ for both.

Answer 1

1. Acetone a is aprotic polar solvent which is often used as solvent for $\ce{S_{N}2}$ reactions (notably in conjunction with $\ce{NaI}$ as in the Finkelstein reaction). For $\ce{S_{N}2}$ reactions steric hindrance and leaving group capability is of great importance, so primary halides will be usually easier replaced by iodine then tertiary.

2. $\ce{AgNO3}$ will probably react as a base forming nitric acid. Even though nitric acid is a rather strong acid this is more likely than $\ce{NO_3-}$ acting as nucleophile. Besides that, $\ce{Ag+}$ is known to have a unique selectivity for halogens and will attack them readily, forming $\ce{AgX}$ and a carbocation, hence promoting $\ce{E 1}$/$\ce{S_{N}1}$.