# Predicting order of nucleophilic substitution reactivity

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.

1. 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.

• possible duplicate - chemistry.stackexchange.com/questions/27734/… Mar 24, 2015 at 8:15
• Question one is basically which one does Sn1 fastest (aprotic solvent and strong nucleophile). So in question 1 you're looking for the least substituted species with the best leaving group. Question 2 is basically which one does Sn2 fastest (protic solvent and weak nucleophile). More subbed and better LG. Mar 24, 2015 at 8:41
• For the Chlorobutane/ene situation think about resonance stabilization in the intermediate/transition state. Mar 24, 2015 at 13:20

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 $\ce{E 1}$/$\ce{S_{N}1}$.