7. Substitution Reactions

Show how you might use S_N2 reactions to convert 1-chlorobutane into the following compounds.

c. 1-iodobutane

d. CH₃—(CH₂)₃—CN

For each pair, choose the nucleophile that would react most quickly in an S_N2 reaction (assume H₂O is the solvent).

(c)

Rank the following compounds in decreasing order of their reactivity toward the S_N2 reaction with sodium ethoxide (Na^{+ –}OCH₂CH₃) in ethanol.

methyl chloride

tert-butyl iodide 

neopentyl bromide

isopropyl bromide

methyl iodide

ethyl chloride

Triethyloxonium tetrafluoroborate, (CH₃CH₂)₃O⁺ BF₄^–, is a solid with melting point 91–92°C. Show how this reagent can transfer an ethyl group to a nucleophile (Nuc:⁻) in an S_N2 reaction. What is the leaving group? Why might this reagent be preferred to using an ethyl halide? (Consult Table 6-2)

Which of the following alkyl halides is most likely to undergo an $S_{N2}$ reaction rapidly?

Given the following alkyl halides: 1. $\mathrm{CH}{3}_{\mathrm{Br}}$ (methyl bromide), 2. $\mathrm{CH}{3}_{\mathrm{CH}}{2}_{\mathrm{Br}}$ (ethyl bromide), 3. $\left(\mathrm{CH}{3}_2\right)\mathrm{CHBr}$ (isopropyl bromide), 4. $\left(\mathrm{CH}{3}_3\right)\mathrm{CBr}$ (tert-butyl bromide), rank them from most to least reactive in an $S{N}_2$ reaction.

Which of the following is the major organic product formed when ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{Br}}_1$ reacts with ${\mathrm{OH}}_1-$ in an $S{N}_2$ reaction?

The rate of the reaction of methyl iodide with quinuclidine was measured in nitrobenzene, and then the rate of the reaction of methyl iodide with ­triethylamine was measured in the same solvent. The concentration of the reagents was the same in both experiments.

b. The same experiment was done using isopropyl iodide instead of methyl iodide. Which reaction had the larger rate constant?

Examine the two $S{N}_2$ reactions below: (A) ${\mathrm{CH}}_3\mathrm{Br}$ with $\mathrm{OH}-$ and (B) ${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}$ with $\mathrm{OH}-$. Which reaction will proceed at a faster rate?

Which energy diagram best represents an $S{N}_2$ reaction?

Which of the following best represents the transition state for the following SN2 reaction: $\mathrm{CH}{3}_0-$ attacking $\mathrm{CH}{3}_1\mathrm{Br}$?

Which of the following statements best describes the mechanism of an $S_{N2}$ reaction?

Predict the compound in each pair that will undergo the S_N2 reaction faster.

(a)

(b)

Arrange the following compounds in order from fastest to slowest $S_{\mathrm{N}}2$ reaction rate when reacting with ${\mathrm{Br}}_{-}$ as the nucleophile: 1-bromopropane, 2-bromopropane, 2-bromo-2-methylpropane.