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Chapter 27: Problem 81

(a) Which of the nucleophiles \(\mathrm{CN}^{-}\) or \(\mathrm{Cl}^{-}\) reacts faster with \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}\) in an \(\mathrm{S}_{\mathrm{N}} 2\) reaction? (b) Which of the substrates, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{I}\) or \(\mathrm{CH}_{3} \mathrm{I},\) reacts faster with \(\mathrm{OH}^{-}\) in an \(\mathrm{S}_{\mathrm{N}}\) 2 reaction?

Short Answer

Expert verified
(a) \(\mathrm{CN}^{-}\) reacts faster with \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}\) in an \(\mathrm{S}_{\mathrm{N}}\) 2 reaction. (b) \(\mathrm{CH}_{3} \mathrm{I}\) reacts faster with \(\mathrm{OH}^{-}\) in an \(\mathrm{S}_{\mathrm{N}}\) 2 reaction.

Step by step solution

01

Understanding the role of nucleophile

In the first question (a), two nucleophiles are given, \(\mathrm{CN}^{-}\) and \(\mathrm{Cl}^{-}\). The reactivity or speed of reaction in SN2 reactions depends mainly on nucleophilicity. Nucleophilicity increases with charge and decreases with electronegativity. In this case, \(\mathrm{CN}^{-}\) (cyanide ion) is less electronegative compared to \(\mathrm{Cl}^{-}\) (chloride ion), therefore \(\mathrm{CN}^{-}\) is more nucleophilic.
02

Deciding the better nucleophile

Having established that, in the given conditions, \(\mathrm{CN}^{-}\) is more nucleophilic, it can be concluded that \(\mathrm{CN}^{-}\) reacts faster with \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}\) in an \(\mathrm{S}_{\mathrm{N}} 2\) reaction.
03

Understanding the role of substrate

In the second question (b), two substrates are given, \(\mathrm{CH}_{3} \mathrm{CH}_{2}\mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{I}\) and \(\mathrm{CH}_{3} \mathrm{I}\). For the SN2 reaction, the reactivity of substrate is largely driven by sterics. The more sterically hindered the reaction center, the slower the reaction.
04

Deciding the better substrate

Given the aforementioned consideration, the primary alkyl halide \(\mathrm{CH}_{3} \mathrm{I}\), having less steric hindrance near the reaction center, will react faster than the secondary alkyl halide \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{I}\) with \(\mathrm{OH}^{-}\) in an \(\mathrm{S}_{\mathrm{N}}\) 2 reaction.

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Most popular questions from this chapter

Answer the following questions for this E1 reaction: $$\begin{array}{c} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{Br}+\mathrm{CH}_{3} \mathrm{OH} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}+\mathrm{Br}^{-}+\mathrm{CH}_{3} \mathrm{OH}_{2}^{+} \end{array}$$ (a) What is the rate expression for the reaction? (b) Draw the reaction profile for the reaction. Label all parts. Assume that the products are lower in energy than the reactants. (c) What is the effect on the rate of the reaction of doubling the concentration of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{Br} ?\) (d) What is the effect on the rate of the reaction of doubling the concentration of \(\mathrm{CH}_{3} \mathrm{OH} ?\)

The iodide ion cannot displace the \(-\mathrm{OH}\) group in ethanol, but excess HI will react to produce ethyl iodide. Explain.

In referring to the molecular mass of a polymer, we can speak only of the average molecular mass. Explain why the molecular mass of a polymer is not a unique quantity, as it is for a substance like benzene.

Identify the nucleophile, electrophile, and leaving group in each of the following substitution reactions. Predict whether equilibrium favors the reactants or products: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{ONa}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{I} \rightleftharpoons\) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}+\mathrm{NaI}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{3}^{+}+\mathrm{KI} \rightleftharpoons\) \(\mathrm{NH}_{3}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}+\mathrm{K}^{+}\)

Answer the following questions for this \(S_{\mathrm{N}} 2\) reaction: $$\begin{aligned} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{NaOH} & \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}+ \mathrm{NaBr} \end{aligned}$$ (a) What is the rate expression for the reaction? (b) Draw the reaction profile for the reaction. Label all parts. Assume that the products are lower in energy than the reactants. (c) What is the effect on the rate of the reaction of doubling the concentration of \(n\) -butyl bromide? (d) What is the effect on the rate of the reaction of halving the concentration of sodium hydroxide?
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