Ch.6 - Alkyl Halides; Nucleophilic Substitution

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.6 - Alkyl Halides; Nucleophilic Substitution

Problem 6

Chapter 6, Problem 6

The reaction of an amine with an alkyl halide gives an ammonium salt.R3N amine + R'—X alkyl halide —> R3(N+)—R' X- ammonium salt The rate of this SN2 reaction is sensitive to the polarity of the solvent. 1. Draw an energy diagram for this reaction in a nonpolar solvent and another in a polar solvent. 2. Consider the nature of the transition state, and explain why this reaction should be sensitive to the polarity of the solvent. 3. Predict whether it will be faster or slower in a more polar solvent.

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Draw the energy diagram for the SN2 reaction in a nonpolar solvent, showing the reactants, transition state, and products. The energy barrier should be relatively high due to the lack of stabilization from the solvent.

Draw a second energy diagram for the SN2 reaction in a polar solvent. The energy barrier should be lower compared to the nonpolar solvent due to better stabilization of the transition state by the polar solvent.

Consider the nature of the transition state: it is a high-energy, charged species. In a polar solvent, this transition state is stabilized by solvation, which lowers the activation energy.

Explain that the reaction is sensitive to solvent polarity because the transition state involves charge separation, which is better stabilized by polar solvents.

Predict that the reaction will be faster in a more polar solvent due to the lower activation energy resulting from better stabilization of the transition state.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

SN2 Reaction Mechanism

The SN2 (substitution nucleophilic bimolecular) reaction involves a nucleophile attacking an electrophile, resulting in the simultaneous displacement of a leaving group. This mechanism is characterized by a single transition state where both the nucleophile and the substrate are involved, leading to a concerted reaction. The reaction rate depends on the concentration of both the nucleophile and the substrate, making it sensitive to steric hindrance and solvent effects.

Transition State and Solvent Polarity

The transition state of a reaction is a high-energy state that occurs during the conversion of reactants to products. In the case of SN2 reactions, the transition state is more polar due to the partial charges developed as the nucleophile approaches the electrophile. Polar solvents stabilize charged transition states through solvation, which can lower the activation energy and thus affect the reaction rate, making the reaction faster in polar solvents compared to nonpolar ones.

Effect of Solvent on Reaction Rate

The polarity of the solvent plays a crucial role in determining the rate of chemical reactions, particularly in SN2 mechanisms. Polar solvents can stabilize the transition state and the charged intermediates, reducing the energy barrier for the reaction. Conversely, nonpolar solvents do not stabilize these charged species effectively, leading to a higher activation energy and a slower reaction rate. Therefore, reactions involving charged transition states are generally faster in polar solvents.

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Related Practice

Textbook Question

a. Propose a mechanism for the following reaction:

b. Use the bond-dissociation enthalpies given in Table 4-2 (page 167) to calculate the value of ΔH° for each step shown in your mechanism. (The BDE for CH₂=CHCH₂―Br is about 280 kJ/mol, or 67 kcal/mol.) Calculate the overall value of ΔH° for the reaction. Are these values consistent with a rapid free-radical chain reaction?

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Textbook Question

For each pair of compounds, predict which one has the higher molecular dipole moment, and explain your reasoning.

c. cis-2,3-dibromobut-2-ene or trans-2,3-dibromobut-2-ene

d. cis-1,2-dichlorocyclobutane or trans-1,3-dichlorocyclobutane

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Textbook Question

The reaction of an amine with an alkyl halide gives an ammonium salt.R3N amine + R'—X alkyl halide —> R3(N+)—R' X- ammonium salt The rate of this SN2 reaction is sensitive to the polarity of the solvent. 1. Draw an energy diagram for this reaction in a nonpolar solvent and another in a polar solvent. 2. Consider the nature of the transition state, and explain why this reaction should be sensitive to the polarity of the solvent. 3. Predict whether it will be faster or slower in a more polar solvent.

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Textbook Question

Kepone, aldrin, and chlordane are synthesized from hexachlorocyclopentadiene and other five-membered-ring compounds. Show how these three pesticides are composed of two five-membered rings.

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Textbook Question

For each pair of compounds, predict which one has the higher molecular dipole moment, and explain your reasoning.

a. ethyl chloride or ethyl iodide

b. 1-bromopropane or cyclopropane

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Textbook Question

For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if your prediction was right; then explain why that compound has the higher boiling point.

a. isopropyl bromide and n-butyl bromide

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