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Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 9 - Substitution and Elimination Reactions of Alkyl HalidesProblem 3a
Chapter 10, Problem 3a

How will the rate of the reaction between bromomethane and hydroxide ion be affected if the following changes in concentration are made?
a. The concentration of the alkyl halide is not changed and the concentration of the nucleophile is tripled.

Verified step by step guidance
1
Understand the reaction mechanism: The reaction between bromomethane (CH₃Br) and hydroxide ion (OH⁻) follows an SN2 (bimolecular nucleophilic substitution) mechanism. In an SN2 reaction, the rate depends on the concentrations of both the alkyl halide and the nucleophile.
Write the rate law for the SN2 reaction: The rate law for an SN2 reaction is given by: r=k⁢[CH3Br]⁢[OH-], where k is the rate constant, [CH3Br] is the concentration of bromomethane, and [OH-] is the concentration of hydroxide ion.
Analyze the change in concentration: In this scenario, the concentration of the alkyl halide ([CH3Br]) remains constant, while the concentration of the nucleophile ([OH-]) is tripled.
Substitute the changes into the rate law: Since the rate is directly proportional to the concentration of the nucleophile, tripling [OH-] will result in the rate being tripled as well. Mathematically, r'=k⁢[CH3Br]⁢(3⁢[OH-])=3r.
Conclude the effect on the reaction rate: The rate of the reaction will increase by a factor of 3 when the concentration of the nucleophile is tripled, while the concentration of the alkyl halide remains unchanged.

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Key Concepts

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

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. In this context, bromomethane (an alkyl halide) reacts with hydroxide ion (a nucleophile) to form methanol and bromide ion. The rate of these reactions can depend on the concentrations of the reactants and the mechanism (SN1 or SN2) involved.
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Reaction Rate and Concentration

The rate of a chemical reaction is influenced by the concentrations of the reactants. According to the rate law, if the concentration of a reactant increases, the rate of reaction typically increases as well, assuming the reaction order with respect to that reactant is greater than zero. In this case, tripling the concentration of the hydroxide ion is likely to increase the reaction rate.
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Order of Reaction

The order of a reaction refers to the power to which the concentration of a reactant is raised in the rate law. For nucleophilic substitution reactions, the order can vary based on the mechanism. For an SN2 reaction, the rate is first order with respect to both the alkyl halide and the nucleophile, meaning that tripling the nucleophile's concentration will significantly enhance the reaction rate.
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Related Practice
Textbook Question

How will the rate of the reaction between bromomethane and hydroxide ion be affected if the following changes in concentration are made?

c. The concentration of the alkyl halide is cut in half and the concentration of the nucleophile is doubled.

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Textbook Question

How will the rate of the reaction between bromomethane and hydroxide ion be affected if the following changes in concentration are made?

b. The concentration of the alkyl halide is cut in half and the concentration of the nucleophile is not changed.

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Textbook Question

Draw the structure of DDE.

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Textbook Question

Methoxychlor is an insecticide that was intended to take DDT’s place because it is not as soluble in fatty tissues and is more readily biodegradable. It, too, can accumulate in the environment, however, so its use was also banned—in 2002 in the European Union and in 2003 in the United States. Why is methoxychlor less soluble in fatty tissues than DDT?

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Textbook Question

Does increasing the energy barrier for an SN2 reaction increase or decrease the magnitude of the rate constant for the reaction?

1238
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