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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 3c
Chapter 10, Problem 3c

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.

Verified step by step guidance
1
Determine the type of reaction mechanism involved. Bromomethane (CH₃Br) reacts with hydroxide ion (OH⁻) via an SN2 mechanism, which is a bimolecular nucleophilic substitution reaction. The rate law for an SN2 reaction is rate = k[CH₃Br][OH⁻], where k is the rate constant.
Understand the relationship between the rate and the concentrations of the reactants. In an SN2 reaction, the rate is directly proportional to the concentration of both the alkyl halide (CH₃Br) and the nucleophile (OH⁻).
Analyze the effect of halving the concentration of the alkyl halide. If [CH₃Br] is reduced to half, the rate contribution from this term will also be halved, as the rate is directly proportional to [CH₃Br].
Analyze the effect of doubling the concentration of the nucleophile. If [OH⁻] is doubled, the rate contribution from this term will also double, as the rate is directly proportional to [OH⁻].
Combine the effects of both changes. Since the rate is proportional to the product of [CH₃Br] and [OH⁻], halving [CH₃Br] and doubling [OH⁻] will result in the overall rate being unchanged (0.5 × 2 = 1).

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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 an alcohol. The rate of these reactions can depend on the concentrations of both the nucleophile and the substrate.
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Nucleophiles and Electrophiles can react in Substitution Reactions.

Rate Law

The rate law of a reaction expresses the relationship between the rate of the reaction and the concentration of its reactants. For a nucleophilic substitution reaction, the rate can be influenced by the order of the reaction with respect to each reactant. Understanding the rate law helps predict how changes in concentration will affect the overall reaction rate.
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Concentration Effects on Reaction Rate

Changes in the concentration of reactants can significantly impact the rate of a chemical reaction. In this case, halving the concentration of bromomethane while doubling the concentration of hydroxide ion alters the balance of reactants. The overall effect on the reaction rate will depend on the specific rate law and the order of the reaction with respect to each reactant.
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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?

a. The concentration of the alkyl halide is not changed and the concentration of the nucleophile is tripled.

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Textbook Question

A small amount of another organic product is formed in a Williamson ether synthesis. What is this product when the alkyl halide used in the synthesis of butyl propyl ether is

a. propyl bromide?

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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

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

Rank the following alkyl bromides from most reactive to least reactive in an SN2 reaction:

1-bromo-2-methylbutane, 1-bromo-3-methylbutane, 2-bromo-2-methylbutane, and 1-bromopentane.

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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?

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