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Ch.4 - The Study of Chemical Reactions
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.4 - The Study of Chemical ReactionsProblem 34a
Chapter 4, Problem 34a

The following reaction is a common synthesis used in the organic chemistry laboratory course.

When we double the concentration of methoxide ion (CH3O–), we find that the reaction rate doubles. When we triple the concentration of 1-bromobutane, we find that the reaction rate triples.
a. What is the order of this reaction with respect to 1-bromobutane? What is the order with respect to methoxide ion? Write the rate equation for this reaction. What is the overall order?

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Step 1: Analyze the problem and identify the reaction components. The reaction involves 1-bromobutane (CH3CH2CH2CH2Br) and methoxide ion (CH3O–) as reactants, producing methoxybutane (CH3CH2CH2CH2OCH3) and bromide ion (Br–). The reaction rate depends on the concentrations of the reactants.
Step 2: Determine the order of the reaction with respect to 1-bromobutane. The problem states that when the concentration of 1-bromobutane is tripled, the reaction rate also triples. This indicates a first-order dependence on 1-bromobutane. Mathematically, this means the rate is proportional to [CH3CH2CH2CH2Br]^1.
Step 3: Determine the order of the reaction with respect to methoxide ion. The problem states that when the concentration of methoxide ion is doubled, the reaction rate also doubles. This indicates a first-order dependence on methoxide ion. Mathematically, this means the rate is proportional to [CH3O–]^1.
Step 4: Write the rate equation for the reaction. Since the reaction is first-order with respect to both 1-bromobutane and methoxide ion, the rate equation can be expressed as: r=k⋅[CH3CH2CH2CH2Br]1⋅[CH3O]1, where k is the rate constant.
Step 5: Determine the overall order of the reaction. The overall order is the sum of the individual orders with respect to each reactant. Since the reaction is first-order with respect to both 1-bromobutane and methoxide ion, the overall order is 1 + 1 = 2.

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Key Concepts

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

Reaction Order

Reaction order refers to the power to which the concentration of a reactant is raised in the rate law of a chemical reaction. It indicates how the rate of reaction is affected by the concentration of that reactant. For example, if doubling the concentration of a reactant doubles the reaction rate, the reaction is first order with respect to that reactant.
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Rate Law

The rate law is an equation that relates the rate of a chemical reaction to the concentration of its reactants. It is typically expressed in the form Rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are the concentrations of the reactants, and m and n are the orders of the reaction with respect to each reactant. Understanding the rate law is essential for determining how changes in concentration affect the reaction rate.
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Overall Reaction Order

The overall reaction order is the sum of the individual orders with respect to each reactant in the rate law. It provides insight into the complexity of the reaction mechanism. For instance, if a reaction is first order with respect to one reactant and second order with respect to another, the overall order would be three. This concept helps in predicting how the reaction rate will change with varying concentrations of reactants.
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Related Practice
Textbook Question

Consider the following reaction-energy diagram.

a. Label the reactants and the products. Label the activation energy for the first step and the second step.

b. Is the overall reaction endothermic or exothermic? What is the sign of ΔH°?

c. Which points in the curve correspond to intermediates? Which correspond to transition states?

d. Label the transition state of the rate-limiting step. Does its structure resemble the reactants, the products, or an ­intermediate?

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Textbook Question

Draw a reaction-energy diagram for a two-step endothermic reaction with a rate-limiting second step.

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Acetonitrile (CH3C≡N) is deprotonated by very strong bases. Write resonance forms to show the stabilization of the carbanion that results.

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Textbook Question

When it is strongly heated, ethyl diazoacetate decomposes to give nitrogen gas and a carbene. Draw a Lewis structure of the carbene.

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Textbook Question

Draw a reaction-energy diagram for a one-step exothermic reaction. Label the parts that represent the reactants, products, transition state, activation energy, and heat of reaction.

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Textbook Question

Acetylacetone (pentane-2,4-dione) reacts with sodium hydroxide to give water and the sodium salt of a carbanion. Write a complete structural formula for the carbanion, and use resonance forms to show the stabilization of the carbanion.

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