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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 38
Chapter 4, Problem 38

a. Draw an approximate reaction-energy diagram for the acid–base reaction of phenol (see below) with 1-molar aqueous sodium hydroxide solution.
b. On the same diagram, draw an approximate reaction-energy diagram for the acid–base reaction of tert-butyl alcohol (see below) with 1-molar aqueous sodium hydroxide solution.

Verified step by step guidance
1
Analyze the given chemical structures and pKa values: Phenol has a pKa of 10.0, indicating it is more acidic than tert-butyl alcohol, which has a pKa of 18.0. Lower pKa values correspond to stronger acids.
Understand the acid-base reaction: In both cases, the hydroxide ion (OH⁻) from the sodium hydroxide solution will act as a base and deprotonate the alcohol group (-OH) of phenol and tert-butyl alcohol, forming their conjugate bases (phenoxide ion and tert-butoxide ion, respectively).
Sketch the reaction-energy diagram for phenol: The diagram should show the reactants (phenol and OH⁻) at a higher energy level than the products (phenoxide ion and water). The energy barrier (activation energy) for the reaction will be relatively low due to phenol's higher acidity.
Sketch the reaction-energy diagram for tert-butyl alcohol: The reactants (tert-butyl alcohol and OH⁻) will be at a higher energy level than the products (tert-butoxide ion and water). However, the energy barrier for this reaction will be higher compared to phenol due to tert-butyl alcohol's lower acidity.
Compare the diagrams: On the same graph, the reaction-energy diagram for phenol will show a lower activation energy and a greater energy difference between reactants and products compared to tert-butyl alcohol, reflecting the stronger acidity of phenol.

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Key Concepts

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

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+) between reactants. In this context, phenol and tert-butyl alcohol act as acids, donating protons to the hydroxide ion (OH-) from sodium hydroxide. The strength of an acid is often measured by its pKa value, which indicates the tendency of the acid to lose a proton; lower pKa values correspond to stronger acids.
Recommended video:
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The Lewis definition of acids and bases.

pKa Values

The pKa value is a quantitative measure of the strength of an acid in solution. It is the negative logarithm of the acid dissociation constant (Ka). For phenol, with a pKa of 10.0, it is a relatively stronger acid compared to tert-butyl alcohol, which has a pKa of 18.0, indicating that phenol will more readily donate a proton in an acid-base reaction.
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Identifying pKa values

Reaction Energy Diagrams

A reaction energy diagram visually represents the energy changes during a chemical reaction. It typically shows the energy of reactants, products, and the transition state. In the context of acid-base reactions, the diagram will illustrate the energy required to break bonds and form new ones, highlighting the stability of the reactants and products based on their pKa values.
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Introduction to free energy diagrams.
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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Textbook Question

Label each hydrogen atom in the following compounds as primary (1°), secondary (2°), or tertiary (3°).

(a) CH3CH2CH(CH3)2

(b) (CH3)3CCH2CH3

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Textbook Question

Treatment of tert-butyl alcohol with concentrated HCl gives tert-butyl chloride.

When the concentration of H+ is doubled, the reaction rate doubles. When the concentration of tert-butyl alcohol is tripled, the reaction rate triples. When the chloride ion concentration is quadrupled, however, the reaction rate is unchanged. Write the rate equation for this reaction.

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Textbook Question

Label each hydrogen atom in the following compounds as primary (1°), secondary (2°), or tertiary (3°).

(c) (CH3)2CHCH(CH3)CH2CH3

(d)

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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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