Textbook Question
Keto–enol tautomerism is a reaction we discuss in detail in Chapter 19. Estimate the equilibrium constant of this reaction (BDE for C―C π bond = 65 kcal/mol ; for C―O π bond = 85 kcal/mol).
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Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
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Mullins 1st EditionCh. 5 - Chemical Reaction Analysis: Thermodynamics and KineticsProblem 59b
Mullins 1st EditionCh. 5 - Chemical Reaction Analysis: Thermodynamics and KineticsProblem 59bChapter 4, Problem 59b
Reactions (a) and (b) are disfavored overall (∆G° > 0), yet they are favored based on ∆H°. Identify the bonds formed and broken for (a) and (b).
(b) 
Verified step by step guidance1
Step 1: Analyze the reaction provided. The reactants are propanal (CH3CH2CHO) and methanol (CH3OH), and the product is a hemiacetal (CH3CH(OH)OCH3). This reaction involves the formation of a hemiacetal from an aldehyde and an alcohol.
Step 2: Identify the bonds broken in the reaction. In the aldehyde (CH3CH2CHO), the C=O double bond is partially broken as the oxygen forms a new bond with the hydrogen from the alcohol. In methanol (CH3OH), the O-H bond is broken as the oxygen forms a new bond with the carbon of the aldehyde.
Step 3: Identify the bonds formed in the reaction. A new C-O bond is formed between the carbon of the aldehyde and the oxygen of the alcohol. Additionally, a new O-H bond is formed on the oxygen of the aldehyde, converting it into a hydroxyl group.
Step 4: Explain why the reaction is favored based on enthalpy (∆H°). The formation of strong bonds, such as the C-O bond in the hemiacetal and the O-H bond, releases energy, making the reaction exothermic and favorable in terms of enthalpy.
Step 5: Discuss why the reaction is disfavored overall (∆G° > 0). Despite the favorable enthalpy change, the reaction may be disfavored due to entropy considerations. The formation of a more ordered product (hemiacetal) from two reactants leads to a decrease in entropy, which can make the Gibbs free energy positive and the reaction disfavored overall.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Gibbs Free Energy (∆G)
Gibbs Free Energy (∆G) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. A positive ∆G indicates that a reaction is non-spontaneous under standard conditions, meaning it requires energy input to proceed. Understanding ∆G is crucial for analyzing the favorability of chemical reactions, particularly in the context of reactions that may be driven by enthalpy changes.
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05:02
Breaking down the different terms of the Gibbs Free Energy equation.
Enthalpy (∆H)
Enthalpy (∆H) is a measure of the total heat content of a system and reflects the energy associated with the bonds formed and broken during a chemical reaction. A negative ∆H indicates that a reaction releases heat (exothermic), which can favor the reaction despite a positive ∆G. Recognizing the role of enthalpy helps in understanding why certain reactions may be thermodynamically disfavored overall yet still proceed due to favorable bond formation.
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04:38Calculating Enthalpies
Bond Formation and Breaking
In chemical reactions, bonds between atoms are broken in the reactants and new bonds are formed in the products. The types of bonds involved (e.g., covalent, ionic) and their strengths significantly influence the overall energy change of the reaction. Identifying which bonds are formed and broken is essential for understanding the reaction mechanism and the associated changes in enthalpy and Gibbs free energy, particularly in reactions that are thermodynamically unfavorable.
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Related Practice
Textbook Question
Reaction (c), on the other hand, is favored (∆G° < 0). Identify the bonds formed and broken and explain this result in light of (a) and (b).
(c)
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Textbook Question
Calculate ∆H° for the following alkene addition reaction, one we discuss further in Chapter 7. Predict the sign of ∆S° . (The BDE for C―C π bond is approximately 65 kcal/mol.)
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Textbook Question
The hydrogenation of alkenes is a reaction we study in Chapter 9.
(b) Is this reaction favored or disfavored in terms of entropy?
Textbook Question
Draw a reaction coordinate diagram, making sure to label reactants (R), products (P), intermediates (I), transition states (‡), activation energies ( Ea) , and ∆G°, for each of the following.
(d) a slightly exothermic, three-step reaction where the third step is rate-determining.
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Textbook Question
(b) Mechanistically, the reaction occurs as shown below. Why is this reaction favored? Based on the stability of the anions, estimate Keq.
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