Textbook Question
Imagine an electrophilic addition where the first step is exothermic. Which carbocation—2°, 3°, or neither—would you expect to form preferentially? Explain.
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Ch. 8 - Alkenes I: Properties and Electrophilic Additions
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
Chapter 7, Problem 23
A wayward chemist proposed the following mechanism for the addition of HBr to an alkene.
(a) Why is this mechanism unlikely?
(b) Compare the reaction coordinate diagrams for the actual mechanism studied in Section 8.3.1 and this alternate mechanism on the same graph.
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Identify the proposed mechanism: The image shows a two-step mechanism where the alkene first reacts with HBr to form a carbocation intermediate, followed by the attack of a hydride ion (H-) on the carbocation.
Explain why the mechanism is unlikely: In typical electrophilic addition reactions of alkenes with HBr, the alkene acts as a nucleophile and attacks the proton (H+) from HBr, forming a carbocation. The bromide ion (Br-) then attacks the carbocation. The presence of a hydride ion (H-) as a nucleophile is highly unusual and unlikely under normal conditions.
Discuss the stability of intermediates: Carbocations are generally stabilized by hyperconjugation and inductive effects. The proposed mechanism suggests a hydride ion attacking a carbocation, which is not a common or favorable reaction pathway.
Compare with the actual mechanism: In the actual mechanism, the alkene first forms a carbocation by protonation, and then the bromide ion attacks the carbocation to form the final product. This is a more straightforward and energetically favorable pathway.
Sketch the reaction coordinate diagram: For the actual mechanism, the diagram would show an initial energy increase to form the carbocation, followed by a decrease as the bromide ion attacks. The proposed mechanism would have an additional energy barrier for the unlikely hydride attack, making it less favorable overall.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electrophilic Addition Mechanism
The electrophilic addition mechanism is a fundamental reaction pathway for alkenes, where an electrophile (like HBr) attacks the electron-rich double bond of the alkene. This process typically involves the formation of a carbocation intermediate, which is then attacked by a nucleophile. Understanding this mechanism is crucial for analyzing the proposed mechanism and identifying its flaws.
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Features of Addition Mechanisms.
Carbocation Stability
Carbocation stability is a key concept in organic chemistry, as the stability of the carbocation intermediate significantly influences the reaction pathway. Carbocations can be classified as primary, secondary, or tertiary, with tertiary carbocations being the most stable due to hyperconjugation and inductive effects. Evaluating the stability of the proposed carbocation in the mechanism helps determine the likelihood of the reaction occurring as suggested.
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Reaction Coordinate Diagrams
Reaction coordinate diagrams visually represent the energy changes during a chemical reaction, illustrating the transition states and intermediates involved. These diagrams allow for a comparison of different mechanisms by showing the energy barriers and the relative stability of intermediates. Analyzing the reaction coordinate diagrams for both the proposed and actual mechanisms will highlight differences in energy profiles and support conclusions about their feasibility.
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Related Practice
Textbook Question
Suggest an alkene that, upon reaction with the appropriate hydrohalic acid, will produce only the alkyl halide shown. [Ignore stereochemistry.]
(d)
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Textbook Question
Which is the most likely transition state for the reactions shown? Explain your answer. [Note the difference in the size of the partial charges or partial unpaired electrons.]
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Textbook Question
Which would you expect to be more selective for carbocation formation, the electrophilic addition of HF or HBr to 2-methylbut-2-ene? Explain your answer.
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Textbook Question
Suggest an alkene that, upon reaction with the appropriate hydrohalic acid, will produce only the alkyl halide shown. [Ignore stereochemistry.]
(b)
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Textbook Question
Provide an arrow-pushing mechanism that rationalizes the formation of each alkyl halide in Assessment 8.19.
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