Textbook Question
Which is a better leaving group, HO⁻ or H2O? Explain your answer.
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Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes
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. 12 - Substitution and Elimination: Reactions of HaloalkanesProblem 70
Mullins 1st EditionCh. 12 - Substitution and Elimination: Reactions of HaloalkanesProblem 70Chapter 11, Problem 70
The following substitution reaction, between a strong base and a 1° haloalkane, occurs in a single step via backside displacement. Yet it is not technically an SN2 reaction. Why?
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Understand the mechanism of an Sₙ2 reaction: It is a bimolecular nucleophilic substitution reaction where the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group, resulting in a single-step backside displacement.
Analyze the given reaction: The problem states that the reaction involves a strong base and a 1° haloalkane, and occurs via backside displacement in a single step. This description aligns with the general features of an Sₙ2 reaction.
Consider the role of the strong base: Strong bases can sometimes act as nucleophiles, but their primary function is to abstract protons. If the strong base is involved in proton abstraction rather than direct nucleophilic attack, the reaction mechanism may differ from a true Sₙ2 process.
Explore alternative mechanisms: The reaction might be classified as an E2 elimination rather than an Sₙ2 substitution. In an E2 mechanism, the strong base abstracts a proton from the β-carbon, leading to the formation of a double bond, while the leaving group departs simultaneously in a single step.
Conclude why it is not technically an Sₙ2 reaction: If the strong base is primarily involved in elimination rather than substitution, the reaction mechanism is better described as E2 rather than Sₙ2, despite the single-step nature and backside displacement.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Substitution Reactions
Substitution reactions involve the replacement of one functional group in a molecule with another. In organic chemistry, these reactions can be classified into two main types: Sₙ1 and Sₙ2. Sₙ2 reactions are characterized by a single concerted step where the nucleophile attacks the electrophile from the opposite side of the leaving group, leading to inversion of configuration.
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Recognizing Substitution Reactions.
Backside Attack
Backside attack refers to the mechanism by which a nucleophile approaches the electrophilic carbon atom from the side opposite to the leaving group. This approach is crucial for Sₙ2 reactions, as it allows for the simultaneous bond formation and bond breaking, resulting in inversion of stereochemistry. However, in certain cases, such as with strong bases and 1° haloalkanes, the reaction may proceed differently, affecting the classification.
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Strong Bases and Reaction Mechanisms
Strong bases can facilitate elimination reactions in addition to substitution reactions. When a strong base reacts with a 1° haloalkane, it may lead to an E2 elimination pathway instead of an Sₙ2 substitution. This duality in reactivity can complicate the classification of the reaction, as the presence of a strong base can favor elimination over substitution, even if a backside attack occurs.
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Related Practice
Textbook Question
When trans-4-bromocyclohexanol is treated with base, an intramolecular substitution reaction occurs to give a cyclic ether. This product does not form when cis-4-bromocyclohexanol is reacted under the same conditions. Explain these observations.
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Textbook Question
Suggest a mechanism for the following reactions.
(a) Substitution:
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Textbook Question
The following chlorocyclohexane undergoes neither Sₙ2 nor E2 under the conditions shown. Why?
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Textbook Question
Using pKₐ values, calculate Keq for the following acid–base reaction.
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Textbook Question
In addition to using mCPBA, epoxides can be synthesized from alkenes in the two-step process shown. Give a mechanism for each step of the process.
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