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 73a
Mullins 1st EditionCh. 12 - Substitution and Elimination: Reactions of HaloalkanesProblem 73aChapter 11, Problem 73a
Suggest a mechanism for the following reactions.
(a) Substitution:
Verified step by step guidance1
Step 1: Protonation of the alcohol group (-OH) occurs. The hydroxyl group is a poor leaving group, but in the presence of HBr, the oxygen atom in the -OH group is protonated, forming water (H2O), which is a much better leaving group. This step involves the donation of a proton (H⁺) from HBr to the oxygen atom.
Step 2: Formation of a carbocation intermediate. After protonation, the water molecule leaves, resulting in the formation of a carbocation (positively charged carbon atom). The stability of the carbocation depends on the structure of the molecule; in this case, the carbocation is primary.
Step 3: Nucleophilic attack by bromide ion (Br⁻). The bromide ion, which is generated from the dissociation of HBr, acts as a nucleophile and attacks the carbocation, forming a new C-Br bond.
Step 4: The final product is formed. The substitution reaction results in the formation of the alkyl bromide (CH3CH2Br) and water (H2O) as the byproduct.
Step 5: Verify the reaction type. This reaction is an SN1 mechanism because it involves the formation of a carbocation intermediate, although primary carbocations are less stable and this reaction may proceed via an SN2 mechanism depending on the specific conditions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nucleophilic Substitution
Nucleophilic substitution is a fundamental reaction in organic chemistry where a nucleophile replaces a leaving group in a molecule. In this case, the hydroxyl group (OH) acts as a leaving group when treated with HBr, allowing the bromide ion (Br-) to take its place. Understanding this mechanism is crucial for predicting the products of reactions involving alcohols and halogen acids.
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Nucleophiles and Electrophiles can react in Substitution Reactions.
Protonation of Alcohols
Protonation of alcohols is a key step in their conversion to better leaving groups. When an alcohol reacts with a strong acid like HBr, the hydroxyl group is protonated to form water, which is a much better leaving group. This step is essential for facilitating the nucleophilic substitution mechanism, as it enhances the reactivity of the alcohol.
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Mechanism of Reaction
The mechanism of a reaction describes the step-by-step process by which reactants are converted into products. In this case, the mechanism involves the protonation of the alcohol followed by the nucleophilic attack of bromide ion on the carbocation formed after the departure of water. Understanding the mechanism helps in predicting the outcome of the reaction and the stability of intermediates.
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Related Practice
Textbook Question
In this, and previous, chapters, we have seen 1,2-alkyl and 1,2-hydride shifts. If both are possible, as in the carbocation shown, which would you expect to occur? Explain your answer.
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Textbook Question
Suggest a mechanism for the following reactions.
(b) Substitution :
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Textbook Question
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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Textbook Question
Using pKₐ values, calculate Keq for the following acid–base reaction.
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Textbook Question
Suggest a mechanism for the following reactions.
(c) Elimination: