Textbook Question
An alternate method for the synthesis of alkynes relies on the double elimination of H―Br from a dihaloalkane under basic conditions. Suggest a mechanism for this reaction that we discuss in Chapter 12.
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Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions
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. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblem 56
Mullins 1st EditionCh. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblem 56Chapter 9, Problem 56
When alkynes are treated with water and bromine a bromoketone is produced. Provide a plausible arrow-pushing mechanism that accounts for the formation of this product.
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Identify the starting material (alkyne) and the reagents (water and bromine). Recognize that the reaction involves electrophilic addition followed by nucleophilic attack and tautomerization.
In the first step, bromine (Br₂) acts as an electrophile. The π-electrons of the alkyne attack one bromine atom, forming a bromonium ion intermediate. This step involves the breaking of the Br-Br bond, leaving a bromide ion (Br⁻) as a byproduct.
Next, water (H₂O) acts as a nucleophile and attacks the more substituted carbon of the bromonium ion, opening the three-membered ring. This step results in the formation of a bromohydrin intermediate.
The bromohydrin undergoes tautomerization. The hydroxyl group (-OH) is protonated by a proton donor (e.g., H⁺), forming a good leaving group (H₂O). The adjacent carbon-carbon bond electrons shift to form a double bond, and the bromide ion (Br⁻) abstracts a proton from the neighboring carbon, resulting in the formation of a bromoketone.
Finally, confirm the product structure as a bromoketone, where the carbonyl group (C=O) is adjacent to a bromine atom. This product is stabilized due to resonance and the electron-withdrawing effect of bromine.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Alkynes and Their Reactivity
Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. Their reactivity is significantly influenced by this triple bond, which can undergo various reactions, including electrophilic addition. Understanding the nature of alkynes is crucial for predicting how they will interact with reagents like water and bromine.
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Alkyne Hydration
Electrophilic Addition Mechanism
Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, leading to the formation of a more stable product. In the case of alkynes, the triple bond acts as a nucleophile, attacking the electrophilic bromine, which initiates the mechanism that ultimately leads to the formation of a bromoketone.
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Arrow-Pushing Notation
Arrow-pushing notation is a visual representation used to depict the movement of electrons during chemical reactions. It illustrates how electrons are transferred between atoms and bonds, helping to clarify the steps in a reaction mechanism. Mastery of this notation is essential for accurately conveying the sequence of events in the formation of products like bromoketones from alkynes.
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Related Practice
Textbook Question
The addition of H―X to alkynes has been shown to occur predominately via anti addition:
Two chemists disagreed on whether or not anti addition would happen on terminal alkynes as well. Suggest an experiment through which you could resolve this dispute.
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Textbook Question
The synthesis of five-membered lactones (cyclic esters) has been accomplished using the electrophilic addition of I―Cl to an alkyne. Suggest a mechanism for this cyclization reaction. (Structure modification of Yao, T.; Larock, R.C. J. Org. Chem. 2005, 70, 1432–1437.)
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Textbook Question
In 1973, Caine and Tuller reported a synthesis of racemic oplapanone, a sesquiterpene isolated from Oplopanax japonicus (a deciduous shrub) involving a reaction we learned in this chapter. Predict the product of the reaction shown. (Caine, D.; Tuller, F. N. J. Org. Chem. 1973, 38, 3663.)
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Textbook Question
A chemist attempted to do the following acetylide alkylation reaction but was unsuccessful in several attempts, producing only the original starting materials in each case. Explain why the reaction didn't work.
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Textbook Question
Trans addition is heavily favored for the addition of Br₂ and Cl₂ to alkynes. With chlorination, however, more of the syn addition product is formed. Rationalize this fact in light of your answer to Assessments 10.50 and 10.51.
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