Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (ii) H2, Pd/C, Pb(OAc)2 , CaCO3 (Lindlar's catalyst). If you expect two products, show both.
(c)
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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 44c(iv)
Mullins 1st EditionCh. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblem 44c(iv)Chapter 9, Problem 44c(iv)
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (iv) HBr (1 equiv.). If you expect two products, show both.
(c) 
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Identify the structure of the alkyne provided in part (c). Determine whether it is a terminal alkyne (with a hydrogen at one end) or an internal alkyne (with substituents on both sides of the triple bond). This will influence the regioselectivity of the reaction.
Recall the reaction mechanism for the addition of HBr to an alkyne. The reaction proceeds via Markovnikov addition, where the hydrogen atom from HBr adds to the carbon of the triple bond that already has more hydrogens, and the bromine atom adds to the other carbon.
Since only 1 equivalent of HBr is used, the reaction will stop at the alkene stage, forming a bromoalkene. The triple bond will be converted into a double bond, and the bromine will be added to the more substituted carbon (Markovnikov's rule).
If the alkyne is unsymmetrical, consider the possibility of regioisomers. Draw both potential products if applicable, showing the placement of the bromine atom and the double bond in each case.
Verify the stereochemistry of the product(s). For alkynes, the addition of HBr does not typically lead to stereoisomers because the reaction proceeds through a planar carbocation intermediate. Ensure the final product(s) are drawn correctly with the double bond and bromine in the appropriate positions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Alkynes
Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. They are unsaturated compounds and are generally more reactive than alkenes and alkanes due to the presence of the triple bond. The reactivity of alkynes allows them to undergo various addition reactions, which is crucial for predicting the products formed when they react with reagents like HBr.
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Electrophilic Addition
Electrophilic addition is a reaction mechanism where an electrophile reacts with a nucleophile, leading to the formation of a more saturated product. In the case of alkynes reacting with HBr, the triple bond acts as a nucleophile, attacking the electrophilic hydrogen atom of HBr. This reaction can lead to the formation of either a vinyl bromide or a bromoalkane, depending on the reaction conditions and the regioselectivity of the addition.
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Markovnikov's Rule
Markovnikov's Rule states that when HX (where X is a halogen) adds to an alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This rule helps predict the major product in reactions involving unsymmetrical alkynes. In the case of HBr addition to an alkyne, this rule can lead to the formation of two different products, depending on the regioselectivity of the addition.
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Related Practice
Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (iii) Na0 , NH3 (liquid), ―33°C . If you expect two products, show both.
(c)
4
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Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (v) HCl (2 equiv.). If you expect two products, show both.
(c)
3
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Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (vii) 1. BH3 2. H2O2, NaOH. If you expect two products, show both.
(c)
6
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Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (vi) H2SO4, HgSO4, H2O. If you expect two products, show both.
(c)
1
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Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (ix) Br2 (2 equiv.). If you expect two products, show both.
(c)
1
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