Textbook Question
We have studied a number of pericyclic reactions previously. Draw the mechanism of the steps shown. The section number where this material was first studied is given for your review.
(c)
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Ch. 22 - Conjugated Systems II: Pericyclic 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
Chapter 21, Problem 12
Reduction of an alkyne using the Lindlar catalyst, a reaction presented in Section 10.6.2, produces only the cis-alkene. Why?
Verified step by step guidance1
Understand the Lindlar catalyst: The Lindlar catalyst is a palladium-based catalyst that is poisoned with lead acetate and quinoline. This poisoning reduces the activity of the catalyst, making it selective for partial hydrogenation of alkynes to alkenes without further reducing the alkene to an alkane.
Recognize the stereoselectivity: The Lindlar catalyst facilitates syn-addition of hydrogen atoms to the alkyne. Syn-addition means that both hydrogen atoms are added to the same side of the triple bond, resulting in the formation of a cis-alkene.
Consider the mechanism: The alkyne adsorbs onto the surface of the Lindlar catalyst, where hydrogen gas is activated. The hydrogen atoms are delivered to the alkyne in a controlled manner, ensuring that the addition occurs on the same face of the molecule.
Understand why trans-alkenes are not formed: The poisoned catalyst prevents over-reduction and does not allow the alkyne to rotate freely during the reaction. This restriction ensures that the hydrogen atoms are added in a syn fashion, preventing the formation of trans-alkenes.
Relate to the reaction conditions: The mild conditions of the Lindlar catalyst reaction are specifically designed to stop at the cis-alkene stage, making it an ideal method for selectively producing cis-alkenes from alkynes.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Lindlar Catalyst
The Lindlar catalyst is a palladium-based catalyst that is partially poisoned with lead salts. This modification allows it to selectively hydrogenate alkynes to cis-alkenes without further reducing them to alkanes. The controlled reactivity of the Lindlar catalyst is crucial for achieving the desired stereochemistry in the reduction process.
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Stereochemistry of Alkenes
Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical properties. In the case of alkenes, the cis and trans configurations arise from the restricted rotation around the double bond. The use of the Lindlar catalyst specifically leads to the formation of the cis-alkene due to the catalyst's ability to stabilize the transition state that favors this configuration.
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Selective Hydrogenation
Selective hydrogenation is a chemical process that adds hydrogen to unsaturated compounds, such as alkynes, while controlling the extent of the reaction. In the context of the Lindlar catalyst, this process is designed to stop at the alkene stage, preventing complete reduction to alkanes. This selectivity is essential for producing specific products with desired properties, such as the cis-alkene in this reaction.
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00:48The definition of hydrogenation.
Related Practice
Textbook Question
Diene A participates in a fast and efficient Diels–Alder reaction with maleic anhydride, the powerful dienophile from Assessment 22.9. However, the related diene B does not undergo a Diels–Alder reaction. Why?
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Textbook Question
In each Diels–Alder reaction shown, predict the product that will result.
(c)
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Textbook Question
Figure 22.16(a) <IMAGE> shows a unique example where the Diels–Alder reaction gives a single product (no enantiomers) regardless of whether the diene attacks the dienophile from the top or bottom.
(a) Show the product of the diene attacking from the bottom and confirm that the same product is obtained.
(b) What is special about this product that makes this true?
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Textbook Question
We have studied a number of pericyclic reactions previously. Draw the mechanism of the steps shown. The section number where this material was first studied is given for your review.
(d)
1360
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Textbook Question
In each Diels–Alder reaction shown, predict the product that will result.
(a)
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