Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

10. Addition Reactions

Halogenation

Organic Chemistry

10. Addition Reactions

Halogenation

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2:55 minutes

Problem 78

Textbook Question

We have seen many examples where halogens add to alkenes with anti stereochemistry via the halonium ion mechanism. However, when 1-phenylcyclohexene reacts with chlorine in carbon tetrachloride, a mixture of the cis and trans isomers of the product is recovered. Propose a mechanism, and explain this lack of stereospecificity.

Verified step by step guidance

Step 1: Begin by recognizing that the reaction involves the addition of chlorine (Cl₂) to the double bond in 1-phenylcyclohexene. Typically, halogens add to alkenes via the halonium ion mechanism, which leads to anti stereochemistry. However, in this case, both cis and trans isomers are formed, indicating a deviation from strict stereospecificity.

Step 2: Analyze the mechanism. When Cl₂ reacts with the alkene, the π-electrons of the double bond attack one chlorine atom, forming a cyclic chloronium ion intermediate. This intermediate is highly strained and reactive, and the positive charge is delocalized over the ring system.

Step 3: Consider the influence of the phenyl group. The phenyl group attached to the cyclohexene ring is bulky and can stabilize the intermediate through resonance. This stabilization can lead to partial opening of the chloronium ion, creating a carbocation-like character at the adjacent carbon. This partial opening allows for nucleophilic attack from either side of the planar carbocation-like intermediate.

Step 4: Explain the formation of cis and trans isomers. The nucleophilic attack by the second chlorine atom can occur from either the same side (leading to the cis product) or the opposite side (leading to the trans product). The lack of stereospecificity arises because the intermediate is not rigidly locked into a single conformation due to the influence of the phenyl group and the solvent (CCl₄), which does not strongly direct the reaction pathway.

Step 5: Summarize the mechanism. The reaction proceeds via the halonium ion mechanism, but the phenyl group and solvent effects lead to partial opening of the intermediate, allowing for attack from both sides. This results in a mixture of cis and trans isomers of 1,2-dichloro-1-phenylcyclohexane.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Halonium Ion Mechanism

The halonium ion mechanism involves the formation of a cyclic halonium ion intermediate when a halogen adds to an alkene. This intermediate is characterized by a three-membered ring structure where the halogen atom is positively charged, leading to anti-addition of the halogen atoms across the double bond. This mechanism typically results in stereospecific products, but certain factors can lead to deviations from this expected outcome.

Stereochemistry of Alkenes

Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the case of alkenes, the geometry around the double bond can lead to different isomers, such as cis and trans forms. Understanding the stereochemical implications of reactions is crucial for predicting the types of products formed, especially when multiple stereoisomers can arise from a single reaction.

Regioselectivity and Stereoselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others, while stereoselectivity indicates the preference for one stereoisomer over another. In the reaction of 1-phenylcyclohexene with chlorine, the formation of both cis and trans isomers suggests a lack of stereoselectivity, which can occur due to the stability of the halonium ion intermediate and the ability of the chlorine atoms to add from either side of the ring, leading to a mixture of products.

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Related Practice

Textbook Question

Draw the product or products that will be obtained from the reaction of cis-2-butene and trans-2-butene with each of the following reagents. If a product can exist as stereoisomers, show which stereoisomers are formed.

4. Br₂ in CH₂Cl₂

Textbook Question

What stereoisomers are obtained when (S)-3-methyl-1-pentene reacts with Cl₂?

Textbook Question

Show how you would make the following compounds from a suitable cyclic alkene.

(c)

Textbook Question

Cyclohexene is dissolved in a solution of lithium chloride in chloroform. To this solution is added one equivalent of bromine. The material isolated from this reaction contains primarily a mixture of trans-1,2-dibromocyclohexane and trans-1-bromo-2-chlorocyclohexane. Propose a mechanism to show how these compounds are formed.

Textbook Question

Using cyclooctyne as your starting material, show how you would synthesize the following compounds. (Once you have shown how to synthesize a compound, you may use it as the starting material in any later parts of this problem.)

(a) cis-cyclooctene

(b) cyclooctane

Textbook Question

a. What is the major product obtained from the reaction of propene and Br₂ plus excess Cl^-?

b. Indicate the relative amounts of the stereoisomers that are obtained.

Textbook Question

When Br₂ adds to a cis alkene that has different substituents attached to each of the two sp² carbons, such as cis-2-heptene, identical amounts of the two threo enantiomers are obtained even though Br^- is more likely to add to the less sterically hindered carbon of the bromonium ion. Explain why identical amounts of the two enantiomers are obtained.

Textbook Question

Propose a mechanism for the following reactions:

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