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Ch.8 - Reactions of Alkenes
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.8 - Reactions of AlkenesProblem 78
Chapter 8, Problem 78

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.
Reaction scheme showing 1-phenylcyclohexene converting to cis and trans isomers of 1,2-dichloro-1-phenylcyclohexane.

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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.
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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.
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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

Ozonolysis can be applied selectively to different types of carbon–carbon double bonds. The compound shown below contains two vinyl ether double bonds, which are electron-rich because of the electron-donating alkoxy groups. Ozone reacts more quickly with electron-rich double bonds and more slowly with hindered double bonds. At −78 °C, this compound quickly adds two equivalents of ozone. Immediate reduction of the ozonide gives a good yield of a single product. Show the expected ozonolyis product, and label the functional groups produced, some of which are not typical from ozonolysis of simple alkenes.

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Textbook Question

The bulky borane 9-BBN was developed to enhance the selectivity of hydroboration. In this example, 9-BBN adds to the less hindered carbon with 99.3% regioselectivity, compared with only 57% for diborane.

a. Show the two organic products generated when the trialkylborane is oxidized with H2O2/NaOH.

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Textbook Question

An inexperienced graduate student treated dec-5-ene with borane in THF, placed the flask in a refrigerator, and left for a party. When he returned from the party, he discovered that the refrigerator was broken and that it had gotten quite warm inside. Although all the THF had evaporated from the flask, he treated the residue with basic hydrogen peroxide. To his surprise, he recovered a fair yield of decan-1-ol. Use a mechanism to show how this reaction might have occurred. (Hint: The addition of BH3 is reversible.)

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Textbook Question

Propose mechanisms to explain the opposite regiochemistry observed in the following two reactions.

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Textbook Question

The bulky borane 9-BBN was developed to enhance the selectivity of hydroboration. In this example, 9-BBN adds to the less hindered carbon with 99.3% regioselectivity, compared with only 57% for diborane.

b. 9-BBN is synthesized by adding BH3 across a symmetric, cyclic diene. What is the structure of the diene?

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