Textbook Question
Predict the major product(s) for each reaction. Include stereochemistry where appropriate.
e. 1-methylcyclopentene + Br2 in saturated aqueous NaCl
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10. Addition Reactions
Halohydrin
5:18 minutesProblem 22b
Textbook Question
Show how you would accomplish the following synthetic conversions.
b. chlorocyclohexane → trans-2-chlorocyclohexanol
Problem-Solving Hint: The opening of a halonium ion is driven by its electrophilic nature. The weak nucleophile attacks the carbon bearing more positive charge.
Verified step by step guidance1
Identify the starting material (chlorocyclohexane) and the target product (trans-2-chlorocyclohexanol). Note that the transformation involves the introduction of a hydroxyl group (-OH) in a trans relationship to the chlorine atom on the cyclohexane ring.
Propose a mechanism to achieve the transformation. A common approach involves the formation of an epoxide intermediate, which can be opened under acidic or basic conditions to introduce the hydroxyl group in the desired stereochemistry.
First, perform a halogenation reaction to form a halonium ion intermediate. Treat chlorocyclohexane with a halogen source (e.g., Br₂) in the presence of water. This will result in the formation of a bromonium ion intermediate, which is electrophilic in nature.
Next, water (a weak nucleophile) will attack the more positively charged carbon of the bromonium ion, leading to the formation of a trans-2-bromo-1-hydroxycyclohexane intermediate. This step ensures the anti addition of the hydroxyl group and bromine across the ring.
Finally, replace the bromine atom with a chlorine atom to achieve the desired product. This can be done using a substitution reaction, such as treating the intermediate with a source of chloride ions (e.g., NaCl) under appropriate conditions to yield trans-2-chlorocyclohexanol.
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Video duration:
5mKey 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 during the reaction of alkenes with halogens. This intermediate is characterized by a three-membered ring structure where the halogen atom is positively charged, making it highly electrophilic. The opening of this ring is crucial for subsequent nucleophilic attack, leading to the formation of alcohols or other products.
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Nucleophilic Attack
Nucleophilic attack refers to the process where a nucleophile, which is a species with a pair of electrons, attacks an electrophilic center, typically a carbon atom with a partial positive charge. In the context of the conversion from chlorocyclohexane to trans-2-chlorocyclohexanol, the weak nucleophile will preferentially attack the carbon that bears more positive charge due to the halonium ion's formation, leading to the desired product.
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Stereochemistry of Alcohols
Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the conversion to trans-2-chlorocyclohexanol, understanding the stereochemical outcome is essential, as the trans configuration indicates that the hydroxyl group and the chlorine atom are on opposite sides of the cyclohexane ring. This spatial arrangement influences the physical and chemical properties of the resulting alcohol.
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