Textbook Question
Give the expected major product for each reaction, including stereochemistry where applicable.
(c)
(d)
10. Addition Reactions
Hydrogenation
3:23 minutesProblem 35
Textbook Question
A misguided chemist attempted to synthesize trans-1,2-dimethylcyclohexane via the hydrogenation of 1,2-dimethylcyclohexene. Explain why this is not possible.
Verified step by step guidance1
Understand the structure of 1,2-dimethylcyclohexene: It is a cyclohexene ring with two methyl groups attached at the 1 and 2 positions. The double bond is between these two carbon atoms.
Recognize the stereochemistry involved: The term 'trans' refers to the configuration where substituents are on opposite sides of the cyclohexane ring. In cyclohexenes, the double bond restricts rotation, so the initial configuration of substituents is crucial.
Consider the process of hydrogenation: Hydrogenation involves adding hydrogen (H₂) across the double bond, converting it into a single bond. This process typically occurs on the same side of the molecule, leading to a 'cis' configuration.
Analyze the stereochemical outcome: During hydrogenation, the addition of hydrogen atoms to the double bond in 1,2-dimethylcyclohexene will result in both methyl groups being on the same side of the ring, forming cis-1,2-dimethylcyclohexane.
Conclude why trans-1,2-dimethylcyclohexane cannot be synthesized this way: The hydrogenation process does not allow for the formation of a trans configuration due to the syn addition mechanism, which inherently leads to a cis product.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Stereochemistry of Cyclohexane
Cyclohexane can exist in different conformations, such as chair and boat, which affect the spatial arrangement of substituents. In the chair conformation, substituents can be axial or equatorial, influencing their interactions and stability. Understanding these conformations is crucial for predicting the stereochemistry of reactions involving cyclohexane derivatives.
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Hydrogenation Reaction Mechanism
Hydrogenation involves the addition of hydrogen (H2) across a double bond, typically using a metal catalyst like palladium or platinum. This reaction converts alkenes to alkanes, but it does not alter the stereochemistry of the substituents on the carbon atoms involved. Therefore, the stereochemistry of the starting alkene dictates the stereochemistry of the product.
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Cis-Trans Isomerism
Cis-trans isomerism refers to the arrangement of substituents around a double bond or a ring structure. In cyclohexene derivatives, the cis configuration has substituents on the same side, while trans has them on opposite sides. Hydrogenation of 1,2-dimethylcyclohexene, which is cis, cannot yield trans-1,2-dimethylcyclohexane due to the preservation of stereochemistry during the reaction.
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