Textbook Question
Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.
(b)
9. Alkenes and Alkynes
Dehydration Reaction
4:46 minutesProblem 20
Textbook Question
If the compound shown in the margin is heated in the presence of H2SO4,
a. what constitutional isomer would be formed in greatest yield?
b. what stereoisomer would be formed in greater yield?
Verified step by step guidance1
Analyze the given compound: The structure shows a cyclohexane ring with a secondary alcohol (-OH group) attached to a carbon that also has a methyl group (-CH3). This is a secondary alcohol.
Understand the reaction conditions: Heating in the presence of H2SO4 (sulfuric acid) typically leads to an elimination reaction (E1 mechanism) where the alcohol is converted into an alkene via dehydration.
Determine the major constitutional isomer: The E1 mechanism proceeds through the formation of a carbocation intermediate. The most stable carbocation will form, which in this case is the secondary carbocation. The elimination will follow Zaitsev's rule, favoring the formation of the more substituted alkene. This means the double bond will likely form between the carbon bearing the -OH group and the adjacent carbon in the ring.
Consider stereoisomer formation: The elimination reaction can produce stereoisomers (E/Z isomers) depending on the spatial arrangement of substituents around the double bond. The major stereoisomer will be the one with the substituents in the most stable configuration, typically the E-isomer (trans configuration) due to reduced steric hindrance.
Summarize the expected products: The major constitutional isomer will be the more substituted alkene formed via Zaitsev's rule, and the major stereoisomer will be the E-isomer due to its stability.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Dehydration Reactions
Dehydration reactions involve the removal of a water molecule from a compound, typically resulting in the formation of a double bond. In organic chemistry, this process is common for alcohols when heated in the presence of an acid, such as sulfuric acid (H2SO4). The reaction can lead to the formation of alkenes, which can have multiple structural isomers depending on the position of the double bond.
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Constitutional Isomers
Constitutional isomers are compounds that have the same molecular formula but differ in the connectivity of their atoms. This means that the atoms are arranged in different ways, leading to distinct compounds with unique properties. In the context of dehydration reactions, the formation of different constitutional isomers can occur based on how the double bond is formed during the elimination of water.
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Stereoisomers
Stereoisomers are compounds that have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. This can lead to different physical and chemical properties. In dehydration reactions, stereoisomer formation is influenced by the orientation of substituents around the double bond, which can result in cis/trans or E/Z isomers depending on the substituents' positions.
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