Textbook Question
The following alkenes can be prepared by dehydration of an appropriate alcohol. Show the structure of the alcohol in each case that would provide the alkene shown as the major product.
e. 1,4-Pentadiene
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14. Compounds with Oxygen or Sulfur
Alcohol Reactions: Dehydration Reactions
Problem 9b
Textbook Question
What alcohols yield the following alkenes as the major product on dehydration?
b. 
Verified step by step guidance1
Step 1: Understand the dehydration reaction. Dehydration of alcohols involves the removal of a water molecule (H₂O) to form an alkene. This reaction typically occurs in the presence of an acid catalyst, such as H₂SO₄ or H₃PO₄, and heat.
Step 2: Analyze the given alkene structure. Identify the carbon atoms involved in the double bond and their substituents. This will help determine the possible alcohol precursor(s).
Step 3: Recall the mechanism of alcohol dehydration. The reaction proceeds via the E1 or E2 mechanism, depending on the type of alcohol (primary, secondary, or tertiary). For secondary and tertiary alcohols, the E1 mechanism is more common and involves the formation of a carbocation intermediate.
Step 4: Work backward from the alkene. To determine the alcohol precursor, add an -OH group to one of the carbon atoms in the double bond. Consider all possible positions for the -OH group that would lead to the given alkene as the major product. Keep in mind Zaitsev's rule, which states that the more substituted alkene is usually the major product.
Step 5: Verify the structure of the alcohol(s). Ensure that the proposed alcohol(s) can undergo dehydration to yield the given alkene. Consider the stability of the carbocation intermediate (if applicable) and the regioselectivity of the reaction.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Dehydration of Alcohols
Dehydration of alcohols is a chemical reaction where an alcohol loses a water molecule, resulting in the formation of an alkene. This process typically occurs under acidic conditions and involves the protonation of the alcohol, followed by the elimination of water. Understanding this mechanism is crucial for predicting the products formed during the reaction.
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Zaitsev's Rule
Zaitsev's Rule states that in elimination reactions, the more substituted alkene is usually the major product. This principle helps predict the outcome of dehydration reactions by favoring the formation of alkenes that are more stable due to greater substitution. Recognizing this rule is essential for determining which alkene will be predominant in a given dehydration scenario.
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1:24Zaitsev’s Rule Concept 2
Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In the context of alcohol dehydration, regioselectivity helps identify which alkene is formed based on the structure of the starting alcohol. This concept is vital for understanding how different alcohols can lead to different alkene products during dehydration.
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