Textbook Question
Although 2-methyl-1,2-propanediol is an unsymmetrical vicinal diol, only one product is obtained when it is dehydrated in an acidic solution.
b. Why is only one product obtained?
8. Elimination Reactions
E1 Reaction
4:54 minutesProblem 47a
Textbook Question
Predict the product of the following rearrangement-prone E1 eliminations.
(a) 
Verified step by step guidance1
Identify the substrate structure and determine if it is prone to rearrangement. Rearrangement typically occurs when a more stable carbocation can form during the E1 mechanism. Look for secondary or tertiary carbocations that can rearrange to a more stable form (e.g., tertiary or resonance-stabilized carbocations).
Understand the E1 mechanism: The first step involves the loss of the leaving group to form a carbocation intermediate. Write the structure of the carbocation formed after the leaving group departs.
Check for possible carbocation rearrangements. This can include hydride shifts (movement of a hydrogen atom with its bonding electrons) or alkyl shifts (movement of an alkyl group with its bonding electrons) to form a more stable carbocation. Draw the rearranged carbocation if applicable.
Once the most stable carbocation is identified, proceed to the elimination step. A base will abstract a β-hydrogen (a hydrogen atom on a carbon adjacent to the carbocation), leading to the formation of a double bond. Identify all possible β-hydrogens and predict the major product based on Zaitsev's rule (the more substituted alkene is usually favored).
Draw the final product(s) of the reaction, ensuring that you account for any rearrangements and the regioselectivity of the elimination step. If multiple products are possible, indicate the major product based on stability and substitution patterns.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
E1 Mechanism
The E1 mechanism is a type of elimination reaction that occurs in two steps: first, the formation of a carbocation intermediate after the leaving group departs, followed by the loss of a proton to form a double bond. This mechanism is favored in polar protic solvents and typically involves tertiary or stabilized carbocations, which can rearrange to more stable forms.
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Carbocation Stability
Carbocation stability is crucial in predicting the outcome of E1 reactions. Carbocations are classified as primary, secondary, or tertiary based on the number of alkyl groups attached to the positively charged carbon. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects, making them more likely to form during the reaction.
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05:58Determining Carbocation Stability
Rearrangement of Carbocations
Carbocation rearrangement occurs when a less stable carbocation transforms into a more stable one, often through hydride or alkyl shifts. This process is significant in E1 reactions, as the stability of the final product can be influenced by the rearrangement, leading to different alkene products based on the most stable carbocation formed during the reaction.
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