Textbook Question
Indicate how each of the following factors affects an E1 reaction:
1. the strength of the base
2. the concentration of the base
3. the solvent
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8. Elimination Reactions
E1 Reaction
Multiple Choice
Which of the following reaction sequences best represents an elimination of an alkyl halide with a carbocation rearrangement?
A
Treatment of -bromobutane with ethanol, forming a carbocation that rearranges to a more stable tertiary carbocation before elimination to yield 2-butene.
B
Treatment of -bromobutane with sodium ethoxide, resulting in direct elimination to form 1-butene without any rearrangement.
C
Treatment of tert-butyl chloride with water, leading to direct elimination to form 2-methylpropene without any carbocation rearrangement.
D
Treatment of 1-chloropropane with potassium tert-butoxide, resulting in a concerted elimination to form propene.
Verified step by step guidance1
Identify the key characteristics of an E1 elimination reaction: it proceeds via a two-step mechanism involving the formation of a carbocation intermediate, which can undergo rearrangement if a more stable carbocation is possible.
Examine the alkyl halide substrate to determine if carbocation rearrangement is feasible. For example, a secondary carbocation can rearrange to a more stable tertiary carbocation through hydride or alkyl shifts.
Look for reaction conditions that favor E1 mechanisms, such as weak bases and polar protic solvents (e.g., ethanol or water), which stabilize the carbocation intermediate and do not strongly favor a concerted elimination.
Analyze the given reaction sequences to see which involves formation of a carbocation intermediate that rearranges before elimination. For instance, treatment of 2-bromobutane with ethanol can form a secondary carbocation that rearranges to a tertiary carbocation, followed by elimination to form 2-butene.
Contrast this with other options that involve strong bases or primary substrates, which typically favor E2 or direct elimination without rearrangement, confirming that the sequence involving 2-bromobutane and ethanol best represents an E1 elimination with carbocation rearrangement.
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