Textbook Question
Predict the products formed by sodium hydroxide-promoted dehydrohalogenation of the following compounds. In each case, predict which will be the major product.
d. cis-1-bromo-2-methylcyclohexane
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9. Alkenes and Alkynes
Dehydrohalogenation
Multiple Choice
Which mechanism best describes the formation of the major product(s) in the dehydrohalogenation of ?
A
(unimolecular elimination)
B
(bimolecular nucleophilic substitution)
C
(bimolecular elimination)
D
(unimolecular nucleophilic substitution)
Verified step by step guidance1
Identify the substrate and reaction conditions: 2-bromobutane is a secondary alkyl halide, and the reaction is a dehydrohalogenation, which involves elimination of HBr to form an alkene.
Recall the common elimination mechanisms: E1 (unimolecular elimination) and E2 (bimolecular elimination). E1 involves carbocation formation and is favored by weak bases and polar protic solvents, while E2 is a one-step concerted mechanism favored by strong bases.
Consider the nature of the base and the substrate: In typical dehydrohalogenation, a strong base abstracts a proton while the leaving group (Br) leaves simultaneously, which is characteristic of the E2 mechanism.
Analyze the stereochemistry and regiochemistry: E2 elimination requires an anti-periplanar arrangement of the β-hydrogen and the leaving group, which is possible in 2-bromobutane, supporting the E2 pathway.
Conclude that the major product formation proceeds via the E2 mechanism because it is a concerted bimolecular elimination favored by the secondary alkyl halide and the reaction conditions.
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