Textbook Question
Indicate how each of the same factors affects an E2 reaction.
1. the strength of the base
2. the concentration of the base
3. the solvent
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8. Elimination Reactions
E2 Mechanism
Multiple Choice
Which of the following is the least reactive substrate in an reaction?
A
(1-bromobutane)
B
(1-bromopropane)
C
(methyl bromide)
D
(tert-butyl bromide)
Verified step by step guidance1
Step 1: Understand that an E2 reaction is a bimolecular elimination where a base removes a proton (β-hydrogen) from the substrate while the leaving group departs simultaneously, forming a double bond.
Step 2: Recall that the reactivity of substrates in E2 depends largely on the degree of the carbon attached to the leaving group (primary, secondary, tertiary) and the availability of β-hydrogens for elimination.
Step 3: Analyze each substrate: methyl bromide has no β-hydrogens, so it cannot undergo E2; primary alkyl bromides (1-bromopropane and 1-bromobutane) have β-hydrogens but are less reactive than tertiary; tert-butyl bromide is tertiary and highly reactive in E2 due to the stability of the transition state and the ease of β-hydrogen removal.
Step 4: Compare the primary substrates (1-bromopropane and 1-bromobutane) by considering steric hindrance and chain length; longer chains can sometimes slightly affect reactivity but generally both are primary and less reactive than tertiary.
Step 5: Conclude that methyl bromide is unreactive in E2 due to lack of β-hydrogens, tert-butyl bromide is most reactive, and among the primary substrates, the least reactive is the one with the least favorable conditions for elimination, which is 1-bromobutane.
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