Textbook Question
Suggest an appropriate base to synthesize the alkene as the major product from the starting haloalkane.
(a)
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8. Elimination Reactions
SN1 SN2 E1 E2 Chart (Big Daddy Flowchart)
8:53 minutesProblem 68
Textbook Question
The following chlorocyclohexane undergoes neither Sₙ2 nor E2 under the conditions shown. Why?
Verified step by step guidance1
Analyze the structure of chlorocyclohexane: Chlorocyclohexane is a cyclohexane ring with a chlorine atom attached to one of the carbons. The chlorine atom is a good leaving group, but the reaction mechanism depends on the conditions provided.
Understand the Sₙ2 mechanism: The Sₙ2 mechanism requires a strong nucleophile and a substrate that is not sterically hindered. In the case of chlorocyclohexane, the cyclohexane ring creates steric hindrance, especially if the chlorine is in the equatorial position, making it difficult for the nucleophile to attack the carbon bearing the chlorine.
Understand the E2 mechanism: The E2 mechanism requires a strong base and an anti-periplanar arrangement of the β-hydrogen and the leaving group. In chlorocyclohexane, the anti-periplanar geometry may not be achievable due to the conformational constraints of the cyclohexane ring, especially if the chlorine is in the equatorial position.
Consider the reaction conditions: If the conditions provided do not include a strong nucleophile (for Sₙ2) or a strong base (for E2), neither mechanism will proceed. Additionally, steric hindrance and conformational constraints further inhibit these reactions.
Conclude why neither mechanism occurs: The combination of steric hindrance, conformational constraints, and potentially unsuitable reaction conditions prevents both the Sₙ2 and E2 mechanisms from occurring in this case.
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Video duration:
8mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Sₙ2 Mechanism
The Sₙ2 mechanism is a bimolecular nucleophilic substitution reaction where a nucleophile attacks an electrophile, leading to the simultaneous displacement of a leaving group. This reaction typically occurs in primary or secondary substrates due to steric accessibility. In the case of chlorocyclohexane, steric hindrance from the cyclohexane ring can prevent effective nucleophilic attack, making Sₙ2 unlikely.
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General Mechanism
E2 Mechanism
The E2 mechanism is a concerted elimination reaction where a base abstracts a proton while a leaving group departs, resulting in the formation of a double bond. This reaction is favored in secondary and tertiary substrates due to steric factors. In chlorocyclohexane, the ring structure may hinder the necessary anti-periplanar arrangement of the leaving group and the hydrogen, thus inhibiting E2 reactivity.
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Steric Hindrance
Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In bulky molecules like chlorocyclohexane, the presence of large groups can obstruct the approach of nucleophiles or bases, making reactions like Sₙ2 and E2 less favorable. Understanding steric hindrance is crucial for predicting the reactivity of organic compounds.
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02:53Understanding steric effects.
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