Textbook Question
Write a balanced equation for each reaction, showing the major product you expect.
(d)
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8. Elimination Reactions
SN1 SN2 E1 E2 Chart (Big Daddy Flowchart)
4:39 minutesProblem 25a
Textbook Question
For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.
a. 1−bromo−1−methylcyclohexane + NaOH in acetone
Verified step by step guidance1
Analyze the structure of the starting material: The compound is 1-chloro-1-methylcyclohexane, which is a secondary alkyl halide due to the carbon attached to the chlorine being bonded to two other carbons.
Identify the reagent: tert-BuO⁻Na⁺ (sodium tert-butoxide) is a strong, bulky base. Bulky bases favor elimination (E2 mechanism) over substitution because steric hindrance makes it difficult for the base to approach the carbon for substitution.
Determine the reaction mechanism: Since the base is strong and bulky, the reaction will proceed via the E2 elimination mechanism. In this mechanism, a β-hydrogen (hydrogen on a carbon adjacent to the carbon bearing the leaving group) is abstracted, and the leaving group (Cl⁻) departs simultaneously, forming a double bond.
Predict the product: The β-hydrogens are located on the carbons adjacent to the carbon bearing the chlorine. Elimination can occur to form a double bond between the α-carbon (the carbon with the chlorine) and one of the β-carbons. Due to steric hindrance, the major product will be the less substituted alkene (Hofmann product), as the bulky base prefers to abstract the less hindered β-hydrogen.
Label the major product: The major product will be 1-methylcyclohexene, as it is the less substituted alkene. The minor product, if formed, would be 1,2-dimethylcyclohexene (the more substituted alkene, Zaitsev product).
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Substitution Reactions
Substitution reactions involve the replacement of one functional group in a molecule with another. In organic chemistry, this often occurs with alkyl halides, where a nucleophile attacks the carbon atom bonded to the halogen, leading to the formation of a new bond and the release of the halide ion. The two main types of substitution reactions are SN1 and SN2, which differ in their mechanisms and kinetics.
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Elimination Reactions
Elimination reactions involve the removal of a small molecule, typically a halide and a hydrogen atom, from adjacent carbon atoms, resulting in the formation of a double bond. These reactions can be classified as E1 or E2, depending on whether they proceed through a carbocation intermediate or a concerted mechanism. The choice between substitution and elimination often depends on the structure of the substrate and the reaction conditions.
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Reaction Conditions and Solvent Effects
The choice of solvent and reaction conditions significantly influences whether a substitution or elimination reaction predominates. Polar protic solvents favor SN1 and E1 mechanisms by stabilizing carbocations, while polar aprotic solvents favor SN2 and E2 mechanisms by enhancing nucleophilicity. In this case, the use of acetone, a polar aprotic solvent, suggests that elimination may be favored, especially with a strong base like NaOH.
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