Textbook Question
Paying close attention to the stereochemical outcome, predict the product of these elimination reactions.
(c)
8. Elimination Reactions
Cumulative Substitution/Elimination
3:03 minutesProblem 93c
Textbook Question
Indicate which species in each pair gives a higher substitution-product-to-elimination-product ratio when it reacts with isopropyl bromide:
c. Cl− or Br−
Verified step by step guidance1
Identify the type of reaction mechanism involved: Isopropyl bromide is a secondary alkyl halide, so the reaction can proceed via either an SN2 (substitution) or E2 (elimination) mechanism. The nucleophile/base will influence the ratio of substitution to elimination products.
Analyze the nucleophiles Cl⁻ and Br⁻: Both are halide ions, but their nucleophilicity and basicity differ. Cl⁻ is a stronger nucleophile than Br⁻ in polar aprotic solvents, which favors substitution reactions. Br⁻, being larger and more polarizable, is less nucleophilic but can still participate in substitution.
Consider the basicity of Cl⁻ and Br⁻: Both are weak bases, but Cl⁻ is slightly stronger than Br⁻. Weak bases generally favor substitution over elimination, as elimination typically requires a stronger base to abstract a β-hydrogen.
Evaluate the solvent effects: If the reaction occurs in a polar aprotic solvent, Cl⁻ will be more nucleophilic than Br⁻, leading to a higher substitution-product-to-elimination-product ratio. In contrast, polar protic solvents may reduce the nucleophilicity of Cl⁻ more significantly than Br⁻, potentially altering the ratio.
Conclude based on the above factors: Cl⁻ is expected to give a higher substitution-product-to-elimination-product ratio compared to Br⁻ when reacting with isopropyl bromide, primarily due to its stronger nucleophilicity in polar aprotic solvents and its slightly higher basicity.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nucleophilicity
Nucleophilicity refers to the ability of a species to donate an electron pair to form a chemical bond. In the context of substitution reactions, stronger nucleophiles are more likely to participate in nucleophilic substitution, leading to a higher ratio of substitution products. The strength of nucleophiles can be influenced by factors such as charge, electronegativity, and solvent effects.
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Substitution vs. Elimination Reactions
Substitution reactions involve the replacement of one functional group in a molecule with another, while elimination reactions involve the removal of a small molecule (like HBr) to form a double bond. The ratio of substitution to elimination products can be affected by the structure of the substrate and the nature of the nucleophile, with steric hindrance and the leaving group's ability playing significant roles.
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Leaving Group Ability
The leaving group ability is a measure of how easily a group can depart from the parent molecule during a reaction. Good leaving groups, such as bromide (Br⁻), stabilize the transition state and facilitate substitution reactions. In comparing Cl⁻ and Br⁻, Br⁻ is generally a better leaving group, which can influence the product distribution in reactions with isopropyl bromide.
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