Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

Problem 18b

Chapter 11, Problem 18b

For each pair, choose the more reactive nucleophile.
(b)

Verified step by step guidance

Step 1: Understand the concept of nucleophilicity. Nucleophilicity refers to the ability of a species to donate a pair of electrons to an electrophile. It is influenced by factors such as charge, electronegativity, steric hindrance, and solvent effects.

Step 2: Analyze the chemical structures provided. The first species is water (H₂O), which is neutral, and the second species is the acetate ion (CH₃COO⁻), which is negatively charged.

Step 3: Consider the effect of charge on nucleophilicity. Negatively charged species are generally more nucleophilic than neutral species because they have a higher electron density available for donation.

Step 4: Evaluate the role of resonance in the acetate ion. The acetate ion has resonance stabilization, which slightly delocalizes its negative charge. While this reduces its nucleophilicity compared to a localized negative charge, it is still more nucleophilic than neutral water.

Step 5: Compare the two species. Water is neutral and less nucleophilic, while the acetate ion is negatively charged and more nucleophilic despite resonance stabilization. Therefore, the acetate ion is the more reactive nucleophile in this pair.

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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 an electrophile during a chemical reaction. It is influenced by factors such as charge, electronegativity, and steric hindrance. Generally, negatively charged species are more nucleophilic than their neutral counterparts, and less sterically hindered nucleophiles are more reactive.

Solvent Effects

The solvent can significantly affect nucleophilicity. In polar protic solvents, nucleophiles are often stabilized by solvation, which can reduce their reactivity. Conversely, in polar aprotic solvents, nucleophiles are less solvated and thus more reactive. Understanding the solvent's role is crucial when comparing the reactivity of nucleophiles.

Electrophilicity

Electrophilicity is the tendency of a species to accept electrons. It is a key factor in nucleophilic substitution reactions, as a more electrophilic center will react more readily with nucleophiles. The strength of the electrophile can influence the overall reaction rate and the choice of nucleophile in a given reaction.

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Textbook Question

Would the following nucleophiles be more likely to participate in an S_N1 or S_N2 reaction?

(a)

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Order the following molecules on the basis of their nucleophilic strength using the pKₐ values of their conjugate acids.

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Even with an excess of cyanide, only one equivalent will react with the following dibromoalkane. To which carbon will the cyanide add? Predict the product and explain your choice.

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Would the following nucleophiles be more likely to participate in an S_N1 or S_N2 reaction?

(b) F^-

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Which S_N2 reaction would you expect to be faster? Explain your answer.

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Textbook Question