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Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 24 - Benzene II: Reactions Influenced by the Aromatic RingProblem 9
Chapter 23, Problem 9

Which proton, Ha or Hb, would you expect to have the lower pKa value?
Chemical structure showing two protons, Hₐ and H_b, with arrows indicating their positions for acidity comparison.

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1
Step 1: Understand the relationship between pKa and acidity. The pKa value is inversely related to the acidity of a proton. A lower pKa value indicates a more acidic proton.
Step 2: Analyze the chemical environment of Hₐ and H_b. Consider factors such as electronegativity of nearby atoms, resonance stabilization, and inductive effects that can influence the acidity of each proton.
Step 3: Evaluate resonance effects. If the removal of a proton leads to a conjugate base that is resonance-stabilized, the proton is likely to be more acidic and have a lower pKa value.
Step 4: Consider inductive effects. Electronegative atoms or groups near the proton can withdraw electron density, making the proton more acidic and lowering its pKa value.
Step 5: Compare the environments of Hₐ and H_b based on the above factors to determine which proton is more acidic and thus has the lower pKa value.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pKₐ and Acidity

pKₐ is a measure of the acidity of a proton in a molecule, with lower pKₐ values indicating stronger acids. The pKₐ value reflects the tendency of a proton to dissociate from its parent molecule. Understanding the relationship between pKₐ and acidity is crucial for predicting which proton is more likely to be released in a chemical reaction.
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Proton Position and Stability

The position of a proton in a molecule can significantly influence its acidity. Protons attached to more electronegative atoms or those that are part of a resonance-stabilized structure tend to have lower pKₐ values. Analyzing the molecular structure helps determine which proton is more stable when dissociated, thus affecting its acidity.
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Resonance and Inductive Effects

Resonance and inductive effects can stabilize the conjugate base formed after a proton is lost, leading to lower pKₐ values. If the removal of a proton results in a resonance-stabilized anion or is influenced by electronegative substituents, the corresponding proton will be more acidic. Understanding these effects is essential for predicting the relative acidity of protons in different environments.
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Related Practice
Textbook Question

Look closely at the resonance structures of the benzylic anions, radicals, and cations in Figures 24.2–24.4. On which benzene carbons did the charge/radical exist in the resonance structures—that is, were they ortho, meta, or para?

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Suggest a phenoxide and an alkyl halide to make the following aryl alkyl ethers.

(a)

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Fluoxetine hydrochloride (Prozac®) is a widely used antidepressant. How might you stereoselectively install the indicated ether functional group (ROR) in (R)-fluoxetine?

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Textbook Question

What is the product of the following reaction?

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Textbook Question

Rationalize the rate difference in carbocation formation for the following molecules.

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Textbook Question

Though the nitro group is electron-withdrawing by resonance, when in the meta position, it doesn’t communicate by resonance with the phenoxide anion. And yet, the pKₐ value of m-nitrophenol is lowered from 10 (the pKₐ value of unsubstituted phenol) to 8.4. Explain this observation.

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