Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Problem 85

Chapter 9, Problem 85

Rank the indicated hydrogen in the following compounds from most acidic to least acidic:

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Step 1: Identify the functional groups in each compound. In compound (a), the hydrogens are alpha to two ketone groups. In compound (b), the hydrogens are alpha to two ketone groups but separated by an additional carbon. In compound (c), the hydrogens are alpha to one ketone group.

Step 2: Recall that the acidity of alpha hydrogens is influenced by the electron-withdrawing effects of nearby carbonyl groups. The more carbonyl groups present and the closer they are to the alpha hydrogens, the more acidic the hydrogens will be.

Step 3: Analyze compound (a). The alpha hydrogens are flanked by two ketone groups directly, which strongly stabilize the conjugate base through resonance, making these hydrogens highly acidic.

Step 4: Analyze compound (b). The alpha hydrogens are also flanked by two ketone groups, but the ketones are separated by an additional carbon. This reduces the resonance stabilization compared to compound (a), making these hydrogens less acidic than those in (a).

Step 5: Analyze compound (c). The alpha hydrogens are flanked by only one ketone group, providing less resonance stabilization compared to compounds (a) and (b). Therefore, these hydrogens are the least acidic.

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Key Concepts

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

Acidity and pKa

Acidity in organic chemistry refers to the tendency of a compound to donate a proton (H+). The strength of an acid is often measured by its pKa value; lower pKa values indicate stronger acids. Understanding the relationship between structure and acidity is crucial, as factors like electronegativity and resonance can significantly influence a compound's ability to release a proton.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, allowing for the delocalization of electrons. This delocalization can stabilize the negative charge that forms when an acid donates a proton, making the compound more acidic. Identifying resonance structures helps predict which hydrogen atoms are more acidic based on their ability to stabilize the resulting anion.

Inductive Effect

The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents on a molecule, which can influence acidity. Electronegative atoms or groups can pull electron density away from the hydrogen atom, stabilizing the conjugate base and increasing acidity. Understanding how different substituents affect the acidity of hydrogen atoms is essential for ranking their acidity in organic compounds.

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