In each pair, choose the most acidic compound. Justify your answer. The most acidic proton in each compound has been indicated.
(b)

Verified step by step guidance

1. Step 1: Identify the acidic proton in each compound. In both compounds, the acidic proton is attached to the nitrogen atom in the amide functional group, as indicated by the blue arrows.
2. Step 2: Analyze the electronic effects of substituents near the acidic proton. In the first compound, there is a bromine atom attached to the carbon adjacent to the carbonyl group. Bromine is an electronegative atom and can exert an inductive effect, pulling electron density away from the nitrogen and carbonyl group, thereby stabilizing the conjugate base formed after deprotonation.
3. Step 3: Compare the second compound. The second compound lacks the bromine substituent, meaning it does not benefit from the additional stabilization provided by the inductive effect of bromine. This makes the conjugate base of the second compound less stable compared to the first compound.
4. Step 4: Recall that the stability of the conjugate base is directly related to the acidity of the compound. A more stable conjugate base corresponds to a more acidic compound.
5. Step 5: Conclude that the first compound, with the bromine substituent, is more acidic than the second compound due to the inductive electron-withdrawing effect of bromine, which stabilizes the conjugate base.

Key Concepts

Acidity and pKa

Acidity refers to the ability 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 acidity and pKa is crucial for comparing the acidity of different compounds.

Resonance Stabilization

Resonance stabilization occurs when a compound can distribute its electron density across multiple structures, leading to increased stability. In the context of acidity, if the conjugate base formed after deprotonation is resonance-stabilized, the original compound is likely to be more acidic due to the lower energy of the conjugate base.

Inductive Effect

The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents attached to a molecule. Electronegative atoms or groups can stabilize a negative charge on the conjugate base through the inductive effect, thereby increasing the acidity of the original compound. Understanding this concept helps in predicting acidity trends among different compounds.