Consider the molecular models shown here, where X represents a halogen atom. (a) If X is the same atom in both molecules, which molecule will be more acidic?

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Identify the functional groups in both molecules A and B. Molecule A has a hydroxyl group (-OH) attached to a carbon that is also bonded to a halogen (X). Molecule B has a carboxyl group (-COOH) attached to a carbon that is bonded to a halogen (X).

Recall that the acidity of a molecule is influenced by the stability of its conjugate base. The more stable the conjugate base, the more acidic the molecule.

Consider the inductive effect of the halogen (X). Halogens are electronegative and can withdraw electron density through the sigma bonds, stabilizing the conjugate base.

Compare the inductive effect in both molecules. In molecule B, the halogen (X) is closer to the carboxyl group, which can enhance the electron-withdrawing effect, making the conjugate base more stable.

Conclude that molecule B will be more acidic than molecule A because the inductive effect of the halogen (X) is stronger in molecule B, leading to a more stable conjugate base.

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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 refers to the ability of a compound to donate a proton (H+) in solution. The strength of an acid is often measured by its pKa value; lower pKa values indicate stronger acids. In the context of the given molecules, the presence of electron-withdrawing groups can stabilize the negative charge on the conjugate base, thus increasing acidity.

Inductive Effect

The inductive effect describes the electron-withdrawing or electron-donating influence of substituents on a molecule. Halogens, being electronegative, exert a strong inductive effect, which can stabilize the conjugate base of an acid. In the comparison of the two molecules, the position and number of halogen atoms can significantly affect their acidity through this effect.

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 conjugate base formed after deprotonation, enhancing acidity. In the context of the two molecules, if one has a carbonyl group adjacent to the acidic site, it may provide additional resonance stabilization, making it more acidic.

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