Final answer:
The acidity of the α-carboxy group is attributed to the ability to ionize and release H+ due to the combination of the carbonyl and hydroxyl groups in -COOH, resonance stabilization of the conjugate base, and the polar nature of the bonds within the group.
Step-by-step explanation:
The acidity of the α-carboxy group is due to several factors. Firstly, the presence of the carbonyl group (C=O) and the hydroxyl group (-OH) on the same carbon atom in carboxylic acids, represented as -COOH, introduces a unique polar nature due to their electronegativity differences. This arrangement leads to the ability of the carboxyl group to ionize and release H+ ions into a solution, making them acidic.
Furthermore, the resonance effect plays a critical role. Resonance stabilization occurs when the negative charge following the release of the hydrogen ion is shared between different atoms within the molecule. Compared to protons on alkanes, protons on a carbon adjacent to a carbonyl group (α-carbon) are much more acidic. This increased acidity is because resonance allows for the distribution of the negative charge over multiple atoms, thus stabilizing the conjugate base formed after deprotonation. This stabilization is not possible in alkanes, whose protons have a pKa around -60, indicating very low acidity. In contrast, the α-carboxy group has a pKa around 20, showing significantly higher acidity.
The carboxylic acid group also contains three polar bonds: C=O, C-O, and O-H. The oxygen atoms are more electronegative than carbon and hydrogen, thereby making the carbon of the C-O an electrophile and the oxygen a nucleophile or base, and the associated hydrogen atom an acidic proton.
Due to these factors, including the ability to release H+, the presence of a strong electronegative element, as well as the resonance stabilization, carboxyl groups provide the acidity to their parent molecule, making them hydrophilic and crucial in biological systems such as amino acids and fatty acids.