Which of the following is the stronger acid: \(\mathrm{CH}_{2} \mathrm{ClCOOH}\) or \(\mathrm{CHCl}_{2} \mathrm{COOH}\) ? Explain your choice.

The inductive effect in organic chemistry is a fundamental concept that describes the transmission of charge across bonded atoms in a molecule due to differences in electronegativity. Electronegativity is the tendency of an atom to attract electrons toward itself. When a highly electronegative atom is bonded to a less electronegative one, the electron density is shifted toward the more electronegative atom.

Consider a simple hydrocarbon chain; if a chlorine atom is attached to one end, it pulls electron density through the sigma bonds. This creates a partial negative charge (δ−) on the chlorine, and a corresponding partial positive charge (δ+) propagates down the chain. This redistribution of electron density along the bonds is known as the inductive effect, and it has important implications for the chemical reactivity and properties of molecules.

For instance, in the context of carboxylic acids, the inductive effect can affect the acid strength by stabilizing or destabilizing the conjugate base that forms when the acid donates a proton (H+). If the inductive effect leads to greater stabilization of the conjugate base, the acid is stronger because it more readily loses its proton.

Electron withdrawing groups (EWGs) are substituents or functional groups attached to a molecule that can pull electron density toward themselves due to their high electronegativity. This characteristic alters the distribution of electrons in the molecule, often affecting its reactivity and physical properties.

Common examples of EWGs include nitro groups (–NO2), cyano groups (–CN), and halogens (like –F, –Cl, –Br). These groups are capable of participating in resonance (delocalizing electrons) or influencing the molecule through the inductive effect mentioned earlier.

In carboxylic acids, the presence of EWGs near the carboxyl group (-COOH) can greatly enhance the acid's strength. They stabilize the negative charge that develops on the oxygen atom of the carboxylate ion after the acid donates a proton. This stabilization facilitates proton detachment, hence, enhances the acidity of the molecule. The more EWGs present, and the closer they are to the acidic functional group, the stronger the acid generally becomes.

Comparing the acid strength of organic compounds often involves looking at the stability of the conjugate base formed after deprotonation. A more stable conjugate base suggests a stronger acid. Stability can be influenced by several factors, including the inductive effect and the presence of electron withdrawing groups.

As shown in the original exercise, dichloroacetic acid ( \(CHCl_{2}COOH\) ) is a stronger acid than monochloroacetic acid ( \(CH_{2}ClCOOH\) ) because it has more chlorine atoms acting as EWGs. These chlorine atoms pull electron density away from the carboxyl group, making it easier for the acid to lose a proton. The effect is cumulative: more EWGs lead to a greater inductive effect, thus a more stable conjugate base, and a stronger acid.

When comparing acid strengths, it's crucial to consider the number and position of EWGs. EWGs closer to the acidic proton have a greater influence on acidity. Other factors can also play a role, such as resonance stabilization and hybridization, but a strong indicator of a stronger acid is the presence of more electronegative atoms or groups within proximity to the acidic hydrogen.