In aldehydes, the carbonyl group is bonded to a carbon and hydrogen, while in ketones it is bonded to two carbon atoms. The carbonyl group is bonded to oxygen are known as carboxylic acid and their derivatives.

Which series represents compounds in increasing order of acidity?

\(CF_3COOH > NO_2CH_2COOH > ClCH_2COOH > CH_3COOH\)

The correct answer is option 4. \(CF_3COOH > NO_2CH_2COOH > ClCH_2COOH > CH_3COOH\).

Understanding Acidity Trends in Carboxylic Acids

The acidity of carboxylic acids \((RCOOH)\) depends on the stability of the resulting conjugate base \((RCOO^-)\) after losing a proton \((H^+)\). Electron-withdrawing groups \((-X)\) attached to the carbon adjacent to the carboxyl group \((Cα)\) stabilize the negative charge on the oxygen, making the acid more likely to donate a proton and increasing its acidity.

Let us look at why option 4 is the correct answer:

1. \(CH_3COOH\, \ vs.\, \ ClCH_2COOH\):

\(CH_3\) is an electron-donating group (\(+I\) effect), slightly increasing electron density around the oxygen, making it less stable when negatively charged and resulting in lower acidity compared to \(ClCH_2COOH\).

\(Cl\) has a \(-I\) effect, withdrawing electron density from the oxygen and stabilizing the negative charge, making \(ClCH_2COOH\) more acidic.

2. \(ClCH_2COOH\, \ vs.\, \ NO_2CH_2COOH\):

\(Cl\) has a weaker \(-I\) effect compared to \(NO_2\). The nitro group \((-NO_2)\) is a stronger electron-withdrawing group due to its resonance structures, attracting more electron density away from the oxygen and creating a more stable negative charge, making \(NO_2CH_2COOH\) more acidic.

3. \(NO_2CH_2COOH\, \ vs.\, \ CF_3COOH\):

\(CF_3\) has an even stronger \(-I\) effect than \(NO_2\) due to its high electronegativity and inductive effect. This further stabilizes the negative charge on the oxygen, making \(CF_3COOH\) the most acidic molecule in the series.

Distance from the functional group:

While the distance from the electron-withdrawing group to the carboxylic acid group can slightly affect its influence, in these cases, the inherent strength of the \(-I\) effect dominates. \(CF_3\) and \(NO_2\) remain significantly more acidic than \(ClCH_2COOH\) even though they are one carbon away from the carboxyl group.

Therefore, the increasing order of acidity based on the electron-withdrawing strength of the substituents is: \(CF_3COOH > NO_2CH_2COOH > ClCH_2COOH > CH_3COOH\)