Question

The compound with highest \(p K_{a}\) is (A) \(\mathrm{Br}_{3} \mathrm{CCOOH}\) (B) \(\mathrm{Br}_{2} \mathrm{CHCOOH}\) (C) \(\mathrm{BrCH}_{2} \mathrm{COOH}\) (D) \(\mathrm{CH}_{3}-\mathrm{COOH}\)

Short answer

The compound with the highest pKa value is (D) CH3COOH, as it has no bromine atoms and thus a weaker electron-withdrawing effect, making it a weaker acid as compared to the other options.

Step-by-step solution

Understand the effect of bromine atoms on acidity

Bromine atoms are highly electronegative and thus, would draw electron density away from the carboxylic acid functional group. It would cause the acidic hydrogen in the COOH group to become more easily donated as a proton (positive charge), resulting in an increase in acidity. In general, adding more electronegative atoms to an acid causes it to become stronger.

Compare the number of bromine atoms in each given option

Now let us look at the number of bromine atoms on each compound. (A) Br3CCOOH: 3 bromine atoms (B) Br2CHCOOH: 2 bromine atoms (C) BrCH2COOH: 1 bromine atom (D) CH3COOH: No bromine atoms

Identify the strongest acid among the given options

Br3CCOOH has the most number of bromine atoms, which means that it will have the highest electron-withdrawing effect, thus making it the strongest acid among the given options.

Identify the compound with the highest pKa value

Since Br3CCOOH is the strongest acid, it will have the lowest pKa value, as pKa is inversely proportional to the acidity of a compound. The compound with the least number of bromine atoms should have the highest pKa value, because it has lesser effect of electron-withdrawing. As CH3COOH has no bromine atoms, so it will have a higher pKa value as compared to other compounds. The correct answer is: (D) CH3COOH

Key concepts

Electron-Withdrawing Effects

Understanding electron-withdrawing effects is crucial in studying the acidity of organic compounds. Atoms or groups that are capable of withdrawing electrons from the rest of the molecule can have a profound impact on its properties. For instance, halogens like bromine are highly electronegative and tend to pull electron density towards themselves.

When attached to the vicinity of acidic protons, such as those in carboxylic acids, these electronegative atoms increase the stability of the resultant anion after the proton is donated. This is done by delocalizing or spreading the negative charge over a larger area, which in organic chemistry, often translates to increased acidity.

Carboxylic acids with more halogen substituents will typically release their protons more readily due to stronger electron-withdrawing effects. This forms an important principle in predicting the acid strength of related organic compounds.

pKa Values

pKa values are a numerical scale used to express the strength of an acid. Essentially, it's the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution. The lower the pKa value, the stronger the acid, because it has a greater tendency to donate its proton to the base.

In the given exercise, the presence of electron-withdrawing bromine atoms influences the stability of the deprotonated form of the acid, thus affecting its pKa value. Acids with higher electron-withdrawing effects due to additional bromine atoms will have lower pKa values indicating their increased strength.

A key takeaway is that pKa values serve as an inverse measure of acid strength: the lower the pKa, the higher the acidity. Organic chemists frequently use pKa values as a reference point to predict reaction outcomes and the behavior of compounds under various conditions.

Carboxylic Acids

Carboxylic acids are a group of organic compounds characterized by the functional group COOH, which consists of a carbonyl and a hydroxyl group. They are widely known for being stronger acids compared to other organic compounds, due to the ability of the functional group to stabilize the negative charge after ionization.

The presence of substituents connected to the carbon atom of the COOH group can significantly alter the acid's strength. As demonstrated by the exercise, the addition of electron-withdrawing groups such as bromine atoms increases the acid strength. In contrast, electron-donating groups would lessen the acid's ability to donate protons and result in weaker acidity.

Understanding the structure and functionality of carboxylic acids is fundamental in organic synthesis, as their acidity plays a pivotal role in numerous chemical reactions, including esterification and the formation of amide bonds in peptide synthesis.