Question

Consider the following acids: (1) \(\mathrm{CH}_{3} \mathrm{COOH}\) (2) \(\mathrm{Cl}_{2} \mathrm{CHCOOH}\) (3) \(\mathrm{ClCH}_{2} \mathrm{COOH}\) (3) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) The correct sequence of acidity is: (a) \(3>1>2>4\) (b) \(2>3>4>1\) (c) \(1>3>4>2\) (d) \(2>4>3>1\)

Short answer

The correct sequence of acidity is (b) 2>3>4>1.

Step-by-step solution

Understanding Acidity

Acidity in carboxylic acids is largely influenced by the presence of electronegative groups located on the carbon chain. These groups stabilize the carboxylate ion formed after the acid donates a proton (H+), making the acid stronger.

Assessing the Effect of Substituents

The electron-withdrawing effect of substituents such as chlorine (3) increases acidity. The more chlorine atoms present and the closer they are to the carboxyl group, the stronger the acid.

Analyzing the Acids

(1) 3 is acetic acid without chlorine substituents, making it the weakest. (2) 1 has two chlorines adjacent to the carboxyl group, greatly increasing acidity. (3) 4 has one chlorine adjacent, increasing acidity relative to acetic acid, but less than two chlorines. (4) 5 has a chlorine further from the carboxyl group, resulting in minimal electron-withdrawing effect.

Ranking Based on Acidity

Based on the electron-withdrawing effects: - (2) 1 is the strongest due to two potent electron-withdrawing chlorine atoms. - (3) 4 follows with one chlorine close to the functional group. - (4) 5 is next, with its less effective placement of the chlorine atom. - (1) 3 is the weakest without any electron-withdrawing chlorines.

Verifying the Correct Sequence

Comparing to the options given in the problem: The strongest acid is (2) followed by (3), then (4), and the weakest is (1). Therefore, the sequence is (b) 2>3>4>1.

Key concepts

Effect of Substituents on Acidity

Have you ever wondered how certain groups attached to a molecule can influence its acidity? In carboxylic acids, substituents can have a significant impact on how acidic the molecule is. Acidity, in simple terms, is a measure of a compound's ability to donate a proton (H"). It's all about how easily a molecule can give away its proton.
One key factor affecting acidity is the nature of the substituents attached to the molecule. Substituents can either donate electrons to the molecule, making it less acidic, or withdraw electrons, enhancing its acidity. When electron-donating groups attach themselves to a molecule, they make the release of the proton less favorable, weakening the acid.
On the flip side, electron-withdrawing groups pull electrons away from the molecule. This action stabilizes the resulting anion after the proton donation, thereby increasing acidity. Remember, stronger acids have a more stable anion once the proton is released. Substituents and their position on the molecule can make a world of difference to the acid's behavior.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are key influencers in the world of chemistry. These are groups that pull electron density away from the rest of the molecule. This electron withdrawal has a profound effect, especially in carboxylic acids, where it increases the molecule's acidity.
Chlorine is a classic example of an electron-withdrawing group. It's highly electronegative, meaning it has a strong affinity for electrons. When chlorine atoms attach to carboxylic acids, they significantly stabilize the negative charge on the carboxylate ion that forms when the acid loses a proton. This stabilization makes it much easier for the acid to donate its proton, thereby increasing its acidity.
Moreover, the position of the EWG relative to the carboxyl group matters. The closer the electron-withdrawing group, such as a chlorine atom, is to the carboxyl group, the more pronounced its effect. This is because proximity enhances the ability to stabilize the negative charge on the conjugate base.

Acidity Ranking

Once you understand the impact of substituents and electron-withdrawing groups, ranking the acidity of various compounds becomes straightforward. By considering factors such as the number and position of these groups, you can determine which carboxylic acid is more acidic.
Take, for instance, the acids in our exercise. An acid with no electron-withdrawing groups, like simple acetic acid \(\mathrm{CH}_3\mathrm{COOH}\) is the weakest. With no stabilizing groups, its ability to donate a proton is limited. In contrast, \(\mathrm{Cl}_2\mathrm{CHCOOH}\) has two chlorine atoms near the carboxyl group, dramatically increasing its acidity.
Thus, we see that \(2>3>4>1\). The formula here indicates that the presence and strategic placement of EWGs can be the deciding factor. This approach to ranking relies on recognizing these minor, yet powerful influencers in molecular structure.