Question

The correct order of decreasing acidity of the acids given below is 1\. \(\mathrm{Cl}_{3} \mathrm{CCH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{COOH}\) 2\. \(\mathrm{H}_{3} \mathrm{CCH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{COOH}\) 3\. \(\mathrm{Cl}_{3} \mathrm{CCH}=\mathrm{CH}-\mathrm{COOH}\) 4\. \(\mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (a) \(1>3>2>4\) (b) \(3>1>2>4\) (c) \(3>4>1>2\) (d) \(3>1>4>2\)

Short answer

The correct order is (b) 3 > 1 > 2 > 4.

Step-by-step solution

Identify Influencing Factors

The acidity of carboxylic acids is influenced by the presence of electronegative groups (like chlorine) and conjugation effects. Electronegative groups increase acidity by stabilizing the carboxylate anion.

Analyze Structure 3

Structure 3, \(\text{Cl}_3\text{CCH} = \text{CH} - \text{COOH}\), has three chlorine atoms directly attached to the carbon adjacent to the carboxylic group. This increases the acidity significantly due to the strong electron-withdrawing effect of the chlorine atoms.

Analyze Structure 1

Structure 1, \(\text{Cl}_3\text{CCH} = \text{CH} - \text{CH}_2 - \text{COOH}\), also contains three chlorine atoms, but they are one carbon further away than in Structure 3. Thus, it is less acidic than Structure 3.

Analyze Structure 2

Structure 2, \(\text{H}_3\text{CCH} = \text{CH} - \text{CH}_2 - \text{COOH}\), has no electronegative substituents, so it is less acidic compared to Structure 1 with three chlorines.

Analyze Structure 4

Structure 4, \(\text{H}_3\text{CCH}_2\text{CH}_2\text{CH}_2\text{COOH}\), is a simple aliphatic carboxylic acid with no electron-withdrawing groups, making it the least acidic among the given compounds.

Determine Decreasing Order

Based on our analysis, the order of decreasing acidity is Structure 3 (most acidic) > Structure 1 > Structure 2 > Structure 4 (least acidic). This corresponds to option (b) \(3 > 1 > 2 > 4\).

Key concepts

Electronegative Groups

Electronegative groups, like chlorine, have a profound effect on the acidity of carboxylic acids. Electronegativity refers to an atom's ability to attract and hold onto electrons. High electronegativity can cause an increase in acidity due to the presence of such groups near the carboxyl group.
For example, when chlorine atoms are attached to the carbon next to the carboxylic group, they withdraw electron density from the carboxylate anion. This withdrawal helps to stabilize the anion form, making it easier for the hydrogen ion to dissociate.
- In carboxylic acids, the more electronegative groups present, the stronger the acid becomes. - These groups need to be close to the carboxyl group for maximum effect.
Thus, the acidity of these acids can be heavily influenced by the kind and number of electronegative atoms involved.

Carboxylate Anion Stability

The stability of the carboxylate anion is a key factor in determining the acidity of carboxylic acids. The carboxylate anion is the resultant ion after a carboxylic acid donates a proton. For this anion to be stable, the negative charge must be effectively delocalized or spread over multiple atoms.
- When electronegative groups are present near the carboxylate, they pull electron density away, helping in charge distribution. - The greater the charge delocalization, the more stable the anion. A more stable anion favors the dissociation of a hydrogen ion, hence increasing the acid's strength.
In essence, if a carboxylate anion is more stable after losing a hydrogen ion, the carboxylic acid is more acidic. This stability is what makes some acids stronger than others.

Electron-Withdrawing Effect

The electron-withdrawing effect is crucial in understanding acidity changes in carboxylic acids. This effect describes how certain groups draw electron density away from other parts of a molecule. This behavior is particularly important in the context of acidity, as it can stabilize the negative charge on the carboxylate anion.
- In molecules like the ones mentioned, chlorine atoms exert a strong electron-withdrawing effect when located near the carboxylic acid group. - The closer an electronegative atom is to the acidic hydrogen, the stronger the electron-withdrawing effect, thereby increasing acidity.
In simpler terms, when groups like chlorine draw electrons toward themselves, they reduce electron density around the carboxylic group. This makes it easier for the hydrogen ion to leave, enhancing the molecule's acidity. This is why acids with strong electron-withdrawing groups are often stronger acids.