Question

Rank the acids in order of increasing acid strength. \(\mathrm{CH}_{3} \mathrm{COOH}\), \(\mathrm{CH}_{2} \mathrm{ClCOOH}\), \(\mathrm{CHCl}_{2} \mathrm{COOH}\), \(\mathrm{CCl}_{3} \mathrm{COOH}\)

Short answer

From weakest to strongest: \text{CH}_3\text{COOH} < \text{CH}_2\text{ClCOOH} < \text{CHCl}_2\text{COOH} < \text{CCl}_3\text{COOH}.

Step-by-step solution

Understanding Acid Strength and Structure

Acid strength can be judged by the ability to donate a proton (H+ ion). In organic acids, the inductive effect of electron-withdrawing groups (like Cl) can stabilize the negative charge of the conjugate base after deprotonation, increasing acid strength. The more electronegative atoms or the greater number of such atoms near the acidic proton, the more acidic the compound is.

Comparing Electron-Withdrawing Effects

Examine the number and position of the chlorine (Cl) atoms in each acid. Chlorine is more electronegative than carbon and will pull electron density towards itself, stabilizing the conjugate base through the inductive effect. More chlorine atoms or chlorines closer to the acidic group (COOH) increase the acid strength.

Ranking Acid Strength

Rank the acids based on the presence and number of chlorine atoms. Acetic acid (CH3COOH) has no chlorine and is the least acidic. Monochloroacetic acid (CH2ClCOOH) has one chlorine, dichloroacetic acid (CHCl2COOH) has two chlorines, and trichloroacetic acid (CCl3COOH) has three. The more chlorines, the stronger the acid, due to an increased inductive effect.

Key concepts

Inductive Effect

The inductive effect is a fundamental concept in organic chemistry that involves the transmission of electrical charge through a chain of atoms in a molecule, resulting from the electronegativity of atoms and the polarizability of bonds.

When an atom in a molecule is bonded to a more electronegative atom, such as chlorine, the electronegative atom will pull electron density towards itself. This creates a partial positive charge on the less electronegative atom, which in turn can influence other neighboring bonds in a domino-like effect. In the context of acids like carboxylic acids, the inductive effect plays a key role in determining acid strength.

The strength of the inductive effect is determined by two primary factors: the electronegativity of the atoms involved and the distance from the site of the effect. Generally, the closer and more electronegative an atom is to the acidic hydrogen, the stronger the inductive effect will be, thereby stabilizing the conjugate base more effectively and making the acid stronger.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are parts of a molecule that, due to their high electronegativity, tend to pull electron density towards themselves. These groups, such as halogens, nitro groups, or carbonyl groups, can profoundly affect the chemical behavior of the molecule.

In the case of acids, EWGs near the acidic proton will enhance the ability of the molecule to donate a proton, thus increasing the acid strength. Chlorine atoms, for example, are excellent electron-withdrawing groups because of their high electronegativity.

• Their presence in a molecule results in a greater pull on the electron cloud.
• Multiple chlorine atoms increase this effect multiplicatively, as seen with the increased acidity from acetic acid to trichloroacetic acid.

When assessing acid strength, the number and position of EWGs can be a strong indicator of how easily the acid can donate a proton.

Conjugate Base Stabilization

The concept of conjugate base stabilization is closely related to the understanding of acid strength. When an acid donates a proton, it forms a conjugate base. The stability of this conjugate base is a critical factor in determining how readily the acid will give up the proton.

A stable conjugate base implies that the molecule can comfortably support the negative charge left behind after proton donation.

• Inductive effects and the presence of electron-withdrawing groups help distribute and stabilize this negative charge.
• The more effectively the negative charge is distributed, the more stable the conjugate base and the stronger the original acid.

Stabilization of the conjugate base through inductive effects and electron-withdrawing groups is therefore essential in understanding the variations in acid strength among different carboxylic acids.