Question

Which of the following is the stronger acid: $\mathrm{CH},\mathrm{ClCOOH}$ or ${\mathrm{CHCl}}_2\mathrm{COOH}$ ? Explain your choice.

Short answer

${\mathrm{CHCl}}_2\mathrm{COOH}$ is the stronger acid due to more chlorine atoms.

Step-by-step solution

Understand the Compounds

Identify the chemical structures of the compounds. $\mathrm{CH}\text{ is a hydrogen atom, }\mathrm{ClCOOH}\text{ and }{\mathrm{CHCl}}_2\mathrm{COOH}$ are chlorinated carboxylic acids. Specifically, $\mathrm{ClCOOH}$ stands for chloromethanoic acid (chloroacetic acid), and ${\mathrm{CHCl}}_2\mathrm{COOH}$ stands for dichloromethanoic acid (dichloroacetic acid).

Consider Inductive Effect

Determine the inductive effect of chlorine atoms. Chlorine is an electronegative element and pulls electron density away through the sigma bonds, enhancing the acidity of the carboxylic acid group. More chlorine atoms generally result in a stronger acid due to a stronger electron-withdrawing inductive effect.

Compare the Acids

Compare the effect of chlorine in each compound. $\mathrm{ClCOOH}$ has one chlorine atom, while ${\mathrm{CHCl}}_2\mathrm{COOH}$ has two chlorine atoms. The greater number of chlorine atoms in ${\mathrm{CHCl}}_2\mathrm{COOH}$ leads to a greater electron-withdrawing effect, increasing the acidity.

Conclusion

Summarize the comparison. Since ${\mathrm{CHCl}}_2\mathrm{COOH}$ has two chlorine atoms compared to the one in $\mathrm{ClCOOH}$, it experiences a stronger inductive effect and is therefore the stronger acid.

Key concepts

Chlorinated Carboxylic Acids

Chlorinated carboxylic acids are a type of carboxylic acid that contain chlorine atoms attached to their carbon chain. In these acids, the presence of chlorine significantly impacts their acidity. The two acids in our exercise are examples of this: chloromethanoic acid ($\text{ClCOOH}$) where one chlorine is attached, and dichloromethanoic acid (${\text{CHCl}}_2\text{COOH}$) where two chlorine atoms are attached.
The chlorine atoms in these compounds are vital for determining their acid strength due to their electronegativity, which we will discuss next. This electronegative nature helps enhance the acid's ability to donate protons into solution, thus increasing its acidity compared to simple carboxylic acids without chlorine substitution.

Inductive Effect

The inductive effect is an important concept when analyzing the strength of chlorinated carboxylic acids. It refers to the ability of an atom or group of atoms to pull electrons towards themselves through sigma bonds, thereby influencing the acidity of the molecule.

• When an electronegative atom like chlorine is near the carboxyl group $(-COOH)$, it draws electron density away from the acid group.
• This electron withdrawal stabilizes the negative charge on the conjugate base after the acid donates a proton ($H^{+}$).
• The more chlorine atoms present, the stronger the inductive effect, leading to more acidic behavior.

This means that in dichloromethanoic acid $({\text{CHCl}}_2\text{COOH})$, which has two chlorine atoms, there is a stronger inductive effect compared to just one in chloromethanoic acid $(\text{ClCOOH})$. Therefore, it results in a stronger acid.

Electronegativity

Electronegativity is a property of atoms that describes their ability to attract electron pairs. Chlorine is a highly electronegative element, which makes it very effective at pulling electron density towards itself.
In chlorinated carboxylic acids, the electronegativity of chlorine plays a crucial role:

• Chlorine, being highly electronegative, enhances the acidity by making the hydrogen atom in the carboxyl group easier to dissociate as a proton.
• This increased electron-withdrawing effect from electronegative atoms like chlorine leads to an increased acidic nature as it stabilizes the conjugate base.

The strength of the acid correlates with the number of electronegative substituents; in our comparison, more chlorine means more pull, enhancing acidity.

Comparative Analysis of Acids

In comparative analysis of acids, various factors are considered, such as inductive effects, number of electronegative groups, and molecular stability post-deprotonation. In this scenario, we compare two chlorinated carboxylic acids to determine which is the stronger acid by analyzing these factors.
Here’s a simplified comparison:

• $\text{ClCOOH}$ (Chloromethanoic acid) with one chlorine atom versus ${\text{CHCl}}_2\text{COOH}$ (Dichloromethanoic acid) with two chlorine atoms.
• More chlorine atoms introduce a greater inductive effect due to increased electron withdrawal, making ${\text{CHCl}}_2\text{COOH}$ the stronger acid.

Therefore, through inductive effect analysis, increased electronegativity, and direct comparison, we determine that ${\text{CHCl}}_2\text{COOH}$ is the stronger acid. Understanding these concepts helps in predicting acidity based on chemical structure effectively.