Question

Which compound in each pair is the stronger acid?

a.

b.

c.

Short answer

a. The compound on the right is more acidic.

b. The compound on the left is more acidic.

c. The compound on the right is more acidic.

Step-by-step solution

Conjugate Base

The species generated succeeding the loss of protons from acid is referred to as its conjugate base.

The more stable is the conjugate base, the higher is the acidity of its corresponding acid.

Acidity

The presence of electronegative atoms on an acidic molecule (whose hydrogen ions donation is easy) increases the stability of the conjugate base.

The electronegative atoms can pull the electron density generated on the molecule by the loss of proton and stabilize it.

The more the number of electronegative atoms is, the higher is the acidity of the molecule.

Acidic strength of the given compounds

a.

Compounds given in option a

Compound 2 is more acidic than compound 1 as the fluorine atom is more electronegative than the chlorine atom. Hence, the conjugate base of 2 has higher stability than that of 1.

b.

Compounds in option b

In this case, both compounds consist of the same number of chlorine atoms.

The acidity of these compounds can be compared by considering the distance between the acidic hydrogen and the electronegative chlorine atoms.

The compound where the chlorine atoms are closer to the acidic hydrogen will be more acidic as chlorine can easily pull the electron density towards it.

In compound 1, the chlorine atoms are attached to the second carbon, while in 2, they are attached to the third carbon.

Therefore, compound 1 will be more acidic.

c.

Compounds in option c

Compound 2 is more acidic as it consists of the electron-withdrawing ${\mathrm{NO}}_2$group that can stabilize the conjugate base of the acid.