Question

Rank the following compounds from strongest acid to weakest acid:

Short answer

The acidic strength of a specific compound is based on its capacity to donate protons in an aqueous solution. Stronger acids have higher dissociation constant than weaker acids.

The higher the${\mathbf{K}}_a$ value, the lower is the${\mathbf{PK}}_{\mathbf{a}}$ value, and such compounds have a high acidic strength.

Step-by-step solution

Acidic strength 

The acidic strength of a specific compound is based on its capacity to donate protons in an aqueous solution. Stronger acids have higher dissociation constant than weaker acids.

The higher the${\mathbf{K}}_a$ value, the lower is the${\mathbf{PK}}_{\mathbf{a}}$ value, and such compounds have a high acidic strength.

Ranking the given compounds from the strongest acid to the weakest acid 

The compounds are arranged from the strongest acid to weakest acid as shown below.

Arrangement of compounds from the strongest acid to weakest acid

• The compound A is the most acidic as it becomes aromatic when it loses a proton.

• Compounds B, C, and D are the second most acidic, because in all the three compounds, the proton is connected to a positively charged nitrogen.

• Compounds B and C are more acidic than the compound D because in B and C, the proton to be lost is connected to an ${\mathrm{sp}}^2$nitrogen, which is more electronegative than the${\mathrm{sp}}^3$nitrogen in D.

• The compound B is more acidic than the compound C as the uncharged nitrogen in C can contribute electrons by resonance to the positively charged nitrogen.

• The compounds E, F, and G are the least acidic. Compounds E and F are more acidic than the compound G, as E and F loses a proton from an${\mathrm{sp}}^2$nitrogen, whereas in G, the proton is lost from a less electronegative${\mathrm{sp}}^3$nitrogen.

• The compound E is more acidic than F as the negative charge in E’s conjugate base can be delocalized onto the second nitrogen.