Question

The correct order of acidic strength of carboxylic acid is:

(A) formic acid < benzoic acid < acetic acid

(B) formic acid < acetic acid < benzoic acid

(C) acetic acid < formic acid < benzoic acid

(D) acetic acid < benzoic acid < formic acid

Answer 1

First convert the acids into their conjugate bases. Check the stability of each one relative to the other. The most stable conjugate base’s acidic form is the most acidic.

Complete step by step solution:

Let us first know the compounds listed in the options:

- Formic acid HCOOH

- Acetic acid $C{{H}_{3}}COOH$

- Benzoic acid ${{C}_{6}}{{H}_{5}}COOH$

These are acids so they will have conjugate bases. A conjugate base of an acid is when the acid has been deprotonated. This comes from the Bronsted-Lowry concept of acid and bases which states: acids are proton donating species and bases are proton accepting species.

So first let us convert all the above mentioned acids into their respective conjugate bases:

- Conjugate base of formic acid is $HCO{{O}^{-}}$

- Conjugate base of acetic acid is $C{{H}_{3}}CO{{O}^{-}}$

- Conjugate base of benzoic acid is ${{C}_{6}}{{H}_{5}}CO{{O}^{-}}$

From the looks of it above, all of them bear a negative charge on one of the oxygen atoms of the carboxylic functional group. This particular functional group can undergo the phenomenon of resonance and therefore both the double bonds are partial in nature. It is as shown below:

But as it is the same for all the three of them we cannot decide their stability solely on this factor. Therefore we go to one of the fundamental properties for help, which is the “inductive effect”.

The inductive effect is only seen in single bonds and the electronegativity of an atom is the only deciding factor. It says if a relatively more electronegative (E.N.) atom is present in a molecule then the electrons in the single bond tend to shift towards it. For this reason the E.N. the atom in question gains a partial negative charge and all the other atoms gain a partial positive charge. Let us look at this property through acetic acid.

- As you can see there is a partial positive charge on the carbon atom of the methyl group. The methyl group is electron-donating in nature. Donation of extra electrons to the already negatively charged carboxylic group makes it unstable. While if we compare this to formic acid, the “H” atom does not donate any electron and therefore it’s carboxylic group is more stable than acetic acid’s. We can conclude here that the conjugate base of formic acid is more stable than that of acetic acid, which implies formic acid is more acidic than acetic acid.

- The acidity is due to the stability of the conjugate base. Here, In Formic acid resonance is stabilised via two equivalent resonating structures. On the other hand, in case of Benzoic acid the conjugate base is stabilised by resonance of benzene ring. Due to the more stable conjugate base of formic acid it is more acidic.

So, the correct answer is “Option D”; acetic acid < benzoic acid < formic acid.

Note: No matter the strength of the inductive effect, resonance is always stronger. The stability of the conjugate bases of respective acids should be compared and not the stability of the acids itself. You have to keep in mind the order given in options and answer accordingly.