Question

Question: Rank the labeled protons $(H_a-H_b)$in mandelic acid, a naturally occurring carboxylic acid in plumsand peaches, in order of increasing acidity. Explain in detail why you chose this order.

Short answer

Answer

The increasing acidity of protons is

${\mathrm{H}}_{\mathrm{a}}<{\mathrm{H}}_{\mathrm{b}}<{\mathrm{H}}_{\mathrm{c}}$

The conjugate base obtained by removing the proton is the most stable, resonance stabilized, and has a negative charge on the oxygen. Hence, the proton is considered the most acidic among all the other protons.

Step-by-step solution

Step-by-Step SolutionStep 1: Acidic protons

Numerous molecules comprise different kinds of protons. The most acidic proton takes part in an acid-base reaction when functioning as an acid.

Protons situated on the carbon are mostly non-acidic as the carbon atom is not electronegative.

Mandelic acid

Mandelic acid belongs to the category of hydroxy acid found in several drugs and cosmetic items. It was earlier employed to cure urinary tract infections.

Acidity of the protons of mandelic acid

The removal of the ${\mathrm{H}}_{\mathrm{a}}$ proton from mandelic acid leads to an unstable conjugate base with a negative charge on carbon. The species obtained by removing the proton can be given as:

Removal of ${\mathrm{H}}_{\mathrm{a}}$proton from mandelic acid

The removal of the ${\mathrm{H}}_{\mathrm{b}}$proton from mandelic acid leads to a more stable conjugate base with a negative charge on oxygen. The species obtained by removing the proton can be given as:

Removal of ${\mathrm{H}}_{\mathrm{b}}$ proton from mandelic acid

The removal of ${\mathrm{H}}_{\mathrm{c}}$ proton from mandelic acid leads to the most stable conjugate base that is resonance stabilized with a negative charge on oxygen. The species obtained by removing the proton can be given as:

Removal of ${\mathrm{H}}_{\mathrm{c}}$ proton from mandelic acid

The conjugate base obtained by removing ${\mathrm{H}}_{\mathrm{a}}$is the least acidic, as it has a negative charge on carbon atoms that makes it unstable.

The conjugate base formed by removing the proton is more acidic than ${\mathrm{H}}_{\mathrm{b}}$ and ${\mathrm{H}}_{\mathrm{a}}$because of the resonance stabilization that imparts extra stability.

Hence, the acidity of the protons can be given as:

${\mathrm{H}}_{\mathrm{a}}<{\mathrm{H}}_{\mathrm{b}}<{\mathrm{H}}_{\mathrm{c}}$