Q36P - Organic Chemistry (Mcmurry)

Question

20-36 Order the compounds in each of the following sets with respect to increasing acidity:

(a) Acetic acid, oxalic acid, formic acid

(b) p-Bromobenzoic acid, p-nitrobenzoic acid, 2,4-dinitrobenzoic acid

(c) Fluoroacetic acid, 3-fluoropropanoic acid, iodoacetic acid

Step-by-Step Solution

Answer

The order of the compounds in respect to increasing acidity can be found for the following sets.

Step 1: (a) Increasing the order of their acidity among acetic acid, oxalic and formic acid

The electron releasing group is methyl and the acetate ion can be destabilized. Formic acid can be more acidic than acetic acid. The electron attracting carboxyl group is Oxalic acid which can stabilize the conjugate base and obtains proton loss. Therefore, it is more acidic compared to formic and acidic acid. The increasing order can be found as,

Acidic acid < Formic acid < Oxalic acid

Step 2: (b) Increasing the order of their acidity among p-Bromobenzoic acid, p-nitrobenzoic acid, 2,4-dinitrobenzoic acid

The electron-withdrawing inductive effect and electron releasing resonance effect have been achieved in bromine of p-bromobenzoic acid. There is no stabilization of the conjugate base. It is weak compared than two in between the three names of one electron-withdrawing nitro group. Thus, p-nitro benzoic acid has 2 withdrawing groups of electrons. The increasing order can be found as,

p-Bromobenzoic acid < p-nitrobenzoic acid < 2,4-dinitrobenzoic acid

Step 3: (c) Increasing the order of their acidity among Fluoroacetic acid, 3-fluoropropanoic acid, iodoacetic acid

The iodoacetic and fluoroacetic acid has halogen on carbon. The withdrawal of inductive electron in order of F>Cl>Br>I. Fluoroacetic acid is stronger than iodoacetic acid. Fluorine on βcarbon has 3-fluoropropanoic acid. There is a decrease in inductive effect with the carboxyl group distance. The increasing order can be found as,

3-fluoropropanoic acid < iodoacetic acid < fluoroacetic acid