Question

The decreasing order of the acidic character is:

A) $I > II > III$
B) $III > II > I$
C) $II > I > II$
D) $III > I > II$

Answer 1

The acidic strength of an acid refers to the ability of the acid to dissociate giving a proton and an anion. To answer this question, you must recall the factors that affect the acidity of an organic compound. The factors are, the strength of the $H - A$ bond, the electronegativity of A and the factors stabilizing the conjugate base formed after the loss of proton.

Complete step by step solution:
An acid is stronger if the $H - A$ bond strength is weaker. Thus, we can say that the stronger the acid is, greater must be the polarity of the $H - A$ bond. The dissociation of acidic hydrogen ions from the molecule is easier in this case.
For this to happen, the molecule must have a strong electron withdrawing group present. Stronger is the electron withdrawing effect, more will be the polarity of $H - A$ bond and thus more will be the acidity. However, the presence of an electron donating group results in reduced electronegative character of $A$ and thereby reducing the acidity.
We know that alkanes are electron donating groups and alkenes are relatively electron withdrawing groups due to the presence of a more electronegative $sp$ hybridized carbon atom. Thus, the correct order of acidic character must be $I > II > III$

The correct answer is A.

Note:
A strong acid is an acid that ionizes completely in a solution whereas a weak acid is that acid which ionizes only partially. Strong acids are corrosive in nature and can cause serious skin burns as well. Weak acids on the other hand are mildly corrosive except those present in foods like citric acid, ethanoic acid etc. The larger is the degree of dissociation, the stronger is the acid.