Animated Solution for Chemistry - Organic Chemistry: The correct decreasing order for acid strength is

Welcome to a classic problem in Organic Chemistry! This question tests your fundamental understanding of how electronic effects, specifically the Inductive Effect, influence the acidic strength of organic molecules. Let's dive deep into the molecular tug-of-war happening inside these substituted acetic acids.

Stability of the Conjugate Base

Whenever you are asked to compare the acidic strength of different compounds, the golden rule is to look at the conjugate base. When an acid donates a proton ($H^{+}$), it leaves behind a negative charge, forming a conjugate base.

For a carboxylic acid like acetic acid (${\text{CH}}_3\text{COOH}$), the conjugate base is the acetate ion (${\text{CH}}_3{\text{COO}}^{-}$). The more stable this negative charge is, the more readily the acid will give up its proton, making it a stronger acid.

How do we stabilize a negative charge? By pulling it away! If we attach an electron-withdrawing group (EWG) to the carbon chain, it will pull electron density towards itself through the sigma bonds. This is known as the $-I$ (minus Inductive) effect. By dispersing the negative charge on the carboxylate oxygen, the $-I$ effect significantly stabilizes the conjugate base.

In our problem, we have four $\alpha$-substituted acetic acids. The parent chain is the same for all: $\text{X}-{\text{CH}}_2\text{COOH}$. The only difference is the substituent '$\text{X}$' attached to the alpha carbon.

The competitors are: 1. Fluoro group ($-\text{F}$) 2. Cyano group ($-\text{CN}$) 3. Nitro group ($-{\text{NO}}_2$) 4. Chloro group ($-\text{Cl}$)

All of these are electron-withdrawing groups, meaning they all exert a $-I$ effect. But who pulls the hardest? We need to recall the standard order of the $-I$ effect power.

The strength of the $-I$ effect depends on the electronegativity and the formal charge of the atoms in the group.

- The Nitro group ($-{\text{NO}}_2$) is a powerhouse. The nitrogen atom is bonded to two highly electronegative oxygen atoms and carries a formal positive charge in its Lewis structure. This makes it incredibly electron-hungry. - The Cyano group ($-\text{CN}$) features an $sp$-hybridized carbon triple-bonded to nitrogen. The $sp$ hybridization means the carbon has $50\%$ s-character, making it highly electronegative, though slightly less so than the nitro group. - Next come the halogens. Fluorine ($-F$) is the most electronegative element on the periodic table, so it exerts a strong $-I$ effect. - Chlorine ($-Cl$) is less electronegative than fluorine, so its $-I$ effect is weaker.

Therefore, the decreasing order of the $-I$ effect is:

Since the acidic strength is directly proportional to the power of the $-I$ effect, the order of acidity will perfectly mirror the order we just established. The stronger the pull, the more stable the conjugate base, and the stronger the acid.

Thus, the correct decreasing order for acid strength is:

This perfectly matches option (c). By mastering the relative strengths of inductive effects, you can easily conquer these types of acidity and basicity ranking questions in JEE!