Animated Solution for Chemistry - Organic Chemistry: Which amongst the following is the strongest acid?

When faced with a question asking to identify the strongest acid among a given set of organic compounds, the most reliable and fundamental approach is to evaluate the stability of their respective conjugate bases.

The logic is beautifully simple: an acid $\text{HA}$ dissociates to give a proton ${\text{H}}^{+}$ and a conjugate base ${\text{A}}^{-}$. If the resulting conjugate base ${\text{A}}^{-}$ is highly stable, the equilibrium of the dissociation reaction shifts heavily to the right. Consequently, the parent compound $\text{HA}$ readily gives up its proton, making it a strong acid.

Let's apply this golden rule to our options by removing an ${\text{H}}^{+}$ ion from each to form their conjugate bases (carbanions):

1. From ${\text{CHBr}}_3$, we get ${\text{CBr}}_3^{-}$ 2. From ${\text{CHI}}_3$, we get ${\text{CI}}_3^{-}$ 3. From ${\text{CHCl}}_3$, we get ${\text{CCl}}_3^{-}$ 4. From ${\text{CH(CN)}}_3$, we get ${\text{C(CN)}}_3^{-}$

Now, the battle is about which of these carbanions is the most stable. The stability of a carbanion is directly proportional to the ability of the attached groups to disperse or withdraw the negative charge away from the central carbon atom.

The Cyano Group

In the first three options, the central carbon is attached to halogens (Bromine, Iodine, and Chlorine). Halogens are electronegative and stabilize the negative charge through the inductive effect ($-I$ effect). Additionally, they can offer some stabilization through $p\pi -d\pi$ back bonding (where the lone pair on carbon delocalizes into the empty d-orbitals of the halogen). However, these effects, while significant, are relatively moderate.

Now, let's turn our attention to the fourth option, ${\text{C(CN)}}_3^{-}$. Here, the central carbon is flanked by three cyano ($-C\equiv N$) groups. The cyano group is an absolute powerhouse when it comes to electron withdrawal. It exerts a very strong $-I$ effect due to the $sp$-hybridized carbon and the highly electronegative nitrogen.

But the real magic happens through the resonance effect ($-R$ effect). The negative charge on the central carbon can delocalize extensively into the $\pi$-system of the three cyano groups.

Imagine the resonance hybrid of ${\text{C(CN)}}_3^{-}$. The negative charge doesn't just sit on the central carbon; it is smeared out and shared among three highly electronegative nitrogen atoms at the periphery of the molecule.

Because ${\text{C(CN)}}_3^{-}$ is by far the most stable conjugate base among the choices, its parent acid, ${\text{CH(CN)}}_3$, is undoubtedly the strongest acid.

Always remember: in the game of acidity, the stability of the conjugate base is the ultimate judge!