Question

Which of the following is strongest acid?

• A. ${\mathrm{CHCl}}_3$
• B. ${\mathrm{CHI}}_3$
• C. ${\mathrm{CHBr}}_3$
• D. $\mathrm{CH}{\left(\mathrm{CN}\right)}_3$

Answer 1

The correct option is D

$\mathrm{CH}{\left(\mathrm{CN}\right)}_3$

The explanation for correct option:

(d) $\mathrm{CH}{\left(\mathrm{CN}\right)}_3$

1. Acidic compound has the ability to donate proton in a chemical reaction.
2. The more stable the equivalent anion generated after proton elimination, the higher the acidity of the substance.
3. The CN group is a strong electron withdrawing group that exhibits the -M and -I effects, stabilising the anion by neutralising its negative charge.
4. After the proton is removed, it produces the most stable anion.
5. So, $\mathrm{CH}{\left(\mathrm{CN}\right)}_3$ will be most acidic.

The explanation of incorrect option:

(a) ${\mathrm{CHCl}}_3$

1. Here, $\mathrm{Cl}$ group also shows electron withdrawing character with -I effect, but it weaker than $\mathrm{CN}$ group.
2. So, the corresponding anion formed will be less stablisized as compared to $\mathrm{CH}{\left(\mathrm{CN}\right)}_3$
3. It will be less acidic than $\mathrm{CH}{\left(\mathrm{CN}\right)}_3$

(b) ${\mathrm{CHI}}_3$

1. Here, $\mathrm{I}$ group is less electronegetive than both $\mathrm{Cl}$ and $\mathrm{CN}$.
2. So, it will show weak -I effect.
3. It will be least acidic compound.

(c) ${\mathrm{CHBr}}_3$

1. Here, $\mathrm{Br}$ group is less electronegetive than $\mathrm{Cl}$ and $\mathrm{CN}$, but more electronegetive than $\mathrm{I}$
2. So, it will stronger -I than in case of $\mathrm{I}$
3. It will be more acidic than ${\mathrm{CHI}}_3$ but less acidic than ${\mathrm{CHCl}}_3$and $\mathrm{CH}{\left(\mathrm{CN}\right)}_3$

Therefore, Option (d) is correct, $\mathrm{CH}{\left(\mathrm{CN}\right)}_3$ will be strongest acid.