Question

The correct order of acidic strength of (i) Alcohol (ii) Acetic acid (iii) 2,4,6-Trinitrophenol (iv) Phenol is

• A. (ii) > (iii) > (iv) > (i)
• B. (iii) > (ii) > (iv) > (i)
• C. (iii) > (iv) > (ii) > (i)
• D. (iv) > (iii) > (ii) > (i)

Answer 1

Answer: (B)

Acidic strength depends on the stability of anion formed after the removal of acidic hydrogen from the compound. Phenol $\left(p{K}_a=10\right)$ will be more acidic than cyclohexanol $\left(p{K}_a=16\right)$ because phenolate ion is stable due to resonance. After acetic acid $(p{K}_a=4.8$ ) loses $H^{+}$, negative charge will resonate between two $O$ atoms that makes it more acidic than phenol, but not more acidic than picric acid. 2,4,6-trinitrophenol (picric acid, $p{K}_a=0.4$ ) is most acidic because $-N{O}_2$ group present in ortho and para positions will make the resonating negative charge more stable due to its -M effect. This order can also be seen from their $p{K}_a$. Lower the $p{K}_a$, the stronger is the acid.