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

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

JIPMERJIPMER 2014Aldehydes Ketones and Carboxylic Acids

Solution:

Acidic strength depends on the stability of anion formed after the removal of acidic hydrogen from the compound. Phenol $\left( pK _{ a }=10\right)$ will be more acidic than cyclohexanol $\left( pK _{ a }=16\right)$ because phenolate ion is stable due to resonance. After acetic acid $\left( pK _{ a }=4.8\right.$ ) 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, $pK _{ a }=0.4$ ) is most acidic because $- NO _2$ 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 $pK _{ a }$. Lower the $pK _{ a }$, the stronger is the acid.