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Why Picric acid (2,4,6-trinitrophenol) is weak acid then expected ?

Picric acid is weak acid than expected due to the presence of three Nitro groups (show strong -I as well as -M effect); this is fact that  6-membered cyclic H-bonding takes place between H of -OH and O atoms of -NO2 groups which lower acidity significantly than the expected acidity.

And second fact that when picric acid releases a proton it becomes an anion for which repulsion results between oxygen atoms of phenoxide ion and oxygen of nitrate group, commonly known as Steric Inhibition of Deprotonation (SID effect). This also results in slight unstability of phenoxide ion and also decrease acidity. Thus picric acid is a weak acid than expected ever.

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