Picric Acid from Salicylic Acid

When Salicylic Acid is nitrated, the end $\ce{COOH}$ group is getting knocked out. How should I reason the knocking. Is it right to say, since $\ce{HCOOH}$ is more acidic than $\ce{HOH}$, hence, $\ce{COO-}$ will be a weaker base than $\ce{OH-}$. And so, a weaker Base being a good leaving group, the $\ce{COOH}$ gets knocked out than $\ce{OH}$ group on the ring?
Or there is other reason for the knocking off of $\ce{COO-}$ group? 




Substitutions in benzene rings occur via *electrophilic aromatic substitution reactions*. Basically, one electrophile substitutes another electrophile. The feasibility of the reaction largely depends on the electrophilicity of the incoming electrophile, but also on the leaving ability of the departing electrophile.


In ordinary electrophilic substitution of unsubstituted benzene, the leaving group is $\ce{H+}$. This is basically the substituent (in this case $\ce{-H}$) having lost an electron. If suppose, the electrophile were to attack the carbon substituted with a carboxylate functional group ($\ce{-COO-}$), the leaving group would be $\ce{CO2}$, which is nothing but **carbon dioxide**.


You must be aware of how stable carbon dioxide is. It has two strong $\ce{C=O}$ bonds and is the product formed from various combustion and decompress reactions as well as a respiratory by-product. This means that carbon dioxide is an excellent leaving group.


In addition, the carboxylate group is located at *ortho* position to the hydroxy group. Considering the fact that hydroxy group is an *ortho/para* directing group, this further supports substitution at $\ce{COO-}$ substituted carbon.


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