Question

Arrange the following in increasing acidic strength
Propan-1-ol ; 2,4,6-Trinitrophenol ; Nitrophenol ; 3,5- Dinitrophenol ; Phenol ; 4-Methyl phenol

Answer 1

Dear student

For a compound to act as an acid, it first of all has to loose a proton. On loosing the proton, the anionic part so formed should be stable. The more stable the anionic part will be, the more will be tendency of the compound to loose the proton and thus more acidic will be the compound.
Now if there is any electron with drawing group on the benzene ring, the OH will easily loose the proton and become stable. In the case of the above mentioned list, 2,4,6-trinitrophenol will be maximum acidic as on loosing the proton, there are three electron withdrawing groups that will stabilise the anionic part. Second will be nitro phenol. Although there is just one nitro group but it shows -R effect and thus stabilizes the compound after loosing the proton. The third will be 3,5 -di nitro phenol. Although in this case, the nitro group can not show resonance since it is at meta position but it will be acting as an electron withdrawing group by its inductive effect. Then comes the phenol only as there is no stabilising or destabilising group. Then there is p-methyl phenol as again the negative charge will be stabilised by resonance. This will be less acidic than phenol as there is CH3 group which is having +I effect. The last will be phenol as it will be forming just a primary carbocation with only +I stabilization. So the order will will
2,4,6-trinitrophenol> nitrophenol> 3,5 -di nitro phenol> phenol> p-methyl phenol> propanol


Regards