Explain why phenol are more acidic than nitro phenol
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Nitro is an electron withdrawing group. It withdraws electrons by two effects namely resonance or mesomeric effect and inductive effect (-I effect). By resonance effect it withdraws electrons predominantly from ortho and para positions of a benzene ring. By inductive effect (-I effect) also it withdraws electrons but this effect is distance dependent unlike mesomeric effect.
Phenol has an acidic proton due to the stabilization of the phenoxide ion by resonance (see figure 1 below) or mesomeric effect. The negative charge on the phenoxide ion is delocalized into the benzene ring stabilizing it. This effect is common to m-nitrophenol too.
In addition, the nitro group present in meta position in m-nitrophenol can not stabilize the phenoxide ion (see figure 2 below) as much as when the nitro group present in ortho or para position by mesomeric effect. But nitro group present in meta position can withdraw electrons by -I effect, though the effect would be less than if the nitro group would have been present in ortho position. Nitro group by -I effect withdraws electrons in the following order ortho > meta > para. This effect is distance dependent. Greater the distance of the nitro group from the carbon containing OH, smaller is the -I effect.
In conclusion, in addition to the stabilization of the phenoxide ion due to mesomeric effect, there is additional stabilization in m-nitrophenol due to -I effect of the nitro group and this tilts the equilibrium to the phenoxide side making it more acidic than phenol and this is also reflected in the pKa values. The pKa values of m-nitro phenol and phenol being 8.3 and 10 respectively (see figure 3 below). [Note: Greater the pKa values less acidic is the compound].
Phenol has an acidic proton due to the stabilization of the phenoxide ion by resonance (see figure 1 below) or mesomeric effect. The negative charge on the phenoxide ion is delocalized into the benzene ring stabilizing it. This effect is common to m-nitrophenol too.
In addition, the nitro group present in meta position in m-nitrophenol can not stabilize the phenoxide ion (see figure 2 below) as much as when the nitro group present in ortho or para position by mesomeric effect. But nitro group present in meta position can withdraw electrons by -I effect, though the effect would be less than if the nitro group would have been present in ortho position. Nitro group by -I effect withdraws electrons in the following order ortho > meta > para. This effect is distance dependent. Greater the distance of the nitro group from the carbon containing OH, smaller is the -I effect.
In conclusion, in addition to the stabilization of the phenoxide ion due to mesomeric effect, there is additional stabilization in m-nitrophenol due to -I effect of the nitro group and this tilts the equilibrium to the phenoxide side making it more acidic than phenol and this is also reflected in the pKa values. The pKa values of m-nitro phenol and phenol being 8.3 and 10 respectively (see figure 3 below). [Note: Greater the pKa values less acidic is the compound].
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