The decrease in electron density of the O−H bond of o,p-dinitrophenol, the polarity of O−H bond decreases. The electron withdrawing group (−NO2)), withdraws electrons and disperses the negative charge. Therefore, −NO2 group stabilizes the phenoxide ion. Hence, o,p-dinitrophenol is more acidic than p-nitrophenol and phenol.
OMe is an electron donating group, hence it increases electron density, there why making it less acidic.
Bu−OH is an aliphatic alcohol. In phenoxide ion the negative charge is delocalized within the ring structure which makes it more stable than the aliphatic alcohols. In aliphatic alcohols there is no such delocalization stabilization. Therefore, phenols are more acidic and the resulting phenoxide ion is stabilized because of resonance.
