Arrange the following compounds in increasing order of their acid strength: Propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol, phenol, 4-methylphenol.

Explanation

(i) Phenols are stronger acids than alcohols, because the phenoxide ion left after the removal of a proton is stabilized by resonance while the alkoxide ion left after the removal of a proton from alcohol is not stabilized. Thus, propan-1-ol is a much weaker acid than any phenol, i.e., 4-methylphenol, 4-methoxyphenol, 4-nitrophenol, 3-nitrophenol, 3,5-dinitrophenol, and 2,4,6-trinitrophenol. we know that electron-donating groups decrease the acidic character and stronger is the electron we know that electron-donating groups decrease the acidic character and stronger is the electron-donating group, weaker is the phenol. since methoxy group (on a benzene ring) is a stronger electron-donating group than methyl group, therefore, 4-methoxyphenol is a weaker acid than 4-methylphenol and both are weaker acids than phenol. Since electron-withdrawing groups increase the acidic character of phenols, and the effect is more pronounced at the p-position than at the m-position, therefore, 4-nitrophenol is a stornger acid than 3-nitrophenol. further as the number of electron-withdrawing groups increases, the acidic strength further increases, therefore, 2,4,6-trinitrophenol is a stronger acid than 3,5-dinitrophenol. it may be noted here than although the two nitro groups in 3,5-dinitrophenol are at m-position w.r.t. OH group, their combined effect is, however, greater than one nitro group at p-postion. therefore, 3,5-dinitrophenol is a stronger acid than 4-nitrophenol. thus, the overall increasing order of acid strength is: propan-1-ollt4-methoxyphenollt4-methylphenolltphenollt3-nitrophenollt4-nitrophenollt3,5-dinitrophenollt2,4,6-trinitrophenol.