Phenols are stronger acids than alcohols because the phenoxide ion left after the removal of 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 much weaker acid than any phenol.
Thus propan- 1 – ol is a much weaker acid than any phenol. We know that electron donating groups decrease the acidic character and stronger is the electron donating group, weaker is the phenol.
Compare to propan – 1 – ol, 4 – methyl phenol is stronger acidic character. But comparing phenol and 4-methyl phenol, phenol is stronger acidic. Since electron withdrawing groups increase the acidic character of phenols and the effect is more pronounced at the para position than at the meta position.
Therefore 4 – nitro phenol is a stronger acid than 3 – nitro phenol. Further as the number of electron withdrawing groups increases the acidic strength further increases. Therefore 2, 4, 6 – trinitro phenol is a stronger acid than 3, 5 – dintiro phenol. It may be noted here that although the two nitro groups in 3, 5 – dinitro phenol are at m – position with respect to OH group, their combined effect is however greater than one nitro group at p – position. Therefore 3, 5 – dinitro phenol is a stronger acid than 4-nitro phenol. Thus, the overall increasing order of acid strength is. Propan – 1 – 01 < 4 – methyl phenol < phenol < 3 – nitrophenol < 3, 5 – dinitro phenol < 2, 4, 6 – trinitro phenol.
