NMR spectra of OH protons of phenols obtained by addition of various metallic salts

Effect of added water on the chemical shifts of OH protons

Abstract

The NMR spectra of phenols in aqueous acetone solution gave broadened signals of their OH protons as the result of proton exchange between the OH group and the H₂O molecule. But both the OH and H₂O protons gave well resolved signals in the presence of small amounts of metallic salts and the signals were shifted towards the low field. The shift values of the OH and H₂O protons varied greatly depending on the quantity of water and kind of phenol; phenols with smaller pK_a's gave larger shift values. In this case, the difference of the shift values between OH and H₂O protons increased with increasing the amount of water added. This result is contrary to the general explanation that the both signals of OH and H₂O protons approach to each other and overlap under the influence of proton exchange. The result mentioned above was considered to be due to the following mechanism: Phenols are dissociated into [H₃O] ⁺ and the phenoxide ion by the addition of water. Because this phenoxide ion has intense affinity to OH protons, the strong hydrogen bonds are formed between the OH group and the phenoxide ion. Accordingly, the signals of the OH protons are shifted towards the low field.