Question

Arrange water, ethanol and phenol in increasing order of acidity and give reason for your answer.

Answer 1

The increasing order of acidity is ethanol < water < phenol. The phenoxide ion obtained after the removal of a proton is stabilised by resonance whereas the ethoxide ion obtained after the removal of a proton is destabilised by the ‘+I’ effect of the —C₂H₅ group. Therefore phenol is a stronger acid than ethanol. On the other hand, ethanol is a weaker acid than water because the electron-releasing —C₂H₅ group in ethanol increases the electron density on oxygen and hence the polarity of the O—H bond in ethanol decreases which results in the decreasing acidic strength. Hence acidic strength increases in the order given above.