Explain why CH3CH2OH (pKa = 15.9) predominates in a solution of pH 4.5.

The pH of the solution is more acidic than the pKa of the OH group (15.9). A compound will mainly exist in its acidic form (with its proton) in solutions that are more acidic than the pKa value of the group that undergoes dissociation (pH is less than pKa).