Hydrogen bonding occurs when hydrogen atoms are attached to highly electronegative atoms like
nitrogen, oxygen, or fluorine. Hydrogen bonding is the result of the extreme polarity of these bonds.
In the case of a hydroxyl group, the electronegative oxygen atom pulls electron density away from
the less electronegative hydrogen atom. This generates a slightly positive charge on the hydrogen
and slightly negative charge on the oxygen. Then, the partially positive hydrogen of one molecule
electrostatically attracts the partially negative oxygen of another molecule, generating a
noncovalent bonding force known as a hydrogen bond.
The hydroxyl hydrogen is weakly acidic, and alcohols can dissociate into protons and alkoxide ions
in the same way that water dissociates into protons and hydroxide ions. Table 5.1 gives pKa values of
several hydroxyl-containing compounds.
Table 5.1. pKa Values of Hydroxyl-Containing Compounds
Dissociation pKa
H 2 O HO− + H+ 14.0
CH 3 OH CH 3 O− + H+ 15.5
C 2 H 5 OH C 2 H 5 O− + H+ 15.9
i-PrOH i-PrO− + H+ 16.5
t-BuOH t-BuO− + H+ 17.0
CF 3 CH 2 OH CF 3 CH 2 O− + H+ 12.5
PhOH PhO− + H+ 10.0BRIDGE
Remember from Chapter 10 of MCAT General Chemistry Review that pKa = –logKa. Strong
acids have high Ka values and low pKa values. Thus, phenol, which has the smallest pKa, is the
most acidic of the alcohols listed in Table 5.1.Looking at Table 5.1, we can see that the hydroxyl hydrogens of phenols are more acidic than those
of other alcohols. This is due to the aromatic nature of the ring, which allows for the resonance