Why does this group act as a weak acid ? Alcohols also have an -OH connected to a carbon atom, and they are not considered as weak acids. The explanation lies in the electron-rich binding with the carbon atom and in a phenomenon know as resonance. The electrons in the valence shells in the atoms of an ionized carboxyl group are shown in two equivalent forms in the next sketch:
Not only is oxygen an atom with a nearly full shell of valence electrons, its double bond with carbon makes the carbon electron-rich also. When the proton leaves to confer a minus charge on the remaining group, electrons can shift so that either oxygen atom can be single bonded to the carbon while the other is double bonded. This ability to switch rapidly from one arrangement to the other is known as resonance. It allows a lower energy state while the unionized form has no such state; this is an aid to ionization. Although it is a weak effect, it is enough to confer acidic properties. The binding of the -OH in an alcohol has none of these features and an alcohol is similar to water in hardly ionizing at all.