5.7 Organic Ions
Some of the organic compounds we've already seen can also form ions. But in molecular systems, ion formation less often involves the transfer of an electron and more often involves the transfer of a proton.
We've seen that carboxylic acids are molecules that have a -CO2H group. That hydrogen atom at the end is part of a strong dipole, so it spends much of its time as a bare nucleus (proton). It can easily come off of the molecule, leaving a -CO2- ion. When this happens, we can say that the acid was deprotonated. Spare protons often attach to water molecules, making H3O+ ions.
Another place spare protons can go is on an amine. Amines, like their analogous compound ammonia, are basic (alkaline), often strongly so. Ammonia will actually pull protons off of water molecules, forming NH4+ and OH- ions. This means the ammonia has been protonated. That is why ammonia solution is actually ammonium hydroxide NH4OH.
Generally, hydrates of nonmetal oxides (for example sulfuric acid H2SO4, which can be formed from sulfur trioxide SO3 and water) form acids that can deprotonate and form a negative ion. This is why carboxylic acids are acidic, since they act like hydrated oxides of carbon.
Since a carboxylic acid can donate a proton to a nearby amine,
what about amino acids, molecules that incorporate both kinds of functional group?
As a matter of fact, amino acids do self-ionize,
forming the structure
R-CH(NH3+)CO2-,
where R means any unspecified group.
A molecule containing both positively and negatively charged parts is called a zwitterion.