Hydrochlorides are the salts resulting from the reaction of hydrochloric acid with an organic molecule. Hydrochlorides are frequently produced through reactions with amine groups and are particularly important in pharmacology for making hydrophobic molecules water-soluble so they can be absorbed into blood and bodily fluids.
Reaction of hydrochloric acid with a basic molecule results in a salt consisting of the base and the proton from hydrochloric acid and the negatively charged chloride ion. When mixed in aqueous solution, the base and chloride ion dissociate in the same way that sodium chloride dissociates into a positively charged sodium ion and a chlorine ion in water.
The positive charge imparted by the proton to the base allows it to be soluble in water. As many organic compounds are not normally soluble, this is valuable in chemistry for allowing solutions of these molecules to be made. In medicine, many drugs are composed largely of organic compounds with low solubility, so converting these compounds into a hydrochloride salt allows them to be easily absorbed by the body. For example, the antidepressant drug paroxetine hydrochloride consists largely of benzene rings that are normally highly hydrophobic, so it would be poorly absorbed into the bloodstream without being first converted to a hydrochloride salt.