R2NH2

R2NH2 commonly refers to the dialkylammonium cation, the conjugate acid of a secondary amine R2NH. In this form, the nitrogen carries a positive charge and is bonded to two alkyl or aryl groups and two hydrogen atoms.

Structure and nomenclature: The nitrogen in R2NH2+ is tetrahedral, bearing four substituents (two R groups and two hydrogens) with a formal positive charge. It is the protonated form of a secondary amine, produced when the lone pair on nitrogen accepts a proton.

Formation and occurrence: R2NH2+ is formed by protonation of a secondary amine, typically by strong acids such as HCl, H2SO4, or other proton sources in solution. In aqueous or nonaqueous media, the resulting salts are usually encountered as R2NH2+X−, where X− is a counterion (for example Cl− or BF4−). The equilibrium R2NH + H+ ↔ R2NH2+ shifts toward the protonated form under acidic conditions and toward the neutral amine under basic conditions.

Properties: The conjugate acid R2NH2+ is, by definition, a relatively strong acid for an ammonium-type species, with pKa values of the corresponding ammonium ion typically around 10–11 in water. The neutral secondary amine R2NH is a Lewis base and can be protonated to give the same R2NH2+ under suitable conditions. R2NH2+ salts are generally soluble in polar solvents, and the neutral amine varies in solubility depending on the R groups.

Reactions and applications: R2NH2+ salts are common reagents and intermediates in organic synthesis, often used to generate secondary amines in situ or to form salts that aid in purification and crystallization. The neutral R2NH (secondary amine) can be alkylated further to give tertiary amines and, with excess alkylating agent, quaternary ammonium salts. See also primary and tertiary amines and ammonium salts.