Amidin

Amidin is an alternative spelling used in some sources for the organic functional group known as amidine. An amidine is characterized by the general structure RC(=NH)NR'R'', where R, R' and R'' are hydrogen or organic substituents. The group is formally derived from imidic and amide features and is closely related to guanidines. In neutral form, the carbon is doubly bonded to a nitrogen (imide-type) and singly bonded to an amino-type nitrogen; resonance can place positive charge on one nitrogen in the amidinium tautomer, which contributes to its high basicity.

Structure and properties: Amidines are typically highly basic and form stable salts with acids. Their conjugate acids (amidinium cations) have pKa values commonly in the range of 10–13, depending on substitution. The C=N–NR' unit allows delocalization of electrons, giving characteristic imidoyl behavior and enabling nucleophilic attack at either nitrogen center in some reactions. Substituents on the carbon or on the amino nitrogen strongly influence basicity, solubility, and overall reactivity.

Preparation: Amidine derivatives are prepared by several routes. A common approach is the condensation of nitriles with ammonia or primary amines under dehydrating conditions. Other methods include conversion of cyanamides with amines or transformations from formamides under dehydrating/rearrangement conditions, and routes starting from amidoximes. The choice of method depends on the desired substitution pattern and the target application.

Reactions and applications: Amidine groups act as bases and nucleophiles, and amidines readily form amidinium salts upon protonation. They can be N-alkylated or N-arylated to give substituted amidines. Benzamidine and related derivatives serve as important building blocks in medicinal chemistry (notably as protease inhibitors) and as ligands in coordination chemistry. Amidine-containing motifs also appear as intermediates in heterocycle synthesis.