A SCHIFF base contains a functional group that possesses a characteristic double bond between carbon and nitrogen in which nitrogen is directly connected to an aryl or alkyl group. Schiff bases were first synthesized in 1864 by a German chemist Hugo Schiff. The general formula of Schiff base is R1CH=NR2, where R1 and R2 are aryl, cycloalkyl, alkyl or heterocyclic groups. These bases possess azomethine group (-C=N) so these are also called as azomethines, imines or anils.
In nature, an imine group (-C=N) is present in the molecular chain of Ancistrocladidine that is produced as a secondary metabolite in plants of family Dioncophyllaceae and Ancistrocladaceae. This linkage of carbon and nitrogen is also similar to protein opsin that is present in the retina of eye. Due to the presence of lone pair of electrons on nitrogen and the nucleophilic character of double bond, Schiff bases are basic in nature. These are crystalline form insoluble salts by reaction with strong acids. Formation of Schiff base is a reversible reaction. These are sensitive to moisture and decompose when exposed to air. Schiff Bases containing alkyl substituents are generally unstable because in aliphatic aldehydes polymerization occurs readily. Schiff bases that possess aryl substituents are stable due to conjugation and can easily be isolated. The nature of amine as well as carbonyl compounds tells about the mechanism and the position of equilibrium. At room temperature, aromatic aldehydes are very reactive towards amine to form a stable Schiff base even without the removal of water.
Several reactions are used to synthesize Schiff base, due to its simplicity and high yield condensation reaction between an amine and a carbonyl compound takes place. In trityl N-haloamines, rearrangement of an alkyl group takes place from carbon to nitrogen by using either PCl5 or a base catalyst known as Stieglitz rearrangement. In Hoesch reaction imine, is formed as an intermediate when nitrile and phenol with HCl and ZnCl2 reacted at low temperature. During the formation of ketones when nitrile reacts with an organometallic compound, imine is formed that again hydrolyzed to form ketone.
It is very interesting to note that Schiff bases find their applications in different fields of science, medicine and technology. In organic chemistry, Schiff bases act as an intermediate in chemical analysis and synthesis. These are used in the synthesis of different organic compounds by the acylation of Schiff bases from acid chlorides, acid anhydrides and acyl cyanides. In Strecker synthesis, Schiff bases react with HCN forming an amino acid precursor. These are also utilized in the preparation of Ruhemanns purple used for identifying fingerprints. In inorganic chemistry, Schiff bases are used in the synthesis of pigments, dyes, lubricating oils, heat resistant polymers, photographic emulsions, antiknocking agents, high temperature stabilizers and liquid crystals. These bases act as ligands with transition metals to form complexes. Substituted Schiff bases are bidentate ligands. These bases also act as corrosion inhibitors. In gas chromatography, Schiff bases are used as a stationary phase.
In the field of biomedicine, Schiff bases are used for the clarification of racemization and transamination reactions in biological systems. Anti-inflammatory, anti-fungal, and anti-bacterial activities have been found in complexes in which ligands are oxygen donor e.g. N-(Salicylidene)-2-hydroxyaniline is identified for showing promising effect against Mycobacterium tuberculosis, while the nitrogen donor ligands show anticandida activities. Schiff bases derived from isatin possess anti-HIV properties. These are also used in the destruction of parasites and protozoa. Schiff bases are an active component of antimalarial drugs. Schiff base derivative N-hydroxy-N-aminoguanidine has an ability to develop depleted hemoglobin, RBCs and WBCs to the balance content. These are also used to reduce the growth of tumor cells. In modern technology, these organic bases are used to control and measure the intensity of radiation in imaging system of optical computers. In photo electrochemical processes these are used as catalysts. Schiff bases are used as photostabilizers and dyes for solar filters due to their photochromic properties.
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