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Acridine is an organic compound and a nitrogen heterocycle with the formula C13H9N. Acridines are substituted derivatives of the parent ring. It is a planar molecule that is structurally related to anthracene with one of the central CH groups replaced by nitrogen. Like the related molecules pyridine and quinoline, acridine is mildly basic. It is an almost colorless solid, which crystallizes in needles. There are few commercial applications of acridines; at one time acridine dyes were popular, but they are now relegated to niche applications, such as with acridine orange. The name is a reference to the acrid odour and acrid skin irritating effect of the compound.

Acridine displays the reactions expected of an unsaturated N heterocycle. It undergoes N alkylation with alkyl iodides to form alkyl acridinium iodides, which are readily transformed by the action of alkaline potassium ferricyanide to N alkyl acridones.

Acridine and its homologues are weakly basic. Acridine is a photobase which has a ground state pKa of 5.1, similar to that of pyridine, and an excited state pKa of 10.6. It also shares properties with quinoline.

Acridines can be reduced to the 9,10 dihydroacridines, sometimes called leucoacridines. Reaction with potassium cyanide gives the 9 cyano 9,10 dehydro derivative. On oxidation with potassium permanganate, it yields acridinic acid (C9H5N(CO2H)2) otherwise known as quinoline 1,2 dicarboxylic acid.

Several dyes and drugs feature the acridine skeleton. Many acridines, such as proflavine, also have antiseptic properties. Acridine and related derivatives (such as amsacrine) bind to DNA and RNA due to their abilities to intercalate. Acridine orange (3,6 dimethylaminoacridine) is a nucleic acid selective metachromatic stain useful for cell cycle determination.

At one time acridine dyes were commercially significant, but they are now uncommon because they are not lightfast. Acridine dyes are prepared by condensation of 1,3 diaminobenzene derivatives. Illustrative is the reaction of 2,4 diaminotoluene with acetaldehyde:

9 Phenylacridine is the parent base of chrysaniline or 3,6 diamino 9 phenylacridine, which is the chief constituent of the dyestuff phosphine (not to be confused with phosphine gas), a byproduct in the manufacture of rosaniline. Chrysaniline forms red coloured salts, which dye silk and wool in a fine yellow; and the solutions of the salts are characterized by their fine yellowish green fluorescence. Chrysaniline was synthesized by O. Fischer and G. Koerner by condensing o nitrobenzaldehyde with aniline, the resulting o nitro p diaminotriphenylmethane being reduced to the corresponding o amino compound, which on oxidation yields chrysaniline. Benzoflavin, an isomer of chrysaniline, is also a dyestuff, and has been prepared by K. Oehler from m phenylenediamine and benzaldehyde. These substances condense to form tetraaminotriphenylmethane, which, on heating with acids, loses ammonia and yields 3,6 diamino 9,10 dihydrophenylacridine, from which benzoflavin is obtained by oxidation. It is a yellow powder, soluble in hot water. This compound was useful to identify the triplet nature of the genetic codes.

Acridine is a skin irritant. Its (rats, oral) is 2,000 mg/kg and 500 mg/kg (mice, oral).