Caffeine (Mr 194.19) is also called, more systematically, 1,3,7-trimethylxanthine, 1,3,7-trimethyl-2,6-dioxopurine, or 3,7-dihydro-1,3,7-trimethyl-lH-purine-2,6-dione and has been referred to as a purine alkaloid.

Caffeine is odourless and has a characteristic bitter taste. It is a white powder (density (d18=4) 1.23) moderately soluble in organic solvents and water. However, its solubility in water is considerably increased at higher temperatures (1% (w/v) at 15 °C and 10% at 60 °C). Its melting point is 234-239 °C and the temperature of sublimation at atmospheric pressure is 178 °C. Caffeine is a very weak base, reacting with acids to yield readily hydrolyzed salts, and relatively stable in dilute acids and alkali. Caffeine forms unstable salts with acids and is decomposed by strong solutions of caustic alkali.

In aqueous solution, caffeine is nonionized at physiological pH. Dimers as well as polymers have been described. The solubility of caffeine in water is increased by the formation of benzoate, cinnamate, citrate, and salicylate complexes. In plants, chloro-genic acid, coumarin, isoeugenol, indolacetic acid, and anthocyanidin have been shown to complex with caffeine.

Caffeine exhibits an ultraviolet absorption spectrum with a maximum at 274 nm and an absorption coefficient of 9700 in aqueous solution. Upon crystallization from water, silky needles are obtained containing 6.9% water (a 4/5 hydrate).

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