Chemical Properties and Biosynthesis
Caffeine is an achiral molecule without stereoisomers.
The two amide groups of caffeine exist predominately as zwitterionic resonance structures where the nitrogen and carbon atoms are double bonded to each other so that both of these nitrogen atoms are essentially planar (in sp2 orbital hybridization). The fused ring system therefore contains a total of ten pi electrons and hence according to Hückel's rule is aromatic.
Caffeine is synthesized in plants from the purine nucleotides AMP, GMP, and IMP. These in turn are transformed into xanthosine and then theobromine, the latter being the penultimate precursor of caffeine.
Being readily available as a byproduct of decaffeination, caffeine is not usually synthesized chemically. If desired, it may be synthesized from dimethylurea and malonic acid.
Pure anhydrous caffeine is a white colorless powder with a melting point of 227–228 °C. Caffeine is moderately soluble in water at room temperature (2 g/100 mL), but very soluble in boiling water (66 g/100 mL). It is also moderately soluble in ethanol (1.5 g/100 mL). It is weakly basic (pKa = ~0.6) requiring strong acid to protonate it.
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