Chemical Properties and Artefact Formation

Most alkaloids have basic properties with pKa values of about 6 to 12, but usually 7-9. The free base is soluble in organic solvents and not in water. Protonation of the nitrogen in the free base usually results in a water-soluble compound. This behaviour is the basis of the selective isolation of alkaloids by liquid/liquid partitioning processes. Quaternary alkaloids are poorly soluble in organic solvents but soluble in water at any pH.

Many alkaloids are difficult to crystallize in the form of the free base, but do crystallize as a salt. Alkaloids are usually colourless; only some highly conjugated compounds are coloured or show strong fluorescence (e.g. berberine and serpentine).

Alkaloids are not very stable; in particular, N-oxidation is quite common. Stability is influenced by solvents, as well as heat and light. Halogen-containing organic solvents such as chloroform and di-chloromethane are widely used in alkaloid research.

Chloroform in particular is a very suitable solvent, because of its relatively strong proton donor character. However, this solvent easily causes the formation of artefacts, e.g. (N-)oxidation occurs easily. Dichloromethane may result in the formation of quaternary N-dichlorometho-compounds. Similar compounds are formed with the minor impurities present in chloroform. Peroxides in ethers may also cause N-oxidation.

Alkaloids are more stable in toluene, ethyl acetate and alcoholic solutions. Carbinolamine functions are often found in alkaloids, either formed during the coupling of a carbonyl group and an amine in the biosynthesis, or as products formed from rearrangements of N-oxides. Carbinolamines readily react with alcohols (e.g. O-methyl pseudostrychnine formed from pseudostrychnine with methanol). Ketones such as acetone and methylethylketone are well-known artefact formers. Berberine, for example, may react with acetone. Ammonia and acetone may react during column chromatography, yielding condensates that give a Dragendorff-positive reaction. Ammonia may also react with aldehydes present in plant materials, giving rise to artificial alkaloids, e.g. the py-ridine-type alkaloid gentianine is formed from swero-side during extraction.

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