Halochromate Salts

On the other hand, isoquinolinium (iQCC),385b p-methylpyridinium (y-PCC)390 and trimethylammonium (TMACC)391 chlorochromates closely resemble the oxidizing behaviour of PCC. p-Methylpyridinium chlorochro-mate has the distinctive advantage over PCC of containing p-methylpyridine that is less toxic than pyridine. 

6-Dicarboxypyridinium chlorochromate (2,6-DCPCC)392 possesses an acidic character that allows the in situ deprotection and oxidation of alcohols, protected as tetrahydropyranyl and trimethylsilyl ethers. 2,2 -Bipyridinium chlorochromate (BPCC)393 contains a ligand that complexes efficiently with the reduced chromium species, generated during the oxidation of alcohols, allowing for a substantial simplification of the work-ups. For this reason, it enjoys a popularity among chlorochromates surpassed by only PCC. 

Other ammonium chlorochromates, occasionally used in the oxidation of alcohols, include pyrazinium-A-oxide (PzOCC),378 naphtyridinium (NapCC),394 pyrazinium (PzCC),394 tripyridinium hydrochloride (TPCC),378a triethylammonium,378b imidazolium and l-methylimidazolium,194e and ben-zyltrimethylammonium (BTMACC)387j chlorochromates. 

Interestingly, the little studied inorganic chlorochromates, potassium397 and magnesium398 chlorochromates are very easy to prepare and are soluble in polar organic solvents, like acetone or acetonitrile. They are able to efficiently oxidize secondary alcohols to ketones, although they provide only low yields of aldehydes on the oxidation of primary alcohols. 

5-Dimethylpyrazolinium fluorochromate,403 isoquinolinium fluorochromate (iQFC)404 and quinolinium bromochromate (QBC)405 have also been described as halochromates able to oxidize alcohols. 

---

The qualitative analysis of trityl compoimds takes advantage of the orange-yellow color of the halochromic salt formed when a trityl com-poimd is treated with a solution of concentrated sulfuric acid in glacial acetic acid. 

The formation from neutral substances (triphenylcarbinol) of coloured, salt-like reaction products which are more or less easily decomposed by water is a phenomenon called halochromism . The halo-chromic salts of triphenylcarbinol are regarded as carbonium salts this follows at once from the above discussion. A quinonoid formula, by which various authors explain the colour, seems less probable. Recently the attempt has been made to attribute complex formulae to the carbonium salts (Hantzsch), in accordance with Werner s scheme for ammonium salts. Such formulae express the fact that, in the ion, the charge is not localised at the methane carbon atom, but spread over the field of force of the whole radicle. The simplest carbonium salt of the group, the yellow perchlorate (K. A. Hofmann), would accordingly have the following structural formula ... 

Sinomenine ac/tromethine [xxvin] (initially called N-methylanhydro-sinomenine [17]) is obtained when sinomenine methiodide is heated for one minute with two equivalents of 2 per cent, aqueous sodium hydroxide. It cannot be recrystallized, even from alcohol, and is best purified through the sodium salt. The name is assigned as a result of the faint yellow halochromism of a solution of the base in concentrated sulphuric acid. Like sinomenine it gives sinomenol and dibenzoylsino-menol on heating with 66 per cent, potassium hydroxide and benzoic anhydride respectively [38]. 

Solvatochromism (12, 13). In solvatochromism the dissolution of a substance is accompanied by color formation, but the cause is not the formation of salts. On the other hand, if strongly colored salts are formed under the influence of mineral or Lewis acids, the process is called halo-chromism. The difference between the two phenomena is also apparent in the changes of the type of absorption spectra. In the case of halochromism absorption curves are radically changed (old bands disappear and new ones are formed), but in the case of solvatochromism the character of absorption curves is usually unchanged, and they are only shifted (as a whole) to shorter or longer wavelengths. Halochromism will be treated below. 

Certain known color reactions cannot be included unequivocally into these groups, because they represent a combination of several of the processes mentioned. For example, in the reaction of primary aromatic amines with aromatic aldehydes (p. 215), Schiff s bases are formed which give strongly colored salts (halochromism) in the presence of acids, the color of which deepens on heating (thermochromism). We should also keep in mind that the color reaction with the organic substance can take place either directly, or it may be neccessary to transform the tested substance... 

---