Tungsten-iodine filament lamp

Sources of ultraviolet radiation include (a) tungsten-filament incandescent lamps (b) tungsten-iodine cycle lamps with quartz envelopes (c) mercury-vapor lamps and (d) the zinc discharge lamp. Odier types are available, but enjoy only limited application. The hydrogen or deuterium lamps are used in the laboratory, but are delicate and costly for process uses. 

A solution of the steroid (50 mg, 0.12 mmol) in cyclohexane (10 ml) containing DIB (40 mg, 0.12 mmol) and iodine (28 mg, 0.11 mmol) was irradiated with two 100 W tungsten-filament lamps for 50 min at 40°C. The reaction mixture was poured into water and extracted with ether the organic layer was washed with aqueous sodium thiosulphate and then water, dried, concentrated and the residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 9 1) to give 6/ ,19-epoxy-5a-cholestan-3/I-yl acetate (45 mg, 90%) m.p. 114-115°C. 

A solution or suspension of the acid (1 mmol) in carbon tetrachloride (75 ml) containing DIB (0.55 mmol) and iodine (0.5 mmol) was irradiated with two 100 W tungsten-filament lamps for 45 min at reflux temperature. Another portion of DIB (0.55 mmol) was then added and irradiation was continued for 45 min at reflux. The reaction mixture was washed with dilute sodium thiosulphate and water, concentrated and chromatographed (silica gel column, 9 1 hexanes-ethyl acetate) to afford the alkyl iodide. Several steroidal acids with the carboxyl group attached at a 1° or 2° carbon atom gave the corresponding iodides in good yields. Acids with a 3° a-C instead of the iodide afforded alkenes similarly, alkenes were formed with a fivefold excess of DIB in the presence of cupric acetate. Aromatic acids also underwent iododecarboxylation, in moderate yields very effective was the otherwise difficult transformation of 1,8-naphthalenedicarboxylic acid to 1,8-diiodonaphthalene (80%) [68]. Cubyl and homocubyl iodides were also prepared in excellent yield [69]. 

More recently, heating by photo-radiation with high intensity photons from quartz iodine or tungsten filament lamps has become popular. One advantage of photoradiation is that often only the substrate is heated significantly. 

A soln. of startg. alcohol in cyclohexane containing 1.7 eqs. acetoxy(hydroxy)di-phenylselenurane and 1 eq. iodine irradiated with two 100 W tungsten-filament lamps for 8 h at 55-60° - product. Y 64%. F.e. inch steroid derivs. s. R.L. Dorta et al.. Tetrahedron Letters 29, 5429-32 (1988). 

Quartz-iodine lamp A tungsten filament high-intensity incandescent lamp which contains iodine in a quartz envelope. Used primarily as a source of visible light. 

Small additions of iodine and bromine are used in halogen lamps. The bulbs are small and made of quartz. Vaporized tungsten from the filament forms tungsten iodide/bromide with the gas and this compound reacts with the filament and redeposit tungsten on it. In these types of lamps the filament can have a much higher temperature than in conventional incandescent lamps, 3200 K compared to 2700 K. 

To overcome these problems the envelope of the tungsten halogen lamp contains a trace of one of the halogen gases, iodine, chiorine, bromine or fluorine. This allows a reversible chemical action to occur between the tungsten filament and the halogen gas. 

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