Pressure Mercury Arcs

Under these conditions, the excitation of most classes of organic compounds (including many solvents ) is ensured. It must be taken into account that, given the large size of the lamp, the amount of photons emitted per surface unity is low. Therefore, these lamps are most useful for external irradiation by using (quartz ) tubes for the irradiated solutions. The heating under operating conditions is modest. 

Low-pressure mercury arcs are manufactured for much more widespread use than preparative photochemistry, and therefore are by far the cheapest light source (particularly if buying them from companies selling optical components can be avoided). Furthermore, these lamps are long-lived ( 10000h, depending on how they are used), consume less energy, and require only an inexpensive transformer and a starter for operation. 

One subcategory of low-pressure lamp that might become more important in the future is the electrodeless discharge lamp, which is energized by an external field. These lamps comprise a quartz tube that has been evacuated, leaving behind a small pressure of argon and mercury or other metal or metal halide. Emission is obtained by placing the lamp in a microwave field, for example. Whilst these lamps are available commercially, they may also be built in-house rather easily [5]. 

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One way of implementing the UV disinfection process at existing activated sludge plants involves suspending the UV lights (in the form of low-pressure mercury arc UV lamps with associated reflectors) above the secondary clarifiers. The effluent is exposed to the UV radiation as it rises over the wire in a thin film. 

In 1970, Hiraoka reported that 2-cyanopyrrole, irradiated in methanol with a low-pressure mercury arc for 20 h, gave a mixture of 3-cyanopyrrole and pyrrole-2-carbaldehyde [70JCS(CC)1306]. l-Methyl-2-cyanopyrrole (38) also gave this reaction (Scheme 15) [71JCS(CC)1610]. In this case, the author isolated the product of the isomerization 39, the product of the shift in C-2 of the IV-methy 1 group 40, and a third product that was assumed to be derived from the addition of methanol to the Dewar pyrrole 41. The reaction depends on the temperature used in fact, no reaction occurred when the reaction was performed at -68°C. This result is in agreement with the presence of a thermal-activated step [78JCS(CC)131]. More... 

The irradiation of isothiazole with a low-pressure mercury arc leads to the formation of thiazole [69JCA(CC)1018], The photochemical behavior of alkyliso-thiazoles has been studied. 3-Methylisothiazole gave 2-methylthiazole in a low yield. 4-Methylisothiazole was converted into 4-methylthiazole, and 5-methyliso-thiazole gave a mixture of 5-methylthiazole (55%) and 4-methylisothiazole (Scheme 38) (72T3141 93JOC3407). Either a ZI (72T3141) or an ICI mechanism was invoked to justify these reactions (93JOC3407). 

A solution of phenol (188 mg, 2 mmol) and benzonitrile (2.06 g, 20 mmol) in McCN (20 mL) was degassed by bubbling nitrogen through it and irradiated with a 16-W low-pressure mercury arc lamp (Applied Photophysics Ltd, APQ40) for 24h. The crude product was separated by flash chromatography (EtOAc/ petroleum ether 1 5) to give yellow crystals yield 79 mg (20%) mp 53-55 C. 

The gas-phase photolysis of 2-furaldehyde in the it -n and ir <-it transitions76 proceeds with fragmentation to CO, furan and C3-hydrocarbons, but a certain amount of resinification is also noted (about 5% quantum yield with excitation of the it - n transition). The latter observation prompted a study of the vacuum liquid-phase photolysis by sunlight or by light from a medium-pressure mercury arc at room temperature24 7S. The resin obtained was submitted to fractionation and structural analysis. On the basis of the results obtained and other mechanistic evidence, the following sequence of events was postulated for the photopolymerization ... 

The checkers employed a Hanovia 450W medium-pressure mercury arc lamp with an uranium glass filter, a Pyrex immersion well, and a 1-L irradiation vessel (all available from Ace Glass). 

The photoreduction of nitrobenzene using p5o ex filtered light from a medium pressure mercury arc was studied in petroleum, toluene, ether, 2-propanol, tert-butyl alcohol, diethylamine, triethylamine, aqueous solutions of 2-propanol and diethylamine and also in aqueous t-butylalcohol containing sodium boro-hydiide 3 >. Varying amounts of aniline, azo- and azoxybenzene were obtained. In the presence of a fourty-fold excess of benzophenone, a six-fold increase in the rate of aniline formation in ethereal solution was observed, and aniline formation was completely suppressed by addition of biacetyl or octafluomaphthalene Since unreacted nitrobenzene could be recovered in these experiments, it is demonstrated that the triplet state of nitrobenzene was quenched. 

Larson et al. (1992) studied the photosensitizing ability of 2, 3, 4, 5 -tetraacetylriboflavin to various organic compounds. An aqueous solution containing aniline was subjected to a medium-pressure mercury arc lamp (X >290 nm). The investigators reported that 2, 3, 4, 5 -tetraacetylribofiavin was superior to another photosensitizer, namely riboflavin, in degrading aniline. Direct photolysis of aniline without any photosensitizer present resulted in a half-life of 23 h. In the presence of riboflavin and 2, 3, 4, 5 -tetraacetylribofiavin, the half-lives were 1 min and 45 sec, respectively. Photoproducts identified in both reactions were azobenzene, phenazine, and azoxybenzene. 

Jang and McDow (1997) studied the photodegradation of benzo[a]anthracene in the presence of three common constituents of atmospheric aerosols reported to accelerate benzo [a] anthracene, namely 9,10-anthroquinone, 9-xanthone, and vanillin. The photo-degradation experiments were conducted using a photochemical reactor equipped with a 450-W medium pressure mercury arc lamp and a water bath to maintain the solution temperature at 16 °C. The concentration of benzo [a] anthracene and co-solutes was 10" M. Irradiation experiments were conducted in toluene, benzene, and benzene-c/e- Products identified by GC/MS, FTIR, and NMR included benzo[a]an-thracene-7,12-dione, phthalic acid, phthalic anhydride, 1,2-benzenedicarboxaldehyde, naphtha-lene-2,3-dicarboxylic acid/anhydride, 7,12-dihydrobenzo[a]anthracene, 10-benzyl-10-hydroan-thracen-9-one, benzyl alcohol, and 1,2-diphenylethanol. 

Larson et al. (1992) studied the photosensitizing ability of 2, 3, 4, 5 -tetraacetylriboflavin to various organic compounds. An aqueous solution containing naphthalene was subjected to a medium-pressure mercury arc lamp (X >290 nm). The investigators reported that 2, 3, 4, 5 -... 

Figure 9. Typical high pressure Mercury-arc spectrum.

The photoreaction of polysilanes with Ceo has also been investigated [35]. Reaction (8.16) shows an example in which the irradiation in benzene with a low-pressure mercury-arc lamp afforded a product that contains 14wt% of Ceo into the polysilane chain. The incorporation of Ceo into the polysilane backbone has not been observed upon irradiation with X > 300 nm, when the cleavage of Si—Si bond does not take place. The adduct obtained from Reaction (8.16) has a lower oxidation potential than C6o( + 0-77 vs + 1.21 V) and a lower reduction potential than polysilane (—1.24 vs — 2 V). 

The electron spin resonance spectra were run in nitrogen-saturated solutions of aromatic compound ca. 10" m) and nucleophile (0-05-0-1 M) in the solvent(s) indicated. Irradiation in the cavity was effected with a high pressure mercury arc. Electrolysis was performed with the platinum cathode in the cavity, tetraethyl-ammonium perchlorate as electrolyte and electric currents of 10-250 /lA. 

UV = Light from medium or high pressure mercury arc filtered through cobalt-nickel sulfate solution Xj - 220-330 nm. 

Another generally useful apparatus for carrying out photochemical reactions on a small scale consists of a 1-kW high-pressure mercury arc, such as the General Electric Company s No. AH-6, focussed on the reaction vessel from the outside. This equipment is pictured in Figure 5. 

A solution of 0.3 g of 2-methy]-3-(2-methy]pheny])-1 -phenylpropanone in 350 mL of cyclohexane is irradiated with a Pyrex-filtered, water-cooled 450-W Hanovia medium pressure mercury arc for two weeks. The major product is purified by preparative GC followed by silica gel chromatography. 

Quantity proportional to the quantum yield of ortho-product formation,30 molded polyethylene films, quartz, medium-pressure mercury arc. d Ref. 30. 6 Ref. 71. 

A heteroaromatic nucleus can serve also as an acceptor for the migrating sulfonyl group. Thus, irradiation of numerous 4-pyrimidinyl esters of alkyl and aryl sulfonic acids (24) in alcohol with low-pressure mercury arcs readily... 

A medium-pressure mercury arc lamp consists of a sealed cylindrical quartz tube with tungsten electrodes on each end. The tube contains a small amount of mercury and starter gas (usually argon). An example of such a lamp is in Figure 3.1. ... 

Irradiation of an acetonitrile solution of octafluoronorbornadiene with a full medium-pressure mercury arc rearranged the carbon skeleton, yielding first octafluorotricyclo[3.2.0.02-7]hept-3-ene and then, on continued irradiation, perfluorobicyclo[3.2.0]hepta-2,6-diene (35).105... 

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