Photo-oxidation of Floating Hydrocarbon Oils in the Presence of some Naphthalene Derivatives B Y WILLIAM H. K. SANNIEZ AND NEITON PILPEL" Department of Pharmacy, Chelsea College, University of London, Manresa Road, London, SW3 6LX Received 6th October, 1976 Samples of pure isopropylbenzene and methylbenzene and of a commercial mineral oil, Dobane, containing various concentrations of 1-hydroxynaphthalene, naphthalene-a-aldehyde and l-nitro- naphthalene were floated on water as layers 0.6 cm thick and irradiated with ultraviolet light over periods of 240 min. By combining measurements of the changes occurring in interfacial tension with chemical and physical analysis of the oil and water phases, it has been possible to identify some of the products of photo-oxidation and to propose mechanisms for the reactions.The oxidation of hydrocarbons is a complex process which involves a large number of concurrent and consecutive, independent and connected stages and this frequently makes it difficult to analyse the re~ults.l-~ Oxidation by photochemical methods can sometimes be useful for providing mild conditions which can be controlled to prevent the onset of complex secondary reacti0ns.l Recent work has shown that oxidation of certain hydrocarbons can also be effected by irradiating them with U.V. light in the presence of suitable photo sensitizer^.^-^ Thus anthracene and certain polynuclear hydrocarbons act as photosensitizers for the homolytic decomposition of acetyl peroxide, yielding the same products as in the thermal reaction. 1 -hydroxynaphthalene acts as a photosensitizer for the oxidation of certain alkanes and alkylbenzene~.~* Chemical reactions in organic compounds can sometimes be investigated by floating them on the surface of an aqueous substrate and observing the changes that occur in their phase potential and surface pressure?* lo* l1 In the present work, hydrocarbon oils containing selected additives were floated on triply distilled water and irradiated with U.V.light for various periods of time. By combining measurements of the changes in their interfacial tensions with physicochemical analysis of the oil and water phases, it was hoped to establish the mechanisms and kinetics of the photochemical reactions. EXPERIMENTAL MATERIALS Puriss isopropylbenzene and AnalaR methylbenzene were further purified by passing them three times under slight pressure through a tightly packed bed of Fuller's earth 2.5 cm thick, which was renewed after each passage. l2 The hydrocarbon oil, Dobane JN from Shell Chemicals, was purified by heating three times with 5 % w/v Fuller's earth at - 100°C for 30 min and then filtered [m.w.232 ; q20 1 I .4 CP ; d$O 0.878 ; interfacial tension (IFT) 31.4 mN m-'1. 1 -hydroxynaphthalene and 1-nitronaphthalene were good grades from BDH and were 123I24 FLOATING HYDROCARBON OILS further purified by crystallisation from water or ethanol + water. Puriss naphthalene-a- aldehyde was obtained from Koch-Light laboratories and was used without further purification. The physicochemical properties of all the materials-density, surface and interfacial tension, refractive index and melting point-agreed closely with reliable literature values.Triply distilled water, surface tension 72.1 mN m-l, conductivity 1.2 x 10-‘ Q-’, was used throughout. FIG. 1.-Irradiation chamber. A, mercury lamp (125 W) ; By cool air blower ; C, silica filter (2 mm thick) ; D, 7.5 cm diameter fan ; E, thermometer; F, 5.0 cm Du Nouy Pot ; G, glass thermostat (water temp. 25 k 2°C) ; H, blocks. TECHNIQUE A N D APPARATUS Dilute solutions of the naphthalene derivatives were prepared in the three hydrocarbon solvents using an electromicrobalance for the weighings. 12cm3 of each sample was floated on 20 cm3 of distilled water in a 5 cm diameter glass pot, producing a layer 0.6 cm thick and irradiated with U.V. light for various periods of time in the apparatus shown in fig.1. The U.V. source was a 125 W Phillips MB/U medium pressure arc equipped with a silica filter which transmitted in the range 290-325 nm.” * During irradiation the temperature of the samples was maintained at 2552°C by circulating a 1 % w/v aqueous solution of chrome alum+cupric sulphate (1 : 1) which was also transparent to light of this wavelength. After irradiation, the interfacial tensions, y, of the samples were measured in sitri with a du Nouy tensiometer using the equation (1 1 where y is the interfacial tension, P the tensiometer reading, C is the circumference of the platinum ring, D is density of water at 20°C and d is the density of hydrocarbon at 2OoC, and the results were corrected to 20°C by the factor -0.04 m Nm-l “el. This technique required the floating hydrocarbon layer to be at least 0.5 cm thick.The peroxides produced in the reactions were determined by iodometric titration 1 3 9 and the other products in the oil and water phases, after 4 h irradiation, were analysed by g.1.c. and thin layer chromatography combined with U.V. spectroscopy. = P(0.725+ r0.0145 P/C2(D-d)]*) RESULTS The effects of irradiation on the interfacial tensions of representative samples are shown in fig. 2(u)-(c). All the systems exhibited decreases in IFT with increasing periods of irradiation; the decreases depended also on the nature and amount of naphthalene derivatives that had been added to the hydrocarbons.W. H. K . SANNIEZ AND N . PILPEL 125 It was found that for all the unirradiated systems there was a linear relationship between the change in interfacial tension and the log concentration c, of the additive, showing that the Gibbs' equation -dy/d log c r = 2.303 RT where r is the surface excess, R is the gas constant and T is the temperature in K ap~1ied.l~ Values for the surface excess are given in table 1.1 0 ,, , u L_ I I IC 1 I I u- 0 I00 2 0 0 300 irradiation time/min FIG. 2.-(u) Plot of interfacial tension against irradiation time for methylbenzene system. A, methylbenzene ; A, methylbenzene+2.2 x mol dm-3, 1-nitronaphthalene ; 0, methylbenzene+ 2.2 x mol dm-3 hydroxynaphthalene ; 0, methylbenzene+2.2 x lod3 rnol dm-3 naphthalene- a-aldehyde. (b) Isopropylbenzene system. mol dm-3 1-nitronaphthalene ; 0, isopropylbenzene+ 2.2 x 1O-j mol dm-3 hydroxynaphthalene ; 0, cumene+2.2 x rnol dm-j naphthalene-a-aldehyde.(c) Dobane system. A, Dobane ; A, Dobane+2.0x rnol dm-3 1-nitronaphthalene ; 0, Dobane+2.0 x mol dm-3 l-hydro- A, isopropylbenzene ; A, isopropylbenzene+ 2.2 x xynaphthalene ; 0, Dobane+ 2.0 x mol dm-3 1-naphthalene-a-aldehyde. For the isopropylbenzene and methylbenzene, though less accurately for the Dobane systems, the slopes -(dy/dt) of the graphs in fig. 2(a) and (b) also remained reasonably constant over the first 2 h of irradiation and when log (-dyldt) was plotted against the log concentration of the additives in these three solvents, straight lines were obtained as shown in fig. 3(a)-(c). TABLE 1 .-SURFACE EXCESS VALUES FOR THE VARIOUS SYSTEMS surface excess I'/pmol m-2 1 -naphthalene solvent I-nitronaphthalene 1-hydroxynaphthalene -a-aldehyde isopropyl- benzene 0.14 0.22 0.24 Dobane 0.001 0.004 0.007 methyl- benzene 0.14 0.13 0.18126 FLOATING HYDROCARBON OILS Fig.4(a)-(c) are representative plots to show how the peroxide concentrations in the oils varied with the period of irradiation, with the nature of the oil and with the amount of additive employed. Initially the Dobane appeared to be more staPle than either isopropylbenzene or methylbenzene, but the occurrence of maxima and points of inflexion in the curves suggests that in all three solvents several different mechanisms could be contributing to the initial formation and, at later stages, to the decomposition of the peroxides formed. -‘r -21----- - I - -_25 -4 -3 -2 0 log concentration FIG.3 . 4 2 ) Plot of -log dy/dt against -log concentration. A, methylbenzene+ 1-nitronaphthalene 0, methylbenzene + 1-hydroxynaphthalene ; 0, methylbenzene+ 1-naphthalene-a-aldehyde. (b) Isopropylbenzene. A, isopropylbenzene + 1-nitronaphthalene ; 0, isopropylbenzene + 1 -hydroxy- naphthalene ; 0, isopropylbenzene + 1-naphthalene-a-aldehyde. (c) Dobane. A, Dobane + 1- nitronaphthalene ; 0, Dobane + 1-hydroxynaphthalene ; 0, Dobane+ 1-naphthalene-a-aldehyde. T.1.c. analysis of the isopropylbenzene and methylbenzene systems after 4 h and of the Dobane system after 3 h of irradiation revealed several fluorescent spots when viewed in light of 350 nm wavelength. Their R, values are shown in table 2. The individual spots were extracted with diethyl ether, redissolved in methanol and their U.V. absorption spectra were recorded.Typical spectra are shown in fig. 5 and 6 . By means of g.1.c. analysis and the use of standard spectra, the main reaction products from the isopropylbenzene and methylbenzene systems were identified and their amounts determined (table 2). The amounts formed in the Dobane systems, however, were mostly too small for them to be analysed.0 I Q E m U 0 151 G 10 5 0 0 I00 200 300 irradiation timelmin FIG. 4.-(a) Graph of peroxide concentration against irradiation time. A, methylbenzene ; A, methylbenzene+2.2 x mol dm-3 1-nitronaphthalene ; 0, methylbenzene+2.2 x lo-' rnol dm-3 1 -hydroxynaphthalene ; 0, methylbenzenef 2.2 x mol dm-3 1-naphthalene-a-aldehyde. (6) A, Isopropylbenzene ; A, isopropylbenzene+2.2 x mol 1-nitronaphthalene ; 0, iso- propylbenzene+2.2 x rnol dm-3 1-hydroxynaphthalene ; 0, isopropylbenzenef2.2 x mol 1-naphthalene-a-aldehyde.(c) A, Dobane ; A, Dobane+ 1.0 x lo-' rnol dmV3 l-nitro- naphthalene ; 0, Dobane+ 1.0 x lo-' mol dm-3 1 hydroxynaphthalene ; 0, Dobane+ 1.0 x 10-1 mol dm-3 1-naphthalene-a-aldehyde. I . "9 8 1.2 - $I I.0- e % 0.8- 0.6 - 0 . 4 1 0 . 2 - 0.0 ' 2 0 0 2 5 0 3 0 0 350 400 wavelength/nm acid ; B, naphthalene. FIG. 5.-U.v. adsorption spectra of oxidation products from the methylbenzene system. A, benzoic128 FLOATING HYDROCARBON OILS -1 1 2 0 0 2 5 0 3 0 0 350 400 waveIength/nm met hox ynapht halene ; D , met hylphenylket one. FIG. 6.-U.V. absorption spectra of oxidation products from the isopropylbenzene system.C DISCUSSION It is apparent from the results that the three hydrocarbon oils on their own are relatively stable to U.V. irradiation in the range 290-325 nm over periods of 240 min. They all absorb U.V. light within this range ’* * but the changes produced in their interfacial tensions and peroxide values are very small. Of the three oils, isopropyl- benzene appears to be marginally the most affected. This is presumably because its benzylic hydrogen is more reactive than that of methylbenzene (due to the proximity of the extra methyl groups) and that of Dobane, which being a commercial oil has been specially blended to be resistant to oxidation.lg In the presence of the additives, however, all three oils are quite rapidly oxidised, as shown by the build up of peroxide/hydroperoxide [fig.4(a)-(c)] and the corres- ponding decreases in their interfacial tensions [fig. 2(a)-(c)]. It is well known 4 9 2o that the oxidation of hydrocarbon oils proceeds via the formation of peroxides and/or hydroperoxides. These subsequently start to de- compose [as demonstrated by the maxima and points of inflexion in fig. 4(a)- (c)] to yield surface active products-alcohols, ketones, etc., some of which have been identified by g.l.c., t.1.c. and spectroscopy in table 2 and fig. 5 and 6 . On more prolonged oxidation, a variety of other products may be formed, including acids, polymers and waxes,lS 2o but the routes followed are then very complex and not readily amenable to the present type of analysis. For isopropyl benzene on its own, the initial slow oxidation could proceed as follows: hv 202 2*PhCH(CH3)2 -+ PhC(CH3)200H+ Phd(CH3)2 + HG2.(3) (hydroperoxide) The corresponding reaction for methylbenzene on its own could be 2o as in reaction (4) hv 202 2PhCH3 -+ PhCH200H + PhdHz + HG2. (4) (hydroperoxide) * Ph = phenylW. H. K. SANNIEZ AND N. PILPEL 129 TABLE PR PRODUCTS OF OXIDATION AND THEIR CONCENTRATION AFTER 240 min IRRADIATION isopropylbenzene a 1 -ni tronapht halene 1 -hydroxynaphthalene naphthalene- a-aldeh yde 1 -nitronaphthalene 1 -hydroxynaphthalene naphthalene- a-aldehyde 1 -nitronaphthalene 1 -hydroxynaphthalene naphthalene- a-aldeh yde spot 1 2 1 2 1 1 2 1 1 1 1 1 2 retention Rf value timelmin 0.75 6.4 0.92 1.2 0.75 6.3 0.91 4.2 0.76 6.4 methylbenzene a 0.72 0.8 0.92 1.2 0.71 0.8 0.72 0.9 Dobane 0.83 1.3 0.84 0.8 0.39 0.85 conc.x identity of spot lO4lmol d m - 3 met hylphenyl 2.0 naphthalene 1.5 methylphen yl 2.4 met hoxy- 1.5 met hylphenyl 3.1 ketone ketone naphthalene ketone benzoic acid 1.7 naphthalene 1.6 benzoic acid 2.1 benzoic acid 2.5 naphthalene 0.6 benzoic acid 1.1 * Products from 12 cm3 of hydrocarbon solvent containing 2.5 x lo-' mol dm--3 additive ; b products from 12 cm3 of hydrocarbon solvent containing 1.5 x 10-1 mol dm-3 additive. The additives 1 -nitronaphthalene, 1 -hydroxynaphthalene and 1 -naphthalene-a- aldehyde act as sensitizers by causing the isopropylbenzene and the methyl benzene to produce more radicals, thus accelerating their oxidation. It is known that aromatic nitro compounds, such as 1-nitronaphthalene, are thermally oxidised to their parent hydrocarbons 21 and if they are similarly decom- posed by U.V.light, this would account for the detection of naphthalene (see table 2) in the systems to which 1-nitronaphthalene had been added. Thus the reaction involving the 1 -nitronaphthalene could be h v 0 2 PhCH3 + ArN02 PhCOOH + *ArH + HN02 (5) (benzoic (naphthalene) acid) and the reaction involving the 1-hydroxynaphthalene could be reaction (6) 0 hr 11 PhCH(CH3)2 + ArOH 3 PhCCH3 + ArOCH, + H20 0 2 (methyl- (met hoxy- phenyl naphthalene) ketone) * Ar = naphthyl130 FLOATING HYDROCARBON OILS to form the methylphenyl ketone and the methoxynaphthalene which were detected in the isopropylbenzene system (table 2). With naphthalene-a-aldehyde, however, the only detectable products were methylphenylketone (from the isopropylbenzene) and benzoic acid (from the methyl- benzene), eqn (6) and (5).Thus, although naphthalene-a-aldehyde accelerates the photo-oxidation of isopropylbenzene and of methyl benzene it does not itself appear to be decomposed.6 This was confirmed by analysis of the test samples over the period of the irradiation. As has been explained in greater detail elsewhere? one can employ the results in fig. 3(u)-(c) to determine the overall orders of reaction (n) for the three oil systems with respect to the initial concentrations of the additives in each. The equation of these graphs can be written as eqn (7) log dy/dt = log K+n log c (additive) (7) where K is a constant for each graph and n is the slope.It was found that the slopes of all the graphs fell between zero and 0.5 showing that virtually zero order kinetics was being followed during this first stage of photo-oxidation, irrespective of the nature of the oil concerned. The similarity in the results obtained for Dobane and for the pure hydrocarbon oils suggests that the first stage in the oxidation of Dobane proceeds by a similar mechanism to that for isopropylbenzene and methylbenzene though for this oil it was not always found possible to analyse the products of the reaction. 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