Simple Reaction Scheme Research Articles

Measurement of the proportion of radiationless decay processes in the photolysis of pentan-2-one

Measurements have been made of the rate of production of acetone and the rate of consumption of the parent ketone in the photolysis, at 313 nm, of pentan-2-one mixed with various amounts of cyclohexene in benzene solution.The results show that the type II reaction of pentan-2-one to form acetone can be partially quenched, as has been noted before, but that the quenching process is accompanied by an increase in the consumption of the ketone. The apparent quenching of the reaction is due to an alternative chemical reaction of the triplet state which reacts with the olefin. The efficiency of the chemical reaction is equivalent to that of energy transfer processes. Comparison of the rates at various concentrations allows the proportion of radiationless decay from triplet pentanone to be estimated, and, from the measured quantum yield, the ratios of all the processes for both singlet and triplet states have been deduced for a simple reaction scheme.

Polymerization of styrene in presence of tetramethylthiuramdisulphide

During the polymerization of styrene (also of methylmethacrylate) in the presence of tetra-methylthiuramdisulphide, there is deviation from the simple reaction scheme. This deviation is explained by the participation of RS . radicals (formed by thermal decomposition of TMTD and by transfer of P . radicals to TMTD) in termination reactions. The total polymerization rate is then well expressed by derived relation (1); it was possible to determine the complex constants B* and C* (for styrene C*→0). The study of the influence of [TMTD] on the molecular weight of polystyrene (determined visco-metrically) enables evaluation of transfer constants C T and of rate constants k 4 by means of the relation (4). The determined values of activation energy E 4 = 11·5 ± 1 kcal. mol −1 and E B* = 6·3 ± 1 kcal, mol −1 lead to E′ 1 = 5·2 ± 2 kcal × mol −1 (for the reaction R S · + M → k 1 P · ) with the assumption that E 3 = E′ 3. The activation energy of initiation E i = 24 ± 2 kcal. mol −1 (the initiation rates R i were calculated from the experimental values of R p for [TMTD] approaching zero) is in good agreement with the activation energy for the consumption of [TMTD] during the vulcanization of rubber.

Analytical investigations of isooctane and diisobutylene slow combustion in an Otto-cycle engine

Samples of end-gas from an Otto-cycle engine operated on isooctane and isooctane-diisobutylene mixtures were analysed for slow combustion products by gas chromatography. The formation of twenty-seven products of isooctane slow combustion were followed in time from before ignition to near complete charge inflammation. Olefins were found to increase late in the cycle much more rapidly than the other major products, cyclic ethers. The cyclic ethers found are principally ring-closed between a primary and a secondary or tertiary position. The results are accounted for by a simple reaction scheme which proposes that alkyl radical pyrolysis and addition of molecular oxygen are competing pathways. Cyclic ether pyrolysis may also contribute to product formation. The addition of diisobutylene inhibits isooctane product formation. It is proposed that the observed inhibition accounts for the reported octane number maximum with these mixtures.

Study of the activity of some sulfur compounds in the modification of vulcanizates by vinyl monomers

AbstractThe kinetics of the bulk polymerization of styrene in the presence of tetramethylthiu‐ramdisulfide ([TMTD]; 2.11 × 10−4‐1.67 × 10−1 mole/1., and some other sulfur compounds at 80, 95, 115, 130, and 140°C, were studied. The rate of polymerization has been measured as a function of the concentration of the studied compound. The formal order of reaction (n) was determined with respect to the concentration of TMTD, which varies with the increase of the concentration of TMTD and temperature of polymerization in the range 0 < n < 0.5 and it is possible to express by the relation: n = d In Rp/d In [I] = n0(1 − C[I]2)/(1 + B[I] + C[I]2). The decrease of the polymerization rate compared with the rate arising from the simple reaction scheme is caused by the participation of radicals on the termination reactions: (P' + RS', RS' + RS'), which permits us to solve this type of polymerization as a retardation polymerization: Rp − A[I]1/2 /(I + B[I] + C[I]2)1/2. (A, B, and C are complex constants dependent on temperature.) The calculated values of the fraction of primary radicals taking part in the termination reaction (assuming the neglect of the reaction (RS' + RS') grow with the increase of the concentration TMTD and temperature. The relatively high values of complex transfer constants Ci'‐(0.57–0.98) and the observed decrease of values of effective activation energies with an increasing concentration of TMTD are in accordance with the given reaction scheme. The observation of other sulfur compounds (zincdiethyldithiocarbamate, dibenzothiazolyldisulfide, dipentamethylenthiuramtetrasulfide, and N‐cyclohexyl‐2‐benz‐thiazylsulfenamide) showed that they had a considerably smaller influence on the polymerization of styrene than did TMTD.

Kinetics of the anodic reaction of molten nitrites on platinum

The kinetics of the anodic reaction occurring on platinum electrodes when molten nitrites are electrolysed has been studied from 240 to 340°C. The following electrolytes were used (i) molten sodium nitrite (ii) mixtures of molten sodium and potassium nitrites (iii) solutions of nitrites in the sodium-nitrate-potassium-nitrate eutectic, covering the concentration range from 10 to 10−1 M. The overall anodic reaction isNO2− → NO2 + e.Current/voltages curves, and decay and build-up of electrode overpotential were measured and the results analysed in the usual way. After the interruption of the electrolysis current a steady residual potential was established on the working electrode.Current/voltage curves fit a Tafel plot in two regions of current density. At lower current density the slope approaches 2·3 (RT/F), whereas at higher ones it becomes 2·3 (RT/2F). Two slopes are also obtained from the logarithmic plot of overpotential decay for the pure nitrites at high current density; the results are satisfactorily coincident with those already mentioned. The transition from the higher to the lower Tafel line occurs at an overpotiential of about 300 mV. From the decay curves the apparent electrode capacitance and its dependence on potential within a certain range of the latter was calculated.The interpretation of results is based upon a simple reaction scheme involving a fast discharge of nitrite ion followed by two alternative reactions for the intermediate formed in the initial discharge, finally yielding nitrogen dioxide. Possibilities related to a low or an intermediate degree of coverage of the electrode surface are discussed.

γ‐Irradiation of poly(ethylene terephthalate). I. Yields of gas and carboxyl groups

AbstractPoly(ethylene terephthalate) was exposed in vacuo γ‐radiation from a Co60 source at a dose rate of 0.02 Mrad/min., and an ambient temperature of 47°C. Gaseous products were analyzed in a mass spectrometer and carboxyl groups estimated by titration with NaOH and by infrared analysis. Initial G values were COOH = 0.77, CO2 = 0.17, CO = 0.11, H2 = 0.015, and CH4 = 0.003. All these values decreased markedly with increasing dose except G(CO)2, which, roughly, was maintained up to 5000 Mrad. It was considered whether the dependence on dose of the yields of the major reaction products could be accounted for by the following set of first‐order reactions: magnified image It was found that the rate of formation of COOH groups at low doses was much too high to fit this simple reaction scheme. However, a better fit was obtained over a range of higher doses (ca. 100–5000 Mrad). A final conclusion could not be reached concerning the validity of the above reaction scheme.

Beobachtung von Elementarprozessen an angeregten Molekülen

Primary dissociation processes on excited cyclohexane have been investigated in d. c. and a. c. (50 cycles and 535 cycles) glow discharges. The C2-, C3- and C4-hydrocarbons formed, have been analysed by gaschromatography. These dissociation products can be explained quantitatively by a simple reaction scheme, which is based on C3 : C3⁻, C4 : C2⁻ and C2 : C2 : C2⁻cleavages of the cyclohexane ring and the following intramolecular disproportionation of the biradicals formed. The composition of the dissociation products is more or less identical for the two different a. c. investigations. Different results are observed for a. c. and d. c. glow discharges. The results indicate that the differences in the probabilities for the dissociation processes between a. c. and d. c. glow discharges are caused by the electrical property of the stationary and the moving striations of the positive column.

The photooxidation of polymers. II. Photolysis of polystyrene

AbstractIn the vacuum photolysis of polystyrene films at 28°C. under 2537 A. radiation the only gaseous product is hydrogen. At the same time the polymer becomes insoluble and discolors. The intensity exponent of the reaction is unity, the quantum yield of hydrogen is 4.3 × 10−2, the overall energy of activation is 2.9 kcal./mole, and the rate of photolysis is independent of molecular weight. No hydrogen is produced when 3650 A. radiation is used. These observations can be interpreted in terms of a simple reaction scheme in which the primary effect of the 2537 A. radiation is to liberate hydrogen atoms. The subsequent reaction of these and the polystyryl radicals simultaneously formed in a number of competing steps results in the appearance of hydrogen, the occurrence of crosslinking, and the production of carbon–carbon unsaturation in the main chain which ultimately leads to conjugation and color. These theories are supported by infrared and ultraviolet spectral measurements and by the observation that the rate of photolysis is accelerated by nitrogen.

Beobachtung von Elementarprozessen an angeregten Molekülen IV. Über Verhältnisse ganzer kleiner Zahlen bei Dissoziationsprozessen von n-Butan

Primary dissociation processes have been investigated on “isolated” excited n-butane. The excitation of the molecules results from electron impact in a glow discharge. The investigations yielded the following results: 1. Formation of new stable substances by rupture of one or two C—C-bonds and the following transfer of Η-atoms (intramolecular disproportionation). 2. These dissociation products can be explained quantitatively by a simple reaction scheme. 3. The observation of ratios of small whole numbers between analogous dissociation processes is taken as a basic for this reaction scheme. 4. These ratios are not identical for alternating current (A.C.) and for direct current (D.C.) glow discharges. A discontinuous variation is observed. 5. In spite of the different results for A.C. and D.C. discharges both results can be explained quantitatively by one reaction scheme. 6. According to this reaction scheme, a single excitation of the molecule is followed by the rupture of two C—C-bonds successively. These results indicate that the general appearance of ratios with small whole numbers and their discontinuous variation is a specific property of the molecular field.

Mass Spectral Studies of Kinetics behind Shock Waves. II. Thermal Decomposition of Hydrazine

The decomposition of hydrazine diluted in argon has been studied over the temperature range 1200° to 2500°K and pressure range 0.04 to 0.25 atm using a shock tube coupled to a time-of-flight mass spectrometer. The time-resolved mass spectra (25 μsec) enable the simultaneous identification and determination of N2H4, NH3, N2, H2, and the NH2 radical, the five major species observed. The material balance is within the experimental error (10%) during reaction up to complete decomposition. The primary process in the dissociation of hydrazine is shown to be the rupture of the N–N bond to give NH2 radicals. The observed rate constants for the disappearance of hydrazine are those expected of a unimolecular reaction: N2H4+M→ lim k1N2H4*,N2H4*→ lim k22NH2,at or near the second-order region where the collisional activation (1) is rate controlling. At the higher temperatures, the results are consistent (within experimental error) with the very simple reaction scheme of Reaction (1) and (2) followed by NH2+NH2→ lim k3NH3+NH,NH+NH→ lim k4aN2H2*→ lim k4bN2+H2.At the lowest temperatures the reaction NH2+N2H4→N2H3+NH3 appears to become important. From the experimental concentrations, a value of k3=2.5×1013 cc/mole−sec (2000∘K)is obtained. An order-of-magnitude estimate gives k4∼1014 cc/mole−sec (2000∘K).

Beobachtung von Elementarprozessen an angeregten Molekülen III. Cyclohexan, 2. Teil

In a previous paper 1 it has been shown that C3: C3, C4: C2, and C2: C2: C2 cleavages of the ring occur by primary dissociation processes of excited cyclohexane molecules. The percentage portions of the reaction products formed are constant within the different groups of cleavage. They have been explained by simple reaction schemes. Now it will be shown that a regularity exists also between these three different dissociation processes. It is remarkable that the C4: C2 and the C2:C2 : C2 dissociation processes are coupled and that the three dissociation processes discussed occur with integral intensities.

Kinetics of anodic and chemical reactions in Kolbe's electrosynthesis

1. Discharging of the carboxylic anion in aqueous solutions takes place at potentials which are more positive than potentials corresponding to oxygen liberation. 2. In aqueous solutions Kolbe's electrosynthesis is made possible by retardation of the discharging of water molecules owing to adsorption of the carboxylic anion on the oxidized platinum surface, or to formation of oxidic anion compounds with platinum. 3. On the basis of a simplified reaction scheme, an approximate relation is given for calculating the yield of ethane from electrochemical data.

On the Mechanism of the Hydrogen Reaction

Two simple reaction schemes for hydrogen evolution, the Volmer‐Tafel and Volmer‐Horiuti mechanisms, are discussed. The results are applied to hydrogen evolution on nickel from alkaline solutions.

Kinetic data and product distributions from fixed bed catalytic reactors subject to catalyst fouling

The paper discusses two problems which arise when dealing with reaction systems in which catalyst fouling occurs. The first problem is concerned with the influence of a profile in the carbon content of the catalyst on the rate coefficient derived from experiments in integral piston flow reactors. It is shown that significant errors may be introduced when it is assumed that the carbon content is uniform throughout the reactor. The second problem is concerned with the conditions under which the selectivity for complex reaction systems is affected by the catalyst fouling. A relatively simple reaction scheme that leads to important variations in the selectivity is solved as an example.

Some Aspects of High Temperature Vulcanization

Abstract When a thin rubber sample is heated to a high curing temperature, the initial kinetics of cure as derived from Newton's rule can be represented by a well-defined time delay at the final curing temperature. This facilitates quantitative studies for which it is also shown that first order changes in crosslink density can be assessed by modulus measurements. The vulcanization rate constants thus determined in the case of a polychloroprene compound indicate the apparently different action of the vulcanization accelerator at conventional and at high temperatures, which allows a fundamental interpretation to be advanced. Partial reversion at high temperature is shown for both the ACS1 and a thiuram-cured natural rubber compound, and using an appropriate graphical method, vulcanization and reversion can be evaluated as first order processes. Two simple overall reaction schemes are developed to fit the experimental data with only three calculated rate constants. Quinoid and peroxide curing systems are also examined.

THE RADIOLYSIS OF LIQUID n-HEXANE

The continuous decrease in hydrogen gas yield during the radiolysis of n- hexane is attributable to the addition of hydrogen atoms to the hexenes concurrently being produced. Unsaturation in the C/sub 7/--C/sub 12/ products arises from the addition reaction of alkyl plus alkenyl radicals. The alkenyl radicals are derived from hydrogen abstraction from hexenes by hydrogen atoms. The over-all kinetics of radiolysis are satisfactorily explained by invoking free radical mechanisms. Ion- molecule reactions appear to play little or no part. Both hot and thermal hydrogen atoms are produced, the former invariably forming hydrogen gas. An explanation for the mechanisms of formation of the socalled molecular products is given. The yield of products can be explained quantitatively by a relatively simple reaction scheme. (auth)

Studies on the hill reaction (II)

AbstractThe influence of the presence of oxygen on the Hill reaction has been investigated by redox potential measurements.A very simple reaction scheme, implying primary formation of a specific reductant reacting subsequently with the oxidant added, seems to fit kinetical and other data of the reaction.Some of the results of investigations on the influence of inhibitors and biochemically important substances are reported and discussed; possible causes for the discrepancies in the literature concerned with the action of inhibitors are indicated.

The Photochlorination and the Chlorine-Sensitized Photo-oxidation of Gaseous Tetrachloroethylene

In the absence of oxygen, the illumination of a carbon tetrachloride solution of chlorine and tetrachloroethylene with blue light has been found to bring about chlorination which proceeds as a chain reaction with a large quantum yield and at a rate proportional to the square root of the intensity of illumination. In the presence of oxygen, the chlorination is strongly inhibited and instead there occurs a chlorine-sensitized oxidation to trichloroacetyl chloride and phosgene; this reaction proceeds in solution with a comparatively small quantum yield and at a rate proportional to the first power of the light intensity. It was found possible to account for the kinetics of the chlorination by either of two simple and plausible reaction schemes, one involving C_2Cl_5 and the other Cl_3.