Free-radical chain mechanism, experiment

We continue our study of chemical kinetics with a presentation of reaction mechanisms. As time permits, we complete this section of the course with a presentation of one or more of the topics Lindemann theory, free radical chain mechanism, enzyme kinetics, or surface chemistry. The study of chemical kinetics is unlike both thermodynamics and quantum mechanics in that the overarching goal is not to produce a formal mathematical structure. Instead, techniques are developed to help design, analyze, and interpret experiments and then to connect experimental results to the proposed mechanism. We devote the balance of the semester to a traditional treatment of classical thermodynamics. In Appendix 2 the reader will find a general outline of the course in place of further detailed descriptions. 

The catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

Ashmore and Levitt have extended their earlier study of the NO2-H2 reaction to higher pressures over the temperature range 371-433 °C. They find clear evidence for a free radical chain mechanism based on sensitization and scavenging experiments over a range of pressures. The course of the reaction was followed by absorption measurements of the NO2 concentration. At high (= 40/1) ratios of hydrogen to nitrogen dioxide surface phenomena are not important, but at 5/1 ratios acceleration of the rate of NO2 loss occurs when the surface-to-volume ratio is increased. 

The first publications dealt with simple alkyloxiranes, The photochemical behavior of oxirane, methyloxirane, and ethyloxirane has been investigated by direct irradiation or mercury-sensitized photolysis. The products formed by cleavage have been analyzed, but the nature of the primary processes has not been touched on. A free-radical mechanism has been proposed. A free-radical chain mechanism has been established on the basis of the products obtained on direct irradiation (254 nm) of pure methyloxirane. Experiments with the aim of clarifying the primary photochemical processes were published recently, with irradiation of methyloxirane in the gas phase at 185 nm. Mainly propanal was obtained with a little acetone and traces of ethanol and propanol. An outline of the reaction mechanism is presented in Eq. 325. 

The free radical chain mechanism of the addition reaction of bromodicyanomethane to alkenes has been corroborated by inhibition experiments with oxygen and t-butylcatechol. The mechanism of equation 66 has been postulated. The ring-closure of the 2-bromoalkylmalononitriles gives rise to mixtures of cis- and trans-cyclopropanes. 

Possible Radical Chain Mechanism. Potentially, the regiochemistry observed in Scheme V could result from a free-radical chain mechanism. Scheme VI. However, a crossover experiment utilizing phosphites 1 and 2 failed to give so much as one percent of cross products 14),... 

Kinetic analysis [1,3-5,7,9], use of inhibitors [1-2,4-5,7,9], CIDNP experiments [16,19] and the stereochemistry [8,12-13,17-18, 30] indicate that the degradation of peracids RCO ,H into alcohols ROH takes place through a free-radical chain mechanism (cf. Schemes 1 and 2). The initiation step (reaction 1) is the thermal homolysis of the weak -O-O- bond of the peracid [32]. The two propagating steps forming the alcohol are ... 

Similar mechanisms were proposed for the aromatization of 2,4,6-tri-phenyl-4//-pyran (151c) to 387b with aryldiazonium tetrafluoroborates,356 with 2,6-di-fe/Y-butylphenoxide radical, and tetracyanoquinone dimethide357 on the basis of kinetic and electrochemical experiments. Another free radical chain pathway for the reaction of 151c with trichloromethyl radical and tetrachloromethane was also postulated353 (Eq. 22). 

The observed first-order rate constant for decomposition is independent of the initial concentration and experiments to distinguish an intramolecular mechanism from a free-radical chain reaction rule out the former. It is rather more difficult to establish whether the a-H atom is transferred to Ta initially or directly to the more basic axial alkyl group (Scheme 1). 

A reaction mechanism is a step-by-step description of the bond-breaking and bond-making processes that occur when reagents react to form products. In the case of halogenation, various experiments show that this reaction occurs in several steps, and not in one magical step. Indeed, halogenation occurs via a free-radical chain of reactions. 

A complete mechanism for the autoxidation of alkylaromatic hydrocarbons by cobalt(n) in acetic acid has not been established,25 6 although a complex rate law has been determined for tetralin. 22 The reaction most likely proceeds by a fiiee radical chain mechanism in which the purpose of the cobalt ions is to provide a hi h steady state concentration of free radicals by catalysis of the decomposition of THP. The free radical nature of the autoxidation of tettalin with the colloidal CoPy catalysts is supported by experiments which showed inhibition of the reaction by 2,6-di-rerr-butylphenol and 2,6-di-rm-butyl-4-methylphenol, and by a shortening of the induction period and increase of the reaction rate when azobis(isobut nitrile) was added to the reaction mixture as a free radical initiator. 

The radical chain mechanism was considered because these reactions have induction periods and because styrene added to the reaction mixture is polymerized242. However, in the reaction of 2-picoline 1-oxide with acetic anhydride, whilst the formation of carbon dioxide and methane demonstrates the presence of free radicals, these are irrelevant to the main reaction, the addition of radical scavengers being without effect on the yield of 2-pyridylmethyl acetate. Further, conversion of 2-picoline 1-oxide by butyric anhydride into 2-pyridylmethyl butyrate is not influenced by the presence of acetate ions. These facts point to the intra-molecular production of radical or ion pairs in a solvent cage as the essential characteristic of the reaction243a. Similar experiments in which i 0-labelled acetic anhydride was used and the effect of solvents studied, point to the same conclusion and favour the formation of a radical pair in a solvent cage for the reaction both... 

The chain mechanism is complicated when two hydrocarbons are oxidized simultaneously. Russell and Williamson [1,2] performed the first experiments on the co-oxidation of hydrocarbons with ethers. The theory of these reactions is close to that for the reaction of free radical copolymerization [3] and was developed by several researchers [4-9], When one hydrocarbon R H is oxidized in the liquid phase at a sufficiently high dioxygen pressure chain propagation is limited only by one reaction, namely, R OO + R H. For the co-oxidation of two hydrocarbons R1 and R2H, four propagation reactions are important, viz,... 

Hydroperoxides oxidize aromatic amines more readily than analogous phenols. Thus, at 368 K cumyl hydroperoxide oxidizes a-naphthylamine and a-naphthol with ku = 1.4 x 10 4 and 1.7 x 10 5L mol-1 s 1, respectively [115,118], The oxidation of amines with hydroperoxides occurs apparently by chain mechanism, since the step of free radical generation proceeds much more slowly. This was proved in experiments on amines oxidation by cumyl hydroperoxide in the presence of /V,/V -diphcnyl-l, 4-phcnylcnediamine (QH2) as a radical acceptor [125]. The following reactions were supposed to occur in solution (80% decane and 20% chlorobenzene) ... 

Experiments demonstrated that photolysis of secondary hydroperoxides in polyethylene does not lead to the initiation of chain oxidation of the polymer [17]. Hydroperoxides are decomposed here either by a molecular mechanism or by free radicals. If formed, they disappear during transformation within a cage reaction from which only a small part of free radicals succeeds to escape. 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

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