Typical Reaction Mechanisms 5 Reaction Mechanisms

Gas-phase reactions play a fundamental role in nature, for example atmospheric chemistry [1, 2, 3, 4 and 5] and interstellar chemistry [6], as well as in many teclmical processes, for example combustion and exliaust fiime cleansing [7, 8 and 9], Apart from such practical aspects the study of gas-phase reactions has provided the basis for our understanding of chemical reaction mechanisms on a microscopic level. The typically small particle densities in the gas phase mean that reactions occur in well defined elementary steps, usually not involving more than three particles.  [c.759]

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6)  [c.417]

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13.  [c.508]

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides.  [c.138]

It should be noted that Scheme 5.1-44 shows idealized Friedel-Crafts allcylation reactions. In practice, there are a number of problems associated with the reaction. These include polyalkylation reactions, since the products of a Friedel-Crafts alkylation reaction are often more reactive than the starting material. Also, isomerization and rearrangement reactions can occur, and can result in a large number of products [74, 75]. The mechanism of Friedel-Crafts reactions is not straightforward, and it is possible to propose two or more different mechanisms for a given reaction. Examples of the typical processes occurring in a Friedel-Crafts alkylation reaction are given in Scheme 5.1-45 for the reaction between 1-chloropropane and benzene.  [c.196]

Novolaks. Novolak resins are typically cured with 5—15% hexa as the cross-linking agent. The reaction mechanism and reactive intermediates have been studied by classical chemical techniques (3,4) and the results showed that as much as 75% of nitrogen is chemically bound. More recent studies of resin cure (42—45) have made use of tga, dta, gc, k, and nmr (15). They confirm that the cure begins with the formation of benzoxazine (12), progresses through a benzyl amine intermediate, and finally forms (hydroxy)diphenyknethanes (DPM).  [c.298]

Peroxides are typical vulcanising agents and the mechanism of cure involves free-radical abstraction of a siUcon methyl group proton and subsequent dimerization of the methyl radicals to form ethylene cross-links (eqs. 29—31) (365,366). Vinyl-containing polymers are often used to control the cross-linking reaction. Commonly used peroxides include di-/-butyl peroxide [110-05-4] benzoyl peroxide [94-36-OJ, di(/-cumyi) peroxide, and di(/-chlorophenyi) peroxide. The choice of peroxide is made based on the desired cure temperature and rate. Table 5 Hsts some common peroxide curing agents, typical cure temperatures, and some recommended processing conditions.  [c.53]

The last type of reserve ceU is the thermally activated ceU. The older types use calcium [7440-70-2] or magnesium anodes newer types use Hthium alloys as anodes. Lithium forms many high melting alloys such as those with aluminum, siHcon, and boron. Furthermore, Hthium can diffuse rapidly within the alloy phase permitting high currents to flow. The electrolyte for both the older and newer chemistries is usually the eutectic composition of Hthium chloride and potassium chloride which melts at 352°C. This electrolyte has temperature dependent conductivities which are an order of magnitude higher than the best aqueous electrolytes. The high conductivity and the enhanced kinetics and mass transport allow the battery to be discharged at a very high rate of several A /cm with complete discharge in 0.5 s. The cathodes for the older calcium anode ceUs are typically metal chromates such as calcium chromate [13765-19-0] CaCrO. The anode reaction product is calcium chloride [10043-52-4] whereas the cathode product is a mixed calcium chromium oxide of uncertain composition. One of the best cathodes for Hthium alloy ceUs is FeS2 which forms a system similar in reaction mechanism to that used in miniature Li—FeS2 ceUs.  [c.537]

The fixer composition has a marked influence on tlie desilvering. At low pH, sulfite can also be reduced at the cathode, thus representing a competitive reaction with the reduction of silver. As a consequence, desilvering of low pH fixers is more difficult since it requires a better control of the electrolysis current. In processing of X-ray materials, fixation is usually performed with a hardening fixer, which has a typical pH value of 4 to 4.5. Desilvering of non-hardening fixers, such as used in graphic art films, is appreciably easier. This accounts for the widespread use of desilvering equipment in graphic arts film processing. By using a pH electrode in the control mechanism of the electrolysis ciurent, STRUCTURIX SILVERFIX succeeds in fast and high-quality desilvering of hardening fixers.  [c.606]

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined.  [c.2135]

The ] Iethanol-to-Gasoline Process. Mobil Corp. laboratories have reported the conversion of methanol and some other oxygenated organic compounds to hydrocarbons using an acidic ZSM-5 zeoUte catalyst (21). In the case of methanol, the highly exothermic reaction sequence includes the formation of dimethyl ether. The process is quite selective in producing an aromatics-rich gasoline-range mixture of hydrocarbons free of oxygen compounds. Several mechanisms have been proposed for this reaction, but none has been shown unequivocally to be correct (r21). The hydrocarbon product typically contains no compounds with more than 10 carbon atoms, and this, in conjunction with the lower than predicted concentrations of certain polyalkjlated benzenes, is constmed as evidence of shape selectivity owing to the moderate pore size of the catalyst. The process has been in commercial Operation for many years in New Zealand, which has no cmde oil deposits, and where methanol is produced from abundant natural gas. It is not competitive with the refining of cmde oil.  [c.459]

Even though numerous patents describing all four classes have been issued, almost all existing OPCs now in use in copiers and xerographic laser printers employ the moleculady doped polymeric charge-transport layers. The transport-active molecules are typically complex aromatic amines having strong electron-donor character (see Eig. 5). It is commonly accepted that these materials transport charges by a hopping mechanism involving charge transfer among discrete sites. These sites are either discrete molecules, pendent groups on the polymer backbone, or segments of a transport-active chain. The hole-transport process is a sequence of redox exchanges among these neutral, uncharged species and the charged, electron-deficient counterparts or cation-radicals. The primary prerequisite is complete reversibiUty of the redox process. The oxidation of transport-active specie, ie, transfer of a hole, to form a cation-radical, and its subsequent reduction back to a neutral molecule must be free of any chemical side reaction which would inevitably result in an immohi1i2ation of the transiting hole. Aromatic amines are rather special in that typically the redox processes are completely reversible.  [c.134]

In systems of LP the dynamic response to a temperature quench is characterized by a different mechanism, namely monomer-mediated equilibrium polymerization (MMEP) in which only single monomers may participate in the mass exchange. For this no analytic solution, even in terms of MFA, seems to exist yet [70]. Monomer-mediated equilibrium polymerization (MMEP) is typical of systems like poly(a-methylstyrene) [5-7] in which a reaction proceeds by the addition or removal of a single monomer at the active end of a polymer chain after a radical initiator has been added to the system so as to start the polymerization. The attachment/detachment of single monomers at chain ends is believed to be the mechanism of equilibrium polymerization also for certain liquid sulphur systems [8] as well as for self-assembled aggregates of certain dyes [9] where chain ends are thermally activated radicals with no initiators needed.  [c.539]

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory.  [c.227]

See pages that mention the term Typical Reaction Mechanisms 5 Reaction Mechanisms : [c.101]    [c.77]    [c.461]    [c.80]    [c.2411]    [c.222]    [c.637]    [c.34]   
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