Ultraviolet light, initiation

This synthesis method can be utilised by any alkene or alkyne, but steric hindrance on internal double bonds can cause these reactions to be quite slow. Conjugated dienes and aromatic alkenes are not suited for the ultraviolet light-initiated process. The use of other free-radical initiators is required in free-radical-initiated reactions involving these species. 

Free radical (homolytic) reactions involve species with an unpaired electron. The ultraviolet-light-initiated reaction of methane with chlorine is an example (Scheme 4). 

The mechanism is a chain reaction (see description in Section 18.4). Ultraviolet light initiates the reaction by dissociating a small number of chlorine molecules into highly reactive atoms. These atoms take part in linked reactions of the following form ... 

The uses of polysilanes as photoresist materials , and as photoinitiators both depend on the photoscission of the polymers by ultraviolet light. Initial steps in this process are chain cleavage to radicals and elimination of silylene, RR Si ... 

Ultraviolet light initiates free radical oxidation at the exposed surface of an elastomeric product to generate a layer of oxidized rubber. Heat, moisture, or high humidity can then initiate crazing of the surface, which subsequently can be abraded off. Such degradation of the surface is more severe with nonblack stocks than with black compounds. Nonblack compounds such as white tire sidewalls thus require higher levels of nonstaining antioxidants than carbon black-loaded formulations. 

Both electron beams and ultraviolet light initiate free-radical polymerizations with very low activation energy. This allows high polymerization rates at room temperature because the rates are not temperature dependent. Once initiated, free-radical polymerizations follow typical paths. It is, however, peculiar to radiation curing of coating materials that the gel states form at very early stages of the reactions. This is due to extensive use of polyfunctional monomers and prepolymers. In fact, it was demonstrated that the gel points occur at around 5% conversion of the prepolymers in typical commercial formulation, yet conversions to about 63% of the prepolymers to polymers are required to obtain dry films. ... 

The ultraviolet-light-initiated reaction of bases with the hexacarbonyls has been carefully studied by Strohmeier and his group (88, 89, 90, 92, 96, 99,104,105, 133-139a). It has been shown that the hexacarbonylmetal is excited and loses carbon monoxide to give a pentacarbonylmetal intermediate, which is rapidly attacked by a donor molecule (134, 137). The quantum yield is one for the preparation of the monosubstituted com-... 

The diacid components for the manufacture of poly(y -phenyleneisophthalamide) and poly(p-phenyleneterephthalamide) are produced by one of two processes. In the first, the diacid chlorides are produced by the oxidation of / -xylene [108-38-3] or -xylene [106-42-3] followed by the reaction of the diacids with phosgene [75-44-5]. In the second, process m- or -xylene reacts with chlorine initiated by ultraviolet light to form the m- or Nhexachloroxylene. This then reacts with the respective aromatic dicarboxyUc acid to form the diacid chloride. 

Hydrogen sulfide reacts with olefins under various conditions forming mercaptans and sulfides (108,109). With ethylene it can react to ultimately give diethyl sulfide (110). With unsymmetrical olefins, the direction of addition can be controlled by the choice of either a free-radical initiator, including ultraviolet light, or an acidic catalyst (110) ... 

Orga.nic Chemistry. The organic chemistry of sulfur dioxide, particularly as it relates to food appHcations, has been discussed (246). Although no reaction takes place with saturated hydrocarbons at moderate temperatures, the simultaneous passage of sulfur dioxide and oxygen into an alkane in the presence of a free-radical initiator or ultraviolet light affords a sulfonic acid such as hexanesulfonic acid [13595-73-8]. This is the so-called sulfoxidation reaction (247) ... 

Tetraneopentyltitanium [36945-13-8] Np Ti, forms from the reaction of TiCl and neopentyllithium ia hexane at —80° C ia modest yield only because of extensive reduction of Ti(IV). Tetranorbomyltitanium [36333-76-3] can be prepared similarly. When exposed to oxygen, (NpO)4Ti forms. If it is boiled ia ben2ene, it decomposes to neopentane. When dissolved ia monomers, eg, a-olefins or dienes, styrene, or methyl methacrylate, it initiates a slow polymerisation (211,212). Results from copolymerisation studies iadicate a radical mechanism (212). Ultraviolet light iacreases the rate of dissociation to... 

Substitution Reactions on the Methyl Group. The reactions that give substitution on the methyl group are generally high temperature and free-radical reactions. Thus, chlorination at ca 100°C, or in the presence of ultraviolet light and other free-radical initiators, successively gives benzyl chloride, benzal chloride, and benzotrichloride. 

Chlorinated paraffins are manufactured by passing pure chlorine gas into a Hquid paraffin at a temperature between 80 and 100°C depending on the chain length of the paraffin feedstock. At these temperatures chlorination occurs exothermically and cooling is necessary to maintain the temperature at around 100°C. Catalysts are not usually necessary to initiate chlorination, but some manufacturers may assist the process with ultraviolet light. Failure to control... 

The chlorination of toluene in the absence of catalysts that promote nuclear substitution occurs preferentially in the side chain. The reaction is promoted by free-radical initiators such as ultraviolet light or peroxides. Chlorination takes place in a stepwise manner and can be controlled to give good yields of the intermediate chlorination products. Small amounts of sequestering agents are sometimes used to remove trace amounts of heavy-metal ions that cause ring chlorination. 

The initiation stage may be activated by free-radical or ionic systems. In the following example a free-radical system will be discussed. In this case a material which can be made to decompose into free radicals on warming, or in the presence of a promoter or by irradiation with ultraviolet light, is added to the monomer and radicals are formed. Two examples of such materials are benzoyl peroxide and azodi-isobutyronitrile, which decompose as indicated in Figure 2.13. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials No reaction Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Heat may cause an explosive polymerization. Strong ultraviolet light can also initiate polymerization Inhibitor of Polymerization Hydroquinone and its methyl ether, in presence of air. ... 

Initiation Irradiation with ultraviolet light begins the reaction by breaking the relatively weak Cl-Cl bond of a small number of Cl2 molecules to give a few reactive chlorine radicals. 

The chlorination of methyl chloroformate in sunlight was first reported by Hentschel, but without a detailed description of either the procedure or the results. The first step of the present procedure for the preparation of trichloromethyl chloroformate utilizes an ultraviolet light source and affords a simple and reproducible way to obtain this reagent. Although trichloromethyl chloroformate may also be synthesized by photochemical chlorination of methyl formate,the volatility of methyl formate causes losses during the reaction and increases the hazard of forming an explosive mixture of its vapor and chlorine gas. The preparation of trichloromethyl chloroformate by chlorination of methyl chloroformate in the dark with diacetyl peroxide as initiator has been reported. However, the procedure consists of several steps, and the overall yield is rather low. 

When MeCOMn(CO)3L2 and L [L = P(OMe)j] are heated under ultraviolet light, the initially formed MeMn(CO)3L2 reacts further to give MeMn(CO)2L3 and MeMn(CO)L4 225). These alkyls undergo CO insertion at 300 psi, yielding MeCOMn(CO)jL2. 

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