Chain-initiating step

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

Some details of the chain-initiation step have been elucidated. With an oxygen radical—initiator such as the /-butoxyl radical, both double bond... 

Step 1 (chain-initiating step — radicals are created)... 

In the chain-initiating step only the bond between two chlorine atoms is broken, and no bonds are formed. 

The Fe(III)/S(IV) reaction has long been of interest because of its importance in the catalytic autoxidation of S(IV). The latter reaction is known to have a complex chain mechanism, and the production of SOr radicals has been considered to be the essential chain-initiating step. It is also widely believed that the direct oxidation of S(IV) by Fe(III) is the source of SO -. There is little agreement among the various papers published on the direct reaction of Fe(III) with S(IV) with regard to its mechanism, and much of this disagreement can be traced to the potential for Fe(III) to bind several S(IV) ligands under the typical conditions of excess S(IV). 

Assume that the steady state of (Br) is formally equivalent to partial equilibrium for the bromine radical chain-initiating step and recalculate the form of Eq. (2.37) on this basis. 

In contrast to reaction (3.71), at high temperatures the thermal decomposition of the methane provides the chain initiation step, namely... 

In the high-temperature oxidation of large paraffin molecules, the chain initiation step is one in which a CC bond is broken to form hydrocarbon radicals namely,... 

HS02, a known radical that has been found in H2-02-S02 systems, is suf-hciendy inert to be destroyed without reforming any active chain carrier. In the lean oxidation of the thiols, even at temperatures around 300°C, all the sulfur is converted to S02. At lower temperatures and under rich conditions, disulhdes form and other products such as aldehydes and methanol are found. The presence of the disulfides suggests a chain-initiating step very similar to that of low-temperature hydrocarbon oxidation,... 

Chain Reactions. In chain reactions the intermediate is formed in a first reaction, called the chain initiation step. It then combines with reactant to form product and more intermediate in the chain propagation step. Occasionally the intermediate is destroyed in the chain termination step. Thus,... 

Chlorine combines with hydrogen forming hydrogen chloride, HCl. The reaction occurs rapidly when exposed to hght, involving a photochemical chain initiation step. 

Step 1 Cl2 - 2 Cl, a chain-initiation step caused by light... 

The free radical produced in the chain-initiation step adds to an alkene to form a new free radical. 

Formation of an enamine radical cation 45 was proposed as the chain initiation step in the autooxidation of enamines and SchifFs bases of a,/ -unsaturated ketones to give unsaturated 1,4-diones37. Pyrrolidine enamine of 10-methyl-A1(9)-octal-2-one (44) reacts with oxygen at room temperature to produce, after acid hydrolysis, 10-methyl-A1 (9)-octalin-2,8-dione (47) in 20% yield. Addition of a catalytic amount of FeCl3, Cu(OAc)2 or CuCl2 causes a pronounced enhancement in the oxidation rate and increases the yield to 80-85% after 1 h. 

The proposed free radical chain mechanism for this reaction is given in Scheme 3. The striking catalytic effect of the metal ions such as Cu2+ and Fe3+ is attributed to their ability to accept an electron from the enamine in the chain initiation step. The autooxidation of the SchifFs bases of a,/ -unsaturated ketones is thought to proceed similarly via the enamine form of the SchifFs bases. 

Since the chain carrier Cl is generated by reaction (29), this process is said to be a chain-initiation step. The energy required to dissociate the CI2 molecule might also be acquired by collision with another molecule or with a hot solid object (such as, a wall) in contact with the gas chain-initiation processes need not be photochemical. 

For those systems in which the rate is independent of oxygen, it seems likely that some branching must occur and possibly predominate over (22) as a chain initiation step since it is otherwise difficult to derive a kinetic equation which does not predict an oxygen dependent rate. 

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