Initiation step, chain reactions

The initiation step can be photoinduced. If a bottle is sitting in sunlight, UV photons (fluorescent lights are also more dangerous than incandescent lights) can cause photodissociation to initiate the chain reaction much faster than in the dark. [Guess why many chemicals are sold in brown bottles ]... 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

The mechanism of aminyl radical generation from PTOC carbamates follows closely the radical chain mechanism of alkyl radical generation from PTOC esters. The chain reaction sequence involves the series of steps shown in Scheme 8. Several methods for inducing N —O bond cleavage are possible. Photochemical decomposition of 29 via visible light irradiation is used to initiate the chain reaction sequence at ambient or subambient temperature reactions have been run as low as -78°C (91TL6493). 

The radical chain mechanism involving allytin belongs to the fragmentation method family and can be schematically presented as in equation 38, which involves five steps (i) initiation step, (ii) reaction with the substrate, forming the carbon-centred radical, (iii) a possible evolution of this radical, (iv) addition of the newly formed radical to the allylic double bond and (v) /3-scission, regenerating the chain-carrying tin radical. 

Fig. 3. Autoxidation of polyunsaturated fatty acids in phospholipid membranes. Addition of oxygen to lipid free radicals is extremely fast. It yields peroxyl radicals ROO which will tend to capture labile hydrogen atoms of neighbouring polyunsaturated lipids. Accidentally produced free radicals will therefore initiate a chain reaction of lipid peroxidation which will propagate along membranes. This process can result in several dozen propagation steps before it is stopped by a termination reaction. Examples of such termination reactions are the recombination of peroxyl radicals and the formation of a stable free radical from a free radical scavenger (scavH). Termination through recombination of low steady-state concentration of alkyl radicals is unlikely in aerobic medium.

Reaction (6-14a) is the initiation step, while reactions (6-14b) and (6-l4c) are atom abstraction propagation reactions. Atom abstraction reactions in free-radical polymerizations are called chain transfer reactions. They are discussed in some detail in Section 6.8. 

Brokaw has re-analyzed the shock-tube results on CO oxidation and concluded that the initiating step is reaction (38) with k s = 2.5 x 10 exp (—48,000/ Rr)l.mole. sec" in agreement with Sulzmann et However, he disagrees with the excited CO2 mechanism and feels that the oxygen atom reacts with H2O impurities to promote oxidation of CO by a chain mechanism involving OH radicals... 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

The way to control a radical chain reaction is to control the initiation and termination steps. Radical chain reactions can be favored by adding radical initiators. Likewise, chain reactions can be greatly diminished by adding compounds called inhibitors that react with radicals to increase chain termination. The sensitivity of the radical reaction to radical initiators and inhibitors provides a convenient way to test for this mechanism. 

Chain reaction (Sections 10.4and 10.10) A reaction that proceeds by a sequential, stepwise mechanism, in which each step generates the reactive intermediate that causes the next step to occur. Chain reactions have chain-initiating steps, chain-propagating steps, and chain-terminating steps. 

A free-radical chain reaction includes a chain-initiating step, chain-propagating steps, and chain-terminating steps. 

Tennination. Any time one of the chain-carrying radicals (in this case a bromine atom or an alkyl radical) is annihilated by combination with another radical, the chain reaction is stopped (Fig. 11.27). If such a step does not happen, a single radical could initiate a chain reaction that would continue until starting material was completely used up. In practice, a competition is set up between the propagation steps carrying the chain reaction and the steps ending it. Such reactions are called termination steps and there are many possibilities. 

The halogen molecules are able to act in this way because they are split into energized separate atoms which have an unpaired electron under these conditions. Such energized particles with an unpaired electron are known as free radicals. Once formed, these radicals initiate a chain reaction in which halogenoalkanes are produced. Studies on these reactions have shown that the reaction can be divided into a sequence of steps, known as the reaction mechanism for the substitution. 

Detailed studies on the mechanisms of addition of organotin hydrides to oleflns and acetylenes remain to be made. Certain inferences can be drawn, however, on the basis of information now available. The catalysis by free radical sources demonstrates that the reaction can proceed by a free radical chain mechanism, since only a few mole per cent of initiator is needed in many cases for complete reaction. An attractive reaction sequence is shown in Eqs. (25-28), in which R is a carbon free radical and Sn is a trisubsti-tuted tin radical. Reaction (26) is the chain initiation step and reactions (27) and (28) comprise the propagation steps. Most of the characteristics of the hydrostannation reaction can be accommodated in such a scheme. For example, free radicals attack terminal oleflns at the terminal carbon atom because the resulting secondary or tertiary radical is more stable than the... 

In the above examples the size of the chain can be measured by considering the number of automobile collisions that result from the first accident, or the number of fission reactions which follow from the first neutron capture. When we think about the number of monomers that react as a result of a single initiation step, we are led directly to the degree of polymerization of the resulting molecule. In this way the chain mechanism and the properties of the polymer chains are directly related. 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

One characteristic of chain reactions is that frequentiy some initiating process is required. In hydrocarbon oxidations radicals must be introduced and to be self-sustained, some source of radicals must be produced in a chain-branching step. Moreover, new radicals must be suppHed at a rate sufficient to replace those lost by chain termination. In hydrocarbon oxidation, this usually involves the hydroperoxide cycle (eqs. 1—5). 

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