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In chemistry, initiation is a chemical reaction that triggers one or more secondary reactions. Initiation creates a reactive centre on a molecule which produces a chain reaction . [ 1 ] The reactive centre generated by initiation is usually a radical , but can also be cations or anions . [ 2 ]
Chemical reactions involving thermal runaway are also called thermal explosions in chemical engineering, or runaway reactions in organic chemistry.It is a process by which an exothermic reaction goes out of control: the reaction rate increases due to an increase in temperature, causing a further increase in temperature and hence a further rapid increase in the reaction rate.
In the simplest example oxygen can be used as it exists naturally in its triplet state (i.e. it is a diradical). This is referred to as air inhibition and is a diffusion-controlled reaction with rates typically in the order of 10 7 –10 9 mol −1 s −1 , [ 3 ] the resulting peroxy radicals (ROO•) are less reactive towards polymerisation.
When they collide with the large, slower-moving particles of the tobacco smoke they push the latter away from the rod. The force that has pushed the smoke particles away from the rod is an example of a thermophoretic force, as the mean free path of air at ambient conditions is 68 nm [2] and the characteristic length scales are between 100 ...
Initiation the process of generating the initial free radical. Propagation the conversion of one active species to another Chain branching steps which end with more than one active species being produced. The photolysis of hydroperoxides is the main example. Termination steps in which active species are removed, for instance by radical ...
For example, di-tert-butyl peroxide (t-Bu OOt-Bu) gives two t-butoxy radicals (t-BuO•) and the radicals become methyl radicals (CH 3 •) with the loss of acetone. Benzoyl peroxide (( Ph C)OO) 2 ) generates benzoyloxyl radicals (PhCOO•), each of which loses carbon dioxide to be converted into a phenyl radical (Ph•).
For chain-growth polymerization, the overall activation energy is = +, where i, p and t refer respectively to initiation, propagation and termination steps. The propagation step normally has a very small activation energy, so that the overall value is negative if the activation energy for termination is larger than that for initiation.
In organosulfur chemistry, the thiol-ene reaction (also alkene hydrothiolation) is an organic reaction between a thiol (R−SH) and an alkene (R 2 C=CR 2) to form a thioether (R−S−R'). This reaction was first reported in 1905, [ 1 ] but it gained prominence in the late 1990s and early 2000s for its feasibility and wide range of applications.