1, 4-Addition mechanism

Potential Solutions No. Concern/Issue and Control Mechanisms Additional Resources... 

Much of the kinetic work in this category has already been described under the section relating to studies of mechanism. Additional data was obtained by Ols-son569, who measured rate coefficients (lO7 ) for dedeuteration and detritiation of thiophen by 57.02 wt. % sulphuric acid at 24.6 °C as follows [2-2H], 3,890 [2-3H], 2,000 [3-2H], 3.72 [3-3H], 2.20. The ratio of reactivities at the 2 and 3 positions (ca. 1,000) is in excellent agreement (bearing in mind the larger p-factor usually obtained with trifluoroacetic acid) with the value of ca. 1,250 which may be deduced from the data in Table 158. The ratio of dedeuteration to detritiation is 1.96 at the 2 position and 1.70 at the 3 position and thus decreases with decreasing reactivity of the reaction site. 

The evidence is in accord with an addition-elimination mechanism (addition of ArPdX followed by elimination of HPdX) in most cases. The reactions are stereospecific, yielding products expected from syn addition followed by syn elimination. Because the product is formed by an elimination step, with suitable substrates the double bond can go the other way, resulting in allylic rearrangement, for example, ... 

The absence of overlapping of bands of various matrix-isolated compounds and the possibility of freezing highly reactive intermediates make this method very convenient for the direct study of reaction mechanisms. Additionally, direct IR spectroscopy of intermediates allows estimation of important structural parameters, e.g. valence force fields, which show the character of bonds in these species. 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Double bond migration occurs either by the Jt-allyl mechanism (abstraction-addition) or by the Horiuti-Polanyi mechanism (addition-abstraction). Pd is thought to favor Jt-allyl and Pt Horiuti-Polanyi mechanisms. 

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