Light atom attack

Figure 5.10 A potential energy contour diagram for a light atom attacking in a reaction with a late barrier, showing energy requirements for reaction and energy disposal in products...

Quantitatively, this follows from the use of mass-scaled coordinates, Appendix 5.B. For a light atom attack the entrance valley is quite compressed compared with the products valley. 

Hence the products relative velocity (i.e., Qi) has a component along the direction of the old bond. For a light atom attack (sin 1), that component can dominate so the products angular distribution reflects the distribution of the orientation of the old bond (with respect to the initial velocity) at the instant of... 

Predictions depend on whether a heavy or a light atom is attacking a molecule, and on whether the overall reaction is exo- or endothermic, or thermoneutral. 

Alkane functionalization on a preparative scale by mercury-photosensitized C-H bond activation has been recently developed by Crabtree [22], Mercury absorbs 254-nm light to generate a Pi excited state which homolyzes a C-H bond of the substrate with a 3° > 2° > 1° selectivity. Radical disproportionation gives an alkene, but this intermediate is recycled back into the radical pool via H-atom attack, which is beneficial in terms of yield and selectivity. The reaction gives alkane dimers and products of cross-dehydrodimerization of alkanes with various C-H compounds ... 

Here, the optimum distribution of energy is S0% in the HCl vibration with most of the balance in relative translation. Further vibrational excitation at the expense of relative translational energy reduces the likelihood of reaction. This is a manifestation of the light atom anomaly , that is the unusual dynamical behaviour that appears when the attacking atom in an exothramic reaction or the product atom in an endothermic reaction, i.e. atom A, is much lighter than the other two, i.e. B and C. 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

In the light of what has been said above, any factors that influence the relative availability of electrons (the electron density) in particular bonds, or at particular atoms, in a compound might be expected to affect very considerably its reactivity towards a particular reagent a position of high electron availability will be attacked with difficulty if at all by, for example, eOH, whereas a position of low electron availability is likely to be attacked with ease, and vice versa with a positively charged reagent. A number of such factors have been recognised. 

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