Site rules breaking

These radicals decompose according to the /3-scission rule, which implies that the bond that will break is one position removed from the radical site, so that an olefin can form without a hydrogen shift. Thus the isopropyl radical gives propene and a H atom, while the //-propyl radical gives ethene and a methyl radical. The /3-scission rule states that when there is a choice between a CC single bond and a CH bond, the CC bond is normally the one that breaks because it is weaker than the CH bond. Even though there are six primary CH bonds in propane and these are somewhat more tightly bound than the two secondary ones, one finds substantially more ethene than propene as an intermediate in the oxidation process. The experimental results [12] shown in Fig. 3.12 verify this conclusion. The same experimental effort found the olefin trends shown in Table 3.2. Note that it is possible to estimate the order reported from the principles just described. 

What about larger molecules Here there are no general rules. It depends on the problem under consideration. It is often possible to leave the CH bonds inactive (unless they are involved in a chemical transformation). A molecule like butadiene (C4H6) then needs 12 active orbitals (12inl2). One can now break all CC bonds. Usually, the active site is only part of a larger molecule and we then need only active orbitals that are localized to that part. For example, a long alkyl chain with an active end group only needs orbitals there to be active. The choice of the active space does not in itself limit the size of molecules that can be studied. 

However, there are exceptions to this rule, when a cation-cation bond across the vacancy site may be formed. This is the case in bulk Si02, where the creation of an oxygen vacancy breaks two 0-Si bonds. The atomic displacements of the two neighbouring silicon atoms are directed inwards in such a way that the Si-Si distance across the vacancy site is reduced by about 0.5 A with respect to its value in the perfect oxide [256]. 

Jencks libido rule [9, 10] attempts to identify situations in which proton transfer can be expected to be concerted with the making or breaking of covalent bonds "Concerted general acid-base catalysis of complex reactions in aqueous solution can occur only (a) at sites that undergo a large change in pK in the course of the reaction, and (b) when this change in pK converts an unfavorable to a favorable proton transfer with respect to the catalyst i.e., the pK of the catalyst is intermediate between the initial and final pK values of the substrate site. [9]. 

In shared workplaces like the constmction site in this case, appliance of the same safety rules and procedures regardless of contractor would benefit all. fir this work situation during lunch break, the installation workers stepped aside to give room to pass when workers carrying the boards entered the landing. So despite the hazard, the mentality was more everyone takes care of his/her own work rather than safe working together . When all workers on a site know and follow the mles and procedures, the evolvement of a safety hazard into a serious accident can be stopped before it is too late. 

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