Chemistry bond scission processes

Note, however, that chain ends 4 and 5 may give different chemistry to those formed in termination by disproportionation (2, see Scheme 8.5) or the processes under (a) above. Chain scission (3 to the double bond will not lead to a MMA propagating species. It is not established whether the presence of these ends will give impaired thermal stability. 

The central cleavage of P-carotene 1 is most likely the major pathway by which mammals produce the required retinoids il), in particular, retinal 2, which is essential for vision and is subsequently oxidized to retinoic acid 3 and reduced to retinol 4. An alternative excentric cleavage of 1 has been reported involving scission of the double bond at C7-C8 producing P-8 -apocarotenal 5 (2a,2b) which is subsequently oxidized to 2 (Fig. 1) (2c). The significance of carotene metabolites such as 2, 3 and 4 to embryonic development and other vital processes such as skin and membrane protection is a major concern of medicinal chemistry. 

In this work we have avoided consideration of specific mechanism for decrease in molecular weight with liquefaction, including in particular the perceived need for thermal scission of C-0 and C-C bonds during conversion. Thus we view conversion simply as a process requiring some kind of nonspecific reduction chemistry. 

The reduction of molecular weight (cracking) of various fractions of crude oil performed over zeolites is an important process in petroleum chemistry and has been studied extensively. Acid-catalyzed cracking, besides the already discussed direct protonation of C-C bonds [Eq. (6.16)], can also involve trivalent carbocations that undergo p-scission through the intermediacy of hypercoordinate carbocations [Eq. (6.21)]. 

The shearing process itself is a simple homolytic scission of a carbon-carbon bond to produce two polymeric radicals. There is no evidence that these radicals are involved in other aspects of lubricant chemistry and are apparently rapidly terminated by either antioxidant or other species in the oil. Backbone carbon-carbon bond strengths of all of the VI improver are much the same so none of the chemistries is inherently more stable than any other. Several workers have proposed that at least some of the shearing takes place at weak links, such as might result from steric hindrance, but there is little or no evidence to support this hypothesis. 

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