Straining system

Filler orientation is not the only consequence of geometric effects in strained systems with an anisodiametric filler. The appearance of normal stresses has been named as another such phenomenon [169,171,185] which, like the abnormal viscosity behavior of suspensions, may not be due to the elasticity of system components. 

In a chain reaction, the step that determines what the product will be is most often an abstraction step. What is abstracted by a free radical is almost never a tetra- or tervalent atom (except in strained systems, see p. 989) and seldom a divalent one. Nearly always it is univalent, and so, for organic compounds, it is hydrogen or halogen. For example, a reaction between a chlorine atom and ethane gives an ethyl radical, not a hydrogen atom ... 

Keyto Species figure (strain) System NOAEL (mg/kg/day) Less Serious (mg/kg/day) Serious (mg/kg/day) Reference Chemical Form... 

It was puzzling to me that the phenomenon appeared to be important chiefly in strained systems, systems where steric eflFects could also be involved. 

Scheme 5.15 shows some examples of the Shapiro reaction. Entry 1 is an example of the standard procedure, as documented in Organic Syntheses. Entry 2 illustrates the preference for the formation of the less-substituted double bond. Entries 3, 4, and 5 involve tosylhydrazone of a, (3-unsaturated ketones. The reactions proceed by a -deprotonation. Entry 6 illustrates the applicability of the reaction to a highly strained system. 

Species/ (Strain) System NOAEL Less Serious Serious Reference Fluid Identity... 

The calculations thus fail to indicate any substantial energy preference for the allowed paths with respect to the forbidden ones. An inspection of the overall shape of the surface confirms, however, that along the allowed CCW path a less steep slope has to be climbed (Fig. 18). The general conclusion is that steric and symmetry factors are so intimately interwoven that it is impossible to distinguish their relative importance in cases where the magnitudes of the two effects are similar. This can perhaps be taken as a warning that orbital symmetry rules should only be applied with some caution to very strained systems. 

Theoretical calculation at the HF/6-31G level99,102 on some S-S dications and their precursors have shown that the electronic structure of 1,4-dithionia-bicyclo[2.2.0]hexane and the sp-sp conformation of the tetramethyldisulfonium dication, the difference in the energy levels of [S] — [S] and [S] + n[S]103 is decreased owing to steric strain and the order of orbitals thus corresponds to case B. In the less strained systems (l,5-dithioniabicyclo[3.3.0]octane, 1,4-dithioniabicyclo[3.2.0]heptane), the order of orbitals corresponds to case C. Interestingly, ap-ap conformer of tetramethyldisulfonium dication was reported to correspond to case A. 

Key toa figure Species (strain) System NOAEL (ppm) Less serious (ppm) Serious (ppm) Reference... 

Cyclizations have also been reported within strained systems such as the conversion of 255 to 256136 or the formation of 258 from 257137. 

Many reducing agents are capable of severing a carbon-halogen bond. Cathodic cleavage provides perhaps the most versatile method, and has been put to excellent use. The electrochemical variation of the Wurtz reaction constitutes a powerful method for the construction of a variety of rings, particularly strained systems. Dramatic examples are provided by the assembly of bicyclobutane (308) [89], bicyclohexene (310) [90-92], [2.2.2]propellane (312) [93], spiropentane (316) [94], j -lactams 318 [95], and a variety of small-ring heterocycles (320) [96,97]. 

The bonds indicated can be single or double Prevents reconnection to strained systems Prevents formation of double bonds over bridgehead atoms... 

In the disconnection tables, when we refer to "prevention against strained system", we mean that "prevents reconnection to rings of less than 8 members and secondary rings". 

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