Strength of Primary Bonds

The molecular theories treated in this chapter have considered the thermally activated breakage of elements or filaments as a source of crack initiation and macroscopic failure. It is the purpose of the following chapters to investigate the mechanical strength of primary bonds and of molecular chains and to study the occurrence of chain breakages. With that information available it will be possible to resume the discussion on the nature of the elements used in molecular theories of fracture. 

The difference in position of attack on primary and secondary aromatic amines, compared with phenols, probably reflects the relative electron-density of the various positions in the former compounds exerting the controlling influence for, in contrast to a number of other aromatic electrophilic substitution reactions, diazo coupling is sensitive to relatively small differences in electron density (reflecting the rather low ability as an electrophile of PhN2 ). Similar differences in electron-density do of course occur in phenols but here control over the position of attack is exerted more by the relative strengths of the bonds formed in the two products in the two alternative coupled products derivable from amines, this latter difference is much less marked. 

On a purely statistical basis, we may expect the ratio of products from 3 to correlate with the number of available hydrogens at the various positions of substitution. That is, 4, 5, 6, and 7 would be formed in the ratio 6 3 2 l (50% 25% 17% 8%). However, as can be seen from Table 4-6, the strengths of hydrogen bonds to primary, secondary, and tertiary carbons are not the same and, from the argument given in Section 4-4E we would expect the weaker C-H bonds to be preferentially attacked by CI-. The proportion of 7 formed is about three times that expected on a statistical basis which is in accord with our expectation that the tertiary C-H bond of 2-methylbutane should be the weakest of the C-H bonds. (See Table 4-6.)... 

In the absence of discrete solvent molecules or a collective solvent coordinate, continuum solvation models do not allow the solvent to enter into the reaction coordinate, and in many cases that misses the primary role of the solvent. The solvent may enter the reaction coordinate only quantitatively, for example by having a slightly different strength of hydrogen bonding to the solute at the transition state than at the reactant, or it may enter qualitatively, for example by entering or leaving the first solvation shell, by... 

In this review, primary isotope effects are defined as concerning reactions in which, at some stage, a bond to the isotopically substituted atom is either broken or formed. A secondary isotope effect will refer to reactions in which no bond to the isotopically substituted atom is broken and none is formed. The strengths of the bonds to the substituted atoms may, however, be altered and, in general, will be to some degree. 

How can we explain the ratios of products that are formed The key is to look at the relative stabilities of the radicals involved in the reaction and the strengths of the bonds that are formed and broken. First, the chlorination of propane. A chlorine radical, produced by photolysis, can abstract either a primary hydrogen atom, from the end of the molecule, or a secondary hydrogen atom, from the middle. For the first process, we have these energy gains and losses. 

These results may be interpreted qualitatively in terms of the difference in the resonance stabilization of the two different product radicals. As the resonance energies of the product radicals are in the order tertiary > secondary > primary, addition at the least substituted position is predicted. The results for pentene-2 can also be explained on this basis if the 3-pentyl radical, CH3.CH2.CH.CH2.CH3, is more stable than the 2-pentyl radical, CH3. OH. CH2. CH2.CH3. These results may also be interpreted from another viewpoint that the important factor is the strength of the bond which is formed rather than the stability of the product radical. 

The orbitals are crucial to the fact that a tert-buty radical reacts faster than primary or secondary radicals with alkenes such as vinylphosphinic esters or acrylonitrile. Thus, the increase in the SOMO energy on going from primary to tertiary radicals has a larger effect on the rate than the decrease in the strength of the bonds that are formed. Alkyl, alkoxyalkyl, aminoalkyl and other similar radicals are therefore nucleophiles. However, radicals with electron-withdrawing... 

In previous sections it was shown that the primary panicle size is approximately determined by the condition that tf = Tf. The theoretical analysis that led to this conclusion can be extended to explain qualitatively the nature and strength of the bonds between the primary particles. During the lime period before Tf i=s r, coalescence is rapid and the... 

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