Strain substituent constants, table

An examination of Table 1 shows that the value of is decreased by the presence of electron-attracting substituents, and increased by bulky groups. The former effect can be attributed to de-stabilization of the carbonyl compound, and the latter to steric strain in the diol. It is therefore of interest to compare the observed values of with the polar and steric substituent constants a and derived by Taft (1952, 1953,... 

When R is primary alkyl, the second-order rate constant k2 is obtained by taking the slope of kobs vs. concentration of the nucleophile. The plot passes through the origin, indicating a pure SN2 mechanism without SN1 participation. The reference pyridinium ion is the 2,4,6-triphenyl derivative (because pyrylium precursors with phenyl substituents are more easily prepared) (82AHC(Suppl 2)1) but numerous other substituents have been introduced into the ring. Rate constant values reported in Table XIX, where release of steric strain has a major influence, are in agreement with the role of structural factors discussed in Section IV,A. 

The equilibrium constants for a series of cycloalkenes decrease in the order norbomene > c -cyclooc-tene > cyclopentene > cycloheptene > cyclohexene, which correlates with the calculated strain energies as well as the kinetically determined relative adsorption constants on Pt (Table 2). Tolman states that electron donation from a filled metal rf-orbital to an empty alkene Tr -orbital is extremely important in determining the stability of these complexes. Steric effects of substituents are relatively unimportant compared to electronic effects, and resonance is more important than inductive interactions. The ability of the metal to back bond is lowered progressively in the series Ni° > Pt° > Rh > Pt" > Ag which reduces the importance of resonance and decreases the selectivity of the metal for different substituted alkenes. 

The rate coefficients for proton removal by hydroxide ion from the protonated amines in Tables 6 and 7 are exceptionally low, particularly in the case of l,8-bis(diethylamino)-2,7-dimethoxynaphthalene [59] for which half-lives in the range of minutes are observed. As strain increases along the series of amines [56] to [59] in Table 6, the basicity increases and the value of the rate coefficient for proton removal by hydroxide ion from the protonated amines decreases. However, for a given change in substituent in the amine, the change in basicity, as reflected in the value of the equilibrium constant (K) between amine and protonated amine in the presence of hydroxide ion, is smaller than the change in rate coefficient for proton transfer (kou-)- For example, in going from l,8-bis(dimethylamino)naphthalene [56] to 1,8-bis(diethylamino)naphthalene [57] the value of K decreases by a factor of... 

The direct reaction is first-order in the concentrations of the metal complex and the olefin and the second-order rate constant depends on the nature of olefin, as indicated by some of the data in Table S.9. Strain in the olefin appears to increase its reactivity, as shown for norbomene and cyclopentene. There may be some steric effects of olefin substituents, but these effects may be attenuated by better electron donation from the substituents. It was found also that the reaction rate with 2,3-dimethyl-2-butene was insensitive to the polarity of the solvent, with relative values of 1 0.6 0.8 in cyclohexane. Tiff and methanol, respectively. This seems to rule out an ionic or polar transition state or intermediate, and the authors favor a concerted cycloaddition mechanism. 

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