Hindrance, stenc

Various combinations of Rf and R (equation 36) have been studied [39, 72, 73, 74, 75], and it appears that the stability of the lithium salt of the hemiketal is the major factor in determming the reaction products formed via paths A, B, or C in equation 37 Other important factors that affect the course of the reacbon are (1) thermal stability of the perfluoroalkyllithium compounds, (2) reaction temperature, (3) mode of addition of the reactants, (4) stenc hindrance, (5) nature of the Y group (in equation 36), and (6) temperature at which the reaction is terminated by acid hydrolysis... 

C C bonds are generally more reactive than secondary or primary C H bonds, leading to preferential nitrolysis of n-alkanes. The nitronium ion, which is linear by itself, does not seem to exercise excessive stenc hindrance in the transition state, where it is substantially bent (as indicated also from its behavior in electrophilic aromatic substitutions). Side products of the nitrolysis are methyl, ethyl, and isopropyl fluoride (formed by the reaction of PF with the cleaved alkylcarbenium ions) or secondary alkylation products, which by themselves are capable of undergoing reaction with the nitronium salt. 

Some experimental results on the rates of photoassociation of various fluorophores are given in Table 6.1 [26]. The figures in the last column are typical in that most of the rate constants are within a factor of 2 of the diffusion-controlled value as calculated by the Smoluchowski theory for slip conditions (Section 3.4.3.1), regardless of solvent and temperature. From spectroscopic evidence, it appears that the dimer has a considerable dipole moment, suggestive of a charge-transfer complex. We may suppose that when these large disc-like molecules collide they usually form such a complex without stenc hindrance and with little or no activation-energy requirement. 

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