Fluorination, direct steric factors

Another series of compounds, the fluorine-substituted 2,2,4,4-tetra-methylpentanes, have proven to be a good artificial blood candidate (69). The compounds are prepared by the direct fluorination of the hydrocarbon precursor (70a) (see Fig. 15). The overall yield of the reaction can be seen from the figure to be in excess of 99%. The central protons are retained preferentially during the fluorination, a result that is attributed to steric rather than electronic factors (70a). 

Analyzing the frequency factor v, Bursey43 was able to show that the details of the reaction mechanisms of the electron impact induced ketene elimination from ortho substituted phenyl acetates and from acetanilides, 36, are dramatically influenced by the nature of the substituent R = F, Cl, Br, J. The results demonstrate that the decrease of v going from the voluminous iodine to the small fluorine is not caused by steric effects (which should operate in an opposite direction) but is the result of an electronic interaction of both substituents, leading to a tighter transition state in the case of the more electronegative fluorine. Additional factors which are not yet completely understood play a decisive part in the fragmentation of the anilides. 

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