Fluorine atomic size

The very low bond dissociation enthalpy of fluorine is an important factor contributing to the greater reactivity of fluorine. (This low energy may be due to repulsion between non-bonding electrons on the two adjacent fluorine atoms.) The higher hydration and lattice enthalpies of the fluoride ion are due to the smaller size of this ion. 

Fluorine is the most electronegative element and thus can oxidize many other elements to their highest oxidation state. The small size of the fluorine atom facihtates the arrangement of a large number of fluorines around an atom of another element. These properties of high oxidation potential and small size allow the formation of many simple and complex fluorides in which the other elements are at their highest oxidation states. 

Uracil is used more effectively, in nucleic acid synthesis within a rat hepatoma than in normal liver. This observation appears to have stimulated the synthesis of 5-fluorouracil (1027) as an antimetabolite mainly because the introduction of a fluorine atom involves a minimal increase in size. In the event, 5-fluorouracil did prove to have antineoplastic activity and it is now a valuable drug for treatment of tumors of the breast, colon or rectum, and to a lesser extent, gastric, hepatic, pancreatic, uterine, ovarian and bladder carcinomas. As with other drugs which interfere with DNA synthesis, the therapeutic index is quite low and great care is required during treatment (69MI21301). 

The size of the group attached to the main chain carbon atom can influence the glass transition point. For example, in polytetrafluoroethylene, which differs from polyethylene in having fluorine instead of hydrogen atoms attached to the backbone, the size of the fluorine atoms requires the molecule to take up a twisted zigzag configuration with the fluorine atoms packed tightly around the chain. In this case steric factors affect the inherent flexibility of the chain. 

As two fluorine atoms approach each other, the eiectrons of each atom feei the attraction of the nucieus of the other atom, just as happens in H2. Remember from Chapter 8 that atomic eiectrons are described by an eiectron configuration, which assigns each eiectron to an atomic orbitai. The eiectron configuration of the F atom is 1 2 2. Orbitais have different sizes and shapes, so when one fluorine atom approaches another, the... 

The fluorine atom, by virtue of its electronegativity, size, and bond strength with carbon, can be used to create compounds with remarkable properties. Small molecules containing fluorine have many positive impacts on everyday life of which blood substitutes, pharmaceuticals, and surface modifiers are only a few examples. 

Since the size of halogen atom Increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest. Consequently the carbon-halogen bond length also Increases from C—to C— I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table 10.2. 

Fluoro-2,2-dinitroethanol, the methylol derivative of fluorodinitromethane, has been used extensively for the synthesis of fluorodinitromethyl compounds. The fluorine atom is similar in size to that of a proton and so rotation in the fluorodinitromethyl group is much less hindered compared to the trinitromethyl group. Consequently, these compounds are far less sensitive to impact than trinitromethyl compounds but only slightly less energetic. The explosive performance of fluorodinitromethyl compounds has been reviewed. ... 

The data on the reactivities of trichloroethylene and tetrachloroethylene further illustrate the competitive effects of substitutions on the 1- and 2-positions of ethylene. Trichloroethylene is more reactive than either of the 1,2-dichloroethylenes but less reactive than vinylidene chloride. Tetrachloroethylene is less reactive than trichloroethylene—analogous to the difference in reactivities between vinyl chloride and 1,2-dichloroethylene. The case of polyfluor-oethylenes is an exception to the generally observed large decrease in reactivity with polysubstitution. Tetrafluoroethylene and chlorotrifluoroethylene show enhanced reactivity due apparently to the small size of the fluorine atoms. 

In brief, the effects of fluorination on the molecular properties stem from the combination of the atomic properties of the fluorine atom strong electronegativity, small size, excellent overlap of the 2s or2p orbitals with the corresponding orbitals of carbon, and very strong bond with carbon. 

Perfluorocarbons bearing a polar hydrophilic head are very active surfactants. Indeed, the presence of fluorine atoms strongly lowers the critical micelle concentration (CMC) of an amphiphilic compound. Moreover, fluorination generally has important effects on micellization phenomena, especially on the size and shape of formed micelles. 

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