Hydrogen atoms, atomic fluorine

Let s examine how the bonding in a fluorine molecule illustrates the octet rule. An independent fluorine atom has seven electrons in its highest energy level ([He]2s 2p ). Like hydrogen atoms, fluorine atoms bond covalently with each other to form diatomic molecules, p2- When two fluorine atoms bond, each atom shares one of its valence electrons with its parmer. The shared electron pair effectively Ms each atom s outermost energy level with an octet of electrons, as illustrated in Figure 2.7a. 

In some force fields the interaction sites are not all situated on the atomic nuclei. For example, in the MM2, MM3 and MM4 programs, the van der Waals centres of hydrogen atoms bonded to carbon are placed not at the nuclei but are approximately 10% along the bond towards the attached atom. The rationale for this is that the electron distribution about small atoms such as oxygen, fluorine and particularly hydrogen is distinctly non-spherical. The single electron from the hydrogen is involved in the bond to the adjacent atom and there are no other electrons that can contribute to the van der Waals interactions. Some force fields also require lone pairs to be defined on particular atoms these have their own van der Waals and electrostatic parameters. 

The unsuhstituted hydrogen atom of the dialkyl hydrogen phosphites can be readily replaced by chlorine, and thence by many other substituents including fluorine. 

The F H- H — H —> F—H + H reaction is a common example of a reaction easily studied by classical trajectory analysis. The potential surface we are interested in is that for FH2. This potential surface may have many extrema. One of them corresponds to an isolated Fluorine atom and a stable H2 molecule these are the reactants. Another extremum of the surface corresponds to an isolated hydrogen atom and the stable H-Fmolecule these are the products. Depending on how the potential surface was obtained there may or may not be an extremum corresponding to stable H2F, but at the least you would expect an extremum corresponding to the transition state of the reaction being considered. 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Replacement of Hydrogen. Three methods of substitution of a hydrogen atom by fluorine are (/) reaction of a G—H bond with elemental fluorine (direct fluorination, (2) reaction of a G—H bond with a high valence state metal fluoride like Agp2 or GoF, and (J) electrochemical fluorination in which the reaction occurs at the anode of a cell containing a source of fluoride, usually HF. 

Electrochemical Fluorination. In the Simons electrochemical fluorination (ECF) process the organic reactant is dissolved in anhydrous hydrogen fluoride and fluorinated at the anode, usually nickel, of an electrochemical ceU. This process has been reviewed (6). Essentially all hydrogen atoms are substituted by fluorine atoms carbon—carbon multiple bonds are saturated. The product phase is heavier than the HF phase and insoluble in it and is recovered by phase separation. 

Often the substitution of fluorine atoms for hydrogen atoms in a polymer chain markedly increases the thermal stabiUty of the base polymer this is tme for polyimides. A typical fluorinated polyimide is prepared from the reaction of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis-(4-amino phenyl)hexafluoropropane according to the following reaction (36) ... 

The fluorine atom confers chemical inertness, but some hydrogen atoms must be in the chain to maintain mbbery properties. Some fluorinated siHcones are also available where superior low temperature properties are requked (see Elastomers, synttietic— fluorocarbonelastomers). 

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. 

Although most deep fluorinations have resulted in fluorocarbons, a few have not, even under more forcing conditions. The results are explained by invoking steric hindrance induced by faster peripheral fluorination shielding more internal hydrogen atoms from attack [15] (equation 1). 

In quinoline and isoquinoline, the benzene ring is more receptive to fluorma-tion, its double bonds being saturated and the hydrogen atoms replaced in preference to those in the pyridine ring As with pyridine and its homologues, ring contraction takes place during fluorination with cesium tetrafluorocobaltate at... 

If we define the positive Z direction as up, the hydrogen atoms lie below this plane, and the fluorine atoms lie above it. The dipole moment points down, toward the hydrogen atoms, which is where we expect the positive charge to be. The same is true for the SS form. 

The values in red are within O.OlA of the experimental value. Using the 6-31G basis set, including diffuse functions on the hydrogen atom, improves the result over that obtained with diffuse functions only on the fluorine atom, although the best result with this basis set is obtained with no diffuse functions at all. 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

Consider abstraction of a hydrogen atom from propan( by fluorine atom. This can generate either of two propy radicals, depending on which hydrogen is attacked. 

The combination of a hydrogen with a fluorine atom leads to... 

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