Carbon-fluorine covalent bond

Electrophilic fluorination is the process by which fluorine is delivered to an electron-donating reactant, such as an alkene, aromatic ring or carbanion, by a formal positive-fluorine reagent to form a carbon-fluorine covalent bond. These reactions are fast and have proven extremely valuable for some important fluorine-18-labelled radiopharmaceuticals. Over the years several reviews on electrophilic fluorination were written. The reader is encouraged to seek out these works for greater detail on the subject [7,68-70]. 

Explain why the electrons in the carbon-fluorine covalent bond tend to move more toward the halogen atom than do the electrons in the carbon-bromine covalent bond. 

Representing a two electron covalent bond by a dash (—) the Lewis structures for hydrogen fluoride fluorine methane and carbon tetrafluoride become... 

Coupling between fluorine and a hydrogen, a carbon or another fluorine that may be separated by many bonds (four, five, six or more) can result from overlap of electronic orbitals occupied by lone pair electrons which are unshared and therefore not involved in normal covalent bonding. The term applied to this effect, through space is somewhat misleading, since all isotropic coupling must be transmitted in some way by electrons, either in bonds or in unshared pairs. 

The period 2 non-metals from carbon to fluorine must fill their 2 s and their three 2p orhitals to acquire a nohle gas configuration like that of neon. Covalent bonding that involves these elements obeys the octet rule. In the formation of the diatomic fluorine molecule, F2, for example, the bonding (shared) pair of electrons gives each fluorine atom a complete valence level. 

The covalent bond that holds molecules of hydrogen, fluorine, and hydrogen fluoride together is a single bond. It involves a single bonding pair of electrons. Some molecules are bonded together with two shared pairs of electrons. These are called double bonds. Carbon dioxide is an example of a covalent molecule that consists of double bonds. 

C2H6 hydrogen peroxide, H202 and acetylene, C2H2. Keep in mind that each carbon atom must have four covalent bonds, each oxygen must have two, and each fluorine and hydrogen must have only one. 

Section 1.5 When two atoms that differ in electronegativity are covalently bonded, the electrons in the bond are drawn toward the more electronegative element. The electrons in a carbon-fluorine bond are drawn away from carbon, toward fluorine. 

Flowever, the electrons of a covalent bond are not necessarily shared equally by the bonded atoms, especially when the affinities of the atoms for electrons are very different. Thus, carbon-fluorine and carbon-lithium bonds, although they are not ionic, are polarized such that the electrons are associated more with the atom of higher electron affinity. This is usually the atom with the higher effective nuclear charge. 

Again, consider the fluoroform molecule, CHF3. Here the four orbitals and four electrons on carbon can form four covalent bonds, one to hydrogen and three to the three fluorine atoms. Again carbon and fluorine have a... 

Atoms other than hydrogen also form covalent bonds by sharing electron pairs, and the electron-dot structures of the resultant molecules are drawn by assigning the correct number of valence electrons to each atom. Group 3A atoms (such as boron) have three valence electrons, group 4A atoms (such as carbon) have four valence electrons, and so on across the periodic table. The group 7 A element fluorine has seven valence electrons, and an electron-dot structure for the F2 molecule shows how a covalent bond can form ... 

The carbon-fluorine bond is a polar covalent bond in the sense that it exhibits a not negligible ionic character. This polarity is reflected in the weight of the ionic VB structure, Fj0n(H3C+F-), which is calculated to... 

The most widely known chemotherapeutic agent directed against TS is 5-fluorour-acil (5-FU). 5-FU was first used clinically almost 50 years ago, yet still remains a mainstay for the treatment of carcinoma of the breast and gastrointestinal tract. In cells, 5-FU is metabolized to 5-FdUMP, which forms a stable inhibitory ternary complex with the co-substrate N5N10-methylene-5,6,7,8-tetrahydrofolate (CH2H4-folate) and TS. In this complex, a covalent bond links the thiol of cysteine 195 of human TS to C6 of deoxyuracil monophosphate (dUMP) and the methylene carbon of the co-substrate is joined to C5 of the nucleotide [55]. The fluorine at C5, unlike the proton, cannot... 

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