Organofluorine Chemistry - The Carbon–fluorine Bond

The Carbon–fluorine Bond

Fluorine has several distinctive differences from all other substituents encountered in organic molecules. As a result, the physical and chemical properties of organofluorines can be distinctive in comparison to other organohalogens.

1. The carbon–fluorine bond is one of the strongest in organic chemistry (an average bond energy around 480 kJ/mol). This is significantly stronger than the bonds of carbon with other halogens (an average bond energy of e.g. C-Cl bond is around 320 kJ/mol) and is one of the reasons why fluoroorganic compounds have high thermal and chemical stability.
2. The carbon–fluorine bond is relatively short (around 1.4 Å).
3. The Van der Waals radius of the fluorine substituent is only 1.47 Å, which is shorter than in any other substituent and is close to that of hydrogen (1.2 Å). This, together with the short bond length, is the reason why there is no steric strain in polyfluorinated compounds. This is another reason for their high thermal stability. In addition, the fluorine substituents in polyfluorinated compounds efficiently shield the carbon skeleton from possible attacking reagents. This is another reason for the high chemical stability of polyfluorinated compounds.
4. Fluorine has the highest electronegativity of all elements: 3.98. This causes the high dipole moment of C-F bond (1.41 D).
5. Fluorine has the lowest polarizability of all atoms: 0.56 10-24 cm3. This causes very weak dispersion forces between polyfluorinated molecules and is the reason for the often-observed boiling point reduction on fluorination as well as for the simultaneous hydrophobicity and lipophobicity of polyfluorinated compounds whereas other perhalogenated compounds are more lipophilic.

In comparison to aryl chlorides and bromides, aryl fluorides form Grignard reagents only reluctantly. On the other hand, aryl fluorides, e.g. fluoroanilines and fluorophenols, often undergo nucleophilic substitution efficiently.