Читать книгу Concise Handbook of Fluorocarbon Gases - Sina Ebnesajjad - Страница 35

Fluorine is the most electronegative of all elements at electronegativity of 4 in Pauling units. Electronegativity of other elements are 3.4 for oxygen, 3.2 for chlorine, 2.6 for carbon and 2.2 for hydrogen (Table 3.1) [3]. Extreme electronegativity of fluorine renders its covalent bonds highly polarized such as in C-F. Consequently, fluorine gas attacks nearly every substance and chemical because of very high reactivity. It even attacks noble gases like xenon producing XeF_x. It is easy to fluorinate hydrocarbons by fluorine gas, but the intensity of this reaction is too severe to control and causes broad decomposition.

The shortest bond is formed between C and H (0.11 nm) followed by C-F at 0.14 nm. Van der Waals radius (r_w) of fluorine substituent is 0.147 nm, shorter than in any other substituent. Van der Waals radius refers to the radius of an imaginary sphere that an atom occupies. The short bond length and r_w prevent the development of steric strain in perfluorinated compounds contributing to high thermal stability [3].

Carbon and fluorine form one of the strongest covalent bonds with an average bond energy around 480 kJ/mol. It exceeds the strength of carbon bond with other halogens (Table 3.2). C-F strength is one of the important reasons for high thermal and chemical stability of organic fluorochemicals. The F atoms have just the right size to create a protective shield (or sheath) over the carbon backbone when it is attached directly to the chain like in polytetrafluoroethylene (PTFE). If the F atoms were any smaller or larger than they are, the sheath would not form a regular uniform cove. This F shield protects the carbon chain from attack and confers chemical inertness and stability to PTFE. Fluorinated chemical groups play a similar role in hydrocarbons [5].

Table 3.1 Atomic properties of fluorine and other elements [4].

Element	Van der Waals, radii, nm	Electronegativity, Pauling
F	0.147	3.98
O	0.152	3.44
N	0.155	3.04
C	0.170	2.55
H	0.120	2.20

Table 3.2 Atomic properties of fluorine and other elements [4].

Element	Average bond strength, kJ/mol	Average bond length, nm
C-F	485	0.139
C-C	356	0.153
C-O	336	0.143
C-H	416	0.109