Introduction of Fluorine

Abstract

Shortly after their first isolation of elemental fluorine in 1886, Moissan and his co-workers treated several organic substrates with this highly reactive gas. All these experiments, either at room temperature or at liquid nitrogen temperature, resulted in sometimes violent explosions. No major defined reaction products could be isolated.A plausible, first explanation for these discouraging results was proposed by W. Bockemüller in the 1930s, on the basis of thermochemical considerations. The energy released by formation of the highly stable carbon-fluorine bonds (∼116 kcal mol −1 ) is considerably greater than the energy needed for dissociation of carbon-carbon (∼83 kcal mol −1 ) or carbon-hydrogen bonds (∼99 kcal mol −1 ) [1]. A second problem is the extremely low homolytic dissociation energy of elemental fluorine (only 37 kcal mol −1 ), which allows the ready initiation of uncontrollable radical chain reactions, even at low temperatures and in the absence of light [2].The first defined fluoroaliphatic compounds obtained by direct fluorination of organic substrates in liquid reaction media were characterized by Bockemüller [3] in the early 1930s and published with his thermochemical analysis. To control the immense reaction enthalpy, the fluorine gas was diluted with nitrogen or carbon dioxide. The organic substrate was dissolved in a cooled inert solvent, for example, CCl 4 or CF 2 Cl 2 . A similar line of work was pursued in the United States by L. A. Bigelow [4], who studied the reaction of arenes with fluorine gas.In an alternative approach, volatile organic substrates were fluorinated in the gas phase on contact with a copper mesh. This work was pioneered by K. Fredenhagen and G. Cadenbach in the early 1930s [5] and then continued by N. Fukuhara and L. A. Bigelow [6] as part of the Manhattan Project (Figure 2.1). Vapor-phase fluorination finally permitted the preparation of (relatively) defined polyfluorination products from aliphatic hydrocarbons, benzene, or acetone.A modern, improved version of this general method, the LaMar (Lagow-Margrave) process, uses a nickel reactor with different temperature zones and Modern Fluoroorganic Chemistry: Synthesis, Reactivity, Applications, Second Edition. Peer Kirsch.