Fluorination, chain-initiating step

The chain-initiating step in fluorination is highly endothermic and thus has a high energy of activation. 

The thermodynamic quantities for iodination of methane make it clear that the chain-initiating step is not responsible for the observed order of reactivities F2 > CI2 > Bt2 > I2. The iodine-iodine bond is even weaker than the fluorine-fluorine bond. On this basis alone, one would predict iodine to be the most reactive of the halogens. This clearly is not the case. Once again, it is the hydrogen atom-abstraction step that correlates with the experimentally determined order of reactivities. The energy of activation of this step in the iodine reaction (140 kJ moF is so large that only two collisions out of every have sufficient energy to produce reactions at 300°C. As a result, iodination is not a feasible reaction experimentally. 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

Since fluorine is less than 1 % dissociated at room temperature, the concentration of fluorine atoms may not be sufficient to initiate a radical chain process. An alternative initiation step lb (Table 1), originally suggested by Miller [31-33], probably occurs but conclusive evidence for this pathway has not been established. 

Assuming the same mechanism applies, calculate AH° for the chain-initiating, chain-propagating, and chain-terminating steps involved in the fluorination of methane. 

The partially fluorinated radicals have been prepared by photolysis of the fluoroalkyl iodides (16, 17) and by the photolysis of the appropriate fluoroalkyl ketone (e. g. CH FCOCH F) (18). Fluorom ethyl and difluorom ethyl radicals have also been formed from the fluoroalkyl iodides using di-Jt-butyl peroxide as initiator (16, 17). Problems can arise in kinetic studies with these radicals because the chain carrying step may involve hydrogen as well as iodine abstraction. In practice this only seems to be of major importance when electronically excited species are present. 

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