Trifluoromethyl radicals structure

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

As expected, fluorine substitution has some consequences on structure and stability of the radicals, which are different from the hydrocarbon counterparts. a-F radicals prefer the pyramidal structure because of minimizing 1 repulsion. The trifluoromethyl radical F3C is essentially tetrahedral and has a significant barrier to inversion of about 25 kcal mol - .39 In contrast, the methyl radical H3C itself is planar. Fluorine /J to the radical site is of minor structural consequence. Thus, the pcrfluoro-/er/-butyl radical exhibits a more planar geometry. 

In addition to the recombination of the two radicals forming dibenzoyl and C2Fb, there are two other processes. In the first of these, hydrogen is abstracted from the aromatic structure of the parent ketone by the trifluoromethyl radicals. 

Fluorine substituents have a dramatic impact upon the structure of alkyl radicals. The methyl radical itself is planar UV, IR, PES and ESR spectroscopy, as well as the highest level of theoretical analysis, all indicate that its conformational properties are best defined as deriving from a single minimum [2], Fluoromethyl radicals, on the other hand, are increasingly pyramidal [3], with the trifluoromethyl radical being essentially tetrahedral [3 - 7], with a significant barrier to inversion [8,9]. 

ESR spectroscopy is perhaps the best method for the unequivocal detection and observation of free radicals, and ESR 13C hyperfine splitting (hfs) constants are considered to be a very useful indicator of a radical s geometry because non-planarity introduces s character into the orbital that contains the unpaired electron. The methyl radical s 13Ca value of 38 G is consistent with a planar structure. Fluoromethyl radicals exhibit increased 13Ca values, as shown in Table 1, thus indicating increasing non-planarity, with trifluoromethyl radical s value of 272 G lying close to that expected for its sp3 hybridization [4]. 

Table IV shows the reactivity ratios rG and r, derived from the probabilities in Table III in accord with a first-order Markov model (2), where it is assumed that the more likely propagating terminal radical structure is 1 (—CHF-) and not 0 (—CH2). This assumption is consistent with gas phase reactions of VF with mono-, di-, and trifluoromethyl radicals, which add more frequently to the CH2 carbon than to the CHF carbon (20). The reactivity ratio product is unity if Bernoullian statistics apply, and we see this is not the case for either PVF sample, although the urea PVF is more nearly Bernoullian in its regiosequence distribution. Polymerization of VF in urea at low temperature also reduces the frequency of head-to-head and tail-to-tail addition, which can be derived from the reactivity ratios according to %defect — 100(1 + ro)/(2 + r0 + r,). Our analysis of the fluorine-19 NMR spectrum shows that commercial PVF has 10.7% of these defects, which compares very well with the value of 10.6% obtained from carbon-13 NMR (13). Therefore the values of 26 to 32% reported by Wilson and Santee (21) are in error.

Even the relatively simple spectrum of the methyl radical tells us quite a lot about its structure. For example, the size of the coupling constant indicates that the methyl radical is planar the trifluoromethyl radical is, on the other hand, pyramidal. The oxygenated radicals CH2OH and CMe20H lie somewhere in between. The calculations that show this lie outside the scope of this book. 

Radical copolymerizadon of substituted norbomenes mth sulfur dioxide produced alternating copolymers in excellent yields in a few hours. The copolymers had an OD of 0.25-0.33/pm at 193 nm. The acidic di(trifluoromethyl)carbinol structure incorporated into the poly(norbomene sulfone) provided an extremely fast dissolution rate in aqueous base. Terpolymerization with carbo-/>butoxynorbomene resulted in an exponential decay of the ssolution rate. In contrast to poly(styrene sulfones), however, poly(norbomene sulfones) exhibited unacceptably fast etching in... 

Precise description of the pyramidal structures would also require that the bond angles be specified. The EPR spectrum of the methyl radical leads to the conclusion that its structure could be either planar or a very shallow pyramid. The IR spectrum of methyl radical has been recorded at very low temperatures in frozen inert gas. ° Under these conditions, a relatively high concentration of reactive species can be obtained, since chemical reactions are prevented by the inertness of the surrounding matrix. The IR spectrum puts a maximum of 5° on the deviation from planarity. Similar studies have suggested a planar arrangement for the mono-chloromethyl radical, although the trichloromethyl radical is found to be pyramidal. Both EPR and IR studies lead to the conclusion that the trifluoromethyl radical is nonplanar. ... 

As an example, consider the trifluoromethyl radical CF3. To determine the s-electron spin density on the carbon, one must measure the hyperflne coupling constant experimentally. This is found to be 271.6 G, which is 24% of a full 2s electron on the carbon. This implies near SP bonding in the radical and indicates that CF3 is tetrahedral and not planar. In contrast, the hyperflne coupling constant for the methyl radical CH3 is 38 G, which indicates only 3% s character. This is consistent with a near-planar structure for CH3. In fact, the time-average structure of CH3 is planar, but a small amount of s hybridization can arise from out-of-plane vibrations of the H atoms. 

The reaction of trifluoromethyl hypofluorite with various alkenes depends on the structure of the alkene and the reaction conditions.66 67 Hexafluoropropene, various substituted ethenes and fluoro-substituted ethenes usually give two types of products, e.g. 10 and 11, 12A and 12B, and 14 and 15, which are formed via radical intermediates.2-3-28-29... 

Styrene and 1,1-diphenylethene are frequently used as target molecules in investigations of the role of reagent structure on its reactivity toward organic compounds. Low-temperature fluorination with trifluoromethyl hypofluorite gives up to six products.32-33 The Hammett correlation value q was established to be — 2.48 and a mechanism suggested in which spin-paired free-radical pairs arc formed, which are then converted by electron transfer into an ion pair.33... 

Further reports on 1,4-dihydro-1,2,4,5-tetrazine structures by x-ray diffraction are found for the 3-(p-chlorophenyl)-6-trifluoromethyl derivative <85JHC643>, l,3,6-triphenyl-l,4-dihydro-l,2,4,5-tetrazine <90JCS(Pl)2527>, 6-bromo-l,4-dihydro-l,4-di(o-tolyl)-l,2,4,5-tetrazin-3(27/)-one, and 3,6-dibromo-1,4-dihydro-l,4-bis(p-methoxyphenyl)-1,2,4,5-tetrazine <94AX(C)l78l>. Finally, there are reports on the crystal structures of the heterocyclic betaine l,4-dimethyl-6-oxo-1,2,4,5-tetrazin-l-ium-3-olate (17) <84TL629> and tetramethyl-p-urazine (18), from which cation radicals could be formed <9UOC1045>. 

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