Triphenylmethyl halides

The first radicals to be studied were, hardly surprisingly, those that were somewhat less reactive, and thus capable of rather longer independent existence. The first such radical to be detected unequivocally was Ph3C- (4), obtained in 1900 on reacting Ph3CCl with finely divided silver (cf. p. 43). The radical reacted with halogens to reform the triphenylmethyl halide (5), or with oxygen from the air to form (6), a peroxide (all radicals react readily with 02 from the air) ... 

As befits their status as compounds well-known to be in equilibrium with carbonium ions in suitable solvents, triphenylmethyl halides and related compounds give particularly unambiguous evidence of reaction involving ionic intermediates. In polar solvents they give... 

This idea was based on the well-known analogous reactions of metal halides with triphenylmethyl halides, and thus seemed plausible. However, detailed studies have shown... 

Swain and Scott56 have measured the knC lkHF ratio in neutral or slightly acidic solutions for the hydrolysis of acetyl and benzoyl halides and compared them with those for triphenylmethyl halides. Values for the triphenyl-methyl halides are — 104 greater than those for acyl halides (Table 10) reflecting the tendency for C-X bond breaking to be more complete than O-C formation at the transition state of trityl hydrolysis and the opposite tendency with the benzoyl halides. The C-F bond is harder to break but the carbon atom is made more electropositive. 

Historically, the triphenylmethyl radical (1), studied by Gomberg in 1987, is the first organic free radical. The triphenylmethyl radical can be obtained by the reaction of triphenylmethyl halide with metal Ag as shown in eq. 1.1. This radical (1) and the dimerized compound (2) are in a state of equilibrium. Free radical (1) is observed by electron spin resonance (ESR) and its spectrum shows beautiful hyperfine spin couplings. The spin density in each carbon atom can be obtained by the analysis of these hyperfine spin coupling constants as well as information on the structure of the free radical. 

Allyl halides heterolyze just as easily as benzyl halides because they also produce a resonance-stabilized carbenium ion. Even faster heterolyses are possible when the charge of the resulting carbenium ion can be delocalized by more than one unsaturated substituent and can thereby be stabilized especially well. This explains the remarkably high SN1 reactivities of the benzhydryl halides (via the benzhydryl cation) and especially of the triphenylmethyl halides (via the trityl cation) ... 

Uncomplexed BX3 interacts with triphenylmethyl halides, (C6H5)3-CY, to give the BX3Y salt (120, 183). Halide ion Y can also be extracted from metal complexes by BX3 to give species that are probably BX3Y salts (51). 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Homolytic rupture of the carbon-halogen bond appears to be the dominant primary process. Lewis and Owen have studied the continuous ultraviolet photolysis of the triphenylmethyl halides in cyclohexane solution. They have proposed a mechanism which involves the formation of the triphenylmethyl radical in an excited electronic state. 

Acetonitrile (bp 82°) is another polar solvent, which promotes the ionization of compounds such as the triphenylmethyl halides. It is also a very useful solvent for the recrystallization of polar compounds such as dicarboxylic acids. It is easily purified by heating to reflux with and then distilling from phosphorous pentoxide. Much of the commercial acetonitrile is dried by azeotropic distillation with benzene. If it is to be used as the solvent for ultraviolet spectroscopy, either one must obtain material which has not been treated with benzene, or the benzene may be removed by azeotropic distillation with water and drying with phosphorus pentoxide. The presence of benzene is indicated by weak absorption around 260 m/ and strong tail absorption beginning at about 220 m/i.. 

A number of other materials can function as catalysts for cationic polymerization, among them iodine, tropylium salts, triphenylmethyl halides, and Lbutyl halides (Eqs. 22.14-22.16). 

Such an ionization of the easily split polar covalent bond ArjC—X upon adsorption, does not represent a phenomenon peculiar for the latter. It could be observed with the same triphenylmethyl halides when dissolved in liquid SO 3 and in other solvents with high dielectric constants. 

Triphenylmethyl halides and tropylium halides ionize to form trityl, (f>3C, and cycloheptatrienyl (tropylium), CyRt", carbocations [Eqs. (P8.20.1) and (P8.20.2)], which are too stable to efficiently polymerize less reactive monomers such as isobutylene and styrene, but polymerization of p-methoxystyrene, vinyl ethers and N-vinyl carbazole, which are more reactive, proceeds rapidly. 

Although it might be expected that reactions which employed triarylmethyl halides would occur very readily, such reactions are rendered potentially more complex by the known nature of the halides and their propensity for involvement in free radical reactions. Whereas normal alkylation proceeds between sodium diethyl phosphite and diphenyl-methyl halides, success, or otherwise, in the use of the triphenylmethyl halides depends to some extent on the individual halide and on the metal in the phosphite salt. Thus, in an early study (in 1939), Arbuzov found that in reactions between silver dialkyl phosphites and triphenylmethyl bromide, dialkyl triphenylmethylphosphonates were indeed formed, but the use of the corresponding alkyl chloride provided the phosphite triester instead (metal dialkyl phosphites possess ambident anions ). A later study confirmed the behaviour of the silver salts towards the chloride, but also showed that, whereas dialkyl phophites with primary alkyl groups yielded phosphonic diesters (as had already been found), those with secondary alkyl groups afforded phosphite triesters moreover, the presence and nature of aromatic substituents were also able to control the course of the reaction. Reactions which involve triarylmethyl halides and sodium dialkyl phosphites may well be of a free radical nature since repeated studies have demonstrated the forma-... 

A similar reaction of 49 with triphenylmethyl halides in the appropriate methylene halide affords the corresponding 1-halosilatranes (equation 67). The reaction rate increases in going from chloro- to bromomethanes (k = 1.56 x 10 M s at 24°C and 2.94 x 10 M s at 20 °C for the Cl and Br derivatives, respectively) . ... 

Triphenylmethyl halides Halogenosilanes from silanes Partial replacement of Si-hydrogen by Si-halogen... 

Adapted from Swain, C. G., Kinetic evidence for a temiolecular mechanism in displacement reactions of triphenylmethyl halides in benzene solution, /. Am. Chem. Soc., 70, 1119 (1948). 

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