Arenediazonium tetrafluoroborates have been prepared and the kinetics of solvolysis have been investigated in water, trifluoroethanol, water-trifluoroethanol mixtures, hexafluoropropan-2-ol, trifluoroacetic acid, and ethanol by a UV method. A heterolytic mechanism involving short-lived aryl cations leads to products derived from nucleophilic capture of the aryl cations by solvent or a solute. Ionic solutes in aqueous trifluoroethanol and trifluoromethoxybenzene in trifluoroethanol have no kinetic effect and neither does replacement of the tetrafluoroborate counter-ion by chloride in trifluoroethanol. Rate constants for any one compound are not very solvent dependent, the reactions generally being characterised by high enthalpies of activation and appreciably positive entropies of activation. Compounds with 4-Cl, 4-F, 4-NO 2 , and 4-MeO substituents proved too unreactive for kinetic studies, but for different reasons. In ethanol, a radical reaction with characteristically different activation parameters competes with the heterolytic path and leads to hydrodediazoniation (reduction) by hydrogen atom abstraction from the CH 2 group of ethanol.

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We have investigated the dediazoniation of 4-nitrobenzenediazonium (4NBD) ions in MeOH/H 2 O mixtures under acidic conditions at 50°C employing a combination of spectrophotometric and chromatographic techniques. The kinetic behaviour is quite complex; in the absence of MeOH, the dediazoniations follow first-order kinetics with a half-life t 1/2 = 1383 min, but the addition of small concentrations of MeOH lead to more rapid but non-first-order kinetics, suggestive of a radical mechanism, with t 1/2 Ϸ 6 min at 10 % MeOH. Further increases in the MeOH concentration slow down the rate of dediazoniation and reactions progressively revert to firstorder behaviour, with clean first-order kinetics at percentages of MeOH higher than 90 % (t 1/2 Ϸ 77 min). The analyses of the reaction mixtures by HPLC indicate up to four dediazoniation products depending on the particular experimental conditions. These are 4-nitrophenol (ArOH), 4-nitroanisole (ArOMe), nitrobenzene (ArH), and 4,4Ј-dinitrobiphenyl (DNB), this last product being detected only at MeOH percentages in the range 0.5-15 %. In the absence of MeOH, ArOH is the only product and formed in quantitative yield; however, at only 20 % MeOH, ArOH is down to less than 10 % and the reduction product, ArH, is obtained in more than 90 %. Upon increasing the MeOH content further, the formation of ArOMe becomes competitive and, at 99 %

Products of thermal and photochemical reactions of eleven arenediazonium tetrafluoroborates in various solvents have been analyzed. All compounds in most solvents undergo unimolecular heterolysis to give singlet aryl cations which are captured by solvent. This mechanism is dominant for arenediazonium ions without electron-withdrawing substituents in all solvents, and the only reaction observed in water. Additionally, appreciable yields of fluoroarenes are obtained by fluoride abstraction by the aryl cation from fluorinated solvents and from tetrafluoroborate in fluorinated solvents. Yields from photochemical processes are very similar to those from thermal reactions indicating that the main reactions proceed through common or very similar intermediates. Aryl cations formed from ion-paired diazonium ions may react with the counterion, but fragmentation of dissociated diazonium ions leads only to solvent-derived product. Some arenediazonium ions in some solvents undergo an alternative radical reaction leading principally to hydrodediazoniation. It is proposed that this reaction involves initial rate-limiting electron transfer from ethanol to the arenediazonium ion followed rapidly by homolysis of the resultant aryldiazenyl radical. Within the same solvent cage, the aryl radical then either abstracts an α-hydrogen from the ethanol radical cation generated in the first step to give the reduction product and protonated acetaldehyde, or combines with it at the oxygen to give a protonated aryl ethyl ether.

A range of tritylamines TrNRRЈ, and 4-methoxy-, 4,4Ј-dimethoxy-, and 4,4Ј,4Љ-trimethoxy-substituted analogues, have been prepared from (substituted) trityl chloride, bromide, or tetrafluoroborate with ammonia or with amines HNRRЈ where R and RЈ are hydrogen, alkyl, or aryl. Crystal structures of 4,4Ј,4Љ-trimethoxytritylamine, N-tritylglycine methyl ester, tritylammonium chloride, and N-tritylglycine have been determined. The central C-N bond of tritylamine is not significantly affected by the introduction of p-methoxy substituents into the trityl group, or by N-alkylation, but is lengthened upon protonation of the amino group. Some degree of planarisation of the three C-C bonds to the central carbon of the trityl group is also associated with this C-N bond lengthening. N-Tritylglycine is shown to be a zwitter-ion in the crystalline state and has pI = 6.4 in aqueous acetonitrile. Base strengths of a range of tritylamines have been measured in aqueous acetonitrile. The pK BH ϩ values (pK a values of the corresponding tritylammonium ions), including ones for N-tritylglycine methyl ester and a range of N-tritylanilines, are remarkable for their similarity at pK BH ϩ = ca. 9, i.e. characteristic of values for simple alkylamines. It is proposed that the (substituted) trityl group sterically inhibits solvation of the protonated tritylglycine ester cation selectively, and prevents significant resonance interaction between the arene ring and the amino group in the anilines.