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The evaluation of the dediazoniation kinetics of various m-and p-substituted benzenediazonium tetrafluoroborates' in 1,2-dichloroethane at 50" in the presence of 18-crown-6, 2 1-crown-7 and dicyclohexano-24-crown-8 demonstrates that the rate constant for the dediazoniation within the complex (k2) is smallest, and the equilibrium constant for complex formation (K) is largest for the complexes with 21-crown-7 (cf Scheme 1). The logarithms of the equilibrium constants ( K ) for complex formation with each of the crown ethers studied correlate well with Hammetf's substituent constants, D , to give reaction constants p= 1.18-1.38.A linear correlation between the logarithms of the rate constants for the dediazoniation within the complex with those of the dediazoniation rate constants of uncomplexed diazonium ions (log k2 vs. log. k l ) , found for most substituted diazoniurn salts, indicates that the dediazoniation mechanism of the complexed diazonium ions is not significantly different from that of the free ions. For very electrophilic diazonium ions (p-Cl, rn-CN), k2 was much larger than expected on the basis of the linear log k2 vs. log k , relationship. Analysis of the dediazoniation products showed that this was due to a change in mechanism from heterolytic to homolytic dediazoniation.The complexation rate of diazonium salts by crown ethers (k,) is practically diffusion controlled and does not change much with the size of the crown ether. The decornplexation rate ( k d ) , however, is significantly lower for complexes with 2 1crown-7, than for those with 18-crown-6 and dicyclohexano-24-crown-8, and is therefore the reason for the variations in the equilibrium constant ( K ) and thus for the fact that complexes of arenediazonium salts with 21-crown-7 are the most stable.The amounts of the N,-NB rearrangement, as well as those of the exchange of the I5N-labelled diazonio group with external nitrogen during dediazoniation of p-toluenediazonium salt were independent of the addition of crown ethers. ') Part xx: see [I].

SummaryIn heterolytic dediazoniations arenediazonium salts form aryl cations. The reaction rates are relatively slow; they depend only to a small extent on the solvent. It is shown that the solvents in which the heterolytic dediazoniation mechanism is predominant have a low nucleophilicity, whereas in solvents of high nucleophilicity homolysis of arenediazonium salts, i.e. the formation of aryl radicals and related intermediates, is favoured. Under comparable conditions, homolytic rates are faster than the corresponding rates of heterolysis. Homolysis is strongly enhanced by addition of nucleophiles which form relatively stable radicals by electron transfer. The ability of additives to catalyze homolysis of arenediazonium salts can be explained using the concept of a nucleofugic3) homolytic leaving group.

The dediazoniation of benzene-(1) and 2,4,6-trimethylbenzenediazonium tetrafluoroborate (2) was investigated in solvents of low nucleophilicity (1,1,1,3,3,3-hexafluoro-2-propanol and fluorosulfonic acid) and was compared with the corresponding data in 2,2,2-trifluoroethanol. The reactions proceed heterolytically, and the solvent effects on the dediazoniation rate, the isotopic Na-Ns rearrangement, and the exchange reaction of the diazonio group of ß-15 labeled diazonium salts with external nitrogen (300 atm) were studied. Diazonium salt 2 shows in HFIP by far the highest amount ever observed of isotopic rearrangement (36.97%) and exchange with external nitrogen (16.50%) at 70% dediazoniation. The corresponding values for diazonium salt 1 in HFIP are lower (10.47 and 6.26%, respectively). The results are interpreted in terms of (at least) two intermediates in the course of the dediazoniation, namely, a tight nitrogen aryl cation molecule-ion pair through which the rearrangement occurs, and a (solvated) aryl cation, free of or solvent separated from nitrogen. The results indicate that the exchange reaction proceeds through the second intermediate.

A detailed investigation of the dediazoniation of 2,4,6-trimethylbenzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol is presented. The dediazoniation rate, the isotopic N,-Np rearrangement, and the exchange reaction of the diazonio group with external nitrogen in p-lsN labeled diazonium salts were monitored under a constant pressure of 300 atm mixed nitrogen/argon gas, as a function of the partial pressure of nitrogen. The results obtained provide evidence additional to that presented in the preceding paper for the presence of two reaction intermediates during the course of the dediazoniation. A statistical treatment of the kinetic data excludes the possibility that one reaction intermediate can account for all the phenomena observed. The reaction mechanism of eq 7 is proposed. Rearrangement occurs at the first intermediate, a tight molecule-ion pair, while the exchange with external nitrogen takes place via the second, free aryl cation, intermediate.The results concerning solvent effects on the decomposition of benzene-and 2,4,6-trimethylbenzenediazonium ion described in the preceding paper' support the existence of two reaction intermediates during the course of the dediazoniation: (1) This paper deals with a detailed kinetic investigation of the dediazoniation of a convenient system, 2,4,6-trimethylbenzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE) under a constant pressure (300 atm) of mixed nitrogen/argon gas, but different partial pressures of nitrogen. The results provide evidence, additional to that already presented,' for the validity of the proposed two-intermediate mechanism.A statistical treatment of the kinetic data excludes the possibility that one reaction intermediate can account for all the phenomena observed. Experimental SectionMaterials, In the study of the rearrangement and exchange reaction reagent grade (Fluka puriss.) TFE was used without further purification. The solvent which was used for kinetic and solubility measurements was recovered and was used repeatedly after purification by the following methods4 TFE was dried over potassium carbonate and distilled. Phosphorus pentoxide was then added to the distillate, and the solvent was distilled again (bp 73-74 "C). The GLC analysis of the recovered solvent showed no impurities. 2-Naphthol was recrystallized from ethanol (mp 121 "C). The disodium salt of 2naphthol-3,6-disulfonic acid (R-salt) was recrystallized twice from water and charcoal. 2,4,6-Trimethylaniline was distilled under reduced pressure (bp 120 OC, 20 mmHg), and the GLC of the distillate showed no impurities. Reagent grade potassium thiocyanate, potassium fluoride, and sodium nitrite were used without purification.2,4,6-Trimethylbenzenediazoni~m-~-~~N Tetrafluoroborate. Fluoroboric acid (2 mL, 50% aqueous solution) and water (2.5 mL) were added to 0.489 g (3.62 mmol) of 2,4,6-trimethylaniline. The solution was cooled in an ice-salt bath and 0.250 g (3.57 mmol) of NaI5N02 (99.2% nitrogen-15, Stohler Isotope Chemicals) in 0.5 mL of water was added slowly ...