**3. Results**

*3.1. Spontaneous Dediazoniation of 2-, 3-, and 4-Methylbenzenediazonium Ions: Effects of Solvent (MeOH/H2O) Composition on the Observed Rate Constant, kobs*

The effects of the solvent composition on the observed rate constant *k*obs for the spontaneous dediazoniation of 2MBD, 3MBD, and 4MBD were explored by modifying the percentage of MeOH in the reaction mixture, as shown in Figure 2. Solvolytic rate constants *k*obs increased smoothly at low percentages of MeOH, but more drastically at high percentages of methanol. The *k*obs values at 0% MeOH, *k*obs = 6 × 10−<sup>4</sup> s<sup>−</sup><sup>1</sup> (2MBD), 8 × 10−<sup>4</sup> s<sup>−</sup><sup>1</sup> (3MBD), and 9.5 × 10−<sup>4</sup> s<sup>−</sup><sup>1</sup> (4MBD) aligned with the reported values obtained by different techniques, including N2 evolution [49] at pH = 1.6–1.8, as well as HPLC and VIS–UV spectrophotometry [14]. The increase in *k*obs, upon changing the methanol content, was modest, less than two-fold, and much lower than those reported in other solvolytic reactions. It is, however, aligned with literature reports that indicate that the rates of heterolytic dediazoniation of a number of arenediazonium ions vary by a factor of only 9 in 19 solvents [1].

**Figure 2.** Effects of solvent composition on the solvolytic rate constants for the spontaneous decomposition of 2-, 3-, and 4-methylbenezenediazonium ions in acidic MeOH/H2O mixtures. Experimental conditions: [ArN<sup>+</sup>2] = ~10−<sup>4</sup> M, [HCl] = 10−<sup>2</sup> M, *T* = 35 ◦C (2MBD, 3MBD), *T* = 60 ◦C (4MBD).

#### *3.2. Effects of Added Electrolytes on Dediazoniation Rate Constants*

The effects of electrolytes ([NaCl] = 0–1 M, [LiCl] = 0–1 M, and [LiCLO<sup>−</sup>4 ] = 0–1.5 M) on *k*obs were determined at two representative solvent compositions (20% and 99.5% MeOH), as seen in Figure 3. At 20% MeOH, *k*obs values, in the absence of salt, were similar to those in Figure 1. The addition of NaCl, LiCl, or LiClO4 did not have a significant effect on *k*obs and the values remained essentially constant. However, when the percentage of MeOH increased to 99.5%, the addition of NaCl and LiCl (up to 1.5 M) did not significantly change *k*obs, but, upon increasing [LiClO4], *k*obs values decreased by approximately 40%. The observed decrease in *k*obs values with the presence of ClO4- ions, though somewhat modest, was unexpected and quite significant because no changes in *k*obs were found in the presence of other salts and the decrease in *k*obs was only detected at high percentages of MeOH. This is an important decrease that deserves further investigation, because we noted that *k*obs values only had a two-fold change when going from 0 to 100% MeOH, as seen in Figure 3.

**Figure 3.** Effects of added electrolytes on *k*obs for dediazoniation of 2MBD, 3MBD, and 4MBD in 20% MeOH/H2O (**A**–**C**) and in 99.5% MeOH/H2O (**D**–**F**) mixtures. *k*obs values were determined spectrophotometrically by monitoring ArN+2 loss at λ = 305 nm (2MBD) and λ = 310 nm (3MBD, 4MBD). Experimental conditions: [ArN<sup>+</sup>2 ] = 10−<sup>4</sup> M, [HCl] = 10−<sup>2</sup> M, *T* = 35 ◦C (2MBD, 3MBD), *T* = 60 ◦C (4MBD).

To obtain further insights into the dediazoniation process, we employed the chromatographic technique, as illustrated in Figure 4, to determine *k*obs for product formation, in the absence and in the presence of added electrolytes, at two selected solvent compositions (20% and 99.5% MeOH). The *k*obs values for product formation were the same as those obtained spectrophotometrically for ArN+2 loss, confirming that products are formed competitively, in keeping with the predictions of the DN + AN mechanism shown in Scheme 2A.

**Figure 4.** Illustrative determination of the rates of dediazoniation product formation in 99.5% MeOH/H2O mixture as determined by HPLC. Rates are determined by fitting the variation in the concentration of a particular dediazoniation product with the time to a first order kinetic Equation (1). (**A**) 2MBD. (**B**) 4MBD. Experimental conditions: [2MBD]0 = [4MBD0] = 10−<sup>4</sup> M, [HCl] = 10−<sup>2</sup> M; *T* = 35 ◦C (2MBD), *T* = 60 ◦C (4MBD).

#### *3.3. Effects of Added Electrolytes on Product Distribution*

Figure 5 shows the effects of the solvent composition on the product distribution of 3- and 4MBD in the absence of added electrolytes. Only the heterolytic products (cresol and methyl phenyl ethers) were formed in significant yields, and the formation of the reduction product Ar-H was only detected in highly alcoholic solutions (4MBD) but its yield was very low when compared with those of heterolytic products; therefore, such a mechanism (homolytic) can be neglected. In all cases, the quantitative conversion to products was achieved in all composition ranges. The results are in agreemen<sup>t</sup> with published data [30,32,34].

**Figure 5.** Variation of the percentage of solvolytic dediazoniation products in MeOH/H2O binary mixtures in the absence of electrolytes. Experimental conditions were the same as those in Figure 2.

To analyze the effects of added electrolytes, we selected two methanol compositions, 20% and 99.5% MeOH. At a low methanol content, the addition of salts (NaCl and LiClO4) up to 1.5 M had a negligible effect on the production distribution, as illustrated in Figure 6. However, this was not the case at high percentages of MeOH, Figure 7.

**Figure 6.** Effects of NaCl on dediazoniation product distribution in 20% MeOH/H2O mixtures as determined by HPLC. (**A**) 3MBD, (**B**) 4MBD. Experimental conditions: [3MBD0] = [4MBD0] = 10−<sup>4</sup> M, [HCl] = 10−<sup>2</sup> M, *T* = 35 ◦C (3MBD), *T* = 60 ◦C (4MBD).

**Figure 7.** Effects of added electrolytes on the percentage of formation of dediazoniation products in 99.5% MeOH/H2O mixtures. (**A**,**B**) 3MBD (**C**,**D**) 4MBD. Note the formation of the ArCl derivative at the expense of ArOMe (**A**,**C**). In the presence of LiClO4 the ArCl derivative is not formed but the yield of ArOMe decreases substantially upon increasing [LiClO4]) (**B**,**D**), attributed to the formation of the arylperchlorate derivative.

At 99.5% MeOH, in the presence of LiCl, the Ar-Cl derivative was detected, in addition to Ar-OH and Ar-OMe, and an increase in [LiCl] of up to 1.5 M had a negligible effect on the product distribution, as seen in Figure 7A,C, so that the quantitative conversion to products was obtained. As expected, when using LiClO4 instead of LiCl, the Ar-Cl derivative was not detected but, surprisingly, a significant reduction in the yields of Ar-OMe was detected, as seen in Figure 7B,D. We note that the total yield decreases because of the presence of an unidentified dediazoniation product, whose yield could not be computed because we made no attempts to fully identify the new product, mainly for safety reasons (see Section 2).
