Solvolysis of o-methylbenzenediazonium tetrafluoroborate in acidic methanol-water mixtures. Further evidence for nucleophilic attack on a solvent separated aryl cation
Abstract:Rate constants for dediazoniation product formation and arenediazonium ion loss and product yields of solvolysis of o-methylbenzenediazonium tetrafluoroborate in acidic methanol-water mixtures at T ϭ 35ЊC are reported. Observed rate constants for diazonium ion loss and product formation are the same, increasing about 45% ongoing from water to methanol, and are not affected by added electrolytes like HCl, NaCl, and CuCl 2 . Only three dediazoniation products are detected, o-cresol, o-chlorotoluene, and o-anisol… Show more
“…When the reaction is carried out in the presence of RMs, Table , k obs values also increase slightly (<5%) and again can be considered constant and independent of the amount of BuOH in the RM, with an average value of k obs = 8.7 × 10 -4 s -1 , which is slightly lower than that obtained in the binary mixture. The values when no butanol is present in the system are in agreement with those previously reported , and the slight increase in k obs observed upon increasing the percentage of BuOH is consistent with previous results in MeOH/H 2 O and EtOH/H 2 O mixtures. , …”
Dediazoniation of o-methylbenzenediazonium tetrafluoroborate was investigated in SDS/BuOH/H2O (SDS = sodium dodecyl sulfate) reverse micelles, RMs, and, for comparison, in binary BuOH/H2O mixtures by employing a combination of spectrophotometric and chromatographic techniques. RMs were characterized by steady-state fluorescence; the data indicate that the aggregation number of the RMs increase upon increasing [SDS], while the radius of the water pool is mainly controlled by the amount of water in the system, and that the thickness of the interfacial region increases upon increasing the amount of BuOH in the system, in agreement with literature reports. Experimental evidence suggests that dediazoniation mainly takes place in the interfacial region of the RMs. Kinetic data show that a turnover from the heterolytic to the homolytic mechanism takes place about pH = 5; the variation of the observed rate constants, k(obs,) with pH following an S-shaped curve. At pH approximately 2, k(obs) values are insensitive to solvent composition both in RMs and in the binary mixture; however, k(obs) values in RMs are slightly lower than those in BuOH/H2O, probably due to the presence of SDS. High-performance liquid chromatography analyses of the reaction mixture indicate, in both RMs and in binary mixtures, the main dediazoniation products are the heterolytic ArOH and ArOBu, their yields depending on the composition of the system, and only small (<10%) amounts of the reduction ArH product were detected. The data at low pH are interpreted in terms of a DN + AN dediazoniation mechanism, i.e., a rate-limiting formation of an extremely reactive aryl cation that further reacts with available nucleophiles in the solvation shell.
“…When the reaction is carried out in the presence of RMs, Table , k obs values also increase slightly (<5%) and again can be considered constant and independent of the amount of BuOH in the RM, with an average value of k obs = 8.7 × 10 -4 s -1 , which is slightly lower than that obtained in the binary mixture. The values when no butanol is present in the system are in agreement with those previously reported , and the slight increase in k obs observed upon increasing the percentage of BuOH is consistent with previous results in MeOH/H 2 O and EtOH/H 2 O mixtures. , …”
Dediazoniation of o-methylbenzenediazonium tetrafluoroborate was investigated in SDS/BuOH/H2O (SDS = sodium dodecyl sulfate) reverse micelles, RMs, and, for comparison, in binary BuOH/H2O mixtures by employing a combination of spectrophotometric and chromatographic techniques. RMs were characterized by steady-state fluorescence; the data indicate that the aggregation number of the RMs increase upon increasing [SDS], while the radius of the water pool is mainly controlled by the amount of water in the system, and that the thickness of the interfacial region increases upon increasing the amount of BuOH in the system, in agreement with literature reports. Experimental evidence suggests that dediazoniation mainly takes place in the interfacial region of the RMs. Kinetic data show that a turnover from the heterolytic to the homolytic mechanism takes place about pH = 5; the variation of the observed rate constants, k(obs,) with pH following an S-shaped curve. At pH approximately 2, k(obs) values are insensitive to solvent composition both in RMs and in the binary mixture; however, k(obs) values in RMs are slightly lower than those in BuOH/H2O, probably due to the presence of SDS. High-performance liquid chromatography analyses of the reaction mixture indicate, in both RMs and in binary mixtures, the main dediazoniation products are the heterolytic ArOH and ArOBu, their yields depending on the composition of the system, and only small (<10%) amounts of the reduction ArH product were detected. The data at low pH are interpreted in terms of a DN + AN dediazoniation mechanism, i.e., a rate-limiting formation of an extremely reactive aryl cation that further reacts with available nucleophiles in the solvation shell.
“…Nonlinear plots are obtained in all cases, reflecting the changes in the nucleophile concentrations (H 2 O and ROH) as the solvent composition changes and the different selectivity values of the aryl cation toward nucleophiles with respect to water, . The profiles are very similar to those obtained when the variation of other halide derivative products such as Cl - was analyzed in the same binary mixtures under analogous experimental conditions. ,, 2 Effect of the percentage of MeOH (A) and EtOH (B) on the 3MBBr HPLC peak areas formed from reaction between 3MBD and Br - ions from CTAB (○) and NaBr (·). [3MBD] = 2 × 10 -4 M, [HCl] = 0.01 M, [CTAB] = [NaBr] = 0.25 M, T = 35 °C.…”
Section: Resultssupporting
confidence: 57%
“…For instance, = 0.41 ± 0.03 on going from 8 to 88% MeOH by weight (16 points) in 0.01 M HCl at T = 35 °C . Similar results have been found for dediazoniations in different alcohols such as BuOH, ethylene glycols, amides, and ureas. ,,,, Selectivities with halide nucleophiles, X - , compared to water range 2−15, depending on the structures of ArN 2 + , X - , and [X - ] …”
Section: Introductionsupporting
confidence: 65%
“…The method also assumes that NaBr behaves as a strong electrolyte at any solvent composition and thus all Br - ions in solution are essentially “free”. We are aware of the possibility of NaBr ion-pairing at high alcohol contents; , such a possibility was checked by measuring the variation in the 3MBBr peak areas with added Br - ions ([NaBr] = 0−0.3 M) at different solvent compositions; linear plots (cc > 0.998) similar to that shown in Figure were obtained in all cases within the concentration range investigated, and thus the possibility of significant NaBr ion-pairing having an effect on the 3MBBr peak area was ruled out. Anyway, the method minimizes the errors caused by possible ion-pairing, if any, because of the comparison of the mentioned HPLC peak areas (see below).…”
Section: Methodsmentioning
confidence: 99%
“…Previous solvolytic studies of a number of ArN 2 + ions in binary MeOH/H 2 O and EtOH/H 2 O mixtures under acidic conditions show that dediazoniation of 3MBD takes place through the D N + A N mechanism, Scheme . − , The selectivity values of aryl cations toward two nucleophiles Nu 1 and Nu 2 , , determined by employing eq 1, are orders of magnitude lower than those typically observed for nucleophiles competing with stabilized carbocations and remain essentially constant upon changing the composition of the system. For instance, = 0.41 ± 0.03 on going from 8 to 88% MeOH by weight (16 points) in 0.01 M HCl at T = 35 °C . Similar results have been found for dediazoniations in different alcohols such as BuOH, ethylene glycols, amides, and ureas. ,,,, Selectivities with halide nucleophiles, X - , compared to water range 2−15, depending on the structures of ArN 2 + , X - , and [X - ] …”
Chemical trapping of bromide ions by 3-methylbenzenediazonium ions, 3MBD, allowed us to investigate
the effects of methanol and ethanol on the fraction of free Br- ions in water−alcohol solutions of
cetyltrimethylammonium bromide, CTAB, and tetradecyltrimethylammonium bromide, TTAB, micellar
systems. For the purpose, the high-performance liquid chromatography, HPLC, peak areas of 3-bromocresol,
3MBBr, obtained from reaction of 3MBD with Br- ions in ROH/H2O/CTAB or ROH/H2O/TTAB mixtures,
A
SURF, and those for 3MBBr obtained in ROH/H2O/NaBr mixtures, A
NaBr, were determined. The A
SURF/A
NaBr ratio reflects the fraction of Br- ions sensed by the 3MBD probe in the water−alcohol bulk phase.
When no alcohol is added, the A
SURF/A
NaBr values are identical to those for the degree of ionization, α, of
CTAB and TTAB micelles obtained by physical methods; meanwhile, at high percentages of alcohol, A
SURF/A
NaBr ∼ 1, indicating that all Br- ions are essentially “free”, consistent with literature reports indicating
that short-chain alcohols totally inhibit micelle formation. Hence the variation of A
SURF/A
NaBr with the
percentage of alcohol is a measure of the fraction of Br- ions not bound to micellar or submicellar aggregates,
thus allowing us to examine the micelle breakdown by employing a chemical trapping method. The data
indicate that a continuous micelle breakdown takes place upon increasing the percentage of ROH. At 50%
ROH, about 30% of the total Br- ions are still associated to the submicellar aggregates (i.e., they are not
free ions), but at ∼80% ROH, no significant aggregation takes place.
The effects of pH on the observed rate constants (k obsd. ) and on the solvolytic dediazoniation product distributions of ethanolysis of 2-, 3-, and 4-methylbenzenediazonium ions (2MBD, 3MBD, and 4MBD, respectively) were determined by a combination of spectrophotometric (UV/Vis) and chromatographic (HPLC) techniques. The variation of both k obsd. and product yields with pH follow S-shaped curves with inflection points at pH ഠ 3.6, depending on solvent composition. With increasing pH, k obsd. values increase by factors of up to about 4 (2MBD), about 3 (3MBD), and about 50 (4MBD) with respect to the k obsd. values at low pH. HPLC analyses of the reaction mixtures show that only heterolytic products are obtained at low pH, indicating that solvolytic dediazoniation takes place through an ionic mechanism, but an increase in
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