Oxidation of Sulfinic and Sulfonic Acids
J. Phys. Chem. A, Vol. 102, No. 34, 1998 6791
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data, the amount of HCHO added is not in overwhelming excess
over the AIMSOA. The rate of reaction is, however, reduced
by a factor of 100 because of the addition of formaldehyde.
However, the age of the solution still affects the rate of reaction,
with the aged solutions reacting faster because of decomposition
of AIMSOA.
The HSO2 species is readily oxidized to bisulfite:
HSO2- + H2O f HSO3- + 2H+ + 2e-
(R17)
-
HSO3 is next also easily oxidized to sulfate (reaction R9).
In Figure 7B the reaction had an overwhelming excess of
HCHO; [HCHO]0/[AIMSOA]0 > 1000. Thus, all the HSO3
Conclusion
-
In the crystalline form, small organic sulfinic and sulfonic
acids have nearly the same structure and exist in their zwitter-
ionic forms. However, upon dissolving in polar solvents, the
sulfinic acid zwitterion loses its stability to its neutral form.
The sulfonic acid zwitterion decomposes to form HSO3-, which
can be quickly oxidized to sulfate. The experimental data
presented in this manuscript show that sulfonic acids are very
stable in solution and decompose only very slowly (cf. half-
life of approximately 3 days). Their decomposition can give
an anomalous rate of oxidation depending upon the length of
time the solutions have been aged. The differences in the rates
of reaction, however, need further attention. The accepted
general mechanism of a sulfinic acid being oxidized to the
sulfonic acid before formation of sulfate may need to be revised.
It does appear that the oxidation of a sulfinic acid may be
produced is consumed by the added HCHO. The age of the
reaction no longer becomes a factor, and the reaction ceases to
respond to it. This is the observation in Figure 7B.
Possible Reaction Pathway for the Oxidation of AIMSA. The
following simple experiment was performed. A 1:1 mole ratio
of AIMSA to bromine was mixed and allowed to stand for more
than 2 h. The expected result was the oxidation of AIMSA to
AIMSOA only as expected from the following stoichiometry:
(NH2)2CSO2 + Br2 + H2O f
(NH2)2CSO3 + 2Br- + 2H+ (R12)
If the oxidation of AIMSA forms AIMSOA as an intermedi-
ate, then we expect very little or no sulfate production at all. A
test for sulfate proved that there was nearly quantitative sulfate
production based on the stoichiometry
-
proceeding through the HSO2 and HSO3-, which then are
oxidized to sulfate. There is need for more extensive experi-
mental data and analysis.
2Br2 + (NH2)2CSO2 + 3H2O f
(H2N)2CdO + SO42- + 4Br- + 6H+ (R13)
Acknowledgment. We acknowledge the National Science
Foundation, through Grant CHE-9632592 for financial support.
with the bromine concentrations being limiting. This experiment
indicates that the oxidation of AIMSA proceeds through more
than one possible pathway with one of them not involving
AIMSOA.
The preparation of AIMSOA, however, had been affected
by the use of a mild oxidizing agent peracetic acid.19 This
oxidation was also carried out in chilled solutions to slow the
reaction and to obtain a quantitative yield of AIMSOA. Product
dependency on the strength of the oxidizing agent is very
common. For example, peracetic acid can oxidize cysteine to
cysteine sulfinic acid while aqueous bromine produces cysteine
sulfonic acid and hydrogen peroxide produces the cysteine dimer
cystine.30
References and Notes
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(R14)
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(R15)
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-
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:
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(R16)
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