Aqueous solution oxidation-reduction equations

Many organic compounds undergo reduction or oxidation at a DME. Consequently, polarographic techniques have been used extensively for determinations of organic compounds and for studying the mechanisms of their electrode reactions. In aqueous solution, the reduction of organic compounds is frequently a 2e process accompanied by protonation as in Equation 3.32 ... 

Balance the oxidation-reduction equation for the oxidation of H2S(aq) by HN03(aq) to produce NO(g) and S(s) in aqueous acidic solution (thus H+ and H20 may be involved). 

Balancing oxidation-reduction equations for reactions occurring in aqueous acidic solutions is usually fairly straightforward since we can use H20 to balance O, and then H+ to balance H. In basic solution,... 

Balancing Oxidation-Reduction Equations in Aqueous Solution... 

Balance the following oxidation-reduction equations. The reactions occur in acidic or basic aqueous solution, as indicated. 

Many of the oxidizing and reducing agents previously described only bring about oxidation and reduction in an acidified aqueous solution. Their half-equations frequently involve water molecules and hydrogen ions. The following procedure describes how such half-equations can be... 

The dichromate (vi) ion is a powerful oxidizing agent. The half-equation below shows how it reacts with reducing agents in acidified aqueous solution. The reduction results in the formation of the green chromium(iii) ion ... 

Write plausible half-equations and a balanced oxidation-reduction equation for the disproportionation of Xep4 to Xe and Xe03 in aqueous acidic solution. Xe and Xe03 are produced in a 2 1 mole ratio, and 02(g) is also produced. 

Balancing the chemical equation for a redox reaction by inspection can be a real challenge, especially for one taking place in aqueous solution, when water may participate and we must include HzO and either H+ or OH. In such cases, it is easier to simplify the equation by separating it into its reduction and oxidation half-reactions, balance the half-reactions separately, and then add them together to obtain the balanced equation for the overall reaction. When adding the equations for half-reactions, we match the number of electrons released by oxidation with the number used in reduction, because electrons are neither created nor destroyed in chemical reactions. The procedure is outlined in Toolbox 12.1 and illustrated in Examples 12.1 and 12.2. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Hi. Lysine. Gamma radiolysis of aerated aqueous solution of lysine (94) has been shown, as inferred from iodometric measurements, to give rise to hydroperoxides in a similar yield to that observed for valine and leucine. However, attempts to isolate by HPLC the peroxidic derivatives using the post-column derivatization chemiluminescence detection approach were unsuccessful. This was assumed to be due to the instability of the lysine hydroperoxides under the conditions of HPLC analysis. Indirect evidence for the OH-mediated formation of hydroperoxides was provided by the isolation of four hydroxylated derivatives of lysine as 9-fluoromethyl chloroformate (FMOC) derivatives . Interestingly, NaBILj reduction of the irradiated lysine solutions before FMOC derivatization is accompanied by a notable increase in the yields of hydroxylysine isomers. Among the latter oxidized compounds, 3-hydroxy lysine was characterized by extensive H NMR and ESI-MS measurements whereas one diastereomer of 4-hydroxylysine and the two isomeric forms of 5-hydroxylysine were identified by comparison of their HPLC features as FMOC derivatives with those of authentic samples prepared by chemical synthesis. A reasonable mechanism for the formation of the four different hydroxylysines and, therefore, of related hydroperoxides 98-100, involves initial OH-mediated hydrogen abstraction followed by O2 addition to the carbon-centered radicals 95-97 thus formed and subsequent reduction of the resulting peroxyl radicals (equation 55). 

Chromium(IV) does not have any aqueous solution chemistry except for the formation of intermediates in the reduction of CrVI to Crm. Chromium(IV) compounds tend to disproportionate into Cr111 and CrVI species (equation 78) and the metal ion in this oxidation state is powerfully oxidizing towards organic compounds. An eight-coordinate complex [CrH4(dmpe)4] is known (Section 35.3.4.1). 

Sulfonium salts of thiepanes are readily formed by electrophilic attack of alkyl halides on the cyclic thioether. Thus, thiepane (35) was found to yield a sulfonium iodide (123), which at elevated temperatures and in the presence of excess methyl iodide underwent ring cleavage to yield 1,6-diiodohexane (isolated as the 1,6-diphenoxy derivative Scheme 24) (53M1206). The alkoxysulfonium salt (124) formed by reaction of (35) with t-butyl hypochlorite (equation 23) was characterized as a stable hexachloroantimonate (67JOC2014). Reduction of thiepane 1-oxide (115) to thiepane has been achieved using an aqueous solution of NaHSC>3 (72JOC919). A hydroxysulfonium salt intermediate (125) has been proposed in the latter reduction reaction which provides a general method for sulfoxide reductions under mild conditions (equation 24). 

In any chemical reaction involving the transfer of electrons there will be two couples involved, one of which undergoes oxidation and the other reduction, so that it will not be possible to study the above reaction (equation 20) in the absence of a second redox couple. To overcome this difficulty of not being able to measure the absolute value of AG° or E° for equation (20), a scale of relative values of E° can be obtained by measuring the potential of a redox couple relative to a common redox couple which is assigned an arbitrary value. In aqueous solution this common redox couple is the standard hydrogen electrode (equation 22), in which the H+(aq) and H2(g) are at unit activity and fugacity, respectively. 

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