Oxidation and. Reduction

Air and water can eat away iron and steel Oxygen binds to IY1°F6 jron molecules to make iron oxide. Layer by layer, the metal changes to a red powder called rust. When painted, rusting slows on cars, bicycles, and tricycles. 

Many different kinds of chemical processes use oxidation and reduction. Food that gets old oxidizes and goes bad. Keeping food refrigerated slows down the rate of oxidation. Humans and animals also rely on this reaction. The food people and animals eat is oxidized in the cells. This helps give the body energy. 

Cows graze all day to give them energy and help their bodies make milk. 

Bleach whitened these socks and removed germs and bacteria. 

Fires are common oxidation reactions. Fire consumes a lot of oxygen. This is because the material that is burning oxidizes at a very fast rate. Flames erupt because the reaction energy is so intense. Fires need oxygen, heat, and fuel. Materials cannot burn when there is no oxygen. 

Magasanik and Chargaff286,287 have shown that cyclitol osazones, for example, lD-c/n ro-inositol phenylosazone (94), consume the expected amount of periodate 

3- bis(phenylhydrazone) 98,288 with sodium periodate, which yields 3-oxo- 

4- bis(phenylhydrazono)pentanedial (99), and which is directly converted into 

Among the many reaction products obtained from hydrazones and osazones that offer promise to synthetic chemists are the carbenes generated from azirines and sulfonylhydrazono-1,5-lactones by irradiation.299 300 

Suami and S. Ogawa, Chemistry of carba sugars (pseudo sugars) and their derivatives, Adv. Carbohydr. Chem. Biochem., 48 (1990) 21-90. 

The standard reduction potentials of some systems used by enzymes in redox reactions are as follows  

As the free-energy change accompanying a redox reaction is given by Eqn. 25, where Eq is the standard reduction potential, n is the number of electrons transferred per mole and F is Faraday s constant, a reaction is thermodynamically feasible if the difference in reduction potentials is positive. Thus in the preceding list NADH will reduce flavins but cytochrome c cannot. 

Single electron transfer generates radicals and although this mechanism is now more common than once thought in non-biological redox reactions, its prevalence in enzyme-catalysed reactions is limited to coenzymes with quinoid-type structures e.g. flavins, coenzyme Q, vitamins C, E and K and to enzymes containing transition metals. Of course, there is a growing interest in metabolic disorders initiated by radical reactions. Reduction by 2-electron transfer can take place by either (a) hydride, H, transfer or (b) discrete electron, e , and proton, H , addition. 

Oxidation and Reduction.—A number of selective oxidation procedures have been reported. Trichloroacetaldehyde on dehydrated chromatographic alumina converts the diol (15) into the 3/3-hydroxy-17-ketone (68%). Primary alcohols are reported to be less readily oxidized than secondary alcohols by this reagent. Similarly, bromine or chlorine with HMPA oxidizes secondary alcohols more readily than primary alcohols. Thus the diol (16) was converted into the ketol (17) 

6-epoxide(s). Allylic alcohols may be isomerized by use of this oxidation procedure followed by reduction with hydrazine (Wharton). Accordingly, the A -3/3-ol (25) was converted via (26) into the isomeric A -5-ol (27). 

Reduction of the 19-tosylates (28) and (30) with lithium aluminium hydride gave the 5,10-methano-compound (29) and the alcohol (31) respectively. Vicinal dibromo-steroids are converted readily into olefins when treated with silver salts in the presence of amines. Diaxial bromohydrins are converted into the epoxides under these conditions. 

Oxidation and Reduction.—Geraniol on treatment with t-butyl hydroperoxide and Ti catalysts in the presence of (+)-diethyl tartrate gave the (2S,3S)-oxide, whereas in the presence of the (—)-tartrate the other isomer was produced.207 t-Butyl hydroperoxide and vanadium catalysts converted (/ )-(—)-linalool into 2,3-epoxycitronellol (89), which on reduction and photosensitized oxidation yielded 

Epoxidations, carbene additions, etc. of 2,3-epoxycitral have been studied.209 A very detailed investigation of the dye-sensitized photoxidation of a-nerol (92) showed the formation of the diols (93) and (94) with no cyclized products.210 Allo-ocimene and myrcene on autoxidation in DMF or DMSO give an odd variety 

Ohloff, W. Giersch, K. H. Schulte-Elte, P. Enggist, and E. Demole, Helv. Chim. Acta, 1980, 63, 1582. 

Schulte-Elte, B. L. Muller, and H. Pamingle, Helv. Chim. Acta, 1979, 62, 816. 

Electrochemical oxidations have been previously mentioned.122 A novel application is the regioselective epoxidation (80% conversion 90% selectivity) at the 6,7-bond of geranyl and neryl esters and phenyl sulphones.214 Another is the one-step conversion of olefins into allylic alcohols via electrooxidative-oxyselenylation-deselenylation, e.g. (97)— (99). This method has been applied to the synthesis of (100) which can be converted (70%) into marmelolactone [(101) from quince], and 

Oxidation and Reduction.—Oxidative rearrangement of olefins with thallium(m) nitrate in methanol provides a simple synthesis of aldehydes and ketones in high yields. Cleavage of the intermediate thallium compound proceeds via a transition state with high carbonium ion character, which leads either to carbonyl compounds (354) by Wagner-Meerwein rearrangement or to glycol methyl ethers (355).  

Epoxidation of olefins under mild conditions is carried out by using an initially formed complex between H2O2 and an isocyanate in non-polar solvents. Using phenyl isocyanate the by-products are 1,3-diphenylurea and carbon dioxide.  

Potassium permanganate in acetic anhydride converts acyclic and larger cyclic rings into a-diketones. Thus cyclododecene is converted into the a-diketone (360) in 48% yield, with (361) and (362) as easily removable byproducts.  

Electron-rich alkenes, e.g. (363), are oxidized rapidly and in good yields by carbon tetrabromide  

Keten diethyl acetal, however, is not oxidized under these conditions. Electrochemical, oxidative 1,2-addition of azide into non-activated double 

Oxidation and Reduction.— The electrochemical oxidation of benzothiazole-2-thiol to its disulphide has been further studied. This conversion may be accomplished with ozone in the presence of potassium iodide to prevent ozoniza-tion. Desulphurization of benzothiazoles as a preparative procedure does not appear to have been exploited to the extent that it has in the field of thiophen chemistry. Noteworthy, however, is the desulphurization of sugars (17 e.g., n = 3, R = H) to l-deoxy-l-(iV-methylanilino)-alditals.  

Treatment of bibenzothiazolin-2-yls (18) with zinc acetate yields zinc complexes, the free azomethines of which are unknown. Reduction of the coloured compounds with sodium borohydride gives colourless complexes, which are decomposed by alkali to give [o-(HS)C H4NHCHR-]-2  

The oxidation of alditols and methyl glycopyranosides, etc. with sodium metaperiodate in deuterium oxide has been followed by n.m.r. spectroscopy  

Acetal migration can lead to the formation of cyclic acetals that are resistant to mild hydrolysis with acid in the Smith degradation of polysaccharides. Lind-berg and his co-workers have shown that the model disaccharides (427) and (428) are degraded to the expected products (429) and (430), respectively, without the formation of significant amounts of the cyclic acetals (431) and (432).  

Oxidation of 4,6 4, 6 -di-C -benzylidene-2,2 -di-0-toluene-j7-sulphonyl-aa-tre-halose with DMSO-phosphorus pentaoxide gave the symmetrical 3,3 -diulose, whereas the 3-ulose-3 -acetate was obtained when DMSO-acetic anhydride was used. i DMSO-acetic anhydride also converted methyl 4,6-0-benzylidene-ot-D-galactopyranoside into a mixture of methyl 2-0-acetyl-4,6-0-benzylidene-a-D-x y/o-hexopyranosid-3-ulose (433) and the 3-0-(methylthio)methyl ether (434).  

DMSO is also used in an improved two-stage procedure for the oxidation of primary and secondary alcohols to carbonyl compounds the alcohol is first converted into the chloroformate, which affords the carbonyl compound on 

Hirano, T. Fukuda, and M. Sato, Agric. and Biol. Chem. Japan), 1974, 38, 2539. 

In contrast, the e availability and e potential are measurable under some conditions, but there is no concentration of free electrons that corresponds to the H+ concentration. Someone calculated that the concentration of free electrons is about 1(T45 M, or about 1 free electron per galaxy. Also in contrast to H+, the measurement of the e- potential is qualitative in natural systems. Because the measuring 

Many chemical reactions involve oxidation. This was originally defined as  

An example of the first type of oxidation involves the burning or combustion of magnesium 

The magnesium has gained oxygen and we say that the magnesium has been oxidized. 

An example of the second type of oxidation involves the reaction between manganese(iv) oxide ( manganese dioxide ) and concentrated aqueous hydrochloric acid  

The hydrochloric acid loses hydrogen and is therefore oxidized. Later, we will see why these two apparently very different reactions are both regarded as oxidation reactions. 

35 OXIDATION AND REDUCTION All the reactions mentioned in the previous sections were ion-combination reactions, where the oxidation number (valency) of the reacting species did not change. There are however a number of reactions in which the state of oxidation changes, accompanied by the interchange of electrons between the reactants. These are called oxidation-reduction reactions or, in short, redox reactions. 

If the reaction is carried out in the presence of hydrochloric acid, the disappearance of the yellow colour (characteristic for Fe3+) can easily be observed. In this reaction Fe3+ is reduced to Fe2+ and Sn2+ oxidized to Sn4+. What in fact happens is that Sn2+ donates electrons to Fe3+, thus an electron transfer takes place. 

In this case the iron metal donates electrons to copper(II) ions. Fe becomes oxidized to Fe2+ and Cu2+ reduced to Cu. 

Electrons are taken up by H+ from Zn the chargeless hydrogen atoms combine to H2 molecules and are removed from the solution. Here Zn is oxidized to Zn2+ and H+ is reduced to H2. 

It is not so easy to follow the transfer of electrons in this case, because an acid-base reaction (the neutralization of H+ to H20) is superimposed on the redox 

In the previous two chapters (Chapters 26 and 27), we showed how kinetic laws describing the rates at which minerals dissolve and precipitate can be integrated into reaction path and reactive transport simulations. The purpose of this chapter is to consider how we can trace the reaction paths that arise when redox reactions proceed according to kinetic rate laws. 

We take two cases in which mineral surfaces catalyze oxidation or reduction, and one in which a consortium of microbes, modeled as if it were a simple enzyme, promotes a redox reaction. In Chapter 33, we treat the question of modeling the interaction of microbial populations with geochemical systems in a more general way. 

I Interpret redox reactions in terms of change in oxidation state. 

Review Vocabulary spectator ion an ion that does not participate in a reaction and is not usually shown In an Ionic equation 

Oxidation and reduction are complementary—as an atom is oxidized, another atom is reduced. 

Real-World Reading Link The light produced by a light stick is the result of a chemical reaction. When you snap the glass capsule inside the plastic case, two chemicals are mixed and electron transfer occurs. As the electrons are transferred, chemical energy is converted into light energy. 

In Chapter 9, you learned that a chemical reaction can usually be classified as one of five types—synthesis, decomposition, combustion, singlereplacement, or double-replacement. A defining characteristic of combustion and single-replacement reactions is that they always involve the transfer of electrons from one atom to another, as do many synthesis and decomposition reactions. For example, in the synthesis reaction in which sodium (Na) and chlorine (CI2) react to form the ionic compound sodium chloride (NaCl), an electron from each of two sodium atoms is transferred to the CI2 molecule to form two Cl ions. 

Matsuura, Bio-mimetic Oxygenation , Tetrahedron, 1977, 33, 2869. Stereoselective Reductions , ed. M. P. Doyle, Halsted Press, Wiley, New York, 1976. 

Corrosion is the conversion of a metal into a metal compound, by a reaction between the metal and some substance in its environment. When a metal corrodes, each metal atom loses electrons and so forms a cation, which can combine with an anion to form an ionic compound. The green coating on the Statue of Liberty contains Cu combined with carbonate and hydroxide anions rust contains Fe combined with oxide and hydroxide anions and silver tarnish contains Ag combined with sulfide anions. 

When an atom, ion, or molecule becomes more positively charged (that is, when it loses electrons), we say that it has been oxidized. Loss of electrons by a substance is 

How many electrons does each oxygen atom gain during the course of this reaction  

Ca(s) is oxidized (loses electrons) 02(g) is reduced ns electrons) and ions combine to form CaO(s) 

In this reaction Ca is oxidized to and neutral O2 is transformed to ions. When an atom, ion, or molecule becomes more negatively charged (gains electrons), we say that it is reduced. The gain of electrons by a substance is called reduction. When one reactant loses electrons (that is, when it is oxidized), another reactant must gain them. In other words, oxidation of one substance must be accompanied by reduction of some other substance. The oxidation involves transfer of electrons from the calcium metal to the O2, leading to formation of CaO. 

Oxidation may be defined as a loss of electrons, and reduction as a gain of electrons. In a reaction such as 

Reactions 2.1 and 2.2 appear similar, but at room temperature, SCI2 is a red liquid containing discrete SCI2 molecules, whose atoms are held together by covalent bonds (Structure 2.1) there cannot be complete transfer of two electrons per sulfur atom from sulfur to chlorine. Consequently, our definitions of oxidation and reduction prevent the classification of both Reactions 2.1 and 2.2 as redox reactions. To obtain a broader definition of oxidation and reduction, we must abandon the implication that complete electron transfer takes place. 

Metabolic energy derives from processes of oxidation and reduction. When energy is consumed in a process, chemical energy is made available for synthesis of ATP as one atom gives up electrons (becomes oxidized) and another atom accepts electrons (becomes reduced). For example, observe the following aerobic metabolism of glucose. 

The carbon in glucose moves from an oxidation state of zero to an oxidation state of +4. Concurrently, elemental oxygen moves from its oxidation state of zero to an oxidation state of -2 during the process. 

Anaerobic catabolic reactions are similar, although the electron acceptor isn t oxygen. The next example shows the fermentation of glucose to lactic acid. 

In this case, one carbon (the methyl carbon of lactic acid) is reduced from the zero oxidation state to -3 while another carbon (the carboxyl carbon of lactic acid) gives up electrons and goes from an oxidation state of zero to +3. In this example, the electron acceptor and electron donor are located on the same molecule, but the principle remains the same One component is oxidized and one is reduced at the same time. 

Reactions that run in the opposite direction of the preceding ones, particularly the first, must exist. Glucose must be made from inorganic 

In this reaction, each of the two magnesium atoms donates two electrons to the two oxygen atoms making up the oxygen molecule. In the process, each magnesium atom becomes Mg + and each oxygen atom becomes 

Because the balanced equation involves two magnesium and oxygen atoms, the previous equations are more appropriately written as 

The two magnesium ions with a positive charge are attracted to the two negatively charged oxygen atoms to form the ionic compound magnesium oxide, MgO. 

The reaction of magnesium and oxygen is an example of an oxidation reaction. The combination of an element with oxygen was the traditional way to define an oxidation reaction. This definition of oxidation has been broadened by chemists to include reactions that do not involve oxygen. Our modern definition for oxidation is that oxidation takes place when a substance loses electrons. Anytime oxidation takes place and a substance loses one or more electrons, another substance must gain the electron(s). When a substance gains one or more electrons, the process is known as reduction. Reactions that involve the transfer of one or more electrons always involve both oxidation and reduction. These reactions are known as oxidation-reduction or redox reactions. 

Redox reactions always consist of one oxidation reaction and one reduction reaction. The separate oxidation and reduction reactions are known as half reactions. The sum of the two half reactions gives the overall reaction. When the half reactions are summed, there is an equal number of elec- 

Try this short test. If you score more than 80 % you can use the chapter as a revision of your knowledge. If you score less than 80 %you probably need to work through the text and test yourself again at the end using the same test. If you still score less than 80 % then come back to the chapter after a few days and read again. 

Define reduction with reference to electron flow (l) 

Chemistry An Introduction for Medical and Health Sciences, A. Jones 2005 John Wiley Sons, Ltd 

Viagra was one of the biggest selling drugs in the last few years its action depends on the presence of minute quantities of an oxide present in our systems. Read on. 

We use the terms reduction and oxidation in many different contexts, but the chemical ones need to be carefully defined. 

Oxides Compounds of oxygen combined with another element Combustion Rapid oxidation that produces heat and (usually) light 

A substance that has combined with oxygen is described as having been oxidized, and the reaction is classified as oxidation. All combustion reactions are oxidations. In the combustion of the methane in natural gas, for example, the carbon is oxidized to carbon dioxide and the hydrogen is oxidized to water. 

The gradual rusting away of iron objects begins with the oxidation of iron. This gradual process is not referred to as combustion, however. 

In the blast furnaces that produce iron from iron ore, one of the important chemical changes is the reaction of the iron oxide in hematite with carbon monoxide. 

If the addition of oxygen to iron is oxidation, how is the removal of oxygen from iron described It is reduction, a term sometimes used to mean to bring something back. To metallurgists hundreds of years ago, reduction meant bringing a metal back from its ore. In today s chemical sense, the iron oxide has been reduced by the removal of oxygen. 

This chapter is concerned with equilibria involving oxidation and reduction processes. Firstly, we review concepts that will be familiar to most readers definitions of oxidation and reduction, and the use of oxidation states (oxidation numbers). 

The terms oxidation and reduction are applied in a number of different ways, and you must be prepared to be versatile in their uses. 

Oxidation and reduction steps complement one another, e.g. in reaction 8.1, magnesium is oxidized, while oxygen is reduced. Magnesium acts as the reducing agent or reduc-tant, while O2 acts as the oxidizing agent or oxidant. 

In a galvanic cell, a spontaneous redox reaction occurs and generates an electrical current (see Section 8.2). 

We have in earlier chapters seen how some chemical compounds are ionic. E.g. are the well-known sodium chloride NaCl in solid form formed from the reaction between solid sodium metal and chlorine gas I the following reaction  

In this reaction a reaction takes place between the Na(s) and the diatomic CI2 molecules whereby the ionic NaCl is formed consisting of Na ions and Cl ions in a lattice. In a reaction there is a transfer of electrons and such reactions are known as oxidation/reduction reactions or simply in short redox-reactions. 

Many important chemical reactions involve oxidation and reduction. E.g. are the chemical reactions involved in the production of energy typical redox-reactions. This also includes the conversion of food into energy in the human body. Oxidation is defined as an increase of the oxidation level while reduction on the contrary is defined as a decrease in the oxidation level explained in the following section. 

The term oxidation refers to a loss of electrons, while reduction means gain of electrons. Chemical reactions involving oxidation and reduction of atoms must be balanced not only in atoms but in electrons as well. 

The oxidation number of an atom is defined as the number of valence electrons in the free atom minus the number controlled by the atom in the compound. If electrons arc shared, control is given to 

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Like the charge on an ion, each atom is assigned an oxidation number. The total of the oxidation numbers of all the atoms is equal to the net charge on the molecule or ion. Thus, for CO,. the charge is 4 -1- 2( 2) = 0. 

Learning these rules will facilitate the process of assigning oxidation numbers. 

A chemical reaction in which electrons are transferred from one atom to another is called an oxidation-reduction reaction, or redox reaction. For example, a thin sliver of zinc metal can be burned to form zinc oxide. 

In this reaction, each zinc atom transfers two electrons to an oxygen atom. The zinc atoms become Zn ions, while the oxygen atoms 

Redox reactions are not limited to reactions in which atoms change to ions or vice versa. For example, consider the synthesis of hydrogen chloride gas from its elements. 

For each of the following reactions, identify what is oxidized and what is reduced. Also identify the oxidizing agent and the reducing agent. 

The oxidation number of an uncombined atom is zero. Therefore, free elements have an oxidation number of zero. 

You only have to look at the ATP yield from the TCA cycle, 12 of them per molecule of acetyl-CoA, to know that oxidative phosphorylation must be important. That s where all the electrons from NADH and FADH2 go after they re made by the TCA cycle. 

Electrons usually aren t floating around in space they re stuck on some atom or other. The simple consequence of this is that when one 

Pyruvate is reduced to lactate. Lactate is oxidized to pyruvate. NADH is oxidized to NAD . NAD is reduced to NADH. Pyruvate and NAD are oxidizing agents. Lactate and NADH are reducing agents. 

One of the most familiar redox reactions is corrosion of a metal (T FIGURE 4.11). In some instances corrosion is limited to the surface of the metal, with the green coating that forms on copper roofs and statues being one such case. In other instances the corrosion goes deeper, eventually compromising the structural int ity of the metaL Iron rusting is an important example. 

When an atom, ion, or molecule becomes more positively charged (that is, when it loses electrons), we say that it has been oxidized. Loss of electrons by a sttbstance is called oxidation. The term oxidation is used because the first reactions of this sort to be studied were reactions with oxygen. Many metals react directly with O2 in air to form metal oxides. In these reactions the metal loses electrons to oxygen, forming an ionic compound of the metal ion and oxide ion. The familiar example of rusting involves the reaction between iron metal and oxygen in the presence of water. In this process Fe is oxidized (loses electrons) to form Fe.  

The reaction between iron and oxygen tends to be relatively slow, but other metals, such as the alkali and alkaline earth metals, react quickly upon exposure to air. FIGURE 4.12 shows how the bright metallic surface of calcium tarnishes as CaO forms in the reaction 

The most frequently reported oxidation reaction of isatins is the oxidation with alkaline hydrogen peroxide to give anthranilic acids. This procedure has been both as a proof of structure of isatins and as a method of synthesis of anthranilic acids. The oxidation has been applied to alkyl 8 10 11 23 33 38-40 46 49 50 118halo n.18.35.36-38-40.47.10 .11 . 119,136,240 alkoxy,26,a8 39,47,75.io7,ii8,i36 trifluoromethyl,33,38,137 and nitro8,120,217 isatins. Use of N-substituted isatins led to N-substituted anthranilic acids.66,71,125,158,169,243 In the oxidation of 5-bromo-l-(y-carbethoxypropyl)-7-ethylisatin, 60 was isolated after treatment with ethanol and acid.11 Oxidation of isatin derivatives 61 led, after treatment with diazomethane, to the acridine derivatives 62.67 Application of this oxidation method to 7-hydroxyisatins gave rise to benzoxazo-lones (63).2,41 

Pummeror, P. Meininger, G. Sehrott, and H. Wagner, Justus Liebigs Ann. Chem. 590, 195 (1954). 

Yamada, T. Konakahara, and H. Iida, Kogyo Kagaku Zasshi 73, 980 (1970). 

Clerc-Bory, M. Clerc-Bory, H. Pacheco, and C. Mentzor, Bull. Soc. Chim. Fr. 1229(1955). 

Oxidation with chromium trioxide in acetic acid,33 chromium trioxide in acetic anhydride-acetic acid,31 3-chloroperbenzoic acid in methylene chloride65 or benzene,74 or peracetic acid in acetic acid74 gave rise to isatoic anhydrides (64). 

The elemental reaction used to describe a redox reaction is the half reaction, usually written as a reduction, as in the following case for the reduction of oxygen atoms in O2 (oxidation state 0) to H2O (oxidation state —2). The half-cell potential, E°, is given in volts after the reaction  

The half-cell potential and the half-cell free energy change are related by the following relationship for reversible conditions  

The Nemst equation describes the dependence of the half-cell potential on concentration  

As an alternative, the tendency for a reduction to occur may also be expressed in terms of a h)q)othetical electron activity based on the standard hydrogen electrode. Activity was functionally defined in Equation (9). The free energy of an electron is related to chemical activity of the electron by 

We connected our earlier definition of activity to a standard state of 1.0 bar or 1.0 M or a mole fraction of unity. None of these make much sense for electrons, but we may define electron 

2-FormyIpyrazine, which is light sensitive, undergoes the Cannizzaro reaction with aqueous sodium hydroxide to give 2-carboxypyrazine and 2-hydroxymethyl-pyrazine (1077). The direct oxidation of 2-formylpyrazine to 2-carboxypyrazine has not been described but 2-amino-3-formylpyrazine was oxidized by aqueous potassium permanganate to 2-amino-3 arboxypyrazine (423). 

From the viewpoint of inorganic chemistry, the reaction of potassium nitrate with sulfur and charcoal can be described as an oxidation-reduction reaction in which electron transfer between reacting species involves a loss or gain of electrons resulting in an oxidation or reduction process respectively. 

Potassium nitrate - - sulfur potassium oxide - - nitrogen - - sulfur dioxide 

Simple ions such as K have an oxidation number of +1, nitrogen has a value of +5 in the nitrate ion, where oxygen is always -2 while elemental sulfur has a value of 0. On the product side, potassium and oxygen retain values of +1 and -2 respectively while nitrogen changes to 0 in the elemental N2. Similarly, the oxygen in SO2 retains a value of —2 while sulfur changes to +4. 

Inserting the oxidation numbers in the reaction above gives  

In the above reaction, sulfur is oxidised to SO2, losing 4 electrons while nitrogen gains 5 electrons in forming elemental nitrogen. 

the ion transferred is Cl , rather than H+. Addition of a strong Lewis acid to such a solvent will result in the appearance in solution of the characteristic positive ion of the solvent. Addition of a base will yield the negative ion. Again, aprotic solvents may be leveling toward acids stronger than the characteristic acid ion of the solvent or, mutatis mutandis, toward bases. It will be seen that this is consistent with the picture based on the proton. 

Generally, bases are recognized by their ability to provide negative charge in the form of actual negative ions or of electron pairs to be shared. Acids are recognized by their ability to provide positive charge or to accept shared electron pairs. In the context of reaction kinetics and mechanisms, a base is termed a nucleophile, and a Lewis acid an electrophile. 

Conant (HaU and Conant, 1927) recognized that certain systems, such as Friedel-Crafts catalysts, can have acidities greater than that of 100% H SO These 

The strength of an oxidant or a reductant that can exist for an indefinite period in any given solvent is obviously limited by the susceptibility of the solvent to oxidation or reduction. One must, however, consider both the thermodynamic and the kinetic aspects of any possible reaction. For instance, in aqueous solution in the absence of catalysts, oxidants that should, thermodynamically, be able to drive the half-reaction 

Liquid ammonia dissolves the alkali metals without immediate reaction and affords a strong reducing agent, as long as catalysts (chiefly finely divided metals) for the reaction 

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

After precomplexation with ji-CD, a variety of alcohols, including aromatic alcohols, were oxidized to their corresponding carbonyl compounds in good yields with NaOCl-KBr in aqueous solution. A substrate-selective and transition metal-free oxidation of benzoic and allylic alcohols with NaOCl oxidant mediated by j8-CD in water was developed. In the presence of one molar equivalent of jS-CD, benzyl alcohol, 4-methoxybenzyl alcohol and some primary aromatic alcohols were oxidated to form benzaldehyde, 4-methoxybenzaldehyde and aromatic aldehydes, respectively, at 50 °C for 1-4 h. When 20% of acetone was added to the reaction system, the yield of aldehyde was dramatically decreased. 

Doussot et al. have reported that regio- and stereoselectivity in the reduction of substituted epoxides and aromatic ketones with NaBU, in the presence of CDs depended mainly on interactions with the wider rim, the secondary hydroxyl side, of the CD. This indicates that an alkoxyborohydride intermediate is formed in the first step of the reaction.  

In aqneons or organic solvents, it is possible that the reagent or substrate can still form a host-guest complex through specific interactions with the CD. Adequate modification of the CD to form new interaction points or adequate change of the cavity can be expected to result in appreciable ee in an asymmetric reaction. 

Reduction of mononitroaienes mediated by P-CD has been reported using hydroxide ion as a leductant. Ordinarily the reducing ability of OH in water is very low as a result of its stabilization by hydration. Reductions by OH have only been observed in aprotic organic solvents. CD includes the reactant (nitrobenzene) and the reaction is carried out near the rim of the cavity. 

Many organic reactions have been carried out with CD cavities in water. In the early stages of CD chemistry, the subject was limited to organic chemistry. Many reactions related to the inorganic held have since been reported. The most attractive advantage of CD is its selectivity , however, the solvation effect cannot be ignored. To develop CD-mediated reactions, it is necessary to clarify the reaction mechanism in detail. Complexation certainly indnces selectivity. Although it is difficult to elucidate the role of the CD, complex orientation in the reaction transition state must be made clear at the molecular level. 

You had a brief introduction to the topic of electrochemistry in Chapter 11 when you reviewed the oxidation-reduction process in which reactions occur by the transfer of electrons. One of the procedures you looked at was the half-reaction method of balancing redox equations. In this chapter, we will be looking at the oxidation and reduction process in even more depth. 

Oxidation and reduction reactions, also known as redox reactions, involve the transfer of electrons from one substance that is being oxidized (losing electrons) to another that is being reduced (gaining electrons). A very common example of a redox reaction, one that you may very well have done in the lab, is the reaction that occurs when you place clean, iron nails into a copper sulfate solution. In this reaction, shown below, iron is oxidized and copper is reduced  

Two electrons are removed from iron by the copper ion. The iron, now an ion, moves into the solution, while copper, now a solid, forms on the surface of the nail. This process can be summarized using redox half-reactions (Chapter 11), such as the one shown here  

The half-reaction shows the transfer of the electrons that occurs in the reaction. 

One of the major problems encountered with the Volta pile was severe corrosion of the metals, and many early experiments were directed towards solving this problem. Thus it is apparent that batteries and corrosion are closely linked and, indeed, both are oxidation and reduction reactions. Oxidation and reduction is also involved in the related process of electrolysis, which underlies electroplating, a method of preventing corrosion. Finally, oxidation and reduction reactions underpin all life processes, although this aspect is not covered here. 

Oxidation of terminal olefins to methyl ketones by aqueous palladium chloride and oxygen is very slow, but addition of micellar sodium lauryl sulphate increases the rate of formation of 2-octanone from 1-octene twentyfold at 50 °C. There is weaker catalysis by the non-ionic surfactant Brij-35 and inhibition by cationic surfactants. Oxidation of diosphenol (35) in basic aqueous tetradecyltrimethylammonium chloride is faster and more effective than in water, giving a higher yield of (36). Two attempts at effecting the enantioselective reduction of aromatic ketones, one in micelles of R-dodecyl-dimethyl-a-phenylethylammonium bromide and the other in sodium cho-late micelles, both give optical yields of less than 2%. Rather more success was obtained in the catalysed oxidation of L-Dopa, 3,4-dihydroxyphenyI-alanine. In the presence of the Cu complex of N-lauroyl-L-histidine in cetyl-trimethylammonium bromide micelles reaction was 1.42 (pH 6.90, 30 °C) to 

Matushita, H. Yamasaki, and A. Takeda, Tetrahedron Lett., 1980, 21, 1063. 

Sugimoto, Y. Matsumura, T. Imanishi, S. Tanimoto, and M. Okano, Tetrahedron Lett., 1978, 3431. 

50 (pH 6.90, 30 °C) times faster than for the corresponding experiment with N-lauroyl-D-histidine. The product (37) is derived from an intermediate o-quinone, and reaction under micellar conditions is strongly catalysed. Cationic micelles enhance the reactivity of L-ascorbic acid in its reduction of methylene blue, with a rate maximum around pH 8. The corresponding reduction of the dye by L-cysteine is also strongly catalysed, but the oxidation of the leuco-product back to methylene blue with oxygen is not greatly affected by micelles.  

Disparlure, the sex pheromone of the female gypsy moth, has been used to control the spread of the gypsy moth caterpillar, a pest that has periodically devastated forests in the northeastern United States by defoliating many shade and fruit-bearing trees.The active pheromone is placed in a trap containing a poison or sticky substance, and the male moth is lured to the trap by the pheromone. Such a species-specific method presents a new way of controlling an insect population that avoids the widespread use of harmful, nonspecific pesticides. Disparlure is synthesized by oxidation of an alkene using chemistry presented in Chapter 12. 

The word mechanism will often be used loosely here. In contrast to the S l reaction of alkyl halides or the electrophilic addition reactions of alkenes, the details of some of the mechanisms presented in Chapter 12 are known with less certainty. For example, although the identity of a particular intermediate might be confirmed by experiment, other details of the mechanism are suggested by the structure or stereochemistry of the final product. 

Oxidation and reduction reactions are very versatile, and knowing them allows us to design many more complex oiganic syntheses. 

Recall from Section 4.14 that the way to determine whether an oiganic compound has been oxidized or reduced is to compare the relative number of C-H and C Z bonds (Z - an element more electronegative than carbon) in the starting material and product. 

Two components are always present in an oxidation or reduction reaction—one component is oxidized and one is reduced. When an organic compound is oxidized by a reagent, the reagent itself must be reduced. Similarly, when an organic compound is reduced by a reagent, the reagent becomes oxidized. 

Throughout history, humans have ingested alcoholic beverages for their pleasant taste and the feeling of euphoria they impart. Wine, beer, and similar products contain ethanol (CH3CH2OH), a 1° alcohol that is quickly absorbed in the stomach and small intestines and rapidly transported in the bloodstream to other organs. Like other 1° alcohols, ethanol is easily oxidized, and as a result, ethanol is metabolized in the body by a series of enzyme-catalyzed oxidation reactions that take place in the liver. In Chapter 12, we learn about oxidation and reduction reactions of organic molecules like ethanol. 

In Chapter 12, we discuss the oxidation and reduction of alkenes and alkynes, as well as compounds with polar C-X CT bonds—alcohols, alkyl halides, and epoxides. Although there will be many different reagents and mechanisms, discussing these reactions as a group allows us to more easily compare and contrast them. 

Formation of the shiny Ag mirror is a positive test for aldehydes. The RCHO must be soluble in aqueous alcohol. This mild oxidant permits —CHO to be oxidized in a molecule having groups more difficult to oxidize, such as 1° or 2° OH s. 

Ketones resist mild oxidation, but with strong oxidants at high temperatures they undergo cleavage of C—C bonds on either side of the carbonyl group to give a mixture of carboxylic acids. 

In the Baeyer-Vllliger reaction, a ketone is oxidized to an ester by persulfuric acid, HjSO, 

When an aryl alkyl ketone is oxidized, the R remains attached to the carbonyl carbon and Ar is bonded to O of the ester group. 

The Clemmensen reaction is used mainly with aryl alkyl ketones, 

RCHz- C- CHzR - RCOOH + R CHzCOOH + RCH2C001 + R COOH  

In this reaction, an electron from each of two sodium atoms is transferred to the CI2 molecule to form two Cl ions. An example of a combustion reaction is the burning of magnesium in air. 

The reaction of magnesium and oxygen involves a transfer of electrons from magnesium to oxygen. Therefore, this reaction is an oxidation-reduction reaction. Using the classifications given in Chapter 10, this redox reaction also is classified as a combustion reaction. 

For oxidation to take place, the electrons lost by the substance that is oxidized must be accepted by atoms or ions of another substance. In other words, there must be an accompanying process that involves the gain of electrons. Reduction is defined as the gain of electrons by atoms of a substance. Following our sodium chloride example further, the reduction reaction that accompanies the oxidation of sodium is the reduction of chlorine. 

1 Following the procedure outlined in Example 5.1, we find the following two half-reactions  

The reduction half-reaction requires five electrons whereas the oxidation half-reaction requires two electrons therefore we must balance electrons so that an equal number of electrons are required in both reactions. The reduction reaction must be multiplied by 2 and the oxidation reaction by 5 to give 10 electrons in each case  

Summing two half-reactions and cancelling lOe found on both the left and right sides of the equation, we obtain the full reaction  

The standard reduction potential for the CriOv VCr couple is +1.38 V, so it can oxidize any couple whose reduction potential is less than +1.38 V. Since the reduction potential for the Fe /Fe couple (+0.77 V) is less positive than Cr2O7 7Cr reduction potential, Fe will be oxidized to Fe by dichromate. The reduction potential for the Clj/Cr couple (+1.36 V) is slightly less positive than that of the couple, so a side reaction 

4 The potential difference of the cell from Example 5.45 was calculated to be +1.23 V. To calculate the potential difference under non-standard conditions we start with the Nemst equation given by the formula  

Reactions in which electrons are transferred from one reactant to another form a subset of systems 

The elemental reaction used to describe a redox reaction is the half reaction, usually written as a reduction  

The dependence of the relative potential for the half reaction on concentration is given by the Nernst equation  

Chromium trioxide, the most versatile of oxidising agents, has been shown to selectively oxidise allylic alcohols to aj5-unsaturated ketones in high yield, when 

Herlem-Gaulier and F. Khuong-Huu-Laine, Compt. rend., 1969, 269 C, 1405. 

15-Oxo-steroids, which are not usually very accessible, may be prepared by the allylic oxidation of 8(14)-enes with potassium chromate. Yields, which vary between 20—60%, are highest when a 12-oxo-group is present.  

Cuilleron, M. F6tizon, and M. Golfier, Bull. Soc. chim. France, 1970, 1193. 

Oxidation of the non-activated C-14 atom of (575) with chromium trioxide has been reported to give the 14 x-alcohol in yields which are dependent upon the concentration of water present in the reaction, optimum (42—44%) yields being obtained with 1.5—2% water.  

It has been seen that many chemical reactions can be regarded as the transfer of electrically charged electrons horn one atom to another. Such a transfer can be arranged to occur tW ugh a wire connected to an electrode in contact with atoms that either gain or lose electrons. In that way, electricity can be used to bring about chemical reactions, or chemical reactions can be used to generate electricity. First, however, consider the transfer of electrons between atoms that are in contact with each other. 

It is easy to see what is meant by oxidation and reduction when there is a transfer of electrons to form ionic bonds (see Section 4.3). However, the concept can be extended to compounds that are bonded together by covalent bonds. For example, when H combines with O to form H2O, 

The oxidation number of any element is zero in the elemental form. Therefore, the oxidation number of O in O2 is zero, and the oxidation number of H in H2 is zero. 

Even though the oxidation numbers of some elements vary, they can usually be determined out by applying the following rules  

The application of these rules can be seen in Table 8.1. For each compound in this table, the oxidation number of each element, except for one, is definitely known. For example, in SOj, each O has an oxidation number of -2. There are 2 Os for a total of -4. The -4 must be balanced with a -1-4 for S. In SO3, the total of -6 for 6 O atoms, each with an oxidation number of -2, must be balanced with a -1-6 for S. In Na3P04, each Na has an oxidation number of -1-1, and each O has -2. Therefore, the oxidation number of P is calculated as follows  

A regioselective reaction forms predominately or exclusively one constitutional isomer (Section 8.5). 

Oxidation reactions result in Reduction reactions result in  

Increasing alkyl substitution on the C=C decreases the rate of reaction. 

For a given molecule, the most Lewis acidic atom = most electron-deficient atom. 

Draw a Lewis structure of borane (BH3). Note that the boron atom does not have an octet. 

Add a curved bouncing arrow to each pair to show the most likely reaction, and draw the resulting 

The following mechanism has been proposed to explain these products. 

BH2 can be easily oxidized to OH without affecting the stereochemistry or regiochemistry. 

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These topics, which are more fully treated in texts on physical chemistry, require some consideration here, because the terms acid , base , oxidation and reduction are used so widely in inorganic chemistry. 

ACIDS AND BASES. OXIDATION AND REDUCTION OTHER CONCEPTS OF ACIDS AND BASES... 

ACIDS AND BASES OXIDATION AND REDUCTION 97 Thus the equation for the reaction is ... 

If the acid of the ammonium salt is an oxidising agent, then on heating the salt, mutual oxidation and reduction occurs. The oxidation products can be nitrogen or one of its oxides and the reactions can be explosive, for example ... 

The two reaction schemes of Figures 3-13 and 3-15 encompass a large proportion of all organic reactions. However, these reactions do not involve a change in the number of bonds at the atoms participating in them. Therefore, when oxidation and reduction reactions that also change the valency of an atom ate to be considered, an additional reaction scheme must be introduced in which free electron pairs are involved. Figure 3-16 shows such a scheme and some specific reaction types. 

Isomerization of double bonds in vitamin D analogs such as calciferol by oxidation and reduction has been carried out via the formation of the tt-allylpalladium complex 334 with PdCl2(PhCN)2 in 70% yield, followed by hydride reduction to afford 335[295],... 

Chapters 9, 10 and 11 describe methods for substitution directly on the ring with successive attention to Nl, C2 and C3. Chapters 12 and 13 are devoted to substituent modification as C3. Chapter 12 is a general discussion of these methods, while Chapter 13 covers the important special cases of the synthesis of 2-aminoethyl (tryptaminc) and 2-aminopropanoic acid (tryptophan) side-chains. Chapter 14 deals with methods for effecting carbo cyclic substitution. Chapter 15 describes synthetically important oxidation and reduction reactions which are characteristic of indoles. Chapter 16 illustrates methods for elaboration of indoles via cycloaddition reactions. 

Although the reduction process is not always a reversible one, oxidation and reduction potential values can be sometimes related to the Hiickel energies of the highest and lowest filled molecular orbital of the dye (108). 

We begin by writing unbalanced equations for the oxidation and reduction half-reactions in part (a). 

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