Organic molecules oxidation-reduction reactions

In the mid-seventies, it was demonstrated that UPD metal adatoms can produce electrocatalytic effects on various electrochemical reactions. These include oxidation of small organic molecules, oxygen reduction, reactions of electroorganic synthesis, electrodeposition of metals, and charge transfers in redox couples. The oxidation of organic molecules that are... 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

Like other compounds, organic molecules undergo acid-base and oxidation-reduction reactions, as discussed in Chapters 2 and 4. Organic molecules also undergo substitution, elimination, and addition reactions. 

Complexation/decomplexation of metal ions or of neutral organic molecules, protonation/deprotonation reactions, and oxidation/reduction processes can all be exploited to alter reversibly the stereoelectronic properties of one of the two recognition sites, thus affecting its ability to sustain noncovalent bonds [30-34, 41]. These kinds of switchable [2 catenanes can be prepared following the template-directed synthetic strategy illustrated in Figure 5, wherein one of the two macrocyclic components is preformed and then the other one is clipped around it with the help of noncovalent bonding interactions. 

The oxidation numbers given in Table 3 can be used to classify organic reactions as either oxidation-reduction reactions or metathesis reactions. Because electrons are neither created nor destroyed, oxidation cannot occur in the absence of reduction, or vice versa. It is often useful, however, to focus attention on one component of the reaction and ask Is that substance oxidized or reduced Assigning oxidation numbers to the individual carbon atoms in a complex molecule can be difficult. Fortunately, there is another way to recognize oxidation-reduction reactions in organic chemistry. 

Oxidation-reduction reactions in water involve the transfer of electrons between chemical species. Metal ions in water are always bonded to water molecules in the form of hydrated ions represented by the general formula, M(H20)xn+ Metals in water may be bound to organic chelating agents. Hardness is due to the presence of calcium ion and to a lesser extent to magnesium ion. 

The different electrocatalytic reactions described below concern the oxidation of molecular hydrogen and of small organic molecules and alcohols, the reduction of protons and of dioxygen, the electrohydrogenation of organic molecules, the reduction of halides and the electroreduction of carbon dioxide. 

In the absence of oxygen, the complete remineralization of complex organic molecules to CO2 is a chemically difficult problem that cannot be accomplished by a single microbial metabolism. Instead, such substrates are degraded into successively smaller pieces via the activities of multiple groups of microorganisms ([19], Fig. 1). Most of these sequential steps are oxidation-reduction reactions... 

Like metal ions, the small organic molecules that act as coenzymes bind reversibly to an enzyme and are essential for its activity. An interesting feature of coenzymes is that many of them are formed in the body from vitamins (see > Table 10.2), which explains why it is necessary to have certain vitamins in the diet for good health. For example, the coenzyme nicotinamide adenine dinucleotide (NAD ), which is a necessary part of some enzyme-catalyzed oxidation-reduction reactions, is formed from the vitamin precursor nicotinamide. Reaction 10.5 shows the participation of NAD in the oxidation of lactate by the enzyme lactate dehydrogenase (LDH). Like other cofactors, NAD is written... 

The reverse of each reaction that we have just given is an oxidation of the organic molecule, and we can summarize these oxidation—reduction reactions as shown below. We use the symbol [O] to indicate in a general way that the organic molecule has been oxidized. 

The reduction of nitrobenzene to aniline is a typical oxidation-reduction reaction in which tin metal, Sn°, is oxidized to stannic ion, Sn, in the form of stannic chloride, SnCl4 hydrochloric acid serves as the source of protons. A plausible mechanism of this reaction is outlined in Scheme 21.3. Generally, the reduction occurs by a sequence of steps in which an electron is first transferred from a tin atom to the organic substrate to give an intermediate radical ion that is then protonated. The oxygen atoms on the nitro group are eventually removed as water molecules. It is left as an exercise to write a balanced equation for the overall reaction and to provide a mechanism for the reduction of N-phenylhydroxylamine (11) into aniline (see Exercises 13 and 14 at the end of this section). 

It has been suggested that biological oxidation-reduction reactions occur by way of one electron transfer As a consequence of univalent transfer of electrons in the oxidation of organic molecules free radicals are produced at the end of the first step. With the help of electron paramagnetic resonance spectrometer it has been established that several enzymic oxidation reactions involve one-electron transfer processes. 

---