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Purdue University - Oxidation-Reduction Reactions
Excellent, comprehensive discussion of redox reactions. Includes an interesting discussion of the history of the oxidation-reduction approach.



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Special points of emphasis

Periodic Properties

The Chemical Bond

Oxidation-Reduction

Oxidation-Reduction in Organic Chemistry

The possible bonding arrangements between carbon and oxygen in organic chemistry translate to a series of oxidation states of carbon. Because oxygen (3.5 on the Pauling scale) is more electronegative than carbon (2.5), electrons shared between the two atoms are said, in oxidation-reduction, to 'belong' to oxygen. The oxidation reduction progression that proceeding from alcohol to aldehyde/ketone to carboxylic acid to carbon dioxide involves a continuous loss of 'electron control' by carbon. At one end of the scale, carbon is bonded to other carbons or hydrogen and singly bound; at the other end, carbon is doubly bound twice to oxygen. A primary alcohol beginning with an oxidation number of -1 (gaining two electrons from bonding with two hydrogens and losing one electron in bonding to oxygen), will have an oxidation state of +1 upon oxidation to an aldehyde and +3 upon further oxidation to carboxylic acid. The oxidation state of carbon in carbon dioxide is +4.

Fluent understanding of the differences between the various oxidation states of carbon in organic compounds is about as clear-cut as one can get in terms of core MCAT preparation.




Work, Energy, and Power

Electricity

Periodic Properties

The Chemical Bond

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Oxidation-Reduction

Electrochemistry

Oxidation-reduction is a formal system for our convenience. In this system, oxidation is defined as a process by which oxidation number increases, corresponding to a loss of electron control. In reduction oxidation number decreases, corresponding to a gain of electron control. In order to assign electron control across covalent bonds, it is essential to know which atoms are more electronegative in the bonds. How electronegativity differences play out in chemical reactions provides the underlying coherence of the oxidation-reduction system of chemistry. Electronegativity reflects the strength of attraction an atom has for the electrons it shares in chemical bonds, while oxidation-reduction is a systematic accounting procedure to reflect the changes in the bonding environment of electrons between products and reagents. When two atoms form a covalent bond, the more electronegative atom is assigned 'electron control' in the oxidation-reduction system. If an atom gains electron control through a chemical process, it is said to be 'reduced,' while the atom that has lost electron control is said to be 'oxidized'. The key to the system is that when a very electronegative atom is reduced, it draws the new electrons inwards towards its strongly attracting nucleus, and the bond becomes polarized. This closing of separation between unlike charges represents a potential energy decrease above and beyond the typical energy decrease that accompanying the formation of an ordinary bonding, molecular orbital. In other words, the formation of polar bonds corresponds to large potential energy decreases (tending toward negative internal energy change, negative enthalpy, negative free energy change), and as a general rule, polar bonds are stronger than nonpolar bonds (more energy is required to break them because the electrons have to be wrenched away from the oxidant). Oxidation-reduction provides a systematic way to account for these tendencies. The more electronegative elements form stronger covalent bonds.



Work, Energy, and Power

Electricity

The Chemical Bond

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Carbohydrates

Oxidation-Reduction

Bioenergetics and Cellular Respiration

Both in terms of the overall chemical transformations and in terms of the stepwise reaction pathways, oxidation-reduction reactions play a central role in metabolism. For example, you can look at the chemical processes of oxidative metabolism through the lens of chemical thermodynamics or you can look at it through the lens of oxidation-reduction. From the thermochemical perspective, oxidative metabolism involves breaking relatively weak carbon-hydrogen bonds and forming stronger carbon-oxygen and hydrogen-oxygen bonds, bonds with greater electronegativity difference than the initial carbon-hydrogen bonds of carbohydrate or lipid. The powerful oxygen nucleus 'desires' to draw electrons in towards itself, an internal energy decrease (which translates to an enthalpy decrease and a free energy decrease, energy ultimately translating to food calories). Because triglycerides have primarily carbon-hydrogen bonds, these molecules have a greater reservoir of electrons which can ultimately participate in bonds with oxygen. From a thermochemistry perspective, we are breaking weak bonds and forming strong bonds. From an oxidation-reduction perspective, oxygen is being reduced. When you can see that this is two ways of saying the same thing, you will have reached the underpinnings of redox.

The successive changes that occur upon the glucose substrate through the various stages of oxidative metabolism (glycolysis, mobilization of pyruvate, the citric acid cycle) involve the oxidation-reduction series comprising alcohol, aldehydes & ketones, carboxylic acids, and carbon dioxide. Oxidative metabolism can be envisioned as a narrative of electron control in which electronegative oxygen is gaining control of bonding electrons that were under the control of carbon. When oxygen forms a bond with a less electronegative element (any except fluorine) the electrons are drawn in toward the powerful oxygen nucleus, leading to the polarity of the bond and corresponding to an extra internal energy decrease beyond ordinary bond energy. The fact that extra energy is lost when electronegative elements form bonds, in other words, the fact that polar bonds are exceptionally strong (requiring the input of large amounts of energy to break), is the reason that energy is liberated in the oxidative metabolism of nutrient molecules. As the system moves from relatively weak C-H and O-O bonds to strong C-O and H-O bonds, the internal energy decrease translates to the free energy decrease that is coupled ultimately with the phosphorylation of ADP to form ATP.




Stoichiometry

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Biological Membranes

The Eukaryotic Cell

Oxidation-Reduction

Oxidation-Reduction in Organic Chemistry

Electrochemistry

The Citric Acid Cycle including the initial pyruvate mobilization is a series of reactions oxidizing the pyruvate from glycolysis to form three molecules of CO2. Through the process NAD+ and FAD act as electron acceptors. These electrons are eventually passed to their final acceptor O2 by means of the electron transport chain. While direct substrate phosphorylation does occur to an extent in the citric acid cycle, the processes of the electron transport chain yield a great deal of energy, forming most of the ATPs. Oxidative metabolism yields far more energy than glycolysis and fermentation because the ultimate electron acceptor is oxygen, rather than organic carbon. While glycolysis and fermentation alone yields only two ATP, the entirety of oxidative metabolism, including glycolysis, oxidation of pyruvate, and the citric acid cycle will yield thirty-six molecules of ATP per molecule of glucose (38 in aerobic bacteria).



Reactions of Alkanes

Reactions of Alkenes

Reactions of Alcohols and Ethers

Reactions of Carboxylic Acids and Derivatives

Oxidation-Reduction

Oxidation-Reduction in Organic Chemistry

Read for comprehension. Catalyzed by isocitrate dehydrogenase, the oxidation of isocitrate by NAD+ converts a hydroxyl group to an aldehyde. This aldehyde is a β-keto acid, a species that is very susceptible to decarboxylization. The decarboxylization of this β-keto acid (oxalosuccinate) forms α-ketoglutarate. In summary, the conversion of isocitrate to α-ketoglutarate yields NADH and CO2.

We continue our discussion of the citric acid cycle with the conversion of α-ketoglutarate into succinate. Next, succinate is oxidized in a dehydrogenation reaction (catalyzed by succinate dehydrogenase), converting succinate into an alkene, fumarate, and converting FAD into FADH2. Fumarate is then hydrated to form the alcohol malate, which is then oxidized to form the aldehyde, oxaloacetate, converting NAD+ to NADH.

That's it! Don't worry if you don't have the cycle memorized to the finest detail. An example of the level that the MCAT might approach the details would be giving a list of compounds from the cycle as the correct answer for substances one may expect to find in the inner compartment of a mitochondrion. Here is a mnemonic for the cycle:

Curly Is Kicking Some Stooge Fanny, MO

This one has a limited life-span though, because nobody watches the Three Stooges anymore.




Electricity

The Chemical Bond

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Oxidation-Reduction

Electrochemistry

Bioenergetics and Cellular Respiration

Photosynthesis

Plants are not on the MCAT, so we can use plant biology as a pressure free zone to practice our comprehension of fundamental principles that are on the test. Photosynthesis utilizes solar energy to drive an oxidation-reduction reaction uphill against positive free energy, the transformation of carbon, oxygen and hydrogen from an initial state composed of carbon dioxide and water into a final state composed of carbohydrate and molecular oxygen. Looking at photosynthesis in terms of bond energy, the conceptual framework underlying oxidation-reduction, the final state in photosynthesis is higher energy than the initial state because the bonds in molecular oxygen and carbohydrate are weaker than the bonds in carbon dioxide and water. Why are the initial state bonds in carbon dioxide and water stronger? When oxygen binds to lower electronegativity elements such as carbon and hydrogen, the total internal energy of the system decreases because oxygen has seized the bonding electrons and has drawn them in towards its strong nucleus. This is why more energy tends to be released when polar covalent bonds are formed than when nonpolar bonds are formed, so it requires more energy to break polar covalent bonds than nonpolar bonds. The bond energy perspective is the underlying coherence of oxidation-reduction: polar bonds are stronger (in redox terms, when polar bonds form, a high reduction potential element is reduced). Summarizing this point, photosynthesis breaks strong bonds and forms weak bonds, fixing energy in the process. Another way of saying this in terms of oxidation-reduction and electrochemistry, photosynthesis transforms light into reduction potential, removing electron control from oxygen and placing it with carbon.

In aerobic respiration, the oxygen gains back electron control, a process with positive cell potential (negative free energy change) that can be coupled with the nonspontaneous phosphorylation of ATP.




The Chemical Bond

Conjugated π Systems and Aromaticity

Coordination Chemistry

Proteins

Oxidation-Reduction

Electrochemistry

Blood

Coordinated iron and copper ions serve vital rolls in the enzyme complexes of the respiratory chain. One important type of prosthetic group are iron-sulfur clusters in which iron is coordinated to sulfhydral groups of cycsteine as well as inorganic sulfides. In these groups the iron atoms interchange from the reduced form, Fe2+ , to the oxidized form, Fe3+. Three of the enzyme complexes of the respiratory chain are cytochromes, which means they are electron transferring proteins containing heme groups. A heme group consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Reduction of the iron corresponds to electron delocalization over the entire porphyrin network.







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