Describe The Concept Of Oxidation Levels And Discuss The Use Of Oxidising And Reducing Agents For The Modification Of Functional Groups In Organic Chemistry.

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Describe The Concept Of Oxidation Levels And Discuss The Use Of Oxidising And Reducing Agents For The Modification Of Functional Groups In Organic Chemistry.

The majority of the reactions of organic compounds, which involve conversion of one type to another, can be classified as oxidation or reduction. This conclusion can be justified on the basis of the definitions of the terms, oxidation and reduction. Oxidation is defined as a loss of electrons and reduction as a gain of electrons. Other definitions have been formulated which deal with such concepts as oxidation involving removal of hydrogen to form multiple bonds or to make new bonds between carbon and a more electronegative element, and reduction involving reactions in which carbon forms new bonds to hydrogen.

It is harder to define oxidation as a loss of electrons in organic compounds compared with metals. In oxidation and reduction of metals, the electronic changes involve transfer, and thus a true net loss or gain of electrons. In covalently bonded compounds, such as the compounds of carbon, such electron transfers do not usually occur. Instead, the carbon atom, even though it retains a covalency of four, changed markedly in the degree of control it exerts over the covalently bound electrons. Thus, it may be that when the electron density about a carbon atom decreases, it has undergone oxidation, and conversely an increase in electron density can be interpreted as a reduction.

Also the oxidation level or the oxidation number of a compound tells us whether an oxidation or reduction reaction has happened. There are a variety of methods of calculating oxidation numbers. In compounds such as methane, CH4, we know that hydrogen has an oxidation number of +1. Assuming that the algebraic sum of all the oxidation numbers must equal zero, the oxidation state of carbon in methane is therefore

-4. When the oxidation number of an atom increases, that atom is said to be oxidised and when the oxidation number decreases, reduction has taken place. Therefore when methane undergoes combustion, carbon dioxide and water are the products. The carbon has been oxidised because its oxidation state has increased from -4 in methane to +4. On the other hand, oxygen has been reduced because its oxidation state has decreased from 0 in its elemental state to -2 in water.

The various oxidising reagents available to the organic chemist have widely different oxidation potentials. Since the various functional groups in organic compounds also have widely different reduction potentials, certain oxidising agents have been found to be specific for converting certain organic functional groups to other functional groups because the respective oxidation and reduction potentials are of the right order of magnitude. Other organic compounds may be too difficult to oxidise (have too low a reduction potential) for one oxidising agent whereas another reagent of higher potential is capable of accomplishing the oxidation.

Oxidation of Hydrocarbons at Sigma Bond.

Since the bond dissociation energies of single bond of alkanes are quite similar, few oxidation procedures permit selective cleavage of specific C-C or C-H bonds without complete oxidation of the entire molecule. Allylic, benzylic and tertiary C-H bonds are weaker than other types and can occasionally oxidised successfully without affecting the remainder of the molecule. For example, cumene, propene, toluene, cyclohexene and similar compounds can be oxidised to useful products.

Cyclic hydrocarbons containing six-membered rings are dehydrogenated when heated in the presence of hydrogenation catalysts such as palladium or platinum.

An indirect route to oxidation products of alkanes is via halogenation followed by hydrolysis of the alkyl halide to the alcohol. The alcohols may be further oxidised under controlled conditions as will be discussed later.

Oxidation of Hydrocarbons at pi (?) Bonds.

Oxidation may result from attack of the oxidising on pi (?) bonds as well as on sigma (?) bonds. Although the pi bonds of alkenes are more reactive with ionic reagents than are the sigma bonds, certain oxidising agents can quite selectively oxidise sigma bonds. Other reagents selectively attack the pi bond. A reagent for accomplishing a cis addition to alkenes is osmium tetroxide; the method, however is both expensive and hazardous since the reagent is very toxic.

A particularly useful group of oxidising agents for converting alkenes to epoxides and glycol derivatives are the peroxy acids. Peroxy acids are compounds characterised by having the peroxide link present and may be considered as derivatives of hydrogen peroxide. They are usually prepared by the addition of hydrogen peroxide to an excess of the acid, or by the reaction of the anhydride of the acid with hydrogen peroxide. They are subject to loss of oxygen on standing. For this reason peroxyformic acid must be used immediately after preparation.

Alkenes react with peroxy acids under mild conditions to produce epoxides or their derivatives. Since the epoxides initially formed are susceptible to attack to form the monoesters of the 1,2-diols, it is not always possible to isolate the epoxides.
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Peroxy esters are also useful oxidising agents in the presence of metal salts. These compounds are capable of converting allylic C-H bonds (as in cyclohexene) to C-O bonds and this reaction constitutes perhaps the most convenient route to allyl alcohol derivatives.

An important use of terminal alkynes is synthesising conjugated alkynes and in general extending the length of a carbon chain involves oxidative coupling. It is well known that terminal alkynes form metal acetylides as the result of the relative acidic alkyne proton.

These metal acetylides may be converted to the corresponding conjugated diynes and may ...

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