Oxidation Level Changes During Reactions

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Chapter: Organic Chemistry : Oxidation States of Organic Compounds

Comparing the oxidation levels of various carbon atoms is excellent for illustrating what oxidation state change must occur at a particular carbon in a given reaction of that compound.


OXIDATION LEVEL CHANGES DURING REACTIONS

Comparing the oxidation levels of various carbon atoms is excellent for illustrating what oxidation state change must occur at a particular carbon in a given reaction of that compound. For example,


The oxidation level of the primary alcohol (1) is less than the aldehyde product (+1); thus this conversion requires an oxidation of the alcohol function to the aldehyde. Any reagent capable of effecting this change must necessarily be an oxidizing agent which is itself reduced. In the preceding reaction the need for an oxidant is noted above the arrow by the [O]. The reverse process, conversion of an aldehyde to a 1 alcohol, is a reduction. Any reagent capable of effecting this change must be a reducing agent and is itself oxidized. A reduction is commonly indicated by a bracketed [H] above the arrow.

By the same analysis conversion of an aldehyde to an acetal involves neither oxidation nor reduction. As a consequence no oxidant or reductant is necessary to carry out this reaction.


Similarly, for the process shown below, the oxidation state of C-1 remains at 1 throughout the sequence; thus the overall sequence involves no change in oxida-tion level at C-1, nor does either step. Modifications of substituents and substitu-tion of one electronegative group for another are generally not redox processes.


One often must consider the balanced reaction in order to be certain of any net changes in oxidation state, and similar procedures for determining the oxidation level can be followed for other covalently bound elements. For example, the conversion of methane into ethane is an oxidation of the carbon atoms since the carbons in methane are at the 4 level while in ethane they are at the 3 oxidation state.


However, the oxidation level of hydrogen changes from being bound to carbon (0) in the reactant to being bound to another hydrogen (1) in the product. Thus hydrogen is formally reduced. The sum of oxidation levels in the reactants (8) is the same as that in the products (8), and the overall process is neither an oxidation nor reduction. This transformation can be thought of as an internal redox process since part of the reactant (carbon) is oxidized and part (hydrogen) is reduced. Generally, such internal redox processes require only a catalyst, not an oxidant or reductant.

On the other hand, if the by-product of the conversion of methane to ethane is H+, then the balanced reaction is written as shown below and a net oxidation is required. An oxidizing agent is thus needed to effect this process. Again the recognition that the organic reactant (methane) and product (ethane) are both alkanes is not sufficient to determine that an oxidant is necessary.


The Grignard reaction is often one of the first reactions encountered for the preparation of organometallic compounds. As such it provides a method for the conversion of an alkyl bromide to an alkane. From the example shown below it is seen that the overall oxidation level change from the organic reactants to the products is from 0 to 2, so a reduction has occurred. Magnesium is the reductant and is itself oxidized from 0 to +2 oxidation state. The actual reduction takes place in the first step of the process in which the C–Br bond is converted to a C–Mg–Br bond. The reaction with water is merely a hydrolysis that does not change the oxidation state of carbon.


Reactions of olefins and acetylenes illustrate that the overall change in oxida-tion level of an organic functional group must be considered when deciding if a net oxidation level change has occurred in a chemical reaction. For example, addition of hydrogen across an acetylene gives a net reduction of each carbon and thus is a reductive process with respect to the alkyne. The same is true for the hydrogenation of an alkene. From the point of view of the alkyne and the alkene, the hydrogen can be considered a reducing agent since it undergoes oxidation during the process.


The conversion of an alkyne to a trans-alkene can be accomplished by heating with lithium aluminum hydride (LAH), by reaction with lithium in liquid ammo-nia (Li, NH3). Thus all of these reagents (H2/P-2 Ni, LAH, and Li, NH3) are reducing agents for alkynes and give alkenes as the reduced products.

In general, any reaction which results in the addition of two hydrogen atoms across a π bond of any type are reductions. Conversions of aldehydes and ketones to alcohols are reductions; thus any reagents which are capable of effecting that conversion must function as reducing agents. Thus NaBH4, LAH, and a large variety of other reagents reduce aldehydes and ketones to alcohols by the net addition of hydrogen across the C–O π bond. By the same logic, con-version of a primary alcohol to an aldehyde (the reverse process) must be an oxidation, and reagents which are capable of effecting this conversion, such as dimethyl sulfoxide (DMSO) and acetic anhydride (Swern oxidation) or pyri-dinium chlorochromate (PCC), are oxidants. Similar considerations hold for other π-bonded functional groups, including acid derivatives and nitriles.

Alkenes also undergo a variety of other addition reactions in which a reagent is added across the double bond. Hydration and hydrohalogenation are classic examples.


Consideration of the oxidation level reveals that while one carbon is reduced (the one to which hydrogen adds), the other is oxidized (the one to which the oxygen adds). There is no net change in oxidation level of the alkene functional group. Likewise the reverse processes of these addition reactions, namely, elimination of HX from alkyl halides and dehydration of alcohols to give alkenes, are not redox processes. Additions of water to alkynes is analogous. In this case, however, the product is a ketone, the oxidation level of the ketone is seen to be the same as the alkyne, and so no net change in oxidation level has occurred.


The conversion of alkenes to 1,2-diols by osmium tetroxide is also an olefin addition reaction. In this case a hydroxy group is added to each carbon of the olefin group, and the addition is termed an oxidative addition since the diol product is at a higher oxidation level than the alkene reactant. Oxidation of the carbon atoms of the alkene takes place in the first step, which is the reaction with OsO4 to produce the intermediate osmate ester.


Zinc serves to further reduce osmium and free the diol product. Similar oxidative additions to alkenes occur with bromine, chlorine, IN3, peracids, and many other electrophiles.


Peracids such as m-chloroperbenzoic acid (MCPBA) clearly illustrate the redox nature of oxidative addition. In this reaction the olefin is oxidized and the MCPBA is reduced to meta-chlorobenzoic acid, which precipitates slowly from solution.


Another common reaction process is one in which one atom or group replaces another atom or group. These are known as substitution reactions. When one electronegative group is substituted for another, no change in oxidation level occurs; thus the reagents which carry out such substitutions are neither oxidants nor reductants.


Such substitutions in saturated compounds can be carried out by a variety of strategies involving different nucleophiles and leaving groups, but the oxida-tion states remain the same. Acyl substitutions are analogous. For this reason carboxylic acid derivatives are treated as a common family of compounds. All have the same oxidation level and all can be converted from one to another by substitution reactions not requiring oxidation or reduction.


Many useful functional group transformations occur in more than one step, and it is not uncommon to find that different redox processes can be found in different steps of the process. However, from the methods of determining oxidation states it is clear that substitution of an electronegative group by a carbon group or a hydrogen atom is a reduction and requires a reducing agent. For example, conversion of an acid chloride to a ketone by a lithium organocuprate reagent involves a reduction of the acid chloride to the ketone oxidation level.


Consequently the copper is oxidized from a cuprate species to an organocop-per. By classifying organocuprates as reducing agents toward acid chlorides, we should expect that they could act as reducing agents toward other functional groups. It is not surprising therefore that as Michael addition reagents they can be used to give net reduction of an α, β -unsaturated ketone.


Reaction of the organocuprate intermediate with water gives the fully reduced product. If the organocuprate intermediate is reacted with bromine, the α-brominated product is formed. This product has the equivalent oxidation level as the starting enone but differs in that an additional carbon substituent is present. Functionally this is equivalent to the addition of HBr to an enone. Thus functionally no net redox has taken place. If individual steps are considered, it is clear that the first step (addition of the organocuprate to the enone) is a reduction and the second step (reaction of the cuprate with bromine) is an oxidation.


No net change in the oxidation level has occurred for the overall process; how-ever, each step in the sequence can involve an oxidation or reduction. This is an important idea to keep in mind—even though no net change in the oxidation level occurs, individual steps in the sequence may have an oxidation or reduction and thus would require oxidants or reductants consistent with the individual step being undertaken.

The realization that many reactions or steps in reactions involve an oxidation or reduction is an important consideration when these reactions are being studied and learned. The change in oxidation level produced is indicative of the transfor-mation and provides an additional organizational category by which reactions can be classified. Reagents can also be classified by their ability to cause oxidation or reduction. For example, from its addition reactions with alkenes and alkynes, bromine can be considered an oxidizing reagent for organic molecules. It is not surprising, therefore, to find that bromine also serves as an oxidant toward other functional groups such as enols, hydrocarbons, aldehydes, and organometallic compounds. Lithium aluminum hydride is well known as a reductant; thus, if it reacts with an organic compound, it is a good bet that some functional group is being reduced by the addition of a hydride. By applying the concepts of oxida-tion and reduction, a new view of organic reactions is possible which is often neglected but extremely important.

 

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