Oxidation Levels

OXIDATION LEVELS

Besides bonding patterns, functional groups also vary with respect to the oxidation states of carbon in those functional groups. Thus another way to classify functional groups is by the carbon oxidation level. Correspondingly, organic reactions can be categorized as to whether an oxidation, a reduction, or no change in oxidation level has occurred in the organic reactants in going from reactants to products. This is a very useful distinction since the reagents used in a given transformation must be compatible with the oxidation change that occurs in the reaction. It is important to remember this fundamental truth—that no oxidation can occur without a corresponding reduction and no reduction can occur without a corresponding oxidation. As a consequence, if a transformation of an organic compound involves a change in its oxidation level, then the reagents necessary to cause that change must be able to undergo the complementary change in oxidation level. To have an oxidation, an oxidizing agent is required which itself gets reduced in the process. Similarly, to carry out the reduction of an organic compound, a reducing agent is needed which itself gets oxidized. Reagents can thus be categorized on the basis of their oxidizing or reducing properties. If no change in oxidation state occurs during a chemical reaction, then reagents used to effect the transformation should themselves undergo no oxidation or reduction. Moreover, if a reagent is not normally an oxidizing agent, then it is not easily reduced and cannot be used to oxidize something else. Conversely, if a reagent is not normally a reducing agent, then it is not easily oxidized and cannot be used to reduce something else.

Oxidation is defined as the loss of electrons. This concept is very straight-forward when dealing with metal ions. Thus the change Mg⁰ → Mg²⁺ is an oxidation because magnesium has lost two electrons in going from the element to the positive ion. Similarly, oxidation of Cu¹⁺ → Cu²⁺ involves loss of an electron from Cu¹⁺ to give the Cu²⁺ species.

Reduction is defined as the gain of electrons. The conversion of Ag¹⁺ → Ag⁰ involves a gain of an electron by the silver ion and thus silver is reduced. Like-wise, the permanganate ion MnO₄⁻ has Mn[VII] but MnO₂ manganese dioxide has Mn[IV]. Thus the gain of three electrons by manganese causes a reduction in oxidation level of from +7 to +4.

Because organic compounds have an overwhelming preponderance of covalent bonds, changes in oxidation state of carbon are not as easily determined by inspection as they are for metal ions. While the definitions of oxidation and reduction for organic compounds are the same as for metal ions (i.e., gain or loss of electrons), the oxidation state of a carbon atom is determined by the types of covalent bonds originating from it. A set of rules has been developed to assign numerical values for the contributions of atoms covalently bonded to carbon to the oxidation state of that carbon. Summation of the contributions of its covalently bonded substituents gives the oxidation state of a particular carbon in a molecule. Furthermore the oxidation levels of various carbons can be compared just as +2 or +3 oxidation states in metal ions can be compared.

These rules are simple and are summarized as follows:

1. Bonds to hydrogen or other elements more electropositive than carbon contribute −1 to the oxidation level.

2. Bonds to other carbon atoms contribute 0 to the oxidation level.

3. Bonds to oxygen or other elements more electronegative than carbon con-tribute +1 to the oxidation level.

4. Multiple bonds to an element count as multiple single bonds to that ele-ment. That is, the carbon–oxygen double bond of carbonyl group (C=O) is oxidatively equivalent to a carbon atom with two single bonds to oxygen originating from it (–O–C–O–).

5. A pair of electrons on carbon contribute −1 to the oxidation level.

6. A positive charge on carbon contributes +1 to the oxidation level.

Given these contributions, the oxidation level of a given carbon can be determined by adding together the contributions of the four attached bonds.