Oxidation and reduction in organic chemistry

The reactions of oxydoreduction (see also Oxidation and Reduction) are of an major importance in Organic chemistry. Nevertheless, the structure of the compounds makes the approach rather different from what one observes in inorganic Chimie or electrochemistry in particular because the chemical bonds in an organic structure are primarily covalent whereas the principles of oxydoreduction treat ionic compounds rather.

Determination of the number of oxidation

One can determine the state of oxidation of an atom by calculating his number of oxidation . The more this one is raised, the more the atom is oxidized .

the sum of all the numbers of oxidation of a molecule is equal to the total load of the molecule , i.e. zero in the majority of the cases in organic chemistry (neutral molecules). Except particular cases, hydrogen has a number of oxidation of +1, oxygen - 2, nitrogen - 3, the halogens - 1 and the number of oxidation of carbon is that which should be determined.

In the examples below, the numbers of oxidation used for calculation are in red and the result in green In the case of ethane, the 6 hydrogen atoms give a total of +6 what results in calculating a number of oxidation from - 3 for each carbon since 6× (+1) + 2× (- 3) = 0. In the case of ethylene, one obtains a number of oxidation from - 2 for two carbons and - 1 for acetylene. According to these calculations, a Alcène is thus an oxidized form of a Alcane and a Alcyne, an oxidized form of a Alcène. In the same way, one can classify in the order ascending of oxidation the alcohols (Méthanol below), the aldehydes (Formaldéhyde) and the carboxylic acid (Acid formic).

Is an epoxy an oxidized form of a Alcène? By comparing the ethylene and the Oxide of ethylene (below), the answer is yes. On the other hand, the formic Acid and the Chlorure of formyl have the same oxidation step.

Limiting: this method is not applicable when the carbon atoms are of very different nature unless regarding the molecule as an assembly of several independent fragments.

Hydrogenation/dehydrogenation

If one compares ethane and ethylene (above), one notes that an oxidation is formally a loss of molecule H2 a hydrogen ( dehydrogenation ), and the reduction is the opposite reaction ( hydrogenation ).

H3C-CH3 → H2C=CH2 + H2

Hydrogenation is a reaction very much used in organic chemistry. It requires the presence of a metal Catalyseur which will activate the Hydrogène gas by dissociating it. This reaction is called catalytic hydrogenation . The catalysts are mainly made up of palladium, of Platine, Nickel or of Rhodium and it can be soluble (homogeneous Catalyseur) or not (heterogeneous Catalyseur) in the reactional medium.

the hydrogénolyse is a particular case of catalytic hydrogenation. In this reaction, it is a simple connection which is hydrogenated what causes the cut (lysis) of the connection. For example (pH is a group Phényle):

R- O-CH2-Ph + H2 → R-OH + CH3-Ph

Examples

  • Reduction of a Alkyne compound in Olefinic hydrocarbon: Organic Syntheses, vol. 68, p.182 (1990).

  • Preparation of a Stilbène by dehydrogenation with DDQ: Organic Syntheses, vol. 49, p.53 (1969).

Oxidation and reduction by class of compounds

See too

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