REDUCTION AND OXIDATION OF ALKENES
Alkenes can be converted to alkanes by reduction with hydrogen gas. A metal catalyst is necessary in order to lower the free energy of activation.
Treating an alkene with ozone then with zinc and water results in cleavage of the alkene across the double bond to give two carbonyl compounds (ketones and/or aldehydes). The reaction is known as an ozonolysis and is an example of an oxidation reaction.
Alkenes can be cleaved by oxidation with hot alkaline potassium perman-ganate. The products obtained are carboxylic acids and/or ketones depend-ing on the substituents present on the alkene.
Alkenes can be converted to 1,2-diols (or glycols) by reaction with osmium tetroxide or by reaction with cold alkaline potassium permanganate. In both cases, the two hydroxyl groups are added to the same face of the alkene – syn-hydroxylation. Osmium tetroxide gives better yields, but is more toxic.
Treatment of alkenes with a peroxyacid (RCO3H) results in the formation of an epoxide. The reaction is a one-step process without intermediates. Epoxides can also be obtained in a two-step process via a halohydrin.
Alkenes are converted to alkanes by treatment with hydrogen over a finely divided metal catalyst such as palladium, nickel, or platinum (Fig. 1). This is an addition reaction since it involves the addition of hydrogen atoms to each end of the double bond. It is also called a catalytic hydrogenation or a reduction reaction.
The catalyst is crucial since the reaction will not take place at room temperature in its absence. This is because hydrogenation has a high free energy of activation (∆G1*) (Fig. 2). The role of the catalyst is to bind the alkene and the hydrogen to a common surface such that they can react more easily.
This results in a much lower energy of activation (∆G2*) allowing the reaction to proceed under much milder conditions. The catalyst itself is unchanged after the reaction and can be used in small quantity.
Both the hydrogen and the alkene are bound to the catalyst surface before the hydrogen atoms are transferred, which means that both hydrogens are added to the same side of the double bond (see Fig. 3) – syn-addition. Note that the hydro-gen molecule is split once it has been added to the catalyst.
Treating an alkene with ozone (Fig. 4) results in oxidation of the alkene and formation of an initial ozonide which then rearranges to an isomeric ozonide. This second ozonide is unstable and potentially explosive and so it is not usually iso-lated. Instead, it is reduced with zinc and water resulting in the formation of two separate molecules.
The alkene is split across the double bond to give two carbonyl compounds. These will be ketones or aldehydes depending on the substituents present. For example, 3-methyl-2-pentene gives an aldehyde and a ketone (Fig. 5).
Alkenes can be oxidatively cleaved with hot permanganate solution to give carboxylic acids and/or ketones (Fig. 6). The products obtained depend on the substituents present on the alkene.
The reaction of alkenes with osmium tetroxide (OsO4) is another example of an oxidation reaction (Fig. 7). However, in this case the alkene is not split. Instead, a 1,2-diol is obtained – also known as a glycol. The reaction involves the formation of a cyclic intermediate where the osmium reagent is attached to one face of the alkene. On treatment with sodium bisulfite, the intermediate is cleaved such that the two oxygen atoms linking the osmium remain attached. This results in both alcohols being added to the same side of the double bond – syn-hydroxylation.
The same reaction can also be carried out using cold alkaline potassium per- manganate (KMnO4) followed by treatment with aqueous base (Fig. 8). It is impor- tant to keep the reaction cold since potassium permanganate can cleave the diol by further oxidation (Fig. 6).
The reaction works better with osmium tetroxide. However, this is a highly toxic and expensive reagent and has to be handled with care. Anti-hydroxylation of the double bond can be achieved by forming an epoxide, then carrying out an acid-catalyzed hydrolysis.
Treatment of an alkene with a peroxyacid (RCO3H) results in the formation of an epoxide (Fig. 9). m-Chloroperoxybenzoic acid is one of the most frequently used peroxyacids for this reaction. The reaction is unusual in that there is no carboca-tion intermediate, and involves a one-step process without intermediates.