Sometimes it can be necessary to cleave a double bond completely, in other words to oxidize not just its π bond (as you have seen with Br₂ and OsO₄) but its σ bond too, as shown in the margin. This can be done in two steps using OsO4 in conjunction with the reagent sodium periodate, NaIO₄. The diol product forms a periodate ester, which decomposes to give two molecules of aldehyde by a cyclic mechanism similar to that for the OsO₄ step. The NaIO₄ also reoxidizes the Os (VI) to Os (VIII) so only a catalytic amount of Os is required.

The process proceeds by two successive oxidations—first of the π, and then the σ bond— with different reagents (which can be added in one step or in two—you can use NaIO4 to cleave any diol, whether or not you made it using OsO₄). But there is another reagent that will achieve double oxidation in one step: ozone. Ozone is a symmetrical bent molecule with a central positively charged oxygen atom and two terminal oxygen atoms that share a negative charge. Ozone is unstable, and is generated immediately before use from oxygen (using a device called an ‘ozonizer’) and bubbled into the reaction mixture. Like OsO₄, it adds to alkenes by a cyclic mechanism: the product is a five membered ring with three oxygen atoms. It is extremely unstable and collapses by breaking a weak O–O bond and a C–C σ bond, but gains two strong C=O bonds in the process.

The immediate products are a simple aldehyde on the left and a new, rather unstable look ing molecule known as a carbonyl oxide on the right. But treatment of this mixture with a very mild reducing agent such as dimethyl sulfide, Me2S, or triphenylphosphine, Ph₃P, removes the ‘spare’ oxygen and reveals the two aldehydes. This cleavage of an alkene by ozone is an important reaction and is known as ozonolysis. Ozonolysis can be used to generate not only aldehydes, but also other functional groups. Completing the reaction with oxidizing agents such as H₂O₂ will give carboxylic acids, and more powerful reducing agents such as NaBH₄ will give alcohols. Here are the overall transformations:

Ozonolysis of cyclohexenes is particularly useful as it gives 1,6-dicarbonyl compounds that are otherwise difficult to make. In the simplest case we get hexane-1,6-dioic acid (adipic acid), a monomer for nylon manufacture.

مواضيع ذات صلة

The Mannich reaction

Base-promoted halogenation

Halogenation

Racemization

Thermodynamically stable enols: 1,3-dicarbonyl compounds

Kinetically stable enols

Summary of types of enol and enolate

The intermediate in the base-catalysed reaction is an enolate ion

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes