Ozonolysis of alkenes

pharmaceutical organic chemistry

1. ozonolysis of Alkenes

2. ozonolysis is a reaction in which the products are more highly oxidized. In this reaction the carbon–carbon double bond is cleaved to produce carbonyl compounds. Alkenes react rapidly with ozone (O3 ) even at –78 °C . Ozone is produced in the laboratory by a device called an ozonator , which forms ozone by passing oxygen gas through an arc discharge. As the ozone forms, oxygen gas containing a few percent ozone is passed through an inert solvent, such as dichloromethane, that contains the alkene. After the reaction is complete, the solution is worked up under reductive conditions such as zinc in acetic acid.

3. Mechanism of Ozonolysis Ozonolysis occurs in several steps. First, an unstable intermediate, called a molozonide , forms by a cyclic concerted addition of the terminal oxygen atoms of ozone to the π bond of the alkene. This step requires a total of three electron pair shifts, as shown below.

4. In subsequent steps, the molozonide rapidly rearranges when the π bond of the alkene and an O-O eroxide bond break. The fragments then recombine to give an ozonide . The individual fragments are reoriented to illustrate the addition reaction in the second step

5. The molozonide has two weak peroxide bonds, but the ozonide has only one. This difference accounts for the direction of the reaction. The rearrangement of the molozonide is exothermic . How ever , ozonides are explosively unstable compounds. For that reason, the reaction mixture is maintained at low temperatures and immediately reduced or oxidized after the reaction is complete.

6. REDUCTIVE WORKUP The ozonolysis reaction mixture is treated with reducing agents such as zinc metal and aqueous acetic acid or dimethyl sulfide. When zinc is used, the by-product is zinc oxide. When dimethyl sulfide is the reducing agent, dimethyl sulfoxide , (CH3)2 SO, is a by-product. In each case, the reducing agent removes one of the oxygen atoms of the ozonide . The other two oxygen atoms of the ozonide are found as carbonyl oxygen atoms in the product as aldehydes or ketones.

7. If the ozonolysis reaction mixture is treated with an oxidizing agent such as hydrogen peroxide, carboxylic acids rather than aldehydes are produced. For example, the ozonolysis of cyclooctene with an oxidative workup yields octanedioic acid ( suberic acid). This compound is one of several dicarboxylic acids excreted in large amounts by persons suffering from diabetes. The choice of reducing or oxidizing workup depends on the goal of the synthesis, which may be to prepare aldehydes (or ketones) or carboxylic acids.