Ozonolysis of alkenes to ketones and aldehydes (reductive workup)
Description: Ozone will cleave carbon-carbon double bonds to give carbonyl compounds such as aldehydes or ketones, after treatment with a reducing agent such as zinc or dimethyl sulfide (“reductive workup”)
Notes: Acid here could be written as “H3O+” , “H+”, “acid”, etc. It’s not crucial.
Notes: Note that any alkene carbon that has a C–H bond is converted into an aldehyde. Any alkene carbon with two C–C bonds is converted into a ketone. Example 3 is the cleavage of a cyclic alkene to give a linear compound. In example 4, reduction with Me2S (DMS) is in no way different from reduction with Zn.
Mechanism: The first step of the reaction is a cycloaddition of ozone with the alkene (Step 1, arrows A, B, and C). The second step is a reverse cycloaddition, resulting in cleavage of the carbon-carbon single bond (Step 2, arrows D, E and F). The oxygen of the carbonyl oxide then performs a 1,2-addition on the other carbonyl (Step 3, arrows G and H) giving a negatively charged oxygen that performs a 1,2-addition on the carbonyl carbon of the carbonyl oxide to give the ozonide. (Step 4, arrows I and J).
For reductive workup the mechanism is generally not considered so important for the purposes of Org 1/ Org 2, but here it is! In the reduction step, zinc attacks one of the oxygens of the ozonide, breaking the weak O–O bond (Step 5, arrows K and L). The resulting anion then performs a 1,2-elimination (Step 6, arrows M and N) to give one of the fragments, followed by a second 1,2-elimination (Step 7, arrows O and P).