Química Orgânica 1 · Estereoquímica das reações de alcenos
Cleave an alkene with ozone
Condições: 2-Methylbut-2-ene with O₃ in CH₂Cl₂ at −78 °C, then dimethyl sulfide, (CH₃)₂S. Ozone is drawn as its O⁻–O⁺=O contributor. Bond the negatively charged end oxygen to the dimethyl carbon and the double-bonded end oxygen to the other alkene carbon. In the fragmentation, follow the branch that leaves the carbonyl oxide at the dimethyl carbon. The dimethyl sulfide reduction is summarized, not drawn arrow by arrow.
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 5 passos escritos
Add ozone across the double bond
Ozone is a 1,3-dipole: its end oxygens can each form a bond at the same time. Three pairs move around one ring, a concerted [3+2] cycloaddition, giving the primary ozonide (molozonide), a five-membered ring with an O–O–O chain. Every atom in it is neutral.
As setas curvas
- A lone pair on the negatively charged end oxygen bonds to the dimethyl carbon.
- The alkene π pair bonds the hydrogen-bearing carbon to the double-bonded end oxygen of ozone.
- The ozone π pair moves onto its central oxygen, which becomes neutral.
Split the unstable molozonide
Two weak O–O bonds make the molozonide unstable even at −78 °C. It falls apart by a reverse [3+2] reaction: the C–C bond, the old alkene σ bond, finally breaks. One fragment is acetaldehyde; the other is a carbonyl oxide (Criegee intermediate), (CH₃)₂C=O⁺–O⁻. The carbonyl oxide goes preferentially to the more substituted carbon, which better supports its positive character. The fragments are redrawn turned toward each other for the next step.
As setas curvas
- The C–C σ pair forms a carbonyl π bond at the hydrogen-bearing carbon.
- The O–O bond pair next to that carbon moves onto the central oxygen of the three-oxygen chain, which becomes negative.
- A lone pair on the oxygen bonded to the dimethyl carbon forms a C=O π bond, making that oxygen positive.
Recombine the fragments
The carbonyl oxide is also a 1,3-dipole. It adds across the C=O of acetaldehyde in a second [3+2] cycloaddition with the opposite orientation, giving the ozonide (1,2,4-trioxolane). Now each carbon carries two oxygens and only one O–O bond remains.
As setas curvas
- A lone pair on the negatively charged oxygen bonds to the aldehyde carbon.
- The aldehyde π pair bonds its oxygen to the carbon of the carbonyl oxide.
- The carbonyl oxide π pair moves back onto its positively charged oxygen, which becomes neutral.
Predict the products of the workup
Dimethyl sulfide reduces the O–O bond of the ozonide and is oxidized to dimethyl sulfoxide (DMSO). Each carbon keeps one of its oxygens as a C=O bond.
Acetone, acetaldehyde and DMSO
The five-carbon alkene has been cut in two at the former C=C: the dimethyl end gives acetone and the hydrogen-bearing end gives acetaldehyde. The alkene hydrogen survives as the aldehyde C–H. One peroxide oxygen ends up in DMSO. This oxygen assignment follows one reasonable reduction pathway; the products are the same whichever oxygen goes to sulfur.
O que você vai praticar
Follow ozonolysis of 2-methylbut-2-ene through molozonide formation, retro-cycloaddition to a carbonyl oxide, and recombination to the ozonide, then identify the carbonyl products of reductive workup.
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