Química Orgânica 1 · Reações de alcenos e alcinos: revisão para a prova
Ozonolysis of 2-methylhex-2-ene to acetone and butanal
Condições: O3, CH2Cl2, −78 °C; then Me2S
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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.
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Ozone adds across the alkene
Ozone is a 1,3-dipole. In one concerted step its two end oxygens bond to the two alkene carbons, giving the five-membered molozonide (a 1,2,3-trioxolane).
As setas curvas
- A lone pair on the negative end oxygen bonds to C3 of the alkene.
- The π pair bonds C2 to the other end oxygen.
- The O=O π pair moves onto the central oxygen, which becomes neutral.
The molozonide falls apart
The molozonide is unstable because of its O–O–O chain. The C–C bond and one O–O bond break, giving butanal and a carbonyl oxide on the former C2.
As setas curvas
- The C–C bond pair becomes the aldehyde C=O π bond.
- The weak O–O bond breaks onto the middle oxygen.
- An oxygen lone pair forms the C=O⁺ π bond of the carbonyl oxide.
What forms after cleavage?
Each former alkene carbon becomes a C=O.
The fragments recombine to the ozonide
The carbonyl oxide and butanal add together again in the opposite orientation, giving the more stable ozonide (a 1,2,4-trioxolane) with only one O–O bond.
As setas curvas
- The negative oxygen bonds to the aldehyde carbon.
- The C=O π pair bonds the aldehyde oxygen to the carbonyl-oxide carbon.
- The C=O⁺ π pair moves onto oxygen, which becomes neutral.
Dimethyl sulfide attacks the peroxide oxygen
The workup reagent dimethyl sulfide uses a sulfur lone pair to attack one oxygen of the weak O–O bond; the other oxygen leaves as an alkoxide.
As setas curvas
- A sulfur lone pair forms the S–O bond.
- The O–O pair moves onto the other oxygen.
Collapse to two carbonyls and DMSO
The alkoxide re-forms a C=O, pushing out the ring oxygen, which in turn forms the aldehyde C=O and expels dimethyl sulfoxide.
As setas curvas
- The alkoxide lone pair forms the ketone C=O.
- The C–O ring bond pair becomes the aldehyde C=O π bond.
- The C–O bond to the sulfur-bound oxygen breaks, releasing DMSO.
Acetone and butanal
Each alkene carbon now carries a C=O: the disubstituted end gives acetone and the CH end gives butanal. Dimethyl sulfide is oxidized to DMSO, so no hydrogen peroxide forms to oxidize the aldehyde.
O que você vai praticar
Draw the Criegee mechanism: cycloaddition of ozone to 2-methylhex-2-ene, fragmentation of the molozonide, recombination to the ozonide, and reduction by dimethyl sulfide to acetone and butanal.
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