Química Orgânica 1 · Reações de alcenos e alcinos: revisão para a prova
Ozonolysis of (E)-3-methylpent-2-ene
Condições: (E)-3-Methylpent-2-ene, O3, CH2Cl2, −78 °C; then (CH3)2S
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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 π bond
Ozone adds as a 1,3-dipole in one step: its negatively charged end oxygen bonds to one alkene carbon, the alkene π pair bonds the other carbon to the far end oxygen, and the O=O π pair moves onto the central oxygen.
As setas curvas
- A lone pair on the negative oxygen forms a C–O bond to the carbon carrying the methyl and ethyl groups.
- The alkene π pair forms the C–O bond at the CH carbon.
- The O=O π pair moves onto the central oxygen, removing its positive charge.
The primary ozonide falls apart
The unstable primary ozonide, with three oxygens in a row, splits in a retro-cycloaddition. The C–C bond and one O–O bond break, giving acetaldehyde and a carbonyl oxide on the more substituted fragment.
As setas curvas
- The C–C pair becomes the C=O π bond of acetaldehyde.
- The weak O–O bond breaks; its pair stays on the terminal oxygen, which becomes negative.
- A lone pair on the ring oxygen bonded to the methyl- and ethyl-bearing carbon forms the C=O⁺ π bond of the carbonyl oxide.
The fragments recombine to the ozonide
The carbonyl oxide adds back across the C=O of acetaldehyde with the opposite orientation. The resulting ozonide has only one weak O–O bond instead of two, so it is more stable than the primary ozonide.
As setas curvas
- The negative terminal oxygen bonds to the aldehyde carbon.
- The aldehyde C=O π pair bonds its oxygen to the carbonyl-oxide carbon.
- The C=O+ π pair moves onto oxygen, removing its positive charge.
What does the workup give?
Each former alkene carbon ends as a C=O carbon.
Ozonide
The ozonide holds both former alkene carbons, bridged by an O–O unit and a single ring oxygen. Dimethyl sulfide then reduces it to butan-2-one and acetaldehyde, forming dimethyl sulfoxide.
O que você vai praticar
Draw ozone cycloaddition to (E)-3-methylpent-2-ene, fragmentation of the primary ozonide into a carbonyl compound and a carbonyl oxide, and their recombination to the ozonide, then predict the carbonyl products after dimethyl sulfide workup.
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