Química Orgânica 1 · Estereoquímica das reações de alcenos
Acetone and formaldehyde from 2-methylpropene
Condições: 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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Add ozone across the double bond
Ozone is a 1,3-dipole. In one concerted [3+2] cycloaddition its two end oxygens bond to the two alkene carbons, giving the primary ozonide (molozonide) with an O–O–O chain.
As setas curvas
- A lone pair on the negative end oxygen bonds to the dimethyl-substituted alkene carbon.
- The alkene π pair bonds the other alkene carbon to the double-bonded end oxygen.
- The ozone π pair moves onto the central oxygen, which becomes neutral.
Split the molozonide
Two weak O–O bonds make the molozonide unstable even at −78 °C. It falls apart in a reverse [3+2]: the old C–C bond breaks, giving a carbonyl group and a carbonyl oxide (Criegee intermediate).
As setas curvas
- The C–C σ pair becomes a C=O π bond at the carbon bonded to the end oxygen.
- The neighbouring O–O pair moves onto the central oxygen, which becomes negative.
- A lone pair on the other end oxygen forms a C=O π bond, making that oxygen positive.
Recombine to the ozonide
The carbonyl oxide is again a 1,3-dipole and adds across the C=O in a second [3+2] with the opposite orientation, giving the ozonide (1,2,4-trioxolane) with only one O–O bond.
As setas curvas
- The negative oxygen bonds to the carbonyl carbon.
- The C=O π pair bonds that oxygen to the carbonyl-oxide carbon.
- The C=O⁺ π pair returns to its oxygen, which becomes neutral.
Dimethyl sulfide attacks the peroxide
In the reductive workup, dimethyl sulfide attacks one peroxide oxygen of the ozonide and breaks the weak O–O bond.
As setas curvas
- A sulfur lone pair bonds to a peroxide oxygen.
- The O–O pair moves onto the other oxygen, which becomes an alkoxide.
Release the first carbonyl
The alkoxide lone pair forms a C=O bond and pushes out the ring oxygen, cleaving one C–O bond.
As setas curvas
- The alkoxide lone pair forms the C=O π bond.
- The C–O bond to the former ring oxygen breaks; that oxygen becomes negative.
Release the second carbonyl and DMSO
The new alkoxide forms the second C=O bond and expels the sulfur-bound oxygen, which leaves as dimethyl sulfoxide.
As setas curvas
- The alkoxide lone pair forms the second C=O π bond.
- The C–O pair to the sulfonium oxygen leaves, giving DMSO.
Reductive or oxidative workup?
The workup decides the oxidation level of the carbonyl products.
Acetone and formaldehyde
The C=C has been replaced by two C=O bonds. Dimethyl sulfide is a reducing agent and takes one oxygen as DMSO, so aldehyde carbons stay at the aldehyde level instead of being oxidised to acids.
O que você vai praticar
Follow ozonolysis of 2-methylpropene through molozonide, carbonyl oxide and ozonide, then the dimethyl sulfide workup that releases acetone and formaldehyde.
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