Química Orgânica 1 · Adição a alcenos e regioquímica
Let a carbocation add to a second alkene
Condições: 2-Methylpropene (isobutylene) with a catalytic amount of concentrated H₂SO₄ and no added water, so the cation meets another alkene molecule instead of water. Two molecules of the same alkene are shown. Follow the dimerization branch to 2,4,4-trimethylpent-1-ene, removing a hydrogen from a methyl group next to the final carbocation. The HO–SO₂ part of sulfuric acid is drawn as one SO₃H group.
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 4 passos escritos
Protonate the first alkene
Protonating the CH₂ end gives the tertiary tert-butyl cation, as in any Markovnikov addition.
As setas curvas
- The first alkene π pair bonds its CH₂ end to H, leaving the empty orbital on the more substituted carbon.
- The O–H pair stays on oxygen, giving hydrogen sulfate.
A second alkene attacks the cation
With no water present, the most available nucleophile is another alkene. The cation is the electrophile, and it bonds to the CH₂ end for the same reason H⁺ did: the new positive charge then sits on a tertiary carbon. Bonding to the other alkene carbon instead would leave a primary cation.
As setas curvas
- The second alkene π pair bonds its CH₂ end to the cation, leaving the empty orbital on its more substituted carbon.
Remove a proton to form the alkene
Instead of adding a nucleophile, this cation loses a proton from a neighboring carbon (the same step that ends an E1 elimination). Hydrogen sulfate becomes sulfuric acid again, so the acid is a catalyst.
Nota: Remove the drawn methyl hydrogen next to the carbocation.
As setas curvas
- A hydrogen sulfate lone pair accepts the drawn hydrogen from a methyl group next to the carbocation.
- The C–H pair forms a new π bond to the cationic carbon, neutralizing it.
2,4,4-Trimethylpent-1-ene and sulfuric acid
Two C₄ alkenes joined head to tail: the CH₂ end of one bonded to the branched carbon of the other, and the new C=C is at the end of the chain. The product still has a C=C bond, so it can add another cation; repeating this addition is cationic polymerization of isobutylene.
O que você vai praticar
Apply the Markovnikov cation-stability rule to a carbon electrophile adding to an alkene, then end the sequence by loss of a proton.
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