Química Orgânica 1 · Adição a alcenos e regioquímica
Hydrate a ring alkene at the branch point
Condições: 1-Methylcyclopentene in dilute aqueous H₂SO₄. In water the proton donor is hydronium; its counterion stays outside the reacting system. Water is the nucleophile, and another water is the final base.
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Protonate the alkene
H bonds to the less substituted alkene carbon so that the positive charge sits on the carbon carrying the methyl group and two ring carbons: a tertiary cation. The other orientation would give a less stable secondary cation.
Nota: Any of hydronium’s three hydrogens can be transferred.
As setas curvas
- The alkene π pair bonds the less substituted carbon to H, leaving the empty orbital on the methyl-bearing carbon.
- The O–H pair returns to oxygen, leaving neutral water.
Water bonds to the cation
Water is a weak nucleophile, but it is the solvent and the cation is very reactive. The new C–O bond leaves oxygen with three bonds and a positive charge (an oxonium ion).
As setas curvas
- A lone pair on water bonds to the cationic carbon.
Deprotonate the oxonium ion
Proton transfer to water regenerates hydronium, so the acid is a catalyst: it is consumed in the protonation and returned here.
Nota: Either hydrogen on the positive oxygen can be removed.
As setas curvas
- A lone pair on water accepts a proton from the positive oxygen.
- The O–H pair stays on oxygen, which becomes a neutral hydroxyl oxygen.
1-Methylcyclopentan-1-ol and hydronium
OH bonds to the carbon bearing the methyl group, and the new H bonds to the neighboring alkene carbon: Markovnikov hydration. Hydronium is regenerated. All three steps are reversible; excess water drives hydration, while removing water with strong acid and heat reverses it (dehydration).
O que você vai praticar
Put the hydroxyl group of an acid-catalyzed ring hydration on the methyl-bearing carbon rather than the neighboring ring CH.
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