Química Orgánica 1 · Reacciones de alquenos y alquinos: repaso para el examen
Hydroboration–oxidation of 2-methylbut-1-ene
Condiciones: BH3·THF; then H2O2, NaOH, H2O
Dibuja este problema tú mismo
Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 8 pasos escritos
Concerted syn hydroboration
In one four-centre step the π electrons bond one alkene carbon to the empty orbital of boron while a B–H hydrogen moves to the other carbon on the same face. No carbocation forms, so nothing can rearrange.
Las flechas curvas
- The π pair forms the C–B bond to boron's empty p orbital.
- The B–H pair delivers the hydrogen to the other alkene carbon from the same face.
Which carbon gets boron?
Sterics and the partial charge in the transition state both decide.
Hydroperoxide adds to boron
In the oxidation, NaOH deprotonates H2O2; the hydroperoxide anion donates a lone pair into the empty p orbital of the trivalent boron.
Las flechas curvas
- The negative oxygen forms the B–O bond.
The alkyl group shifts from boron to oxygen
The carbon on boron migrates with its bonding pair to the neighbouring oxygen while the weak O–O bond breaks and hydroxide leaves. The migrating carbon keeps its configuration.
Las flechas curvas
- The C–B pair moves with the carbon onto the adjacent oxygen.
- The O–O bond pair leaves with hydroxide.
Hydroxide adds to boron
Hydroxide adds to the boron of the alkoxyborane, preparing the B–O bond to the alkyl oxygen to break.
Las flechas curvas
- A hydroxide lone pair forms a new B–O bond.
The alkoxide is released
The B–O bond to the alkyl oxygen breaks with the pair staying on that oxygen, releasing the alkoxide from boron.
Las flechas curvas
- The B–O pair stays on the alkyl oxygen as an alkoxide.
Water protonates the alkoxide
Water gives a proton to the strongly basic alkoxide, giving the neutral alcohol and hydroxide.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
2-methylbutan-1-ol
OH is on the former CH2 carbon and H on the more substituted carbon: anti-Markovnikov hydration. The carbon bearing the methyl group is a new stereocentre, formed as a racemate because borane adds to both alkene faces equally.
Qué vas a practicar
Draw concerted syn hydroboration of 2-methylbut-1-ene with boron on the CH2 carbon, then the oxidation steps that replace boron by OH, giving the anti-Markovnikov alcohol 2-methylbutan-1-ol.
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