Química Orgánica 1 · Reacciones radicalarias y selectividad
2-Bromo-3-methylbutane from 2-methylbut-2-ene with HBr/ROOR
Condiciones: HBr, ROOR (peroxide), heat or hν
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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 6 pasos escritos
Initiation: split the peroxide
Heat or light breaks the weak O–O bond of the peroxide (ROOR, drawn as di-tert-butyl peroxide, assumed) homolytically into two tert-butoxyl radicals. The O–O bond (about 150 kJ/mol) is the weakest bond present.
Las flechas curvas
- One electron of the O–O bond stays on the first atom.
- The other electron of the O–O bond stays on the second atom, so the bond splits evenly into two radicals.
Initiation: make the bromine radical
A tert-butoxyl radical removes the hydrogen from H–Br. The O–H bond formed is much stronger than the H–Br bond broken, so this step is fast and delivers Br•, the real chain carrier.
Las flechas curvas
- The unpaired electron on oxygen supplies one electron of the new bond to hydrogen.
- One electron of the H–Br bond completes the new O–H bond.
- The other H–Br electron stays on bromine, giving a bromine radical.
Propagation: Br• adds to the alkene
The bromine radical adds to the CH end of the trisubstituted alkene. The unpaired electron is left on the carbon carrying two methyl groups, a tertiary radical, the most stable choice.
Las flechas curvas
- The unpaired electron on bromine supplies one electron of the new bond to the less hindered alkene carbon.
- One π electron of the C=C completes that bond.
- The other π electron stays on the dimethyl-substituted carbon, which becomes the new radical centre.
Why anti-Markovnikov?
In the radical mechanism bromine adds first, to the carbon that gives the more stable carbon radical; hydrogen arrives last. So Br ends up on the less substituted carbon, the opposite of ionic HBr addition, where H+ adds first.
Propagation: the carbon radical takes H from HBr
The carbon radical abstracts the hydrogen atom from a second HBr, completing the anti-Markovnikov addition and regenerating Br• to add to the next alkene.
Las flechas curvas
- The unpaired electron on the dimethyl-substituted carbon supplies one electron of the new C–H bond.
- One electron of the weak Br–H bond completes the C–H bond.
- The other Br–H electron stays on Br, which carries the chain on.
2-Bromo-3-methylbutane, tert-butanol and radicals
Bromine is on the CH carbon and the new hydrogen on the tertiary carbon: 2-bromo-3-methylbutane, formed as a racemate.
Qué vas a practicar
Draw the peroxide-initiated radical addition of HBr to 2-methylbut-2-ene that places bromine on the CH carbon, giving 2-bromo-3-methylbutane.
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