Química Orgánica 2 · Enoles, enolatos y reacciones en alfa
Hell–Volhard–Zelinsky bromination of propanoic acid to 2-bromopropanoic acid
Condiciones: Br2, PBr3 (cat.); then H2O
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El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
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The acid OH attacks PBr3
Catalytic PBr3 first converts a little propanoic acid into propanoyl bromide. The O–H oxygen lone pair attacks phosphorus and bromide leaves.
Las flechas curvas
- An O–H lone pair bonds to phosphorus.
- A P–Br pair leaves as bromide.
Bromide adds to the carbonyl
Bromide attacks the carbonyl carbon, now activated by the positive oxygen next to it.
Las flechas curvas
- Bromide forms a bond to the carbonyl carbon.
- The C=O π pair moves onto oxygen.
Collapse to propanoyl bromide
The oxyanion re-forms C=O and expels HOPBr2, an excellent leaving group, giving the acyl bromide.
Las flechas curvas
- An oxygen lone pair re-forms C=O.
- The C–O pair leaves with HOPBr2.
HBr protonates the acyl bromide
The HBr formed in the reaction (assumed present) protonates the carbonyl oxygen. Acyl bromides enolize far more readily than the acid itself, which is why PBr3 is needed.
Las flechas curvas
- The carbonyl oxygen takes the HBr proton.
- The H–Br pair stays on bromide.
Form the enol
Bromide removes the α-hydrogen; the C–H pair forms C=C and the C=O⁺ π pair returns to oxygen, giving the enol of propanoyl bromide.
Las flechas curvas
- Bromide takes the α-proton.
- The C–H pair forms the enol C=C.
- The π pair returns to oxygen.
The enol attacks Br2
The enol C=C bonds to bromine with a push from the O–H lone pair, and bromide leaves: bromination happens on the α-carbon.
Las flechas curvas
- An oxygen lone pair forms C=O⁺.
- The C=C π pair forms the new C–Br bond.
- The Br–Br pair leaves as bromide.
Deprotonate the carbonyl
Bromide removes the O–H proton, giving neutral 2-bromopropanoyl bromide and regenerating HBr.
Las flechas curvas
- Bromide takes the O–H proton.
- The O–H pair stays on oxygen.
HBr protonates the acyl bromide again
In the aqueous workup the acyl bromide is hydrolysed. HBr, the strongest acid present, first protonates its carbonyl oxygen so that water can add.
Las flechas curvas
- The carbonyl oxygen takes the HBr proton.
- The H–Br pair stays on bromide.
Water adds to the carbonyl
Water attacks the activated carbonyl carbon; the C=O⁺ π pair moves onto oxygen, giving a neutral O–H.
Las flechas curvas
- A water lone pair bonds to the carbonyl carbon.
- The π pair moves onto the protonated oxygen.
Water removes the oxonium proton
A second water molecule takes the proton from the oxygen that just added.
Las flechas curvas
- A water lone pair takes the proton.
- The O–H pair stays on oxygen.
Lose bromide
An O–H lone pair forms C=O⁺ and pushes bromide out of the acyl carbon.
Las flechas curvas
- An oxygen lone pair forms the C=O⁺ bond.
- The C–Br pair leaves as bromide.
Water removes the last proton
Water deprotonates the protonated carbonyl, giving 2-bromopropanoic acid.
Las flechas curvas
- A water lone pair takes the proton.
- The O–H pair stays on the carbonyl oxygen.
Why PBr3?
Carboxylic acids barely enolize; acyl bromides enolize readily.
2-Bromopropanoic acid
Bromine is now on the α-carbon: 2-bromopropanoic acid (racemic, from the planar enol). HBr and HOPBr2 are by-products.
Qué vas a practicar
Draw conversion of propanoic acid to propanoyl bromide with PBr3, enolization and α-bromination of the acyl bromide, and hydrolysis to 2-bromopropanoic acid.
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