Química Orgánica 2 · Herramientas de oxidación y reducción
KMnO4 oxidation of propan-1-ol to propanoic acid
Condiciones: KMnO4, NaOH, H2O, Δ; then H3O⁺
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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 alcohol attacks manganese
The alcohol oxygen bonds to the electrophilic manganese(VII) of permanganate and an Mn=O π pair moves onto oxygen, giving a manganate ester with a positively charged oxygen.
Las flechas curvas
- An alcohol oxygen lone pair forms the O–Mn bond.
- An Mn=O π pair moves onto oxygen.
Hydroxide removes the O–H proton
Hydroxide takes the proton from the positively charged ester oxygen, giving the neutral manganate ester.
Las flechas curvas
- A hydroxide lone pair takes the proton.
- The O–H pair stays on the ester oxygen.
Elimination gives propanal
Hydroxide removes a hydrogen from the carbinol carbon; that C–H pair becomes the C=O π bond while the O–Mn pair moves onto manganese, reducing it.
Las flechas curvas
- A hydroxide lone pair takes the carbinol hydrogen.
- The C–H pair becomes the C=O π bond.
- The O–Mn pair moves onto manganese.
Hydroxide adds to propanal
In hot aqueous base the aldehyde does not survive: hydroxide adds to its carbonyl carbon and the π pair moves onto oxygen.
Las flechas curvas
- A hydroxide lone pair forms the C–O bond.
- The C=O π pair moves onto oxygen.
Water protonates: the aldehyde hydrate
Water protonates the alkoxide, giving the hydrate (a 1,1-diol), which has a C–H on an alcohol carbon and can be oxidised like an alcohol.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
The hydrate attacks a second permanganate
One hydrate OH bonds to manganese of another permanganate and an Mn=O π pair moves onto oxygen.
Las flechas curvas
- A hydrate oxygen lone pair forms the O–Mn bond.
- An Mn=O π pair moves onto oxygen.
Hydroxide removes the O–H proton
Hydroxide deprotonates the positively charged ester oxygen, giving a neutral manganate ester of the hydrate.
Las flechas curvas
- A hydroxide lone pair takes the proton.
- The O–H pair stays on the ester oxygen.
Elimination gives propanoic acid
Hydroxide removes the last hydrogen on the carbon; the C–H pair becomes a C=O π bond while the O–Mn pair moves onto manganese.
Las flechas curvas
- A hydroxide lone pair takes the hydrogen on carbon.
- The C–H pair becomes the C=O π bond.
- The O–Mn pair moves onto manganese.
Hydroxide deprotonates the acid
In the basic solution the carboxylic acid is deprotonated at once, so propanoate is what the flask contains before workup.
Las flechas curvas
- A hydroxide lone pair takes the acidic O–H proton.
- The O–H pair stays on oxygen as carboxylate.
Why does permanganate go past the aldehyde?
The aldehyde formed in water is in equilibrium with its hydrate.
Acidic workup protonates propanoate
Adding aqueous acid after the oxidation protonates propanoate, giving the neutral carboxylic acid.
Las flechas curvas
- A carboxylate lone pair takes the proton from hydronium.
- The O–H pair stays on water.
Propanoic acid
The CH2OH carbon has been oxidised twice: propan-1-ol has become propanoic acid, and permanganate has been reduced to lower-valent manganese (brown MnO2 after workup).
Qué vas a practicar
Draw the manganate-ester oxidation of propan-1-ol to propanal, hydration of propanal, a second manganate-ester oxidation of the hydrate to propanoic acid, its deprotonation in base, and protonation in the acidic workup.
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