Química Orgânica 2 · Rotas de síntese: reagentes, grupos protetores e planejamento em várias etapas
Acid-catalysed hydration of pentanenitrile to pentanamide
Condições: Pentanenitrile (from 1-bromobutane and NaCN in DMSO), concentrated aqueous HCl (about 37%), about 40 °C, short reaction time; then diluted with ice water
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Hydronium protonates the nitrile nitrogen
The lone pair on the sp nitrogen takes a proton from hydronium, giving a nitrilium ion whose carbon is strongly electrophilic.
As setas curvas
- The nitrogen lone pair takes the proton.
- The O–H pair stays on oxygen, leaving water.
Why protonate first?
A neutral nitrile carbon is only weakly electrophilic; water is a weak nucleophile.
Water adds to the nitrilium carbon
A water lone pair forms a C–O bond to the nitrile carbon and one π pair of the C≡N⁺ bond moves onto nitrogen, which becomes neutral.
As setas curvas
- The water lone pair forms the new C–O bond.
- One π pair moves onto nitrogen.
Water removes a proton: the imidic acid
Water takes a proton from the oxonium oxygen, giving the neutral imidic acid, R–C(OH)=NH, and hydronium.
As setas curvas
- The water lone pair takes the proton.
- The O–H pair stays on oxygen.
Hydronium protonates the imine nitrogen
The imidic acid tautomerises to the amide. First the C=N nitrogen takes a proton from hydronium; the cation is stabilised by the lone pair on the OH oxygen.
As setas curvas
- The nitrogen lone pair takes the proton.
- The O–H pair stays on oxygen, leaving water.
Water removes the O–H proton: the amide
Water removes the proton from the hydroxyl oxygen. That O–H pair becomes the C=O π bond, and the C=N π pair moves onto nitrogen, giving the amide.
As setas curvas
- The water lone pair takes the O–H proton.
- The O–H pair forms the C=O π bond.
- The C=N π pair moves onto nitrogen as a lone pair.
Pentanamide
Water has added across the C≡N bond, giving pentanamide and regenerating hydronium. Under these mild conditions (about 40 °C, short reaction time) the amide survives and is isolated after dilution with ice water; heating under reflux in aqueous acid would hydrolyse it further to pentanoic acid and ammonium ion. Together with the cyanide SN2 on 1-bromobutane, this converts a four-carbon halide into a five-carbon amide (or, with prolonged heating, a five-carbon carboxylic acid).
O que você vai praticar
Draw how aqueous acid adds water across the C≡N bond of pentanenitrile, through the nitrilium ion and the imidic acid, to give pentanamide, and recognise that mild conditions stop at the amide while prolonged heating in aqueous acid continues to pentanoic acid.
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