Química Orgânica 2 · Hidrólise e esterificação
Hydrate a nitrile under basic conditions
Condições: Benzonitrile in aqueous sodium hydroxide with heating. Follow the first stage, hydration to benzamide. Longer heating would hydrolyze the amide further to benzoate and ammonia; that stage is not requested. Two water molecules supply the protons; Na⁺ is a spectator.
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
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Hydroxide adds to the nitrile carbon
The nitrile carbon is electrophilic, like a carbonyl carbon, because nitrogen pulls π density toward itself. Only one of the two π bonds is used.
As setas curvas
- A hydroxide lone pair bonds to the nitrile carbon.
- One C≡N π pair moves onto nitrogen.
Protonate the nitrogen
The negatively charged nitrogen is a strong base, and water (pKa approximately 15.7) is the most plentiful proton source. The neutral product, an imidic acid, is a tautomer of an amide.
As setas curvas
- A nitrogen lone pair takes a proton from water.
- The O–H pair stays on the water oxygen, which becomes hydroxide.
Remove the oxygen proton
An imidic acid is a much less stable tautomer than an amide, so its O–H proton is considerably more acidic than the amide N–H (pKa approximately 17). Hydroxide removes it readily.
As setas curvas
- The hydroxide formed in the previous step takes the O–H proton from the imidic acid.
- The O–H pair stays on the imidic acid oxygen.
Place the charge on nitrogen
The anion is delocalized over oxygen, the carbon bonded to it and nitrogen. The nitrogen-centered contributor shows where the next proton goes.
As setas curvas
- A lone pair on the negatively charged oxygen forms a C=O π bond.
- The C=N π pair becomes a lone pair on nitrogen.
Protonate nitrogen again
Water (pKa approximately 15.7) is a slightly stronger acid than an amide N–H (pKa approximately 17), so protonation of the amide anion is favorable and hydroxide is returned.
Nota: A second water molecule now supplies a proton. Its two hydrogens are equivalent.
As setas curvas
- A nitrogen lone pair takes a proton from water.
- The O–H pair stays on the water oxygen, regenerating hydroxide.
Primary amide and regenerated hydroxide
The oxygen from hydroxide became the amide oxygen, and nitrogen gained two hydrogens from water. The net change is addition of one H₂O across C≡N to give benzamide. Hydroxide is consumed and regenerated in this stage; it is used up only if the amide is hydrolyzed further to benzoate.
O que você vai praticar
Follow hydroxide addition to a nitrile through the imidic acid and amide anion to the primary amide, including each proton transfer.
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