Química Orgânica 2 · Substituição acílica e escolha do derivado
Benzoyl chloride from benzoic acid with oxalyl chloride and DMF
Condições: (COCl)2, DMF (cat.)
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O mecanismo, passo a passo
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DMF oxygen attacks oxalyl chloride
The carbonyl oxygen of DMF (electron-rich through amide resonance) attacks one carbonyl of oxalyl chloride; the C=O π pair moves onto oxygen.
As setas curvas
- A DMF oxygen lone pair forms the new O–C bond.
- The C=O π electrons move onto oxygen.
Chloride leaves
The O⁻ re-forms C=O and expels chloride, giving an acyloxy-iminium-type cation.
As setas curvas
- An oxygen lone pair re-forms C=O.
- The C–Cl pair leaves as chloride.
Chloride adds to the DMF carbon
Chloride attacks the activated former DMF carbon; the C=O⁺ π pair moves onto oxygen.
As setas curvas
- A chloride lone pair forms the new C–Cl bond.
- The C=O⁺ π pair moves onto oxygen.
Nitrogen pushes out the oxalate fragment
The nitrogen lone pair forms C=N⁺ and expels the chlorooxalate anion, giving the chloroiminium (Vilsmeier) ion.
As setas curvas
- The nitrogen lone pair forms the C=N⁺ π bond.
- The C–O bond breaks; its electrons go to the carboxylate oxygen.
Chlorooxalate fragments to CO2, CO and chloride
In one concerted fragmentation the carboxylate oxygen forms a C=O, the C–C bond breaks, the second oxygen forms a third C–O bond and chloride leaves, releasing CO2 and CO. Loss of two gases makes the activation irreversible.
As setas curvas
- An oxygen lone pair forms the second C=O of carbon dioxide.
- The C–C bond electrons become the carbon lone pair of CO.
- An oxygen lone pair forms the third C–O bond of CO.
- The C–Cl pair leaves as chloride.
Benzoic acid adds to the Vilsmeier ion
The OH oxygen of benzoic acid attacks the iminium carbon; the C=N⁺ π pair moves onto nitrogen.
As setas curvas
- The acid OH oxygen lone pair forms the new O–C bond.
- The C=N⁺ π pair moves onto nitrogen.
Chloride removes the proton
Chloride takes the proton from the oxonium oxygen, releasing HCl gas.
As setas curvas
- A chloride lone pair bonds to the O–H proton.
- The O–H pair returns to oxygen.
Nitrogen expels chloride
The nitrogen lone pair re-forms C=N⁺ and pushes out chloride, giving an acyloxyiminium ion: the acid is now activated.
As setas curvas
- The nitrogen lone pair forms the C=N⁺ π bond.
- The C–Cl pair leaves as chloride.
Chloride substitutes at the acyl carbon, releasing DMF
Chloride attacks the activated benzoyl carbon; as the new C–Cl bond forms, the C–O bond to the iminium oxygen breaks and becomes the C=O π bond of DMF while the C=N⁺ π pair returns to nitrogen. DMF is an excellent neutral leaving group, so addition and loss are drawn together.
As setas curvas
- A chloride lone pair forms the new C–Cl bond.
- The C–O bond electrons become the C=O π bond of DMF.
- The C=N⁺ π pair returns to nitrogen.
Role of DMF
DMF is consumed to make the Vilsmeier ion and released in the last step.
Benzoyl chloride
Benzoyl chloride forms with DMF (regenerated), CO2, CO and HCl. All by-products are gases, which makes this the mildest common way to make acid chlorides.
O que você vai praticar
Make the Vilsmeier chloroiminium ion from DMF and oxalyl chloride, use it to activate benzoic acid, and let chloride give benzoyl chloride while DMF is regenerated.
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