Química Orgânica 2 · Substituição acílica, aminas e sais de diazônio
Acid-catalysed lactonisation of 2-(hydroxymethyl)benzoic acid
Condições: 2-(Hydroxymethyl)benzoic acid, dilute aqueous HCl (catalytic H3O+), warm
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O mecanismo, passo a passo
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Hydronium protonates the carbonyl oxygen
Protonation of the carboxyl C=O makes its carbon electrophilic enough for the weakly nucleophilic alcohol oxygen to attack.
As setas curvas
- A carbonyl oxygen lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen, releasing water.
The CH2OH oxygen closes the ring
The alcohol oxygen of the neighbouring CH2OH group attacks the activated carbonyl carbon. Counting the carbonyl carbon, the two benzene carbons between the groups, the CH2 and the oxygen gives a five-membered ring.
As setas curvas
- The alcohol oxygen lone pair forms the ring O–C bond.
- The C=O π pair moves onto the protonated oxygen.
Water removes the ring O–H proton
The ring oxygen carries a positive charge; water removes its proton, giving a neutral cyclic tetrahedral intermediate.
As setas curvas
- A water lone pair takes the proton.
- The O–H pair stays on the ring oxygen.
Hydronium protonates an OH group
One of the two OH groups on the tetrahedral carbon is protonated, turning it into water, a good leaving group.
As setas curvas
- A hydroxyl lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen, releasing water.
Water leaves
The other OH lone pair re-forms the C=O bond as water departs, giving the protonated lactone.
As setas curvas
- An oxygen lone pair forms the new C=O π bond.
- The C–O bond breaks and water leaves with the pair.
Water removes the last proton
Water takes the proton from the protonated carbonyl oxygen, giving phthalide and regenerating hydronium.
As setas curvas
- A water lone pair takes the proton.
- The O–H pair becomes a lone pair on the carbonyl oxygen.
Why does the lactone win?
In water, a carboxylic acid and a separate alcohol give very little ester, because water is present in huge excess. This hydroxy acid nevertheless closes mainly to the lactone.
Phthalide
The carboxyl carbon and the CH2 oxygen are now joined in a five-membered cyclic ester fused to the benzene ring; water is released and hydronium is regenerated.
O que você vai praticar
Draw the acid-catalysed ring closure of 2-(hydroxymethyl)benzoic acid to the five-membered lactone phthalide, with hydronium and water as the only acid and base.
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