Química Orgânica 2 · Aminoácidos e conectividade dos peptídeos
L-Alanine methyl ester hydrochloride from alanine, SOCl2 and methanol
Condições: SOCl2, MeOH
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Protonate the amine
Thionyl chloride reacts with the methanol solvent to give HCl (assumed acid). The basic amine is protonated first; as the ammonium salt it cannot attack the activated carboxyl group, so no amide forms.
As setas curvas
- The amine lone pair takes the proton of HCl.
- The H–Cl pair stays on chlorine as chloride.
Protonate the carbonyl oxygen
A second HCl protonates the carbonyl oxygen, making the carbonyl carbon much more electrophilic toward the weak nucleophile methanol.
As setas curvas
- A carbonyl oxygen lone pair takes the proton.
- The H–Cl pair stays on chlorine.
Methanol adds
Methanol attacks the protonated carbonyl carbon and the C=O π pair moves onto the protonated oxygen, giving a tetrahedral intermediate.
As setas curvas
- A methanol oxygen lone pair attacks the carbonyl carbon.
- The C=O π pair moves onto oxygen, neutralizing it.
Methanol removes a proton
The solvent methanol removes the proton from the oxonium ion of the added methanol.
As setas curvas
- A methanol lone pair takes the O–H proton.
- The O–H pair stays on the methoxy oxygen.
Protonate an OH group
The protonated methanol passes the proton to one of the two OH groups, turning it into water, a good leaving group.
As setas curvas
- An OH lone pair takes the proton.
- The O–H pair stays on methanol oxygen.
Water leaves
The remaining OH lone pair forms a C=O+ π bond and pushes out water, giving the protonated ester.
As setas curvas
- An oxygen lone pair forms the C=O π bond.
- The C–O pair leaves with oxygen as water.
Deprotonate the ester
Methanol removes the proton from the carbonyl oxygen, giving the neutral methyl ester and regenerating the acid catalyst.
As setas curvas
- A methanol lone pair takes the proton.
- The O–H pair stays on the carbonyl oxygen.
Why the amine is protonated
The ammonium salt is the isolated product.
Alanine methyl ester hydrochloride
The product is L-alanine methyl ester as its hydrochloride, with water and HCl in methanol. The stereocentre was never touched, so its configuration is retained; the ester protects the carboxyl group for peptide coupling.
O que você vai praticar
Draw the HCl-catalysed Fischer esterification of alanine in methanol, from amine protonation to the methyl ester ammonium salt.
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