Why is acetic anhydride used in aspirin synthesis?

Perché acetil acido salicilico, aka Aspirin, has an acetato group, and acetic anhydride has one that is a good leaving group.

The typical lab-ready reaction for this synthesis is:

Basically, the acid acts as a catalizzatore (non protonating water though, because acetic anhydride is violently reactive with water), and the hydroxyl on Acido 2-idrossibenzoico agisce come a nucleofilo. It doesn't work too quickly though, so the heat is needed to speed it up.

The acetic anhydride essentially ends up transferring its acetyl group to 2-hydroxybenzoic acid.

The meccanismo è il seguente:

You can also tell from the full procedure (and from the name "anhydride", resembling "anhydrous", meaning "lacking water...") that the water easily cleaves acetic anhydride (into acetic acid).

Come risultato, it's easy to get rid of all the acetic anhydride when you're done with it. It makes cleaning up easier, because although acetic anhydride is actually quite hazardous, diluire acetic acid is pretty innocuo, and it's generally easy to find DI water nearby to further dilute the acetic acid if necessary. Ultimately, relatively green chemistry.

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