I have a reaction between an amine and acetyl phosphate. The results can be that the amine can be phosphorylated or acetylated. NMR experiments suggest that at high pH what happens mostly is acetylation, whereas at low pH it seems that phosphorylation is predominant. This would imply that at pH 12 (when the phosphate is fully charged) the amine is acetylated prdominantly probably because the fully charged phosphate is an OK leaving group. Therefore, the mildly charged phosphate (when the reaction happens at pH 7) should be a worse leaving group (and acetate a better one) and that's why phosphorylation seems to be the likely outcome. Does this make sense to you chemists of the world? Thank you so much!
Dear Eloi Camprubí Casas,
You are probably right. At higher pH, phosphate group retards the nucleophillic attack of amine at the phosphorus center. It produces an amide through the acetylation reaction. While at lower pH, the phosphorus is prone for nucleophillic attack by amine molecule due to excessive protonation.
Sushil has a point that you need to consider: differences in leaving groups are only one of the factors you need to consider - there are also differences in electrophilicity.
Consideration of the relative leaving ability of PO4-3 and HPO4-2 is complicated by an alternative Sn1-like mechanism, via PO3-1, per the enclosed article.
All of the above factors are subject to change with pH, so whatever you observe is not as easily explained as we might wish - in fact, people still argue about the role of PO3-1.
What about the behaviour of the amine at these pH values? At high pH it should be more active, particularly if it is a primary or secondary amine, and at pH 7 less active, in equilibrium between its protonated and neural forms.
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