Three phsiologically relevant $\mathrm{p}K_\mathrm{a}$ values are associated with psilocybin: 1.3, 6.5 and 10.4. Suggest the appropriate charged species and mark the $\mathrm{p}K_\mathrm{a}$ values next to the corresponding protons in your structure.

pKa values

How come one of the hydroxides are more acidic than the other in the phosphate group? Is because of electron repulsion? Can the substituents on a phosphorus group not move around? Why are the two hydroxide groups on the phosphorus atom not the same functional group?


1 Answer 1


Because the rendering is not really accurate. It would have been better to render the two dissociation constants without trying to associate them with specific hydrogen atoms.

Given the neutral phosphate group with both hydroxyl hydrogens attached, either one has an equal probability of being removed as a proton, corresponding to the lower of the two $\mathrm{p}K_\mathrm{a}$ values. The drawing simply picked one of the protons at random. Then with that proton removed and the functional group now negatively charged, the second hydroxyl proton has become harder to remove and that is what the higher $\mathrm{p}K_\mathrm{a}$ represents.


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