Why picric acid is stronger than HBr in DMF but weaker than HBr in water?

1 Answer

Here's how it works.


Acidity is an equilibrium process. We measure acidity by determining the position of equilibrium of the ionization reaction

#"X-H" ⇌ "H"^+ + "X"^"-"#

In a protic solvent like water, the #"Br"^"-"# ion is stabilized by ion-dipole interactions. This pulls the position of equilibrium to the right.


In the picrate ion, the negative charge is delocalized over the ring and the nitro groups, so the attractions to polar water molecules are much reduced.

Thus, the #"pK_text(a)# values of #"HBr"# and of picric acid in water are -9 and 0.42, respectively.

Polar aprotic solvents like DMF preferentially solvate cations.

Thus, the #"Br"^"-"# ion is only slightly stabilized by interactions with DMF.

However, the large picrate anion, with its delocalized charge, is highly polarizable and is stabilized by ion-dipole interactions with the carbonyl group of DMF.

Thus, picric acid is a stronger acid than #"HBr"# is in DMF.