Equilibria with positive Hammett ρ values
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
1044
2025-08-04
498
Equilibria with positive Hammett ρ values
To take a simple example, let’s just see what happens to ρ if we simply move the carboxylic acid away from the ring. The ρ value for ionization gets less. This is just what you would expect—the further it is from the aromatic ring, the less the acid cares about how electron rich or poor the ring is. With two saturated carbons between the benzene ring and the carboxylic acid, there is almost no effect on pKa. But restore electronic communications with a double bond, and ρ goes back up again.
If the negative charge on the anion can actually be delocalized round the ring, as it can in substituted phenols, we should expect the size of ρ to increase. Both the phenol and the anion are delocalized but delocalization is more important for the anion. The effect is even more significant for the ionization of anilinium salts as the acid ArNH3+ does not have a delocalized lone pair but the conjugate base (ArNH2) does.
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