Which Is More Acidic? Study guide
The more acidic compound is the one with the better-stabilized conjugate base: compare resonance, electronegativity and induction, hybridization, and aromaticity, then check the measured pKa. The study guide covers the full toolkit.
Pick the more acidic compound of each pair below (DMSO pKa data from Reich and Bordwell).
Molecule A
Molecule B
Which is more acidic?
Common Questions
What is pKa?
pKa is the negative log of the acid dissociation constant (Ka). A lower pKa means a stronger acid that more readily donates a proton. Each unit decrease in pKa represents a 10-fold increase in acidity.
How do you compare acid strength using pKa?
The compound with the lower pKa is the stronger acid. Compare pKa values directly: an acid with pKa 4 is 1,000 times stronger than one with pKa 7.
What factors affect pKa?
Electronegativity of the atom bearing the charge, resonance stabilization of the conjugate base, inductive effects from nearby electron-withdrawing or donating groups, hybridization of the orbital (more s-character = more acidic), and aromaticity of the conjugate base.
What is DMSO and why measure pKa in it?
DMSO (dimethyl sulfoxide) is an aprotic solvent that does not hydrogen-bond to anions. Measuring pKa in DMSO reveals intrinsic acidity differences that can be masked in water by strong solvation of small anions.