SolutionsAcid-Base Properties
1. Substituent effects
Arrange in order of decreasing acidity: propanoic acid, acetic acid, chloroacetic acid, trichloroacetic acid and fluoroacetic acid.
Trichloroacetic (0.64) > fluoroacetic (2.59) > chloroacetic (2.86) > acetic (4.74) > propanoic (4.88).
Electronegative groups stabilize the carboxylate and increase acidity; electron-releasing alkyl groups decrease it. Several halogens add up their effects.
2. Distance matters
Which acid is stronger, 2-chlorobutanoic or 4-chlorobutanoic acid? Why?
2-Chlorobutanoic acid (pKa 2.84 versus 4.52). The inductive effect of chlorine is transmitted through the bonds and fades quickly with distance: at the α position it strongly stabilizes the carboxylate; three carbons further away, it hardly does.
3. An acid-base extraction
You have an ether solution of benzoic acid and naphthalene. How would you separate them by taking advantage of the acidity of the acid?
- Shake the ether solution with aqueous NaHCO3 (or dilute NaOH): the acid becomes sodium benzoate, ionic and water-soluble.
- Neutral naphthalene stays in the organic phase; separate the two layers.
- Acidify the aqueous layer with HCl: benzoic acid precipitates and is filtered off.