SolutionsAcid-Base Properties
1. Why more acidic?
Why is phenol (pKa ≈ 10) much more acidic than cyclohexanol (pKa ≈ 18)?
Because the negative charge of the phenoxide ion is delocalized by resonance onto the ortho and para positions of the ring, which stabilizes it. In cyclohexoxide the charge stays localized on the oxygen.
2. Substituents
Order by acidity: phenol, 4-nitrophenol and 4-methylphenol.
4-Nitrophenol (pKa 7.2) > phenol (10.0) > 4-methylphenol (10.3). The electron-withdrawing nitro group delocalizes the phenoxide charge even more; the electron-donating methyl destabilizes it somewhat.
3. Separation
An ether solution contains benzoic acid, phenol and cyclohexanol. In which order and with which reagents would you separate them?
First extract with aqueous NaHCO3: sodium benzoate goes into the water. Then with aqueous NaOH: sodium phenoxide goes into the water. Cyclohexanol remains in the ether. The acid and the phenol are recovered by acidifying each aqueous phase with HCl.