SolutionsAcid-Base Properties
1. From pKb to pKa
The pKb of methylamine is 3.4. What is the pKa of the methylammonium ion? And that of the anilinium ion, if the pKb of aniline is 9.4?
pKa = 14 − pKb: 10.6 for CH3NH3+ and 4.6 for C6H5NH3+.
The more basic the amine, the less acidic its conjugate acid.
2. Order by basicity
Order from most to least basic in water: ammonia, methylamine, dimethylamine, trimethylamine and aniline.
(CH3)2NH (pKb 3.3) ≈ CH3NH2 (3.4) > (CH3)3N (4.2) > NH3 (4.8) > C6H5NH2 (9.4).
Alkyl groups donate electron density and increase basicity, but in water trimethylamine is less basic than expected because its conjugate acid, with a single N–H, is less well solvated. Aniline is much less basic because its lone pair is delocalized into the ring.
3. Which way does the equilibrium lie?
State which way each equilibrium lies:
- CH3NH2 + CH3COOH ⇌ CH3NH3+ + CH3COO− (pKa of acetic acid = 4.8)
- (iPr)2NH + CH3CH2CH2CH2Li ⇌ (iPr)2N− Li+ + CH3CH2CH2CH3
- C6H5NH3+ + HO− ⇌ C6H5NH2 + H2O
- To the right: acetic acid (pKa 4.8) is much stronger than the methylammonium ion (pKa 10.6); K ≈ 105.8.
- Completely to the right: the amine (pKa ≈ 35) is much more acidic than butane (pKa > 50). This is how lithium diisopropylamide (LDA), a very strong, poorly nucleophilic base, is prepared.
- To the right: anilinium (pKa 4.6) is a much stronger acid than water (15.7).
4. Acid-base extraction
An ether solution contains cyclohexylamine and cyclohexanol. How would you separate them?
Wash the solution with dilute aqueous HCl: the amine goes into the aqueous phase as cyclohexylammonium chloride and the alcohol stays in the ether. Then make the aqueous phase basic with NaOH; the free amine separates and is extracted with ether.