Amine Basicity

Amines are weak Brønsted bases. When protonated, they form ammonium ions (RNH₃⁺, R₂NH₂⁺, R₃NH⁺) with characteristic pKa values around 9-11. The specific basicity depends on electron availability at nitrogen, modulated by alkyl donation, aromatic delocalization, and steric effects.

Key pKa Values

Higher pKa of conjugate acid = stronger base (harder to remove the H⁺ from the ammonium = ammonium is less willing to lose H = amine is more willing to accept H = stronger base).

Alkyl Amines: More Basic Than Ammonia

Alkyl groups donate electron density to nitrogen (induction + hyperconjugation), making the lone pair more electron-rich and more available to accept a proton. Trend in gas phase: (CH₃)₃N > (CH₃)₂NH > CH₃NH₂ > NH₃.

In water, the trend is blurred by solvation effects: the ammonium cation is stabilized by H-bonding to water, and more H-N bonds on the ammonium mean more H-bond donors to water. This extra solvation stabilizes primary ammoniums more than tertiary. The net result in water: R₂NH (secondary) ≈ RNH₂ (primary) ≈ R₃N (tertiary) > NH₃. The differences are small (pKa varies by only ~1 unit).

Aryl Amines: Much Less Basic

Aniline (C₆H₅NH₂) has pKa 4.6 - about 4.6 units less basic than methylamine. Why? The nitrogen lone pair on aniline delocalizes into the aromatic ring via resonance. Three resonance contributors place partial negative charge on the ortho and para positions.

Consequence: the lone pair is partially tied up in the ring, less available to accept a proton. Protonation would require breaking the resonance - energetically costly.

Substituents on aniline modulate basicity:

• p-Methoxyaniline: methoxy donates into the ring; nitrogen lone pair is less “needed” by the ring; amine is more basic than aniline. pKa ~5.3.
• p-Nitroaniline: nitro withdraws; nitrogen lone pair is more delocalized; amine is much less basic. pKa ~1.
• p-Methylaniline (p-toluidine): methyl donor; slightly more basic. pKa ~5.1.

Amide Nitrogen: Essentially Not Basic

Amides (R-CO-NR’₂) have the nitrogen lone pair delocalized into the adjacent carbonyl (~40% double bond character). The lone pair is not available for protonation. Protonation would disrupt resonance and go against the dipolar contributor.

As a result, amides are NOT basic under physiological conditions. The pKa of a protonated amide is around 0 to -1 (depending on substrate), meaning the amide has to be in strong acid to protonate significantly.

This is why the peptide backbone nitrogens in proteins do NOT act as bases at pH 7.4, while the alpha-amino groups of unattached amino acids (CH(NH₂)(COOH)R) DO act as bases at that pH.

Basicity of Nitrogen Heterocycles

• Pyridine (C₅H₅N): pKa of conjugate acid ~5.2. The nitrogen lone pair is in the SP² orbital in the plane of the ring, NOT in the aromatic pi system. This lone pair is available for protonation. Pyridine is a modest base (less than alkyl amines but more basic than aniline).

• Pyrrole (C₄H₄NH): pKa of conjugate acid ~0.4. Here the nitrogen lone pair IS part of the aromatic pi system (contributes 2 of the 6 electrons for Huckel’s rule). Protonating this nitrogen would break aromaticity. Pyrrole is essentially not basic at the nitrogen. Protonation happens at the C2 or C5 ring carbon instead.

• Imidazole (C₃H₄N₂): pKa ~7.0. Two nitrogens, one like pyrrole (pi contributor) and one like pyridine (sp² lone pair available). The basic nitrogen is the pyridine-like one. Imidazole’s pKa ~7 is close to physiological pH, which is exactly why histidine’s imidazole side chain can act as both acid and base in enzyme active sites.