24.3 Basicity of Amines

Basicity of Amines

Basicity measurement with pKa values

Amines act as bases by accepting a proton ($H^+$) to form ammonium ions. To compare how basic different amines are, you look at the $pK_a$ of the conjugate acid (the ammonium ion that forms after protonation).

The relevant equilibrium in water is:

$RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-$

The $pK_a$ is defined as:

$pK_a = -\log(K_a)$

A higher $pK_a$ means the conjugate acid is weaker, which means it holds onto its proton more tightly. That tells you the original amine is a stronger base because it's better at grabbing and keeping that proton.

What controls the $pK_a$? It comes down to how well the conjugate acid is stabilized:

• Electron-donating groups increase electron density on nitrogen, making the amine more willing to accept a proton. This raises the $pK_a$ (stronger base).
• Electron-withdrawing groups pull electron density away from nitrogen, making the amine less willing to accept a proton. This lowers the $pK_a$ (weaker base).

Basicity comparison of amine types

Alkylamines vs. ammonia: Alkylamines like methylamine ($pK_a \approx 10.6$) and ethylamine ($pK_a \approx 10.7$) are more basic than ammonia ($pK_a = 9.25$). Alkyl groups are electron-donating through induction, which increases electron density on nitrogen and stabilizes the positive charge of the conjugate acid.

The general trend: $3° > 2° > 1° > NH_3$ in terms of inductive stabilization, though steric effects can complicate this in practice (more on that below).

Arylamines: Aniline ($pK_a \approx 4.6$) is dramatically less basic than alkylamines. The nitrogen lone pair delocalizes into the aromatic ring through resonance, which makes it less available for protonation. This is the dominant effect, not simply that the ring is "electron-withdrawing."

Substituents on the aromatic ring shift basicity further:

• Electron-donating groups ($-OCH_3$, $-NH_2$) at the ortho or para position increase basicity by pushing electron density toward nitrogen
• Electron-withdrawing groups ($-NO_2$, $-CN$) at the ortho or para position decrease basicity by pulling electron density away

Heterocyclic amines: These vary widely depending on structure.

• Pyridine ($pK_a \approx 5.2$) is less basic than alkylamines because the nitrogen lone pair sits in an $sp^2$ orbital, which holds electrons more tightly. The lone pair is not part of the aromatic $\pi$ system, though, so pyridine is still more basic than aniline.
• Imidazole ($pK_a \approx 7.0$) is more basic than pyridine. When the pyridine-like nitrogen gets protonated, the resulting positive charge is stabilized by resonance across both nitrogen atoms in the ring.
• Pyrrole ($pK_a \approx -3.8$) is extremely weak as a base because its nitrogen lone pair is part of the aromatic sextet. Protonating that nitrogen would destroy aromaticity.

Factors affecting amine basicity

Hybridization plays a major role. An $sp^3$-hybridized nitrogen (as in alkylamines) holds its lone pair in an orbital with more p-character, making the electrons more available for donation. An $sp^2$-hybridized nitrogen (as in pyridine) holds its lone pair in an orbital with more s-character, keeping the electrons closer to the nucleus and less available. That's why alkylamines are more basic than pyridine.

Resonance can dramatically reduce basicity. If the nitrogen lone pair is delocalized into a $\pi$ system (as in aniline or pyrrole), it's less available for protonation. Conversely, if protonation creates a cation that benefits from resonance stabilization (as in imidazole), basicity increases.

Inductive effects from nearby electronegative atoms or groups pull electron density away from nitrogen. For example, $CF_3CH_2NH_2$ is less basic than $CH_3CH_2NH_2$ because the fluorines withdraw electron density through the sigma bonds.

Steric effects matter for bulky amines. Tri-tert-butylamine, despite having three electron-donating alkyl groups, is actually a weaker base than expected because the bulky groups physically block the proton from reaching nitrogen.

Amine purification through acid-base extraction

One of the most practical applications of amine basicity is acid-base extraction, which separates amines from neutral organic compounds. Here's how it works:

1. Dissolve the mixture in an organic solvent (like $CH_2Cl_2$).
2. Wash with dilute aqueous acid (like dilute $HCl$). The amine gets protonated to form a water-soluble ammonium salt, which moves into the aqueous layer. Neutral compounds stay in the organic layer.
3. Separate the layers.
4. Add a strong base (like $NaOH$) to the aqueous layer. This deprotonates the ammonium ion, regenerating the free amine.
5. Extract the now-neutral amine back into a fresh organic solvent.

The pH you need depends on the amine's basicity. Strongly basic amines (high $pK_a$) protonate easily even in mildly acidic solutions, while weakly basic amines (like aniline) may need more strongly acidic conditions to ensure complete protonation. You can even separate two amines of different basicity from each other by carefully controlling the pH of the aqueous wash.