Basicity
Amines are organic compounds containing a nitrogen atom bonded to one or more carbon groups and bearing a lone pair of electrons. This lone pair makes amines Lewis bases and Brønsted–Lowry bases, capable of accepting a proton (H⁺) to form an ammonium ion. The basicity of an amine is quantified by the pKa of its conjugate acid (the ammonium salt): a higher pKa means a stronger base, because the protonated form is less willing to release the proton back. For example, methylamine (CH₃NH₂) has a conjugate-acid pKa of about 10.6, making it a significantly stronger base than ammonia (pKa 9.25). A fascinating and often counterintuitive aspect of amine basicity is that the order of basicity changes dramatically depending on whether you measure it in the gas phase or in aqueous solution. In the gas phase, basicity increases smoothly with the number of alkyl substituents: tertiary > secondary > primary > ammonia. This trend reflects the pure inductive electron-donating effect of alkyl groups, which stabilise the positive charge on the protonated nitrogen. In water, however, the order is scrambled — typically secondary > primary > tertiary > ammonia — because solvation of the charged ammonium ion by water molecules introduces an opposing steric effect. Understanding these competing effects — inductive donation, solvation stabilisation, and steric hindrance — is essential for predicting reactivity in synthesis, drug design, and biological chemistry. Nitrogen-containing bases are central to life itself, appearing in amino acids, DNA bases, alkaloids, and pharmaceuticals. This article dissects each effect in turn, compares gas-phase and aqueous data, and shows you how to reason through basicity trends systematically.
- Inductive Effect (Electron Donation by Alkyl Groups)
- Solvation Effect (Hydrogen Bonding to Water)
- Steric Effect (Bulk Around Nitrogen)
- The Aqueous Basicity Order and pKa Values
- Aromatic Amines: Resonance Delocalisation
Inductive Effect (Electron Donation by Alkyl Groups)
Alkyl groups are electron-donating by the inductive effect: the C–N bond polarises slightly toward N because alkyl carbons push electron density along the sigma framework. Each additional alkyl substituent increases the electron density on nitrogen, making the lone pair more available to accept a proton and also better stabilising the positive charge on the ammonium ion formed after protonation. This is why, in the gas phase (where there is no solvent to complicate matters), basicity increases monotonically: (CH₃)₃N > (CH₃)₂NH > CH₃NH₂ > NH₃, with measured proton affinities of 948, 929, 899, and 854 kJ mol⁻¹ respectively. The gas phase represents the 'pure' electronic effect uncomplicated by any solvent interactions.
Solvation Effect (Hydrogen Bonding to Water)
In aqueous solution, the protonated amine (ammonium ion) is stabilised by hydrogen bonds to surrounding water molecules. Each N–H bond in the ammonium ion can act as a hydrogen-bond donor. A primary ammonium ion R–NH₃⁺ has three N–H bonds available for H-bonding, a secondary ammonium R₂NH₂⁺ has two, and a tertiary ammonium R₃NH⁺ has only one. Therefore, solvation stabilises the conjugate acid in the order primary > secondary > tertiary. Greater solvation stabilisation of the conjugate acid makes the proton less likely to be released, which means greater effective basicity of the amine. This solvation effect opposes the inductive effect and is responsible for the anomalously lower aqueous basicity of triethylamine compared with diethylamine, despite triethylamine being the stronger gas-phase base.
Steric Effect (Bulk Around Nitrogen)
Larger alkyl groups create steric hindrance around the nitrogen atom. Bulky substituents physically impede water molecules from approaching and hydrogen-bonding to the ammonium ion, reducing the solvation stabilisation of the conjugate acid. They also slightly hinder the approach of the proton itself. Steric effects become particularly important for branched or large alkyl groups. For example, di-tert-butylamine is a much weaker base in solution than would be predicted from the inductive effect alone, because the large tert-butyl groups block solvation of the N–H in the ammonium ion. Steric effects reinforce the solvation effect in pushing tertiary amines toward lower aqueous basicity relative to secondary amines.
The Aqueous Basicity Order and pKa Values
Combining all three effects, the observed aqueous basicity order for simple methylamines is: (CH₃)₂NH (pKa 10.7) > CH₃NH₂ (pKa 10.6) > (CH₃)₃N (pKa 9.8) > NH₃ (pKa 9.25). Notice that secondary amine wins because it benefits from significant inductive stabilisation (two methyl groups) while retaining two N–H bonds for good solvation. Tertiary amine has maximum inductive effect but is penalised most severely by loss of solvating N–H bonds and steric shielding. For aromatic amines like aniline (C₆H₅NH₂, pKa 4.6), basicity drops dramatically because the lone pair is delocalised into the aromatic π system, reducing its availability to accept a proton — an entirely different resonance/conjugation effect.
Aromatic Amines: Resonance Delocalisation
In aniline, the nitrogen lone pair overlaps with the π system of the benzene ring, as shown by resonance structures that place positive charge on nitrogen and negative charge at the ortho and para positions of the ring. This delocalisation reduces the electron density on nitrogen, making it a far weaker base than alkylamines. Electron-withdrawing groups (e.g., –NO₂) on the ring further reduce basicity by pulling even more electron density away from nitrogen, while electron-donating groups (e.g., –OCH₃) increase basicity slightly. The pKa of 4-nitroaniline is only 1.0, compared with 4.6 for aniline and 5.1 for 4-methoxyaniline, illustrating how powerfully ring substituents tune basicity through resonance and inductive pathways.
The Key Equation
- \ce{R_3N} — Tertiary amine (—): Nitrogen bearing three organic (alkyl or aryl) substituents and one lone pair
- \ce{R_3NH^{+}} — Ammonium ion (conjugate acid) (—): The protonated amine; its pKa measures the basicity of the parent amine
- K_b — Base dissociation constant (mol dm⁻³): Equilibrium constant for amine acting as a base; related to pKa of conjugate acid by pKa + pKb = 14 at 25 °C
- \text{pK}_a — Conjugate acid pKa (dimensionless): Negative log of the acid dissociation constant of R₃NH⁺; higher pKa = stronger base
Worked Example
Problem. Arrange the following amines in order of increasing basicity in aqueous solution, and explain your reasoning: (a) ammonia NH₃, (b) methylamine CH₃NH₂, (c) dimethylamine (CH₃)₂NH, (d) trimethylamine (CH₃)₃N, (e) aniline C₆H₅NH₂.
Answer. Increasing aqueous basicity: aniline < ammonia < trimethylamine < methylamine < dimethylamine
Origins · 1 minAugust Wilhelm von Hofmann and how this was discovered
Hofmann was the first chemist to systematically investigate the chemistry of amines, isolating methylamine, dimethylamine, and trimethylamine from degradation products of various organic substances. He established the classification of amines into primary, secondary, and tertiary categories — the very framework still used today — and developed the Hofmann elimination and Hofmann rearrangement reactions. His pioneering work in the mid-19th century laid the foundation for understanding nitrogen compounds in organic chemistry. Hofmann was so influential in British chemistry that he trained a generation of chemists at the Royal College of Chemistry in London before returning to Germany, where he founded the German Chemical Society (Deutsche Chemische Gesellschaft) in 1867.
Related Concepts
- Amines: Structure and Classification — Understanding primary, secondary, and tertiary amine structures is a prerequisite for analysing how substituent number affects basicity trends.
- Amines as Nucleophiles — The lone pair that makes amines basic also makes them nucleophiles; the same electronic and steric factors that influence basicity affect nucleophilicity and reactivity.
- Acid–Base Equilibria: pKa and pKb — Basicity is quantified using pKa and pKb; a firm grasp of these equilibrium constants is essential for interpreting and comparing amine base strengths.
- Inductive and Resonance Effects in Organic Molecules — The electron-donating inductive effect of alkyl groups and the electron-withdrawing resonance effect in aniline are the core electronic concepts driving amine basicity differences.
- Aromatic Amines and Aniline Chemistry — Aniline's dramatically reduced basicity due to lone pair delocalisation into the aromatic ring represents an important extension of the basicity trends discussed in this article.