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Basicity of Aliphatic Amines

Basicity of Aliphatic Amines

Amines can behave as Brønsted–Lowry bases by accepting a proton from the acid to form corresponding conjugate acids. Due to a lone pair of nonbonding electrons, aliphatic amines can also act as Lewis bases by forming a covalent bond with an electrophile.
To measure the basicity of amines, two conventions are generally used. The first defines Kb as the basicity constant for the deprotonation reaction of water by the amine, as presented in Figure 1. Conventionally, lower Kb indicates higher...

Leveling Effect and Non-Aqueous Acid-Base Solutions

Leveling Effect and Non-Aqueous Acid-Base Solutions

This lesson defines the leveling effect in acidic and basic solutions and its role in aqueous and non-aqueous solutions. It is essential to understand the competing nature of various species in a chemical system.
The Leveling Effect of a Solvent
A generic acid (HA) reacts with the generic base (B-) to yield the corresponding conjugate base (A-) and conjugate acid (HB):

Acid Strength and Molecular Structure

Acid Strength and Molecular Structure

Binary Acids and Bases
In the absence of any leveling effect, the acid strength of binary compounds of hydrogen with nonmetals (A) increases as the H-A bond strength decreases down a group in the periodic table. For group 17, the order of increasing acidity is HF < HCl < HBr < HI. Likewise, for group 16, the order of increasing acid strength is H2O < H2S < H2Se < H2Te. Across a row in the periodic table, the acid strength of binary hydrogen compounds increases with increasing...

Acidity and Basicity of Alcohols and Phenols

Acidity and Basicity of Alcohols and Phenols

Like water, alcohols are weak acids and bases. This is attributed to the polarization of the O–H bond making the hydrogen partially positive. Moreover, the electron pairs on the oxygen atom of alcohol make it both basic and nucleophilic. Protonation of an alcohol converts hydroxide, a poor leaving group, into water—a good one. The two acid–base equilibria corresponding to ethanol are depicted below.

Basicity of Heterocyclic Aromatic Amines

Basicity of Heterocyclic Aromatic Amines

Heterocyclic amines, where the N atom is a part of an alicyclic system, are similar in basicity to alkylamines. Interestingly, the heterocyclic amine having a nitrogen atom as part of an aromatic ring has much less basicity than its corresponding alicyclic counterpart. For this reason, as presented in Figure 1, piperidine (pKb = 2.8) is significantly more basic than pyridine (pKb = 8.8).

Relative Strengths of Conjugate Acid-Base Pairs

Relative Strengths of Conjugate Acid-Base Pairs

Brønsted-Lowry acid-base chemistry is the transfer of protons; thus, logic suggests a relation between the relative strengths of conjugate acid-base pairs. The strength of an acid or base is quantified in its ionization constant, Ka or Kb, which represents the extent of the acid or base ionization reaction. For the conjugate acid-base pair HA / A−, the ionization equilibrium equations and ionization constant expressions are

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Related Experiment Video

Updated: Jun 8, 2026

Determination of the Gas-phase Acidities of Oligopeptides

Determination of the Gas-phase Acidities of Oligopeptides

Published on: June 24, 2013

Gas-phase basicities around and below water revisited.

Ivo Leito¹, Ilmar A Koppel, Peeter Burk

¹Institute of Chemistry, University of Tartu, 14a Ravila Str., Tartu 50411, Estonia.

The Journal of Physical Chemistry. A

|September 14, 2010

Summary

This study resolves discrepancies in gas-phase basicity measurements for weak bases by comparing Fourier transform ion cyclotron resonance (FT-ICR) and high-pressure mass spectrometry (HPMS) data, finding computational methods support HPMS results.

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Characterization, Quantification and Compound-specific Isotopic Analysis of Pyrogenic Carbon Using Benzene Polycarboxylic Acids (BPCA)

Characterization, Quantification and Compound-specific Isotopic Analysis of Pyrogenic Carbon Using Benzene Polycarboxylic Acids (BPCA)

Published on: May 16, 2016

Area of Science:

Physical Chemistry
Computational Chemistry
Spectroscopy

Background:

Gas-phase basicity (GB) measurements for weak bases exhibit discrepancies between high-pressure mass spectrometry (HPMS) and Fourier transform ion cyclotron resonance (FT-ICR) techniques.
Previous FT-ICR studies reported a contracted basicity scale compared to HPMS, necessitating clarification.

Purpose of the Study:

To reconcile the long-standing discrepancy in gas-phase basicity (GB) values between HPMS and FT-ICR methods.
To computationally and experimentally analyze GB for weak bases around and below water.

Main Methods:

Utilized Fourier transform ion cyclotron resonance (FT-ICR) for experimental analysis.
Employed Gaussian quantum chemistry composite methods (W1 and G2) for computational analysis.
Re-examined experimental equilibria for polyfluorinated weak bases.

Main Results:

Computational results, particularly at the W1 level, strongly support published HPMS data.
Identified competing protonation pathways (oxygen vs. fluorine) in polyfluorinated compounds, with fluorine protonation being kinetically favored but thermodynamically less stable.
Observed irreversible loss of HF and slow rearrangement of fluorine-protonated cations.

Conclusions:

The study partially explains discrepancies between HPMS and FT-ICR data, especially for fluorinated compounds.
FT-ICR's narrower dynamic range can lead to inaccuracies for certain bases compared to HPMS.
Careful consideration of protonation pathways and experimental technique is crucial for accurate gas-phase basicity measurements.