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Universal Trends between Acid Dissociation Constants in Protic and Aprotic Solvents
Michael Busch1,2, Elisabet Ahlberg3, Kari Laasonen2
1Institute of theoretical chemistry, Ulm University, Albert-Einstein Allee 11, 89069, Ulm, Germany.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|July 6, 2022
Summary
This study computed pKa values for 182 compounds across 21 non-aqueous solvents. Molecular charge and Kamlet-Taft parameters predict pKa, revealing solvent effects on neutral acids but not cationic acids.
Area of Science:
- Physical Chemistry
- Computational Chemistry
Background:
- pKa values in non-aqueous solvents are crucial but poorly understood.
- Existing data is limited to fundamental properties and common solvents.
Purpose of the Study:
- To compute pKa values for a large set of compounds in diverse non-aqueous solvents.
- To establish universal trends in pKa values across different solvents.
- To identify key molecular and solvent properties influencing pKa.
Main Methods:
- Utilized a computationally efficient procedure to calculate pKa values.
- Computed pKa for 182 compounds in 21 different solvents.
- Employed Kamlet-Taft solvatochromic parameters for analysis.
Main Results:
- Established universal trends in pKa values across all tested solvents.
- Demonstrated that molecular charge and acidic group charge, along with Kamlet-Taft parameters, predict pKa with semi-quantitative accuracy.
- Observed that neutral acids (e.g., alcohols) are highly sensitive to solvent properties.
- Found that pKa of cationic acids (e.g., ammonium ions) are largely independent of solvent choice.
Conclusions:
- A predictive model for pKa values in non-aqueous solvents has been developed.
- Solvent effects significantly impact neutral acids but minimally affect cationic acids.
- This work expands the understanding of acid-base chemistry in non-aqueous environments.
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