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1 2020

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Wpływ wiązań wodorowych na kwasowość związków chemicznych

Brzeski Jakub, Makowski Mariusz

Wiadomości Chemiczne

2020

[Z] 74, 9-10

629--644

Both intra- and intermolecular hydrogen bonds increase the acidity of the systems in which they occur. Stabilization of the anion formed after deprotonation with a single intramolecular hydrogen bond causes a significant increase in the strength of acid in question, compared to the system in which such interaction does not exist. Hydrogen bonds, through their influence on the acid-base properties of chemical compounds, play a key role in organic chemistry, coordination chemistry, biochemistry and medicine. This paper comprise a review of the most important publications on the impact of hydrogen bonds on the acidity of chemical compounds and the relationship between the specificity of hydrogen bonds and the strength of the resulting acid. The relationship between intermolecular hydrogen bond energy and the pKa value of a given complex is thoroughly discussed in this paper. It turns out that the energy of the hydrogen bond is not related to a single value of pKa of neither the donor nor acceptor of this bond, but rather to the relative difference of these quantities. Namely, the strongest bonds are formed between those systems for which the pKa’s of a donor and acid conjugated to an acceptor differ the least. The feature that clearly correlates with the pKa value of the acid turns out to be the hydrogen bond length. The results of crystallographic studies have shown that the pKa values of C-H acids strongly correlate with the length of C-H ••• O hydrogen bonds. It is worth noting here that the correlation is much better for systems in which the formation of a hydrogen bond is not sterically hindered. In the abundance of donor and acceptor groups in the structure of an acid and its corresponding base, the anion formed after deprotonation is stabilized by phenomenon known as networking. Spreading the negative charge over a larger area of the molecule increases the stability of the anion and thus significantly increases the strength of the corresponding acid. Acids, whose acidity is mainly based on the networking are called SHEAs (single-centered hydrogen-bonded enhanced acidity acids). In addition, the effect of hydrogen bonds on the acidity of specific biochemical systems, namely nucleobases, has been discussed based on the papers by Wetmore and collaborators. It turns out that intra- and extracellular water molecules should not be overlooked when assessing the acidity of biomolecules.