Prototropic forms of hydroxy derivatives of naphthoic acid within deep eutectic solvents

by Selleckchem | Jul 29 2023

Deep eutectic solvents (DESs) are not only recognized as benign and inexpensive alternatives to ionic liquids, they offer a unique solvation milieu due to the varying H-bonding capabilities of their constituents. Proton-transfer involving a probe and its prototropic forms strongly depend on the H-bonding nature of the solubilizing media. The presence of prototropic forms of three probes, 1-hydroxy-2-naphthoic acid (1,2-HNA), 3-hydroxy-2-naphthoic acid (3,2-HNA), and 6-hydroxy-2-naphthoic acid (6,2-HNA) is investigated in two DESs, named ChCl:urea and ChCl:glycerol, constituted of H-bond acceptor choline chloride and different H-bond donors, urea and glycerol, respectively, in a 1 : 2 mole ratio under ambient conditions. While 1,2-HNA and 3,2-HNA exhibit an intramolecular H-bonding ability, 6,2-HNA does not. In contrast to common polar solvents, where the monoanionic emitting form of 1,2-HNA is also supported along with the neutral one, in both the DESs only the neutral emitting form exists. Addition of acid to the two DESs, respectively, fail to generate the monocationic form of the probe. Addition of a base to ChCl:urea results in the generation of the monoanionic form; even a very high strength of the base fails to generate the monoanionic emitting form in ChCl:glycerol. Relatively higher H-bond donating acidity of ChCl:glycerol results in added hydroxyl getting involved in H-bonding with alcohol functionalities of ChCl:glycerol leading to the absence of proton extraction to create the monoanionic form of the probe. Only the monoanionic emitting form of 3,2-HNA is present in ChCl:urea; in ChCl:glycerol, due to its higher H-bond donor acidity, the neutral emitting form is also detected. Addition of high strength of acid to ChCl:urea does result in formation of the neutral emitting form. Addition of an aqueous base results in the formation of the dianionic form of 3,2-HNA in ChCl:urea; however, in ChCl:glycerol, the added base fails to convert the neutral form of this probe to the monoanionic form as efficiently as that in ChCl:urea. The monoanionic (carboxylate) form of 6,2-HNA exits in ChCl:urea, whereas the neutral form is present in ChCl:glycerol due to its higher H-bond donating acidity. Addition of an acid can induce a shift in prototropic equilibrium towards the neutral form of 6,2-HNA in ChCl:urea; no change is observed in the behavior of this probe in ChCl:glycerol as the acid is added. Both the DESs support the dianionic form of 6,2-HNA in the presence of the base; the added base helps extract both -OH and -COOH protons of this probe. The H-bond donor component of the DES is clearly established to play a critical role in the prototropic behavior of the probe.

Comments:

Deep eutectic solvents (DESs) are considered as alternative solvents to ionic liquids, offering several advantages such as being benign and inexpensive. DESs have unique solvation properties due to the varying hydrogen-bonding capabilities of their constituents.

In a study, the presence of three probes, namely 1-hydroxy-2-naphthoic acid (1,2-HNA), 3-hydroxy-2-naphthoic acid (3,2-HNA), and 6-hydroxy-2-naphthoic acid (6,2-HNA), was investigated in two DESs: ChCl:urea and ChCl:glycerol. These DESs are composed of a hydrogen-bond acceptor, choline chloride, and different hydrogen-bond donors, urea and glycerol, respectively, in a 1:2 mole ratio under ambient conditions.

The probes 1,2-HNA and 3,2-HNA have an intramolecular hydrogen-bonding ability, while 6,2-HNA does not. In typical polar solvents, both the neutral and monoanionic emitting forms of 1,2-HNA are present. However, in both DESs, only the neutral emitting form is observed. Even upon addition of acid to the DESs, the monocationic form of the probe is not generated.

In ChCl:urea, the monoanionic emitting form of 3,2-HNA is present, while in ChCl:glycerol, both the neutral emitting form and the monoanionic form are detected. The higher hydrogen-bond donor acidity of ChCl:glycerol leads to the involvement of the additional hydroxyl group in hydrogen-bonding with the alcohol functionalities of glycerol. This interaction prevents the extraction of the proton required to form the monoanionic form of the probe. Adding a strong acid to ChCl:urea results in the formation of the neutral emitting form.

Upon addition of a base, ChCl:urea generates the monoanionic form of 3,2-HNA, while ChCl:glycerol is less efficient in converting the neutral form of this probe to the monoanionic form. The presence of a relatively higher hydrogen-bond donor acidity in ChCl:glycerol leads to differences in the behavior of the probe.

In ChCl:urea, the monoanionic (carboxylate) form of 6,2-HNA is present, whereas the neutral form is observed in ChCl:glycerol due to its higher hydrogen-bond donor acidity. Adding an acid to ChCl:urea can shift the prototropic equilibrium towards the neutral form of 6,2-HNA, while no change is observed in ChCl:glycerol upon acid addition. Both DESs support the dianionic form of 6,2-HNA in the presence of a base, which helps extract both the -OH and -COOH protons of the probe.

Overall, the hydrogen-bond donor component of the DES plays a critical role in determining the prototropic behavior of the probes in the solvents. The differences in the hydrogen-bonding properties of DESs can influence the proton-transfer reactions and the presence of different prototropic forms of the probes.