Acidity Constants of Boronic Acids as Simply as Possible: Experimental, Correlations, and Prediction
Abstract
:1. Introduction
- -
- Collecting acidity constant data for monosubstituted phenylboronic acids with simple substituents;
- -
- Checking correlations with corresponding compounds (carboxylic acids);
- -
- Checking correlations with data for solvents other than water;
- -
- Selection of the simplest methods for determining pKa;
- -
- Performing measurements for a series of missing compounds and assessing their accuracy;
- -
- Correlation checking using the above results as a uniform database.
2. Results and Discussion
2.1. Correlation of Acidity: Boronic vs. Benzoic Acids
- The substituent does not form intramolecular hydrogen bonds with the B-OH group. In this case, the steric hindrance makes it difficult to form a tetrahedral form, and the acidity of the compound is lower than that of the corresponding para isomer. This can be observed for such substituents as CH3, NO2, and OCF3, and the reduction in acidity is approximately by one pKa unit. A much stronger reduction occurs in the case of the bulk CF3 group—here, the change in pKa is over 2 pKa units. It is worth noting that the steric effect is opposite to that observed for carboxylic acids, where the acidity of the ortho isomer is higher than that of the para isomer.
- The substituent forms intramolecular bonds with the B-OH group. This occurs in the case of such substituents as F, CHO, and OR [54], and the acidity of the ortho isomer is higher than that of the para isomer by from 0.3 to 0.9 units. The increase in acidity can be explained by the stabilization of the tetrahedral form B(OH)3−, which has a more favorable hydrogen bond geometry than in the case of the planar B(OH)2 group.
2.2. Hammett Equation
2.3. Other Solvents
2.4. Determination of pKa Values of New Compounds: Comparison of Experimental Methods
2.5. Determination of pKa by Calculations
3. Experimental
3.1. pKa Determination
3.2. Potentiometric Method P1
3.3. Potentiometric Method P2
3.4. Spectrophotometric Method
4. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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X | Hammett Const. [27] | |||||||
H | 4.21 [25], 4.16 [28] | 8.9 [29], 8.7 [30], 8.64 [31], 8.86 [32], | ||||||
8.72 [21], 8.90 [33], 8.70 [34] | ||||||||
ortho | meta | para | ortho | meta | para | meta | para | |
OH | 2.85 [28] | 4.09 [25] | 4.59 [25] | 8.55 [35] | 0.12 | −0.37 | ||
3.94 [28] | 4.48 [28] | |||||||
F | 3.27 [36] | 3.88 [25] | 4.16 [25] | 7.83 [37] | 7.50 [37] | 8.66 [37] | 0.34 | 0.06 |
7.89 [38] | 8.09 [38] | 8.77 [38] | ||||||
7.85 [38] | 8.15 [38] | 8.71 [38] | ||||||
8.7 [39] | 8.6 [40] | |||||||
9.1 [41] | ||||||||
9.0 [39] | ||||||||
Cl | 2.94 [42] | 3.82 [42] | 3.98 [42] | 8.3 [29] | 0.37 | 0.23 | ||
3.01 [28] | 3.70 [28] | 4.00 [28] | ||||||
3.84 [25] | 4.00 [25] | |||||||
Br | 2.85 [42] | 3.81 [42] | 4.00 [25] | 8.8 [40] | 0.39 | 0.23 | ||
2.81 [28] | 3.78 [28] | 3.93 [28] | ||||||
3.99 [25] | ||||||||
I | 2.84 [28] | 3.79 [28] | 3.98 [28] | 8.9 [43] | 0.35 | 0.18 | ||
3.88 [25] | 4.00 [25] | |||||||
CN | 3.14 [44] | 3.59 [25] | 3.5 [25] | 7.5 [39] | 7.7 [39] | 0.56 | 0.66 | |
CH3 | 3.81 [28] | 4.24 [28] | 4.36 [28] | 9.7 [29] | 9.0 [29] | 9.3 [29] | −0.07 | −0.17 |
4.27 [25] | 4.35 [25] | 9.00 [33] | 9.26 [33] | |||||
8.74 [34] | 8.95 [34] | |||||||
OCH3 | 4.02 [28] | 3.92 [28] | 4.41 [28] | 9.0 [45] | 8.7 [45] | 9.3 [45] | 0.12 | −0.27 |
4.12 [25] | 4.49 [25] | 9.0 [40] | 8.5 [40] | 9.3 [29] | ||||
9.0 [39] | 8.7 [41] | 9.32 [46] | ||||||
9.7 [29] | 8.4 [39] | |||||||
CHO | 4.56 [47] | 3.77 [48] | 7.3 [45] | 7.5 [45] | 7.7 [45] | 0.35 | 0.42 | |
4.5 [49] | 7.31 [46] | 7.8 [40] | 7.6 [40] | |||||
7.80 [46] | 7.80 [46] | |||||||
COCH3 | 3.86 [25] | 3.74 [25] | 8.0 [45] | 7.7 [45] | 0.38 | 0.50 | ||
8.0 [40] | 7.7 [40] | |||||||
NO2 | 2.17 [42] | 3.45 [42] | 3.44 [42] | ca. 8.5 [46] | 7.1 [40] | 7.4 [39] | 0.71 | 0.78 |
2.14 [28] | 3.50 [25] | 3.40 [25] | 7.3 [46] | 7.15 [46] | ||||
CF3 | 2.73 [50] | 3.75 [25] | 3.77 [51] | 9.58 [52] | 7.88 [52] | 7.39 [37] | 0.43 | 0.54 |
3.90 [50] | 3.77 [50] | 9.45 [52] | 7.85 [52] | 8.1 [39] | ||||
8.5 [39] | 7.87 [37] | 7.82 [52] | ||||||
7.9 [39] | 7.90 [52] | |||||||
OCF3 | 9.53 [53] | 7.79 [53] | 8.11 [53] | 0.38 | 0.35 | |||
9.49 [53] | 7.96 [53] | 8.03 [53] |
Substituent | pKa H2O (Spectr) | pKa H2O (Pot) | pKa H2O/Dioxane |
---|---|---|---|
2F | 7.89 | 7.85 | 10.14 |
3F | 8.09 | 8.15 | 10.46 |
4F | 8.77 | 8.71 | 10.97 |
2,3F | 6.99 | 6.93 | 9.51 |
2,4F | 7.75 | 7.73 | 10.02 |
2,5F | 7.06 | 7.01 | 9.34 |
2,6F | 7.37 | 7.41 | 9.15 |
3,4F | 7.74 | - | 10.34 |
3,5F | 7.60 | 7.52 | 9.78 |
2,3,4F | 6.97 | 7.01 | 9.06 |
2,3,5F | 6.34 | 6.38 | 8.47 |
2,3,6F | 5.60 | - | 8.66 |
2,4,5F | 7.06 | 6.98 | 9.00 |
2,4,6F | 7.10 | - | 9.03 |
3,4,5F | 7.34 | 7.32 | 9.44 |
2,3,4,5F | 6.23 | 6.17 | 8.91 |
2,3,4,6F | 6.17 | 6.19 | 8.39 |
Substituent | Position | pKa | Hammett Constant | ||
---|---|---|---|---|---|
Spectrophotometric S | Potentiometric P1 | Potentiometric P2 | |||
F * | ortho | 7.89 ± 0.01 | 7.85 ± 0.07 | - | - |
meta | 8.09 ± 0.01 | 8.15 ± 0.11 | - | 0.34 | |
para | 8.77 ± 0.01 | 8.71 ± 0.10 | - | 0.06 | |
Cl | ortho | 8.08 ± 0.01 | 8.07 ± 0.02 | 7.96 ± 0.02 | - |
meta | 7.93 ± 0.02 | 7.95 ± 0.03 | 7.81 ± 0.01 | 0.37 | |
para | 8.67 ± 0.04 | 8.24 ± 0.04 | 8.17 ± 0.02 | 0.23 | |
Br | ortho | 8.09 ± 0.03 | 8.18 ± 0.03 | 8.04 ± 0.03 | - |
meta | 7.90 ± 0.04 | 7.94 ± 0.03 | 7.77 ± 0.01 | 0.39 | |
para | 8.19 ± 0.01 | 8.22 ± 0.05 | 8.19 ± 0.03 | 0.23 | |
I | ortho | 8.63 ± 0.03 | 8.51 ± 0.04 | 7.92 ± 0.02 | - |
meta | 8.13 ± 0.05 | 8.01 ± 0.03 | 7.79 ± 003 | 0.35 | |
para | 8.56 ± 0.01 | 8.42 ± 0.05 | 8.06 ± 0.01 | 0.18 | |
CN | ortho | 7.01 ± 0.10 | 6.89 ± 0.08 | - | - |
meta | 7.50 ± 0.02 | 7.40 ± 0.04 | 0.56 | ||
para | 7.32 ± 0.04 | 7.24 ± 0.04 | 0.66 |
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Sporzyński, A.; Adamczyk-Woźniak, A.; Zarzeczańska, D.; Gozdalik, J.T.; Ramotowska, P.; Abramczyk, W. Acidity Constants of Boronic Acids as Simply as Possible: Experimental, Correlations, and Prediction. Molecules 2024, 29, 2713. https://doi.org/10.3390/molecules29112713
Sporzyński A, Adamczyk-Woźniak A, Zarzeczańska D, Gozdalik JT, Ramotowska P, Abramczyk W. Acidity Constants of Boronic Acids as Simply as Possible: Experimental, Correlations, and Prediction. Molecules. 2024; 29(11):2713. https://doi.org/10.3390/molecules29112713
Chicago/Turabian StyleSporzyński, Andrzej, Agnieszka Adamczyk-Woźniak, Dorota Zarzeczańska, Jan T. Gozdalik, Paulina Ramotowska, and Wiktoria Abramczyk. 2024. "Acidity Constants of Boronic Acids as Simply as Possible: Experimental, Correlations, and Prediction" Molecules 29, no. 11: 2713. https://doi.org/10.3390/molecules29112713