Effect of hydrogen bonds on pKa values: importance of networking.
Effect of hydrogen bonds on pKa values: importance of networking.
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DOI：
10.1021/ja3037349
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发表时间：
2012-06
影响因子：
15
通讯作者：
Alireza Shokri;Azardokht Abedin;A. Fattahi;S. Kass
中科院分区：
文献类型：
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作者：
Alireza Shokri;Azardokht Abedin;A. Fattahi;S. Kass
在 DMSO 中测量无环脂肪族七醇 ((HOCH(2)CH(2)CH(OH)CH(2))(3)COH) 的 pK(a)，并报告其气相酸度。发现这种叔醇的酸性比 DMSO 中的叔丁醇高 10(21) 倍，比乙酸高一个数量级（即 pK(a) = 11.4 vs 12.3）。这可归因于共轭碱中带电氧中心通过三个氢键的 21.9 kcal mol(-1) 稳定作用以及由不带电的伯羟基和仲羟基之间的额外三个氢键产生的另一个 6.3 kcal mol(-1) 稳定作用。第一和第二溶剂化壳的电荷离域可用于促进酶促反应。还计算了一系列多元醇的酸度常数，发现氢键和吸电子取代基的组合可提供预计在 DMSO 中呈极酸性的酸（即 pK(a) < 0）。这些氢键增强的酸代表了一类有吸引力的布朗斯台德酸催化剂。
The pK(a) of an acyclic aliphatic heptaol ((HOCH(2)CH(2)CH(OH)CH(2))(3)COH) was measured in DMSO, and its gas-phase acidity is reported as well. This tertiary alcohol was found to be 10(21) times more acidic than tert-butyl alcohol in DMSO and an order of magnitude more acidic than acetic acid (i.e., pK(a) = 11.4 vs 12.3). This can be attributed to a 21.9 kcal mol(-1) stabilization of the charged oxygen center in the conjugate base by three hydrogen bonds and another 6.3 kcal mol(-1) stabilization resulting from an additional three hydrogen bonds between the uncharged primary and secondary hydroxyl groups. Charge delocalization by both the first and second solvation shells may be used to facilitate enzymatic reactions. Acidity constants of a series of polyols were also computed, and the combination of hydrogen-bonding and electron-withdrawing substituents was found to afford acids that are predicted to be extremely acidic in DMSO (i.e., pK(a) < 0). These hydrogen bond enhanced acids represent an attractive class of Brønsted acid catalysts.