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Effect of hydrogen bonds on pK a values: Importance of networking

Shokri, A ; Sharif University of Technology | 2012

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  1. Type of Document: Article
  2. DOI: 10.1021/ja3037349
  3. Publisher: 2012
  4. Abstract:
  5. The pK a of an acyclic aliphatic heptaol ((HOCH 2CH 2CH(OH)CH 2) 3COH) 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
  6. Keywords:
  7. Acid catalyst ; Acidity constants ; Charge delocalization ; Effect of hydrogen ; Electron-withdrawing substituents ; Enzymatic reaction ; Gas-phase acidities ; Hydroxyl groups ; Solvation shell ; Tert-butyl alcohols ; Tertiary alcohols ; Alcohols ; Stabilization ; Hydrogen bonds ; Acetic acid ; Alcohol ; Dimethyl sulfoxide ; Hydroxyl group ; Polyol ; Chemical structure ; Electron ; Enzyme mechanism ; Hydrogen bond ; PKa ; Acids ; Acylation ; Alkalies ; Dimethyl Sulfoxide ; Hydrogen Bonding ; Hydrogen-Ion Concentration ; Molecular Structure ; Phase Transition
  8. Source: Journal of the American Chemical Society ; Volume 134, Issue 25 , 2012 , Pages 10646-10650 ; 00027863 (ISSN)
  9. URL: http://pubs.acs.org/doi/abs/10.1021/ja3037349