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Solution enthalpies of amides in the organic solvents at infinite dilution: hydrogen bond analysis


M. Varfolomeev1 and I. Rakipov1

1Kazan (Volga region) Federal University, Russia

Keywords: hydrogen bond, calorimetry
property: enthalpy of solution
material: amide

One of the main tasks of investigations in the field of solution calorimetry of nonelectrolytes is the more deep insight into the nature of intermolecular interactions in condensed media and the application of this knowledge to other fields of physical organic chemistry such as the solvent effects on chemical equilibrium and kinetics, reactivity of organic molecules, spectral properties, etc. Intermolecular interactions significantly influence on solution behavior of nucleic acids and proteins. N-H and C=O groups of nucleic acids and proteins can form strong hydrogen bonds with each other or with solvent causing a change in the secondary structure of these molecules. Quantification and analysis of hydrogen bonds in complex biological molecules is a very complicated task, so for their research simple model compounds are used. Amides of carboxylic acid are the most appropriate facilities for these purposes.

In the present work hydrogen bonding between amides and different organic non-electrolytes was studied. Vapor pressure measurements of amides were carried out by transpiration method. Enthalpies of vaporization and sublimation of amides were determined. Consistency test of experimental and literature data on vaporization and sublimation enthalpies of amides were carried out. Enthalpies of solution of formamide, N-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide and N,N-dimethylacetamide in ethers, ketones, esters, nitriles, amines, alcohols, chloroform and water were determined at infinite dilution using semi-adiabatic calorimeter. Enthalpies of solvation of solute molecules in organic solvents were evaluated based on thermochemical and calorimetric results. Relations among thermodynamic data and structures of studied compounds were discussed. Hydrogen bond enthalpies of amides with different solvents were determined in accordance with procedure proposed in the work [1]. It was shown that proton acceptor abilities of studied amides are practically equal. Some unusual results in thermodynamic parameters of hydrogen bonding of amides with basic solvents were observed. Process of specific interaction of amides with aliphatic alcohols and water is complicated. Complexes of different structure and composition are formed during the dissolution of amides in alcohol or water media. Enthalpies of specific interactions in these systems change sign depending on the amide. Influence of structure of amides on their hydrogen bonding with alcohols and water were discussed.

This work was supported by Russian Federal Program "Scientific and scientific-pedagogical personnel of innovative Russia 2009-2013".

References
  1. B. Solomonov, V. Novikov, M. Varfolomeev, N. Mileshko, J. Phys. Org. Chem. 18, 49 (2005)

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