Enthalpies of solution of N,N,N′,N′-tetramethylurea in amides, dimethylsulphoxide, and acetone at 298.15 K

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Abstract

The enthalpies of solution ΔsolHmm were determined for N,N,N′,N′-tetramethylurea in formamide, N-methylformamide, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulphoxide, and acetone. Measurements were made at 298.15 K and molalities m = (0.004 to 0.031) mol · kg−1 with a precise isoperibol ampoule-type calorimeter. Standard enthalpies of solution ΔsolHm and transfer ΔtrHm from one solvent to another were computed. The enthalpies of solution of the solute in the hydrogen-non-bonding media were found to be endothermic and weak depending on the nature of methylation in a solvent molecule. It was concluded that the solvent proton-donor ability and existing steric hindrances for hydrogen bonding and other intermolecular interactions play the key role in solvation of tetramethylurea.

Introduction

Tetramethylurea, referred to below as TMU, plays an important role among alkyl-substituted derivatives of urea. This is determined by the fact that TMU is widely used both in chemical technologies as an efficient ligand in the complexation chemistry [1] and in pharmacology as a precursor for the synthesis of polyfunctional pharmaceuticals (such as the Mebicarum and some other glycoluril-containing drugs) [2], [3]. Interest in TMU also stems from peculiarities of its hydration as well as solvation in the amphiprotic organic media [4], [5], [6], [7]. Previously [7], it was ascertained that configurational effects associated with the specifics of molecular structure of the solvent play the main role in the formation of the molecular packing of TMU–alkanol solvate complex. As a consequence, the enthalpic effects of transfer of TMU, ΔtrHm, from n-alkanols (C3,C4) to their branched analogues, 1-PrOH  2-PrOH, and 1-BuOH  t-BuOH, are opposite in sign.

Meanwhile, some questions important for physical chemistry (and biochemistry) concerning the state of this compound in solution and the structure of its solvation environment still remain unanswered. In particular, of interest is a question of the specific features of solvation of TMU molecules by amides, since the latter can be regarded as molecular analogues towards the solute studied. Some of the liquid amides, such as the aprotic dipolar N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA), like TMU, have a –CON(CH3)2 grouping in molecules and proton-accepting properties close to those for the solute condensed under ordinary conditions.1 At the same it is expected that a TMU molecule, due to the presence of two –N(CH3)2 groups, brings about more “resonance form” and, as a consequence, has more hydrogen-bonding ability on an oxygen atom in a –Cdouble bondO moiety than either amide under comparison [4], [7].

Up to now, as far as we know, virtually no data on the thermodynamic (energy- and packing-related) characteristics of solution (solvation) of TMU in amides, with the exception of ΔsolHm values for the specified solute in mixtures of DMF with N-methylformamide (NMF) at 298.15 K reported earlier by Somsen and Rouw [9]. In the present paper, we report the experimental (ΔsolHmm) and standard enthalpies of solution of TMU in formamide (FA), NMF, DMF, DMA, DMSO, and acetone (the last two solvents were chosen as the comparison ones) at 298.15 K. The enthalpic effects of transfer of the solute considered from one solvent to the other are discussed.

Section snippets

Experimental

Characterization data for solvents under study are listed in table 1. TMU was purified as in our previous studies [7], [10]. FA, NMF, and DMF were distilled twice under reduced pressure from NaOH according to [11]. DMA was dried with 0.4 nm molecular sieves (which had been dried in vacuum above 473 K for more than 15 h) for two days and fractionally distilled at reduced pressure. DMSO and acetone were used without further purification. The water content of these liquids (see table 1) was

Results

Experimental data on the enthalpies of solution of TMU in the solvents studied are listed in table 2, together with the average deviations. It can be seen from the table that the ΔsolHmm values virtually (within the experimental error) do not depend on the solute molality. Thus, the molar enthalpies of TMU solution at infinite dilution, i.e. the ΔsolHm(ΔsolHm) quantities have been calculated as average values, ΔsolHmmav, in the range of our measured results using the procedure described in

Discussion

As can be seen from table 2, upon N(C)-sited sequential introduction of CH3 groups into a “parent” FA molecule, the trend of ΔsolHm changes gradually comes to show the features of solvation of TMU in methyl-substituted amides. The most striking feature is the negative-to-positive sign-inversion of the quantity considered in NMF  DMF (DMA) transitions, i.e., the dissolution of TMU in N,N-methyl-substituted amides is already a weak-endothermic process. The behavior of TMU in DMA is also rather

Conclusion

The results can be summarized briefly as follows.

Firstly, caused by the solvation of TMU the degree of the solute–solvent molecular affinity in FA is substantially higher than that in each of the methyl-substituted amides including NMF, as well as in acetone and DMSO, showing the hydrogen-bonding effects play the decisive role in the solvation (amide-based) environment of the solute.

Secondly, the nature of methyl-substitution in an amide molecule has a weak effect on the TMU solvation, as the

Acknowledgement

The financial support of this work by the Russian Foundation for Basic Researches is gratefully acknowledged (Grant No. 04-03-32957).

References (23)

  • R.G. Kostyanovsky et al.

    Mendeleev Commun.

    (2002)
  • Y. Uosaki et al.

    J. Chem. Thermodyn.

    (1992)
  • E.V. Ivanov et al.

    Thermochim. Acta

    (2006)
  • E.V. Ivanov et al.

    J. Chem. Thermodyn.

    (2007)
  • M.N. Rodnikova et al.

    Russ. J. Phys. Chem.

    (2005)
  • E.G. Atavin et al.

    J. Struct. Chem.

    (2005)
  • D.H.C. Chen et al.

    Fluid Phase Equil.

    (2000)
  • Y. Marcus

    Ion Solvation

    (1985)
  • A. Rouw et al.

    J. Chem. Soc. Faraday Trans. 1

    (1982)
  • V. Ivanov et al.

    Thermochim. Acta

    (2007)
  • H. Sijpkes et al.

    J. Chem. Thermodyn.

    (1989)
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