Interaction of Water with Alkyl-Substituted Amides - Advances in

1 Interaction of Water with Alkyl-Substituted Amides P. ASSARSSON Molecular Association in Biological and Related Systems Downloaded from pubs.acs.org by 186.227.93.97 on 09/21/15. For personal use only.

Sterling Forest Research Center, Union Carbide Corp., Tuxedo, Ν. Y. F. R. E I R I C H Polytechnic Institute of Brooklyn, Brooklyn, Ν. Y.

Phase diagrams from freezing point depressions show true compound formations for simpler amides—e.g., water-N -methylacetamide forms a compound at a mole ratio of 2 to 1, water-N,N-dimethylcetamide at 3 to 2 and 3 to 1, and water-N-methylpyrrolidone at 2 to 1. The heats of mixing and heat capacities at 25°C. of a number of water-amide systems were determined. All mixing curves were exo thermic and possess maxima at definite mole ratios, while the heat capacities for the most part show distinct curvature changes at the characteristic mole ratios. Both experimental results point to the stability of the particular complexes even at room temperature. This is further supported by absolute viscosity studies over the whole concentration range where large maxima occur at these same mole ratios for disubstituted amides and N-substituted pyrrolidones.

Amides i n general have been the subject of considerable discussion in the literature, prompted among other things by their versatility as solvents for organic and inorganic compounds, and serving as models i n elucidating the physical properties of the peptide bond. Furthermore, amide-water mixtures are commonly used binary solvents or reaction media in chemical rate and equilibrium studies. The above applications are then ample reasons for taking a critical look at the physical properties of aqueous solutions of amides, trying to establish the mechanism and energetics of the interaction pattern throughout the complete concentra tion range. The solution process of any particular amide may be arbi trarily divided and discussed in terms of the amide dipole association 1

Molecular Association in Biological and Related Systems Downloaded from pubs.acs.org by 186.227.93.97 on 09/21/15. For personal use only.

2

MOLECULAR ASSOCIATION IN BIOLOGICAL AND RELATED SYSTEMS

with water molecules, the influence of the hydrophobic alkyl substituents on the strength of the amide group, and the possible structural changes of water in the solvation sphere. Those amides which are soluble in all proportions in water at room temperature follow a general trend in that the fully substituted amides are capable of bringing into solution a greater number of alkyl carbons than the mono- or unsubstituted ones—e.g., the upper limit for N-disubstituted acetamides is a pair of propyl groups, while for the N-monosubstituted acetamide the butyl group appears to represent the maximum. Acetamide, on the other hand, is only partially soluble in water, and i n contrast to most other amides, the heat of solution for this binary mixture is endothermic. It is evident that the breaking of hydrogen bonds in bulk and their reforming between the amide dipole i n solution and water are the predominant features governing this particular solubility pattern. The preference of the peptide group, found in N-methylacetamide, to associate selectively with water rather than with itself has been demonstrated through investigation i n the near-infrared region by spectroscopic investigation following the N — H absorption band in the 1.5-micron region (8). The atomic coordinates for the planar amide bond were first charted by Pauling and co-workers (9) in their investigation of the structure of proteins, and later confirmed in x-ray studies (4,7). Ample experimental evidence also exists, mainly from N M R data on N-alkyl substituted amides, in support of the partial double bond character of the N C O bond, although estimates of the amount of double bond character vary from the 40% originally proposed by Pauling to lower values of about 25% (5). The electrical moment of the amide bond calculated from the individually known bond polarities has a value of about 3.4 D , for Nmethylacetamide, or roughly 1 D less than found experimentally. This discrepancy has been referred to as another indication of a double bond character in the nitrogen-carbon bond (11). The pertinent dimensions of the planar peptide bond in trans-configuration are shown opposite. [Actually the complete planarity of amides containing the — N H group may be questioned in view of the shallow pyramid shape shown for formamide by microwave spectroscopy (5).] The basicity of the amide bond, when the substituents are changed, remains almost constant or increases slightly with increasing size of the substituents. These results were obtained from infrared studies and potentiometric titrations of N,N-disubstituted amides in several organic solvents (1) and corroborated by the magnitude of the experimentally obtained electrical moments (12). 2


1.

ASSARSSON AND EIRICH

Water with Amides

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The work introduced here concerns the aqueous solutions of amide groups or peptide dipoles studied with the help of substituted amides which are soluble over the entire concentration region. Specifically, attention has been focused on N,N-disubstituted acetamides where the — R groups have been varied in pairs from methyl to propyl or isopropyl, N-monosubstituted acetamides with — R either a methyl or ethyl group, and N-substituted pyrrolidones with — R either a methyl or isobutyl group. These choices allow one to study the mechanism of hydration of the peptide dipole unaffected by coulombic charges. Various experimental techniques have been utilized, aimed at probing the energetics of water binding and possibly elucidating where "complexing" occurs between the amide group and water. Secondly, when possible, we have been looking for noticeable changes in the water structure which might accompany the solution process. Results Experimental. VISCOSITIES. The absolute viscosities in centipoises (cp.) for several amide-water systems at 25 °C. were obtained using calibrated Ubbelohde viscometers and calculated from rj = p[Kt — L/t] where p is density of mixture, and K and L are instrument constants. The results, plotted against the mole fraction of water, are shown in Figures 1 and 8. There are two rather striking features—the large magnitudes of the maxima in the case of the fully N-substituted acetamides or pyrrolidones which extend from about 1 to 2 cp. for the pure amides to between 4 and 5 cp. at the maxima, and the three families of curves, according to which specific mole fraction ratios the respective maxima occur. The fully 2V-substituted acetamides, N,N-dimethylacetamide ( D M A ) , N,Ndiethylacetamide ( D E A ) , N,N-di-n-propylacetamide ( D n - P A ) , and N,Ndiisopropylacetamide ( D i - P A ) , together with N,N-diethylformamide with

4


a smaller maximum (no figure) have their maxima close to a mole ratio of 3 waters to 1 amide (3 to 1). These results may be compared with previously published data on D M A and are in fair agreement with respect to that maximum at a mole ratio of 2.7 to 1 (10).


VISCOSITY AT 2 5 ° C PHASE DIAGRAM

Figure 1.

DENSITY AT 2 5 ° C HEAT OF MIXING

N,N-Dimethylacetamide-water system

The second family of curves are those of the JV-substituted pyrrolidones, N-methylpyrrolidone ( N M P y ) and N-terf-butylpyrrolidone ( N t B P y ) , with their respective maxima at a common mole ratio of 2 to 1. The third family are of the self-associating N-monosubstituted acetamides, N-methylacetamide and iV-ethylacetamide, which show shallow maxima at a mole ratio of about 1 to 1. The general nature of the curves shows strong, positive deviations from ideal behavior, as is often seen with associating binary liquid mixtures, in particular where one of the components is water. The interpretation of this phenomenon lacks a well founded theoretical treatment, but it is not too unreasonable to attribute the maxima of the fully substituted amides to discrete hydrodynamic entities. The unusual specificity of the maxima is a good indication that we are dealing with a highly associating or complexing system. O n the other hand, the shallow maxima at 1 to 1, observed for the N-monosubstituted acetamides, appears to be more a sign of an optimum ratio i n the process of forming



Figure 3.

Water with Amides

N,N-Di-n-propylacetamide-water system

5

6



polymeric species when chains are formed by interposing water molecules through hydrogen bonding between the amides. V O L U M E CHANGES. The density changes of several amide-water systems have been measured using 10-cc. specific gravity bottles. These experimental values were compared with an ideal density defined i n terms of additive molar volumes and are shown as a dotted line i n Figures 1 through 8 for the respective amide-water mixtures.

—i •20

i

i

.40

.60

Figure 4.

i .80

1 x,

i .20

I .40

i .60

I .80

I 1.00

N N-Diisopropylacetamide^ivater system 9

The positive deviations in the density changes, or conversely negative volume changes upon mixing, are consistent with what one would expect from the exothermic heats of mixing shown below. The maximum deviations of these changes are all of approximately the same order— e.g., in terms of A V they amount to about 1.46 cc. per mole for D M A . They decrease slightly within the homologous acetamide series to about 1.40 cc. per mole for D E A , 1.11 cc. per mole for D i - P A , and 1.05 cc. per mole for Dr-Pa. The greatest deviations are found around a mole ratio of 2 to 1 for all amide-water systems studied. The partial molar volumes at infinite dilution of the amides show a consistent volume decrease for the amides on transfer into the aqueous phase, which becomes greater with increasing size of the substituents and amounts to about 2.5 cc. per mole per pair of methylenic groups in the case of the acetamide series. This value may be compared with that found in the alcohol series (6), where a more negative value of about 2.5 cc. per mole per methylenic group is found, and also to similar dem i x


1.


.20

.40

.60

Figure 5.

Water with Amides

.80

x,

.20

AO

7

.60

.80

1.00

N-Methylacetamide-water system

creases i n the tetraalkylammonium series (13). This volume deficiency has often been interpreted in terms of interstitial solvation—i.e., fitting the hydrophobic portion into cavities supposed to exist in the liquid water structure. However, other explanations are possible, such as comparable "fitting" losses which can be approximated purely on the basis of geometrical considerations of the packing of spheres of different sizes (2). HEATS OF MIXING. In order to obtain data on the energetics of the

interaction between the amide group and water, the heats of mixing, A H , and associated heat capacities were determined on several systems. The experiments were conducted i n a semimicro adiabatic calorimeter using a calibrated thermistor as one leg i n a Wheatstone bridge circuit to sense the temperature changes. A l l the amides used i n this s t u d y — D M A , D E A , Dn-PA, N M A , and NMPy—show exothermic heats over the entire concentration region, especially large for D M A and N M P y . It is suggestive, though possibly fortuitous, that the maxima again occur at the definite mole ratios found i n either by viscosity or phase diagrams since the A H values are also functions of the heats of evaporation, AH , of the two components. A better quantity to describe the molecular interaction would be the heat of solvation; however, most AHevap values for the amides are unknown. Despite this, the large maximum of D M A at 2 to 1 and the skewed successively lower maxima at 3 to 1 for D E A and D n - P A would seem to prove that two molecules of water interact strongly with the peptide dipole. Further m i x

n i i x

ey&v


8


J 1 0

1 -40

I

I

.60

.80

Figure 6.

VISCOSITY AT 2 9 ° C PHASE DIAGRAM

Figure 7.

I X

I .20

I

I

I

I

.40

.60

.80

1.00

N-Ethylacetamide-*vater system

DENSITY AT 2 5 ° C HEAT OF MIXING

N-Methylpyrrolidone-water system

1.


9

Water with Amides

supporting this conclusion is the fact that the maxima for N M A and N M P y also occur at a mole ratio of 2 to 1. H E A T CAPACITIES. The heat capacities ( C in c a l . / m o l e - ° C , not shown here) exhibit some unusual features, in that they show marked curvature changes at the characteristic mole ratios found thus far for each of the respective amides or pyrrolidones, again rendering it likely that definite stoichiometric complexes occur in the amide-water mixtures. PHASE DIAGRAMS. In an endeavor to prove the reality of definite water binding or complexing in these systems, the phase diagrams for a few selected amide-water mixtures—i.e., of N M A , D M A , D n - P A , and N M P y — w e r e studied. The freezing point depressions were measured with a conventional thermocouple circuit. Some of the resulting phase diagrams, shown in the figures, clearly establish that the simpler amides form compounds. The D M A - w a t e r system is complex, with a peritectic point at a mole fraction of about 0.44 of water, followed by maxima, interpreted as compounds, at mole ratios of 3 to 2 and 3 to 1. Both N M A and N M P y exhibit compound formation at mole ratios of 2 to 1, while, as i n the case of D M A , there is a common eutectic point at 4 to 1. The 3 to 1 compound for D M A , and the 2 to 1 compound for N M P y , are of the same ratios as found for the viscosity maxima, perhaps indicating that a similar molecular association ratio is retained to some extent even


p

at 2 5 ° C .

The same explanation or analogy should, of course, not be valid for N M A , where the viscosity maximum was interpreted as being caused by

•20

. 40

Figure 8.

.60

. 80

K|

.20

.40

.60

N-tert-Butylpyrrolidone-water system

.80

1.00

10


polymeric species. The freezing point depression curve for D n - P A fails to show any compound at 3 to 1 and is discontinuous i n the middle region because of the formation of a noncrystallizable glass. The absence of any compound here is probably not so much caused by lack of specific association between the amide group and water, but the bulky N-propyl substituents may be incompatible i n a simple crystal lattice, while yet short enough not to be able to participate i n van der Waals associations which occur i n the crystallization of longer hydrocarbon chains.


Conclusions W e conclude that a definite molecular interaction exists between the peptide dipole and water. The common feature among the amides and pyrrolidones investigated here is the evidence—thermodynamic and from viscosity data—that two molecules of water are being bound per N C O group. The logical site of this association is through hydrogen bonding with the lone electron pairs of the carbonyl oxygen; this argu ment runs parallel with spectroscopic studies from N M R data on the protonation of the amide group (3). The fully substituted amides appear to be able to hold also a third water molecule i n the immediate solvation sphere by an apparently less energetic interaction. It is not possible on the basis of known bond angles and bond distances of the water molecule to bridge the first two hydrogen-bonded water molecules into a ring structure. It is more likely that there is interaction with the partially positive nitrogen. The fact that one does not find evidence of a third water molecule associated with the N-substituted pyrrolidones is prob ably caused by the presence of the ring with its special geometry as evi denced by the greater densities of these compounds. Acknowledgment The authors wish to express their gratitude for grants made available by the National Institutes of Health, Grant G M 09553, Research Grants Branch, and Grant D E 01769, National Institute of Dental Research, which also supported one of the authors ( P.A. ) through graduate school at Polytechnic Institute of Brooklyn.

Literature Cited (1) Adelman, R.,J.Org. Chem. 29, 1296 (1963). (2) Assarsson, P., Eirich, F.R.,in press. (3) Bunton, C. Α., Figgis, B., Nayak, B., "Advances in Molecular Spectros copy," Vol. 3, Macmillan, New York, 1962. (4) Corey, R., Donohue, J.,J.Am. Chem. Soc. 72, 2899 (1950). (5) Costain, C.C.,Dowling, J. M., J. Chem. Phys. 32, 158 (1960).

1. ASSARSSON AND EIRICH

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Franks, F., Ann. Ν.Y.Acad.Sci.125, 277 (1965). Katz, J. L., Ph.D. Thesis, Polytechnic Institute of Brooklyn, 1957. Klotz, J., Franzen, J., J. Am. Chem. Soc. 82, 5241 (1960). Pauling, L., Proc. Natl. Acad. Sci. U.S. 18, 293 (1932). Petersen, R.,J.Phys. Chem. 64, 184 (1960). Smyth, C. P., "Dielectric Behavior and Structure," pp. 53, 73, McGrawHill, New York, 1955. (12) Thompson, B., LaPlanche, L., J. Phys. Chem. 67, 2230 (1963). (13) Wen, W., Saito, S.,J.Phys. Chem. 68, 2639 (1964). RECEIVED October 6, 1967. Molecular Association in Biological and Related Systems Downloaded from pubs.acs.org by 186.227.93.97 on 09/21/15. For personal use only.

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Water with Amides

Interaction of Water with Alkyl-Substituted Amides - Advances in

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