Energetic vs structural effects of aminoalkyl substituents in the morpholine

https://doi.org/10.1016/j.jct.2018.03.001Get rights and content

Highlights

  • Energetic characterization of N-aminoalkylmorpholine derivatives.

  • Conformational analysis of N-aminoalkylmorpholinederivatives.

  • Measurementof the massic energies of combustion for N-aminoalkylmorpholine derivatives).

  • Measurement of enthalpy of vaporization for N-(2-aminoethyl)morpholine and N-(3-aminopropyl)morpholine.

  • Experimental and computational gas-phase enthalpies of formation.

Abstract

Experimental and computational studies were performed in this work with the aim of evaluating and understanding the energetic effect inherent to the substitution of the hydrogen of the morpholine amino group by aminoalkyl substituents: N-(2-aminoethyl)morpholine and N-(3-aminopropyl)morpholine.

The standard enthalpies of vaporization and the standard energies of combustion of the two morpholine derivatives obtained, respectively, from Calvet microcalorimetry and combustion calorimetry measurements, are reported. These data were used to derive the standard enthalpies of formation of the morpholine derivatives, in the liquid and gaseous phases, at T = 298.15 K. The computational study involved the energetic analysis of the most stable conformers on the potential energy surfaces and the determination of their gas-phase standard enthalpies of formation at the reference temperature of 298.15 K. All the computational calculations were performed using the G3(MP2)//B3LYP composite method.

The combination of experimental and computational data determined in this work for morpholine derivatives, together with other available in the literature for related molecules, enabled the analyses of the energetic effects associated with the substitution of the hydrogen of the morpholine amino group by substituents aminoalkyl and alkyl, as well as the establishment of incremental schemes for the determination of the gas-phase enthalpies of formation.

Introduction

A molecular energetic study of N-(2-aminoethyl)morpholine and N-(3-aminopropyl)morpholine (molecular formulae in Fig. 1) is reported in this work, following-up previous researches [1], [2] on the study of related heterocyclic compounds.

In general, saturated six-membered ring compounds have a high structural flexibility, being known for the possible existence of several conformers with different energies and different relative proportions. In the case of the morpholine, its molecular structure can adopt chair and twist-boat conformations, where the N–H bond can be in axial or equatorial arrangements relative to the ring backbone, resulting in at least four minima conformers [1]. Only the chair conformers, with the N–H bond in equatorial or axial positions have meaningful energy contribution, with conformer compositions of 0.78 and 0.22, respectively.

The N-substituted morpholine conformers are in most of the cases determined by the possible space arrangements of the substituents in the amino group. The structural knowledge of the morpholine derivatives is an important tool for the clearest understanding of the reactivity of the morpholine derivatives and their interaction with other molecules in different environments.

Both morpholine derivatives reported in this work make part of Schiff bases with relevant antibacterial activity. For example, the 1-[(2-morpholin-4-ylethylimino)methyl]-naphthalen-2-ol [3] (derived from 2-hydroxy-1-naphthyaldehyde [4] and N-(2-aminoethyl)morpholine molecule) and the 2-bromo-4-chloro-6-[(3-morpholin-4-ylpropylimino)methyl]phenol [5] (derived from 2-bromo-4-chloro-6-methylphenol and N-(3-aminopropyl)morpholine) are versatile tridentate ligands available to quelate metallic centres.

Concerning the industrial applications, the N-(3-aminopropyl)morpholine is used as a highly reactive diluent for some epoxy formulations due to its low viscosity, e.g., coatings and adhesives, being its functionality evidenced as a chain extender or cross-linking agent in resins and as an accelerator for slower curing amine containing systems [6]. It is also relevant to highlight that some products prepared by reacting N-(3-aminopropyl)morpholine with polyisobutenyl chloride are useful as detergents, anti-icing and dispersant additives for liquid fuels and lubricating oils [7].

The morpholine has been used as a small C/H/N/O-containing model of biomass-derived fuels with the aim of contributing to the understanding of fuel nitrogen conversion and of the formation of potential hazardous emissions from such compounds [8].

In the current study, experimental and computational studies were performed in order to evaluate and understand the energetic effect inherent to the substitution of hydrogen of the morpholine amino group by bulky functional groups (2-aminoethyl and 3-aminopropyl). The standard (p° = 0.1 MPa) molar enthalpies of vaporization and the standard (p° = 0.1 MPa) internal energies of combustion of the morpholine derivatives obtained, respectively, from Calvet microcalorimetry and combustion calorimetry measurements, are reported. These data were used to derive the standard (p° = 0.1 MPa) molar enthalpies of formation of the two aminoalkylmorpholine derivatives, in the liquid and gaseous phases, at T = 298.15 K. Computational calculations of the enthalpies of atomization reactions in the gaseous phase involving these molecules were performed, using the G3(MP2)//B3LYP composite method. The computational studies were also extended to the calculation of gas-phase enthalpies of formation of N-propylmorpholine and N-(aminomethyl)morpholine.

The structural changes and the inherent energetic effects, associated with the substitution of the hydrogen of the morpholine amino group by the aminoalkyl substituents will be analyzed.

Section snippets

Materials and purification processes

The liquid samples of N-(2-aminoethyl)morpholine, CAS Registry No. 2038-03-1, and N-(3-aminopropyl)morpholine, CAS Registry No. 123-00-2, were acquired from TCI®, with molar fraction purities superior to 0.99.

Prior analysis by gas-liquid chromatography of N-(3-aminopropyl)morpholine revealed that the compound had a molar fraction purity enough for calorimetric measurements (higher than 0.999) to be used without any additional purification. The N-(2-aminoethyl)morpholine was purified by

Computational methods

Electronic structure calculations were performed with the Gaussian-03 software [17] using the composite method G3(MP2)//B3LYP [18] a variation of the Gaussian-3 (G3) theory [19].

Only atomization reactions were considered to estimate the computational gas-phase enthalpies of formation of the morpholine derivatives. In these reactions all homolytic bonds in the compound are broken in atomization [20]. This approach has the advantage that only requires the experimental values of the standard molar

Experimental gas-phase enthalpies of formation

The standard massic energies of combustion in the liquid state, Δcu°(l), at T = 298.15 K, obtained from combustion calorimetry for N-(2-aminoethyl)morpholine (C6H14N2O) and N-(3-aminopropyl)morpholine (C7H16N2O) were the following (−31864 ± 10) J·g−1 and (−33305 ± 11) J·g−1, and correspond to the reactions represented by Eqs. (1), (2), respectively. Each of these values match up to the mean of one set of six combustion experiments (typical combustion results are reported in Tables S2 and S3 of

Conclusions

The gas-phase standard molar enthalpies of formation, at T = 298.15 K, of N-(2-aminoethyl)morpholine and N-(3-aminopropyl)morpholine) have been determined using calorimetric techniques: ΔfHm°(g) = (−144.6 ± 4.9) kJ·mol−1 for N-(2-aminoethyl)morpholine and ΔfHm°(g) = (−170.1 ± 5.1) kJ·mol−1 for N-(3-aminopropyl)morpholine). These values are in good agreement with the estimated values from computational studies using the G3(MP2)//B3LYP, giving enhanced support to the computational methodology

Acknowledgements

This work was developed within the scope of the projects UID/QUI/0081/2013, POCI-01-0145-FEDER-006980, and NORTE-01-0145-FEDER-000028 (Sustained Advanced Materials, SAM), awarded to CIQUP, financed by Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and co-financed in the framework of Operational Programme for Competitiveness and Internationalisation, COMPETE, with community funds (FEDER) and National Funds of MEC. VLSF thanks FCT, European Social Fund (ESF), and National Funds of

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