Energetic vs structural effects of aminoalkyl substituents in the morpholine
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, , 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: = (−144.6 ± 4.9) kJ·mol−1 for N-(2-aminoethyl)morpholine and = (−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
References (29)
- et al.
Enthalpies of combustion of the three aminopyridines and the three cyanopyridines
J. Chem. Thermodyn.
(1984) - et al.
Enthalpies of combustion of the three aminopyridines and the three cyanopyridines
J. Chem. Thermodyn.
(2003) - et al.
Thermochemistry of arene chromium tricarbonyls and the strenghts of arene-chromium bonds
J. Organomet. Chem.
(1975) - et al.
Thermochemical study of 2-, 4-, 6-, and 8-methylquinoline
J. Chem. Thermodyn.
(1995) - et al.
Experimental and computational thermochemical studies of 9-R-xanthene derivatives (R = OH, COOH, CONH2)
J. Chem. Thermodyn.
(2012) - et al.
Energetic and structural properties of 4-nitro-2,1,3-benzothiadiazole
J. Chem. Thermodyn.
(2012) - et al.
Energetics and reactivity of morpholine and thiomorpholine: a joint experimental and computational study
J. Chem. Eng. Data
(2014) - et al.
Energetic effects of alkyl groups (methyl and ethyl) on the nitrogen of the morpholine structure
J. Therm. Anal. Calorim.
(2017) - et al.
Synthesis, crystal structures, and antibacterial activities of two copper(II) complexes derived from 1-[(2-morpholin-4-ylethylimino)methyl]-naphthalen-2-ol
Transition Met. Chem.
(2009) - et al.
The influence of the hydroxy and methoxy functional groups on the energetic and structural properties of naphthaldehyde as evaluated by both experimental and computational methods
Struct. Chem.
(2015)
Synthesis, crystal structures, and antibacterial activities of Schiff base nickel(II) and cadmium(II) complexes with tridentate Schiff bases
Russ. J. Coord. Chem.
Combustion chemistry and fuel-nitrogen conversion in a laminar premixed flame of morpholine as a model biofuel
Combust. Flame
Cited by (5)
Thermochemical study of anthranilate derivatives: Effect of the size of the alkyl substituent
2021, Journal of Chemical ThermodynamicsPhase Transition Enthalpy Measurements of Organic Compounds. An Update of Sublimation, Vaporization, and Fusion Enthalpies from 2016 to 2021
2022, Journal of Physical and Chemical Reference DataEfficient Estimation of Formation Enthalpies for Closed-Shell Organic Compounds with Local Coupled-Cluster Methods
2018, Journal of Chemical Theory and Computation