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Abstract

Contents

Introduction

Insufficiency oil reserves and increase the cost of it are important problems now. Necessary to search other sources of organic compounds. Methane can be an alternative to oil. The chief source of methane is natural gas, but it can also be produced from coal. Abundant, cheap, and clean, methane is used widely as a fuel in homes, commercial establishments, and factories; as a safety measure, it is mixed with trace amounts of an odorant to allow its detection. It is also a raw material for many industrial materials, including fertilizers, explosives, chloroform, carbon tetrachloride, and carbon black, and is the principal source of methanol. [1].

Processing of alkane associated with the search for methods of activation stable molecules. Low reactivity of alkanes stipulated by absence of π- or n-electrons and the small σ-polarity of C-H and C-C in the molecules of these compounds

1.Property of alkanes

An alkane is an acyclic saturated hydrocarbon. In other words, an alkane is a long chain of carbon linked together by single bonds. Alkanes are aliphatic compounds. The general formula for alkanes is CnH2n+2; the simplest possible alkane is therefore methane, CH4. The next simplest is ethane, C2H6; the series continues indefinitely. Each carbon atom in an alkane has sp3 hybridization. [2].

Methane is a colorless, odorless gas with a wide distribution in nature. It is the principal component of natural gas, a mixture containing about 75% CH4, 15% ethane (C2H6), and 5% other hydrocarbons, such as propane (C3H8) and butane (C4H10). The "firedamp" of coal mines is chiefly methane. Anaerobic bacterial decomposition of plant and animal matter, such as occurs under water, produces marsh gas, which is also methane.

At room temperature, methane is a gas less dense than air. It melts at –183°C and boils at –164°C. It is not very soluble in water. Methane is combustible, and mixtures of about 5 to 15 percent in air are explosive. Methane is not toxic when inhaled, but it can produce suffocation by reducing the concentration of oxygen inhaled. A trace amount of smelly organic sulfur compounds (tertiary-butyl mercaptan, (CH3)3CSH and dimethyl sulfide, CH3–S–CH3) is added to give commercial natural gas a detectable odor. This is done to make gas leaks readily detectible. An undetected gas leak could result in an explosion or asphyxiation. (The attached scratch-and-sniff sheet from Madison Gas & Electric Company is for your use outside of class.)

In the chemical industry, methane is a raw material for the manufacture of methanol (CH2OH), formaldehyde (CH2O), nitromethane (CH2NO2), chloroform (CH2Cl), carbon tetrachloride (CCl4), and some freons (compounds containing carbon and fluorine, and perhaps chlorine and hydrogen). The reactions of methane with chlorine and fluorine are triggered by light. When exposed to bright visible light, mixtures of methane with chlorine or fluorine react explosively. The principal use of methane is as a fuel [3].

2.Organometallic alkane CH activation

The first reaction of alkane with a metal complex was opened in 1969 [4], in which the intermediately formed organometallic compound, ie, a derivative containing a metal-carbon bond.

The atom-efficiency of one of the most widely used catalytic reactions for forging C-C bonds, the Tsuji-Trost reaction, is limited by the need of preoxidized reagents. This limitation can be overcome by utilization of the recently discovered palladium-catalyzed C-H activation, the allylic C-H alkylation reaction which is the topic of the current review. Particular emphasis is put on current mechanistic proposals for the three reaction types comprising the overall transformation: C-H activation, nucleophillic addition, and re-oxidation of the active catalyst. Recent advances in C-H bond activation are highlighted with emphasis on those leading to C-C bond formation, but where it was deemed necessary for the general understanding of the process closely related C-H oxidations and aminations are also included. It is found that C-H cleavage is most likely achieved by ligand participation which could involve an acetate ion coordinated to Pd. Several of the reported systems rely on benzoquinone for re-oxidation of the active catalyst. The scope for nucleophilic addition in allylic C-H alkylation is currently limited, due to demands on pKa of the nucleophile. This limitation could be due to the pH dependence of the benzoquinone/hydroquinone redox couple. Alternative methods for re-oxidation that does not rely on benzoquinone could be able to alleviate this limitation. [5]

The direct conversion of methane to other chemicals is a subject of continuing interest. One of the products that can be made by this means is acetic acid, a commodity chemical widely used in the chemical industry. Periana et al.[6] have reported that Pd 2+ cations in 96% sulfuric acid will catalyze the direct oxidation of methane at 453 K to acetic acid, with methyl bisulfate (a precursor to methanol) and carbon dioxide as the only byproducts. It was observed, though, that during the course of the reaction Pd2+ is reduced and precipitates from solution as Pd-black, resulting in the loss of the active catalyst. The authors noted that while H2SO4 can oxidize metallic Pd back to Pd2+ , the rate at which the reaction proceeds is insufficient to maintain Pd in solution. Zerella and Bell [7] have confirmed these results and have shown that the retention of Pd2+ in solution is controlled by the balance between the oxidizing and reducing potentials of the reaction system. These investigators reported that, by adding oxygen to the methane feed and controlling the total reactant pressure, virtually all of the Pd2+ can be retained in solution. These conditions also lead to an enhancement in the yield of acetic acid relative to what is observed in the absence of oxygen. [8]

Insights into the elementary processes involved in the activation and oxidation of methane catalyzed by Pt and Au cations in sulfuric acid have been gained from quantum chemical studies. Ziegler and co-workers [9, 10] have studied the mechanism acid and have concluded that methane activation occurs preferentially via oxidative addition. Their calculations also confirmed that Pt2+– CH3 can be oxidized to Pt4+&ndashCH3 by SO3. Goddard and co-workers [11, 12] have studied the same system and have investigated the effects of different ligands on the stability and activity of the catalyst. They showed that C-H bond activation may occur via electrophilic substitution or oxidative addition, depending on the ligands on the platinum center. Jones et al.[13] have reported experimental and theoretical results for methane conversion to methanol by a mixture of selenic acid and metallic gold in 96 wt % sulfuric acid. Their studies show that methane undergoes an electrophilic substitution reaction, and that the catalytic cycle involves Au+&ndashAu 3+ or Au 2+&ndashAu 4+ pairs. Methane activation involves the abstraction of a proton by a bisulfate group.

The biological oxidation of saturated hydrocarbons was studied and modeled based on metal complexes in works [14 – 18].

Reactions with palladium cations is promising area of catalysis by metal. Two mechanisms of alkane activation by palladium complexes are possible. Both begin with the formation of alkane complex with palladium. The first mechanism is the oxidative addition of methane to C-H bond is illustrated in Figure 1 and the second is H/D exchange is illustrated in Figure 2.

oxidative addition of methane to C-H bond
 

Figure 1 Oxidative addition of methane to C-H bond

H/D exchange

Figure 2 H/D exchange

3. Quantum–chemical methods

Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems. It involves heavy interplay of experimental and theoretical methods.

The programs used in computational chemistry are based on many different quantum-chemical methods that solve the molecular Schrodinger equation associated with the molecular Hamiltonian. Methods that do not include any empirical or semi-empirical parameters in their equations – being derived directly from theoretical principles, with no inclusion of experimental data – are called ab initio methods. This does not imply that the solution is an exact one; they are all approximate quantum mechanical calculations. It means that a particular approximation is rigorously defined on first principles (quantum theory) and then solved within an error margin that is qualitatively known beforehand. If numerical iterative methods have to be employed, the aim is to iterate until full machine accuracy is obtained (the best that is possible with a finite word length on the computer, and within the mathematical and/or physical approximations made).

The simplest type of ab initio electronic structure calculation is the Hartree–Fock (HF) scheme, an extension of molecular orbital theory, in which the correlated electron–electron repulsion is not specifically taken into account; only its average effect is included in the calculation. As the basis set size is increased, the energy and wave function tend towards a limit called the Hartree–Fock limit. Many types of calculations (known as post-Hartree–Fock methods) begin with a Hartree–Fock calculation and subsequently correct for electron–electron repulsion, referred to also as electronic correlation. As these methods are pushed to the limit, they approach the exact solution of the non-relativistic Schrodinger equation. In order to obtain exact agreement with experiment, it is necessary to include relativistic and spin orbit terms, both of which are only really important for heavy atoms. In all of these approaches, in addition to the choice of method, it is necessary to choose a basis set. This is a set of functions, usually centered on the different atoms in the molecule, which are used to expand the molecular orbitals with the LCAO ansatz. Ab initio methods need to define a level of theory (the method) and a basis set.

Among the most popular and versatile methods available in condensed-matter physics, computational physics, and computational chemistry is Density functional theory (DFT). DFT is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure (principally the ground state) of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by using functionals, i.e. functions of another function, which in this case is the spatially dependent electron density. Hence the name density functional theory comes from the use of functionals of the electron density. [19]

One of the programs which are implemented raschetі is GAMESS. GAMESS (US) can perform a number of general computational chemistry calculations, including Hartree–Fock, d ensity functional theory (DFT), generalized valence bond (GVB), and Multi-configurational self-consistent field (MCSCF). Correlation corrections after these SCF calculations can be estimated by configuration interaction (CI), second order Moller–Plesset perturbation theory, and coupled cluster theory. Solvent effect can be considered using quantum mechanics/molecular mechanics through discrete effective fragment potentials or continuum models (such as PCM). Relativistic corrections can be calculated, including third order Douglas-Kroll scalar terms. [20]

One of the programs which are implemented raschetі is GAMESS. GAMESS (US) can perform a number of general computational chemistry calculations, including Hartree–Fock, d ensity functional theory (DFT), generalized valence bond (GVB), and Multi-configurational self-consistent field (MCSCF). Correlation corrections after these SCF calculations can be estimated by configuration interaction (CI), second order Moller–Plesset perturbation theory, and coupled cluster theory. Solvent effect can be considered using quantum mechanics/molecular mechanics through discrete effective fragment potentials or continuum models (such as PCM). Relativistic corrections can be calculated, including third order Douglas-Kroll scalar terms.

While the program does not directly perform molecular mechanics, it can do mixed quantum mechanics/molecular mechanics calculations through effective fragment potentials or through an interface with the TINKER code. The fragment molecular orbital method can be used to treat large systems, by dividing them into fragments. [20]

Graphical program for working with quantum chemistry computations is Chemcraft . It is a convenient tool for visualization of computed results and preparing new jobs for the calculation. Chemcraft is mainly developed as a graphical user interface for Gamess (US version and the PCGamess) and Gaussian program packages. For working with other formats of calculations, the possibility to import/export coordinates of atoms in text format can be easily used. Chemcraft does not perform its own calculations, but can significantly facilitate the use of widespread quantum chemistry packages. Chemcraft works under Windows and Linux (but the Linux version has some disadvantages). [21]

Interface of ChemCraft

Figure 3 – Interface of ChemCraft

Conclusion

Although the transition metal compounds differ quite a complicated structure and expensive in their use primarily attracts high selectivity together with high activity catalysts. That is why the study of the properties of transition metals and their reaction mechanisms are relevant scientific objectives, the consideration which the subject of this master's work.

Thus, in the master's work addressed the following tasks:

  1. The studies of methane activation reaction, which is catalyzed by palladium cations (II), surrounded by various ligands.
  2. Calculated structure of the source of palladium complexes and their derivatives of methane.
  3. We find the structure of the transition states of methane activation by two possible mechanisms .
  4. The estimation of activation barriers of reactions studied, which correspond to two possible mechanisms.

Further work aims to study the activation process as palladium complexes with different ligands and other metal complexes.

References

  1. What is an Alkane? [Электронный ресурс] – Режим доступа: http://encycl.opentopia.com/term/Alkane#Occurrence
  2. Methane [Электронный ресурс] – Режим доступа: http://www.merriam-webster.com/concise/methane
  3. Methane [Электронный ресурс] – Режим доступа: http://www.scifun.org/chemweek/methane/methane.html
  4. Шилов А.Е., Шульпин Г.Б. Активация и каталитические реакции алканов в растворах комплексов металлов: - Успехи химии, 1989, №1,сс.38 -57
  5. Casper Junker Engelin and Peter Fristrup Palladium Catalyzed Allylic C-H Alkylation: A Mechanistic Perspective - Molecules 2011, 16, 951-969; doi:10.3390/molecules16010951
  6. Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. J. Science 2003, 301, 814.
  7. Zerella, M.; Mukhopadhyay, S.; Bell, A. T. Chem. Commun. 2004, 1948
  8. Shaji Chempath and Alexis T. Bell Density Functional Theory Analysis of the Reaction Pathway for Methane Oxidation to Acetic Acid Catalyzed by Pd 2+ in Sulfuric Acid – JACS Articles 03.21.2006
  9. Gilbert, T. M.; Hristov, I.; Ziegler, T. Organometallics 2001, 20, 1183.
  10. Hristov, I. H.; Ziegler, T. Organometallics 2003, 22, 1668.
  11. Xu, X.; Kua, J.; Periana, R. A.; Goddard, W. A. Organometallics 2003, 22, 2057.
  12. Kua, J.; Xu, X.; Periana, R. A.; Goddard, W. A. Organometallics 2002, 21, 511.
  13. Jones, C. J.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A. Angew. Chem., Int. Ed. 2004, 43, 4626.
  14. Gilbert, T. M.; Hristov, I.; Ziegler, T. Organometallics 2001, 20, 1183.
  15. Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346.
  16. Rudakov, E. S.; Yaroshenko, A. P.; Rudakova, R. I.; Zamashchikov, V. V. Ukr. Khim. Zh. 1984, 50, 680.
  17. Губин С. П., Шульпин Г. Б. Химия комплексов со связями металл — углерод. Новосибирск: Наука, 1984..
  18. Мацура В.А., Украинцев В.Б., Потехин В.В. Окисление молекулярным кислородом и гидрирование бензилового спирта в присутствии коллоидного палладия in situ// Журн. общей химии. 2000. Т.70. Вып. 12. С. 2058.
  19. Computational chemistry [Электронный ресурс] – Режим доступа: http://en.wikipedia.org/wiki/Computational_chemistry#Ab_initio_methods
  20. GAMESS [Электронный ресурс] – Режим доступа: http://en.wikipedia.org/wiki/GAMESS_%28US%29
  21. ChemCraft [Электронный ресурс] – Режим доступа: http://www.chemcraftprog.com/