Hydrocarbon Three Dimensional Structure and Molecular Geometry

For a study of hydrocarbon three dimensional structure, understanding of a tetrahedral molecular geometry is very important. Tetrahedral molecular geometry states that a metal atom is situated at the centre of the molecule with four substituents bonded with it and are located at the four corners of the tetrahedron. When all the substituents bonded with the central atom are the same atoms then it forms a perfect tetrahedral structure with a bond angle of 109.50 that is also represented mathematically as cos-1(-⅓). The perfectly symmetrical tetrahedral molecules belong to the Td point group but mostly tetrahedral molecules have very low symmetry, that is the molecules whose substitute atoms are not the same. That is why the unsymmetrical molecules are chiral in nature. Therefore hydrocarbon three dimensional structure having tetrahedral geometry is given below.

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The above two illustrations show the perfect symmetrical three dimensional tetrahedral structure having same atoms in the tetragone as well as non symmetrical three dimensional tetrahedral structure having different atoms in the tetragone. One of the common examples of the symmetrical tetrahedral geometry is CH4 and its heavier analogue and the unsymmetrical tetrahedral geometry is CH3Cl. The tetrahedral structure of carbon shows that the carbon atom shares four attachments with other atoms and the bond angles for all the atoms attached in symmetrical molecular structure is 109.50  and the polarity is zero. And for asymmetrical molecular structure the angles shared by different atoms with the central carbon atom and hence it becomes a stereocenter but overall for all the tetrahedral structure of carbon is tetragonal, that is, a pyramid with all its faces being equilateral triangle or nearly equilateral is shaped. In order to attain tetrahedral structure of carbon, it follows sp3 orbitals. 

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Methane: Shape and Structure

Methane is a group 14 hydride and the simplest alkane known with a molecular formula CH4 and its tetrahedral structure follow Sp3 orbitals. In molecular shape of methane the molecular arrangement is expressed as lewis structure. It is clearly shown by the methane 3D structure given below.

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The molecular shape of methane is a complete tetragone having Td  point group and zero dipole moment with a tetrahedral  of carbon. Thus methane shape can also be identified as trigonal pyramidal. The molecular shape of methane suggests that four atoms of hydrogen are combined with one carbon atom. Now one hydrogen atom is short of one electron to attain a fully stable S shell having 2 atoms and a carbon atom is short of 4 electrons to make a stable outer electron shell that carries 8 atoms. Thus all the four atoms of hydrogen share one of the four electrons of carbon individually completing its S shell. Similarly the carbon atom by sharing all the electrons of the four hydrogen atoms surrounding it completes its outer shell with eight electrons. Thus carbon and hydrogen atoms share a strong C-H covalent bonding.

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Though methane makes a major constituent of natural gas but because of its gaseous state its recovery and storage under a certain temperature and pressure becomes the main challenge. Now the natural gas that is used in homes as room freshener is blended with an odorant which is usually tert-butylthiol as safely measured. The concentration of methane that ranges from 5.4-17% in air at normal pressure is not subjected to inflammation. Methane in solid forms exists and nine modifications of the same are known. When gaseous methane is cooled at normal pressure crystalline methane is formed. It forms a cubic system and the hydrogen atoms present in it can move freely as  the positioning of the hydrogen atoms in a methane molecule is not fixed.  

Reaction of Methane

1. Partial Oxidation : Formation of methanol from methane by oxidation reaction is not possible as the reaction proceeds towards formation of carbon dioxide with water even in the absence of sufficient oxygen. Though enzymes like methane monooxygenase can convert methane to methanol but this reaction is not feasible for a large scale industrial production. Over the years some homogeneous as well as heterogeneous catalyzed systems have been developed but all of them have their own certain drawbacks. Thus the oxidation of methane to methanol is carried out with the help of certain protected products that can shield themselves from getting over oxidized. Few of them are copper zeolite and iron zeolite that stabilize the alpha oxygen active site.

2. Acid-Base Reaction : Methane, like other hydrocarbons, is a very weak acid with pKa value is estimated at around 56. Thus it suggests that it cannot be deprotonated easily in the solution and its conjugate base is formed in the form of methyllithium. Methane under low pressure gas mixture forms a number of positive ion derivatives. Some of them are methyl cation CH3+, methane cation CH4+ and protonated methane also known as methenium  CH5+. Methanium can be synthesised as a diluted solution by the reaction of methane with superacids. These positive ion derivatives are mostly detected in outer space and very rarely in the earth's atmosphere. Though the C-H bond in methane and in its higher derivatives are very stable, still certain catalysts  are induced in the reaction involving this methane or its higher derivatives to initiate C-H bond activation.

3. Combustion Reaction : The combustion of methane is a multiple step reaction that is summed up to a single reaction and the heat of combustion generated by burning of methane is 55.5 Mj / Kg. The combustion reaction will be:

CH4 + 2O2 → CO2 + 2H2O

The complete symmetrical combustion reaction of methane is explained by Peters four step chemistry and the reactions are:

CH4 + 2H + H2O → CO + 4H2 (STAGE 1)

CO + H2O ↔ CO2  + H2 (STAGE 2)

H + H + M →   H2 + M (STAGE 3)

O2 + 3H2 ↔ 2H + 2H2O (STAGE 4)

4. Methane Radical Reaction : In a suitable condition, methane tends to react with the halogen radicals. The reaction is as follows:-

 X• + CH4 → XH + CH3• (STAGE 1)

CH3• + X2  → CH3X + X• (STAGE 2)

Where X can be any halogene, that is , fluorine (F), chlorine (Cl), bromine (Br) or iodine (I) and the reaction mechanism is called halogenation and is completed in two steps. This reaction in stage one is initiated by UV light or any catalyst initiator like peroxide where a halogen radical targets an hydrogen atom from an alkane group resulting in formation of an hydrogen halide molecule and an alkane radical. Then the alkane radical further reacts with an halogen molecule in stage two in order to form an haloalkane and produce a new halogen radical as a byproduct. 

Properties of Methane

Physical Properties of Methane

Chemical Properties of Methane

Uses of Methane

Methane is widely used in industrial processes and many industrial pipelines carry natural gas that has methane as a major component. Methane is also transported as LNG (liquified natural gas) that is used in the cooling component of refrigerators. Though methane gas at ambient temperature is lighter than air, its density becomes larger as it is converted into cold gas. Therefore leakes of methane in the form of cold gas from the refrigerator cooling container is initially heavier than air.

1. Fuel : methane is majorly used as fuels in ovens, water heaters, turbines, automobiles and many other things. When refined liquid methane undergoes combustion with liquid oxygen, it acts as fuel for rockets as in BE-4 and raptor engines. The heat of combustion of methane at about 891 KJ/mol is less than any other hydrocarbon but it produces more heat with combustion per unit of its mass than any other organic molecule. This is because it has more no. of hydrogen atoms as compared to any other hydrocarbon. Therefore by burning about 22% of methane mass 55% of heat of combustion is produced. Also methane produces less percentage of carbon monoxide as compared to any other hydrides during its combustion. Therefore it is also considered as natural gas and is used as a fuel for cooking in homes. This natural gas has an energy content of 39 megajoules per cubic meter. Therefore methane is converted into liquefied natural gas for its easy handling, transportation and use. It is  a major component of natural gas, thus it is used for production of electricity. The methane in the natural gas is burnt as a fuel in the gas turbine or the steam generator. Methane has a great advantage as rocket fuel or kerosene due to its lower molecular weight. This enables methane small exhaust molecules that deposit less soot in the internal surface of the rocket motors and hence the booster reuse becomes easy. Due to the lower molecular weight of the exhaust it also increases the kinetic energy driven from the factor of heat energy that provides propulsion to the rocket by increasing its specific impulse. Methane is also very compatible and binds easily with liquid oxygen as it has a temperature range of 91-112 K which is a nearby range of liquid oxygen (54-90 K).

2. Chemical Feedstock : natural gas is mostly composed of methane that is largely used for the production of hydrogen gas at industrial scale. Steam reforming which is also commonly known as steam methane reforming is an industrial process that is commonly used to produce large scales of hydrogen gas and is commercially viable at industrial level. Most of these hydrogen are taken up by petroleum refineries for their internal use, for processing and production of chemicals and largely used to synthesize ammonia at industrial scale. In 2013, 50 million metric ton of hydrogen was produced worldwide from natural gas by process of steam methane reforming. The synthesis of ammonia in industries is carried out in these two steps.

1. In the presence of a metal base catalyst like nickel and at a high temperature of about 700 - 11100C, methane reacts with steam to yield carbon monoxide and water gas famously known as syngas. The reaction is strongly endothermic and absorbs a lot of heat energy which is about ΔHr = 206 KJ/mol and this reaction is known as water-gas shift reaction where carbon monoxide is a byproduct.

CH4 + H2O ⇋ CO + 3H2 (syngas)

2. In the second stage the byproduct carbon monoxide from the first stage is further made to react with steam in order to produce hydrogen gas and carbon dioxide as the final byproduct of very little amount. This is purely exothermic reaction and release heat energy which is about ΔHr = -41KJ/mol

CO + H2O ⇋ CO2 + H2 

Methane also undergoes free radical chlorination reaction with halogen radicals to produce chloromethane. For this reaction methanol is a better and typical precursor.