Saturated hydrocarbons are further classified into alkane (open chain of carbon atoms) and cycloalkane (closed chain of carbon atoms). Ethane can also be written as CH3-CH3. Ethane is colorless and odorless gas at standard temperature and pressure. The melting and boiling point of ethane are -182.8 °C and -89 °C, respectively. The flashpoint of ethane is -135 °C and hence, its vapors ignite easily by an ignition source. The molar mass of ethane is 30.07 g/mol. Ethane is obtained from natural gas and petroleum industrially. It can also be prepared from ethene, ethyl chloride, and sodium acetate in the laboratory. Here are some methods of preparation of ethane: CH2 = CH2 + H2 —–Pt/Pd/Ni——> CH3 − CH3 CH3 − CH2Cl + H2 —— Zn/H+ —–> CH3 − CH3 + HCl 2CH3Cl + 2Na ——Dry ether——> CH3 − CH3 + 2 NaCl 2 CH3COONa + 2H2O —–Electrolysis——> CH3 − CH3 + 2NaOH + H2 + 2CO2 Let us discuss the basic concepts of ethane such as its Lewis structure, polarity, hybridization of carbon atom in ethane, and its Molecular orbital (MO) diagram to understand its chemical bonding in terms of molecular orbitals.
C2H6 Lewis Structure
Lewis structure is a 2D representation of the compound, which represents only the valance shell electrons of the atoms in the molecule. It is based on the octet rule i.e. every atom tends to complete its octet ( 8 electrons) either by gaining or losing electrons except Hydrogen and Helium as they complete their duplet. Let us draw a Lewis structure of ethane step by step. Step 1: Determining the total number of valence electrons in the molecule. The valence electron for carbon (1s22s22p2) and hydrogen (1s1) is 4 and 1, respectively. In ethane, we have two carbon atoms and 6 hydrogen atoms and hence, the total number of valence electron are (2 X 4) + (1 X 6) = 14. Step 2: Drawing the Lewis structure: A carbon atom has 4 valance electrons and it needs 4 more electrons to complete its octet. Therefore, it forms 4 bonds, one with neighboring carbon atoms and three with three hydrogen atoms by sharing electrons. The Lewis structure of ethane would be:
We can count the total number of valance electrons in the Lewis structure of ethane, which is equal to 14. The total number of electrons around each carbon atom is 8 electrons and hence, it has completed its octet. Every hydrogen atom is surrounded by two electrons, leading to duplet formation. If we observe, we can see that all 14 valance electron has been used for bond formation. Therefore, there is no lone pair of electrons on any atom of ethane. Now, let us move towards from 2D representation to 3D representation of the molecule i.e., molecular geometry of ethane.
C2H6 Molecular Geometry
The molecular geometry of a compound is determined by valance shell electron pair repulsion (VSEPR) theory. According to this theory, the shape and geometry of the molecule depend on the number of bonding electrons and lone pair of electrons. In ethane, carbon is a central atom and it has no lone pair of electrons. The geometry and shape of ethane will be the same owing to the absence of lone pair of electrons. So, the geometry/shape of ethane can be predicted from the following table: We have observed already that carbon, a central atom forms 4 bonds, one with the nearest carbon atom and 3 with hydrogen atoms in the Lewis structure. The four bond pair of carbon corresponds to the tetrahedral geometry/shape of each carbon atom whereas one bond pair of every hydrogen atom corresponds to linear geometry/shape. Hence, the three-dimensional structure of ethane would be: The tetrahedral geometry of ethane leads to a bond angle (either H-C-H or H-C-H) of 109.5 °. The bond length of the C-C and C-H bond is 153.52 pm and 109.40 pm, respectively. After studying the lewis structure and molecular geometry, we move towards the hybridization of carbon atoms in the ethane molecule.
C2H6 Hybridization
Hybridization is the mixing of two or more atomic orbitals of similar energy, which leads to the formation of hybrid orbitals. The hybridization term is used in the valence bond theory (VBT) to explain the shape, formation, and directional properties of bonds in polyatomic molecules. Now, we will use valence bond theory to understand the hybridization of ethane. The ground state electronic configuration of C is 1s22s22p2, with two unpaired electrons in p orbital and one pair of electrons in s orbital of valence shell. However, carbon forms four bonds, and paired electrons do not participate in bond formation. Hence, one of the 2s electrons gets excited to p orbital and the excited state electronic configuration of the carbon atom would be 1s22s12p3. This excitation energy is obtained from the overlapping of carbon and hydrogen orbitals. One 2s orbital and three 2p orbitals mix as they are of nearly similar energy and form four sp3 hybrid orbitals of equivalent energy. Therefore, the hybridization of both carbon atoms in ethane is sp3 with tetrahedral geometry. Out of four sp3 hybrid orbitals, three of them will overlap axially with three 1s orbitals of the hydrogen atom and one will overlap axially with the sp3 hybrid orbital of another carbon atom. It leads to the formation of four sigma bonds associated with each carbon atom. The orbital diagram of ethane is given below:
C2H6 Polarity
The electronegativity of C and Hydrogen is 2.6 and 2.2 on the Pauling scale, respectively. The electronegativity difference between carbon and a hydrogen atom is 0.4, which is very less and hence, the C-H bond in ethane is nonpolar. It leads to the nonpolar nature of the ethane molecule. The polar solvent dissolves polar molecules and the nonpolar solvent dissolves nonpolar molecules. Therefore, ethane is more soluble in toluene and benzene and very less or negligible soluble in the water.
C2H6 Molecular Orbital (MO) Diagram
The molecular orbital theory, a quantum mechanical model, is used to draw the molecular orbital (MO) diagram of the ethane molecule. It is based on the linear combination of atomic orbitals, which lead to the formation of the molecular orbital. The molecular orbital diagram of ethane would be:
The molecular orbital is formed from the combination of atomic orbitals, which must have nearly the same energy and are symmetrical about the molecular axis. To understand the MO diagram of ethane, we consider it as a homonuclear diatomic A2 molecule. We assume A as the CH3 group and will consider only valance shell electrons for clarity. The total number of valence electrons in ethane is 14 and hence, each CH3 group will have 7 electrons. These seven electrons can be distributed in various atomic orbitals such as σ, π, and nσ. As two atomic orbitals combine to form two molecular orbitals, one is bonding and the other is nonbonding. Three atomic orbitals combine to form three molecular orbitals and so on. Hence, the formation of molecular orbitals can be understood as follows: Once the molecular orbitals are formed, the distribution of electrons into molecular orbitals takes place by following the Aufbau principle, Hund’s rule of maximum multiplicity, and Pauli’s exclusion principle. Therefore, the electrons are filled in molecular orbitals in order of their increasing energy such as σs σs′ πy = πz σx πy′ = πz′ σx′. According to Pauli’s exclusion principle, one atomic orbital can have a maximum of two electrons with opposite spins. The same is also true for molecular orbitals. The pairing of electrons in the degenerate molecular orbitals will take place only when all the degenerate molecular orbitals are singly occupied. Hence, the electronic configuration of ethane molecule for valance shell electron would be: (σs)2(σs′)2(πy2 = πz2)(σx)2(πy′2 = πz′2)
Conclusion
Ethane or C2H6 is a saturated hydrocarbon and the second member of the alkane family. It is inert at room temperature and pressure and hence, it is known as paraffin. However, it shows some chemical reactions under certain conditions. The nature of chemical reactions can be well understood by studying the basic concepts of the reactant molecule. Here, we have learned to draw a Lewis structure of ethane molecule by following the octet rule and then, found their molecular geometry, which came out to be tetrahedral for both carbon atoms. Afterward, we discussed the hybridization and polarity of ethane. In the last, we studied the molecular orbital diagram of ethane by considering the CH3 group as an atom for better understanding. In short, we have studied every aspect of bonding in ethane. Please feel free to ask any query related to the binding nature of the ethane molecule. Thank you for reading this article.
