Chemistry of Carbon Bonds - Part I

Carbon has a unique place in our lives. Each living cell, food, wood, paper, petro-chemicals, cooking gas, perfumes, etc are all made up of carbon. Chemistry of carbon compounds is known as organic chemistry. Organic chemistry encompasses study of all carbon-hydrogen compounds. These are also called hydrocarbons. Inorganic carbon chemistry is the study of oxides, nitrides and allotropes of carbon.

There are many differences between organic and inorganic compounds. Organic compounds are mostly insoluble in water. They have low melting points and exist by forming covalent bonds. Inorganic compounds on the other hand usually dissolve in water and have high melting points. Inorganic compounds can be formed due to either ionic or covalent bonds. Another characteristic of organic compounds is that they are inflammable.

Carbon has 6 protons, 6 neutrons and 6 electrons. Its chemical symbol is ¹²C ₆. Its electronic configuration is 2 electrons in the K-shell, and 4 electrons in the L-shell. In principle it should either give up or borrow 4 electrons while forming compounds. But it doesn’t form ionic bonds at all. It likes to share its four electrons with other atoms and form covalent bonds instead. Carbon covalent bonds are the strongest in nature.

What we will study in this chapter :  

1. How carbon forms tetravalent bonds
2. Why are there so many carbon compounds

1. How carbon forms tetravalent bonds  
Before we look into details of the type of covalent carbon bonds, let us know a little about the electron orbits. We have seen earlier that electrons move around the positively charged nucleus in various orbits. The first orbit closest to the nucleus is taken as the first orbit n=1 and is called the K-shell, the next higher orbit is assigned n=2 and is called the L-shell, and so on.

The moving electron is also assigned an orbital (angular momentum) number l, and l takes values
l = 0,1 …to (n-1).  Shapes of the electron orbit depend on the l number.
l = 0 is called the s orbital and is spherical. 
l = 1 is called the p orbital and is dumb-bell shaped. 
The dumb bells can be oriented in space in three different directions. So the dumb bells themselves are three types p_x, p_y, p_z. 
l = 2 is called d orbital and has a very complex shape in the three dimensional space. 
In principle, n is called the principle quantum number and l is called the orbital quantum number.

In addition to orbital quantum number, electron has an intrinsic spin assigned to it. There are only two ways an electron spin quantum number is oriented : either up  or down  . Electrons like to pair up. So if there is a single electron orbiting with spin up, it will try to seek another electron which has spin down.

Table below gives some idea of the quantum number assigned to various orbits.  

In the electronic configuration the first number shows the main shell (K, L, M, etc.), the second letter shows the shape of the orbital (s, l, d, g, h, etc) and the next number shows how many electrons are there in the shell. The orbital,  s, p, d, etc are also known as sub-shells.  

The six electrons of carbon are distributed as   

From this picture, we should feel that carbon is a divalent compound and should borrow two electrons in its p_x and p_y orbital. But this does not happen!! Carbon does not form compounds like CH₂. Instead carbon shows tetra valency.  Since the 2s and the 2p orbitals are very close in energy, one electron from the 2s orbital jumps to the 2p_z orbital.

The picture is as given below :  

We might expect that a carbon atom to form three bonds of one kind using the p orbital electrons and one bond of another kind using the s orbital electron. But this does not happen!! The one 2s and three 2p orbitals mix together and give rise to four new altogether different types of orbitals. This is called hybridization and is seen only in carbon atom. The four orbitals are at an angle of 109⁰28’. This is called tetrahedral or sp³ (pronounced as sp three) type of hybridization. There are sp² and sp types of hybridization also.

A good example of sp³ hybridization is the methane CH₄ molecule. The methane molecule has a tetrahedral shape. The C atom is at the center of the tetragon (three dimensional equilateral triangle) and the four H in the four corners of the tetragon. Each carbon bond in methane makes an angle of 109⁰28’ with the other bonds.   

            Structure of a methane molecule

The sp³ hybridization is generally denoted as follows :

In  a sp² hybridization, the two of the four carbon bonds are parallel. The sp² hybridization is generally denoted as follows :  

Thus the sp² hybridization leads to carbon double bonds. The angle between the three directions of the bonds is 120⁰.  A molecule of ethene  (CH₂  CH₂)  is a good example of sp² hybridization.

For sp hybridization the molecule of ethyne is an example. Here three of the bonds lie parallel to each other   (CH  CH). The angle between the  direction of the bonds is 180⁰.

When a carbon atom forms a compound, it always forms covalent bonds. There are two types of covalent bonds : sigma and pi bond. When the covalent bonds are linear or aligned along the plane containing the atoms, the bond is known as sigma () bond. Sigma bonds are strong and the electron sharing is maximum. Methane CH₄ is a good example for sigma bond and it has four of them. 

       Sigma () bond

When the electron orbitals overlap laterally, the bond is called the pi () bond. In pi bonds the resulting overlap is not maximum and these bonds are relatively weak. A molecule of ethylene or ethene (C₂H₄) has 4 bonds and 2  bonds.  In pi bonds the resulting overlap is not maximum and these bonds are relatively weak. A molecule of ethylene or ethene (C₂H₄) has 4 bonds and 2  bonds.  

            Pi () bond

From the above discussions it is clear that the sigma () bonds are stronger than the pi () bonds, as the overall of the electron orbital between C and it neighboring atom is maximum. In case of - bonds, the overlap between C-C atoms is lateral and hence not maximum. Thus carbon compounds formed due to double and triple bonds are called unsaturated carbon compounds.  

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