Organic chemistry is the study of carbon compounds.
There are more than 35 million organic compounds known till date and this number is increasing rapidly. This is because of the unique tendency of carbon to form bonds with successive carbon atoms known as catenation. Organic compound exhibit covalent bonding and are stable compounds. Organic compounds which consist of mostly carbon and hydrogen are referred as hydrocarbons. The basic hydrocarbon is methane (CH4).
The atomic number of carbon is 6 and the electronic configuration is as follows:
Ground State configuration: [1S2 2S2 2Px1 2Py1 2Pz0]
As per the electronic configuration, there are 4 electrons in the valence shell; out of which 2 are paired and 2 are unpaired. So the valance of carbon must be two as there are two unpaired electrons. But actually the valence of carbon is four. Now the question arising here is why? and how?
Let’s see an example. Suppose a student has passed B.Pharm examination and now he wants to get a job. He finds two opportunities: one in the same city where he lives and second in the nearby city. The first job is paying him Rs. 10000/month and will offer no future growth as such, but the second job is paying him Rs. 25000/month and there is good scope for him to grow and enrich his expertise. Which job he should choose? Almost everyone will agree that he should get the second job as it will make him more stable financially and offer good career prospective. But for getting that job, that student has to travel to that nearby city to attend the interview. For this he has to pay from his pocket. He has to make arrangements for his living there on his own. The student happily agrees to do so as the benefits he will get are far more lucrative.
Let’s apply the same analogy for carbon.
Everything in this universe tries to gain stability and carbon is no exception. When an atom forms a bond with another atom the energy is released hence the stability of the system increases. So by forming four bonds carbon can gain double stability than the case when it forms two bonds. Now we will see how carbon does that? Carbon inspects any possibility for having more unpaired electrons and it finds that if one of the electrons of 2S orbital is promoted to the empty 2Pz orbital, the valence shell will contain four unpaired electrons which then can form four bonds with other atoms. So carbon promotes 1 electron from 2S to 2Pz on account of its internal energy (paying from its pocket).
This is called excited state configuration, it is: [1S2 2S1 2Px1 2Py1 2Pz1]
Now carbon has four unpaired electrons so it will form four bonds with hydrogen atoms to form methane (CH4). As per above description three bonds out of four must have the bond character of 2P-S, as P orbitals are perpendicular to each other these bonds should also be at right angle to each other. The fourth bond being S-S (spherical) will not have any directional specifications. But from the x-ray crystallographic studies, it is evident that all four bonds in methane are placed at the corners of a regular tetrahedron, at the same bond angle of 109.5˚ and having same bond length also.
This can be explained with the concept of hybridisation. If we mix one S and three P orbitals to form four equal energy orbitals with similar shape, then the structure of methane can be justified. The mixing and rearrangement of atomic orbitals to form equal number of hybrid orbitals with equal energy and same orientation is called hybridisation. The hybridisation carbon undergoes depends upon the number of atoms with which it is attaching.
If it is attaching with four other atoms, it needs four similar hybrid orbitals. In this case, carbon undergoes SP3 hybridisation (1S+ 3P). Similarly for attaching with three or two other atoms, it will undergo SP2 or SP hybridisation respectively.
The concept of hybridisation plays very important role in organic chemistry.For better understanding of the concept, you can check my video on this topic:
Hybridization.. why and how? Video
Comments
Post a Comment