Reason for the formation of large number of organic compounds : Catenation

Reason for the formation of large number of organic compounds

 What makes the carbon so special ?

What is it that sets carbon apart from all other elements in the periodic table ?

Why are there so many organic compounds ?

The answer lies in carbon’s position in the periodic table. Carbon is in the centre of second row elements

Li > Be >  B > C > N > O > F

First think why molecules are formed from atoms ? It is because of the reason that atom combines with same or with other atoms to form molecule so as to complete its octet and attain lower energy stable and hence become stable. That is the reason why noble gases are considered as inert gases, they generally do not combine with itself or with other atoms because they have complete octet. But what about other atoms ? They have incomplete octet, so they must combines with same or other atoms to form molecule for better stability.

Elements on the left hand side of carbon have less than 4 electrons in the valence shell (Li-1, Be-2, B-3) so they have more tendencies to loose electron to attain noble gas configuration for stability. That’s why they generally forms compounds with Li+, Be2+, B3+ by losing 1, 2, 3 electrons respectively. Elements present downside in the same group too have similar tendency as that of Li, Be and B, hence form compounds in the following states; Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Al3+, Ga3+, etc.

Elements on the right hand side of carbon have more than 4 electrons in the valence shell (N-5, O-6, F-7). To complete their octet, valance electron must be subtracted from 8 that’s why the valency of N is (8-5) i.e. 3, O is (8-6) i.e. and that of F is (8-7) i.e. 1. It is much easier to gain 3, 2, 1 electrons to complete their octet as compared to loosing 5, 6, 7 electrons to complete their octet. So these elements have more tendency to gain electrons and form compounds in the following states; N3, P3, As3, Sb3, Bi3, O2, S2, Se2, Te2, Po2, F, Cl, Br, I.

As elements present on the left hand side of carbon loose electrons to form compounds and elements of right hand gain electrons to form compounds so compounds formed are ionic in nature.

But think about carbon and the elements present down side, which are present in the middle of each period and have equal tendency to loose or gain electrons as they have 4 electrons in their octet. This led carbon and other elements of this group (Si, Ge, Sn & Pb) to share electrons with itself and other elements of periodic table to complete octet. As these compounds are formed by sharing of electrons so they are considered to be covalently bonded.

Carbon by sharing its electrons with other carbon atoms leads to formation of long chain carbon compounds which may be single, double or triple bonded, cyclic or acyclic, linear or branched. This self-linking property of carbon is called catenation. All the atoms of 14th group show the property of catenation but it decreases down the group because of weak overlapping due to large size and follows order :

C >> Si >> Ge > Sn > Pb

Carbon may also form multiple bonds with N, P, O, S etc. forming large number of functional group, which we will discuss later.

This is not the end of compound formation, carbon forms many abnormal compounds with elements of s, p & d blocks. So for sake of simplicity we are constructing an organic chemist’s periodic table with the most important elements emphasized.

Elements, which are in dark box, are generally involved in making organic compounds along with deuterium (D), which is an isotope of hydrogen (H).

As there are large number of atoms in periodic table which have valence electrons, atomic orbital of carbon may overlap with them and share its electron to form large number of compounds. But for that many other factors such as size, activation energy, electronegativity, electron affinity, catenation etc. are responsible which all come under one word “Position” i.e. position for carbon in the periodic table. This word “position” include everything related with molecule formation therefore the main reason behind large number of organic compounds is the position of carbon in the periodic table.


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