ORGANIC COMPOUNDS, CHARACTERISTICS OF CARBON ATOMS

MENCLATURE OF ORGANIC COMPOUNDS 

 HISTORY : 

In 1675, Nicholas Lemery had devided chemical substances into 3 parts.

1. Mineral substance : which are obtained from minerals. eg. gold, silver, iron etc. 
2. Vegetable substance : which are obtained from vegetables. eg. sugar, citric acid etc. 
3.  Animal substance : which are obtained from animals. eg. albumin, gilatin etc. After some time when many of the chemical substance were discovered, it was found that some of them can be obtained from both vegetables and animals. So this classification was failed. So chemical substance were then divided into two parts :

           (i) Organic compounds : which are obtained from living organism. 
          (ii) Inorganic compounds : compounds which are obtained from any other sources except                        living organisms.

VFT(Vital Force Theory) : 

By Berzelius in 1815. Upto 1815, any organic compound could not be synthesized in lab. So Berzelius suggested that there is a mysterious force in living organisms which was named as Vital Force and said that organic compounds cannot be synthesized in lab. This theory was called as VFT. But in 1828 a German scientist Wholar synthesized an organic compound in lab. Which was 'urea'. So VFT was failed. Urea was synthesized in lab by heating of Ammonium cyanate (NH4CNO)

Organic Compounds : 

Hydrocarbons and their derivatives are called as organic compounds.

Why are organic compounds found in larger no.? or Why are they studied as a separate subject ?

ANS :

1. Catenation Property : Carbon atom has a property by which it can join with other C–atoms and form a long chain or a ring of different size and shapes.If covalency of atom is more, then catenation property is also more.
2. Organic compound shows isomerism.
3. Exhibits Homologous Series. 
4. Same Empirical Formulae. 
5. Polymerisation.

Characteristics of C-Atoms

1.Tetra valency :

 Atomic number of carbon atom is 6 and it have four valency electrons so C-Atom is tetravalent. It is explained by promotion rule 
                 In ground state (here covalency of carbon is 2) 

               First excited state (here covalency of carbon is 4)

2.Tendency to form multiple bonds :

 Carbon atom forms following type of bonds, such as

3.Tetrahedral shape : 

The four covalent bond are directed towards the four corners of a regular tetrahedron C 109°28' 

Bond angle 109°28'  or 109.5'

4.Catenation :

 Self linking property of C-atom is known as catenation. It is responsible for the variety and large number of organic compounds. It may also give rise to open chain and closed chain nature of compounds. Bond energy for catenation of C is maximum.

Bond energy in Kcal : C — C          Si—Si          N — N       P — P 

                                        85                  54                 39            50

5.Hybridisation : 

The orbitals of different shape but almost of equal energies blend up to give the same number of new orbitals of another shape and of identical energies.

σ  (sigma) bonds :

 The molecular orbital formed by the overlapping of two-s atomic orbtials or one s and one p atomic orbitals or co-axial overlapping of p-orbitals is called a σ bond.

Note : (i) Overlapping of hybrid orbitals also give  σ bonds.  σ bonds are stronger, as they are resulted from the effective axial overlapping.

 (ii) More the directional character (p) in covalent bond more is the strength of the bond.

                   sp3 - sp3 > sp3 - sp2 > sp2 - sp2 > sp - sp

 𝝅 (Pi) bonds :

 𝝅 bond is formed by the + lateral overlapping of two p-atomic orbitals. It is weaker than  σ bond, as there is only partial overlapping.   

Note : (A)  electrons are mobile hence  𝝅 bond is more reactive.  𝝅 bond is formed by the collateral overlapping of sp2 orbitals. 

(B) sp2 hybridised orbitals overlap with each other and with s orbitals of six H-atoms forming C–C and C-H σ  bonds. 

(C) Six 2p unhybridised orbitals of 6 C-atom in benzene form 3  𝝅 bonds by lateral overlapping with each other. These six  𝝅 electrons are free to move over all the six carbon atoms. Since delocalised electrons have lower energy than localised. 

(D) The relative sizes of hybrid orbital follows the order sp3 > sp2 > sp 

(E) The electronegativity of hybrid orbitals follows the order sp > sp2 > sp3