ORGANIC CHEMISTRY: AN INTRODUCTION TO BASIC CONCEPTS

WHAT IS ORGANIC CHEMISTRY?

Let us start with the question "What is Organic chemistry?".
The simple answer is: It is the chemistry of carbon containing compounds, which are otherwise known as organic compounds.
So it is pretty easy to recognize that we should start our journey of organic chemistry by exploring the chemical nature of carbon.

WHAT IS CARBON?

So the next question is: What is carbon?
    * Carbon is an element with atomic number (Z) = 6.
    * Its ground state electronic configuration can be represented as: 1s²2s²2p² (or) 1s²2s²2p_x¹2p_y¹2p_z⁰

   

    * It is the first element in Group-14 of Long form of Periodic table.
    * It is a non metal.
    * On Pauling's scale, its electronegativity value is around 2.5.
    * It usually forms covalent bonds.
    * Its valency is 4 since there are four electrons in the outer shell i.e., it can form four covalent bonds with other atoms.

WHY THERE ARE MILLIONS OF ORGANIC COMPOUNDS? 

It is well known that there are millions of organic compounds around, which are either originated from the nature or prepared synthetically.
Examples of organic compounds include carbohydrates, proteins, enzymes, vitamins, lipids, nucleic acids , synthetic polymers, synthetic fabrics, synthetic rubbers, plastics, medicines, drugs, organic dyes and so on.
Now the immediate question is: Why the carbon atom is so special and forms millions of compounds?
To answer this question, we should know about catenation.
Catenation is the ability of atoms of same element to bond covalently among themselves and form long chains or rings.
Carbon has a stronger tendency to catenate since it is a smaller atom and can form stronger covalent bonds with other carbons. The C-C bonds are stronger due to effective overlapping of atomic orbitals.

It also forms stronger bonds with other elements like hydrogen, oxygen, nitrogen, halogens, sulfur, phosphorus etc.
The organic compounds can also exhibit isomerism due to different structural and spatial arrangement of atoms or groups leading to formation of huge array of compounds.
These arguments explain why the carbon can form millions of compounds and organic chemistry is flourishing like nothing.

HOW DOES CARBON FORM CHEMICAL BONDS?

LEWI'S DOT MODEL 

Carbon is an appreciably electronegative element and tends to form four covalent bonds by using all the four electrons in its valence shell i.e., the second shell for which the electronic configuration can be written as 2s²2p². Hence the combining power or the valency of carbon is 4. It can form 4 bonds with other atoms.
It is possible to understand the bonding in carbon compounds by using Lewi's dot model. According to this model, each atom participating in the bonding contributes one electron to form an electron pair which is shared between the two contributing atoms. Thus a covalent bond is formed. If atoms share two electron pairs, a double bond is formed. And a triple bond is formed when three electron pairs are shared.
The purpose of participating in bond formation is to get the nearest inert gas configuration and thus by getting stability. Most of the atoms try to get eight electrons or octet configuration in the valence shell. This is also called as octet rule.
The structures of some simple organic molecules are explained as shown below.
Methane, CH₄: The carbon atom contributes four valence electrons to make four bonds with hydrogen atoms. Each hydrogen also contributes one electron for the bond formation.
Thus there are 4 C-H bonds in the methane molecule and carbon gets octet configuration in the valence shell.
Note that the valency of hydrogen atom is one. It can form only one bond since there is only one electron in this atom. It also gets Helium's configuration during bond formation.
Also note that in Lewi dot models, only the valence electrons are shown. The bond pairs can also be shown by lines.

Ethane, C₂H₆: In Ethane molecule each carbon forms 4 bonds again. Among them three are C-H bonds, while the fourth one is a C-C bond.

Ethylene, C₂H₄: In this molecule, there is a double bond between two carbon atoms due to sharing of two pairs of electrons. Each carbon also forms two bonds with hydrogen atoms.

Acetylene, C₂H₂: There is a triple bond between two carbon atoms in acetylene molecule. It is formed due to sharing of three electron pairs. Each carbon also forms a single bond with hydrogen atom.

 Methyl fluoride, CH₃F: Since there are 7 electrons in the valence shell of Fluorine, it require one electron to complete octet. Hence it contributes one electron for bond formation with carbon as shown below.
Note that the only the bond pairs are shown as lines in the second representation.
There are three lone pairs and one bond pair around fluorine atom. The bond pair is shown as a line.

In the same way, carbon atom forms bonds with other halogen atoms.
Formaldehyde, CH₂O: There are six electrons in the valence shell of oxygen. It forms two bonds by contributing two of its valence electrons and thus by completing the octet. In formaldehyde, the oxygen atom forms two bonds with a carbon atom.

 

Methyl alcohol, CH₃OH: However, the oxygen atom can also form just one bond with the carbon as in case of methyl alcohol. It forms the second bond with hydrogen.

Hydrogen cyanide, HCN: The carbon atom can also form bonds with nitrogen. In hydrogen cyanide there is a triple bond between carbon and nitrogen. The nitrogen atom contributes three of its valence electrons for the formation of this triple bond.

Methyl amine, CH₃NH₂: In this molecule, the carbon and nitrogen atoms are sharing only one pair of electrons.

However this model could not explain the exact geometry of organic molecules. For example, methane molecule is tetrahedral, whereas ethylene is a planar molecule. These structures with exact bond angles cannot be explained by this model. Therefore it is necessary to understand the structures of these molecules by using valence bond theory as explained in the next section.

VALENCE BOND THEORY 

According to valence bond theory, four unpaired electrons are required to form four covalent bonds. But there are only 2 unpaired electrons in the valence shell of carbon in the ground state.
However it is possible to get 4 unpaired electrons by transferring one of the electrons from 2s orbital into the empty 2p orbital. This process is called excitation and carbon is said to be in the excited state. Now the electronic configuration of carbon in the excited state becomes 2s¹2p³.
A small amount of energy, which is available during the chemical bond formation, is sufficient for a carbon atom to undergo excitation.

It is now possible for carbon atom to form 4 bonds in the excited state.
However, carbon undergoes hybridization before forming actual chemical bonds with other atoms.
Hybridization is the process of intermixing of two or more pure atomic orbitals of almost same energy to form same number of identical and degenerate new orbitals known as hybrid orbitals.
The carbon atom can undergo three types of hybridizations i.e., sp³ or sp² or sp.

SP³ HYBRIDIZATION OF CARBON

In sp³ hybridization, one 2s and three 2p orbitals of excited carbon intermix together and form 4 hybrid orbitals which are oriented in tetrahedral geometry in space around the carbon atom. Each sp³ hybrid orbital is occupied by one electron.

Each of these sp³ orbitals can form σ-bond with other atom. Thus carbon is forming four single bonds with other atoms in tetrahedral geometry. The bond angles are usually equal to or nearer to 109^o28'.
E.g. In methane molecule, CH₄, the carbon atom undergoes sp³ hybridization and forms four σ-bonds with hydrogen atoms.

Note that whenever carbon atom undergoes sp³ hybridization, it forms 4 σ-bonds i.e., 4 single bonds.

SP² HYBRIDIZATION OF CARBON

In sp² hybridization, there is intermixing of one 2s and two of the 2p orbitals of carbon in the excited state to form three hybrid orbitals. These are oriented in trigonal planar geometry. Each sp² hybrid orbital is occupied by one electron. The remaining pure 2p orbital with one electron lies at right angle to the plane of hybrid orbitals.

 The sp² hybrid orbital form 3 σ-bonds in trigonal planar geometry. Thus the bond angles are about 120^o. The remaining pure 'p' orbital will form a π-bond. Thus carbon forms total four bonds i.e., three σ-bonds and one π-bond.
E.g. In ethylene molecule, C₂H₄, each carbon atom undergoes sp² hybridization. Each carbon forms 2 σ-bonds with hydrogens and one σ-bond with another carbon. The remaining pure 'p' orbitals on two carbons overlap sidewise to form a π-bond. Thus there is a double bond between two carbons.

Note that whenever carbon atom undergoes sp² hybridization, it forms 3 σ-bonds and 1 π-bond i.e., two single bonds and one double bond.

SP HYBRIDIZATION OF CARBON

In sp hybridization, one 2s and one 2p orbitals of excited carbon intermix to form two sp-hybrid orbitals in linear geometry. Each sp hybrid orbital is occupied by one electron. The remaining pure 2p orbitals ( for our convenience, let us say: 2p_y and 2p_z) orient at right angles to the sp-hybrid orbitals. These are also occupied by one electron each.

The two sp hybrid orbitals form 2 σ-bonds in linear geometry. Thus the bond angle will be about 180^o. The remaining pure 'p' orbitals will form two π-bonds. Thus carbon again forms total four bonds i.e., two σ-bonds and two π-bonds.
E.g., In acetylene molecule, C₂H₂, each carbon undergoes sp hybridization and forms one σ-bond with a hydrogen atom and one σ-bond with another carbon. The two carbon atoms also form two π-bonds with each other due to sidewise overlapping of pure p-orbitals. Thus a triple bond is formed between two carbon atoms in acetylene molecule.

Note that whenever carbon atom undergoes sp hybridization, it forms 2 σ-bonds and 2 π-bonds. It may either form one triple bond as in case of acetylene or two double bonds e.g. allenes.
The following diagram summarizes the bonding pattern by carbon atom in different hybridizations.