Introduction To Organic Chemistry

ORGANIC CHEMISTRY INTRODUCTION: - Organic chemistry is the behavioral study of those chemicals which are not only used in our daily life but also touches all walks of our life. Our very own existence, growth and survival depends on organic chemicals. Domain of organic chemistry is so vast that it not only extends to whole universe around us but also exists deep inside us down to the level of microscopic cell and tissue. That is why study of these chemicals though is very simple yet very complex but very important. DEFINITION: -

Definition of organic chemistry is being constantly modified. In the eighteenth century it was the study of chemicals found in organism. It was so called because it was thought that organic chemicals could not be synthesized in the laboratory. However now large number of chemicals is not only prepared in laboratory but also produce industrially. During the process of research it was discovered that all organic compounds contain carbon. Therefore modern definition is the study of carbon compound. Although there are quite a few compound which contain carbon and are not organic. But the name organic stuck because it is still the study of chemical found in living being

CONSTITUENT ORGANIC

1) Carbon –

Present in all organic compounds

2) Hydrogen - It is not found in few compound but present in most compounds of organic chemicals In addition to these element nitrogen, oxygen, chlorine, bromine, sulfur, phosphor, etc as well as some metals are also present in organic chemicals COMPOUND: -

A compound is formed when at least two atoms are bonded to each other. However there are several compound which contain more than two atoms

BONDS: -

a) These are forces that keep the two atoms together. b) The attractive and repulsive electrostatic forces between negatively charged electrons and positively charged nuclei bond the two atoms c) Principals of higher stability to posses lower energy is the cause of formation of bond. d)

Electrons posses energy. Lower energy increases stability Hence electrons are constantly thriving for opportunity to reduce their energy. The energy is minimum when the electrons are nearest to nuclei and maximum when farthest from electron. When two atoms come closer electrons of one atom are nearest to nuclei and farthest from the electron of other atom. This conditions suit the electron and nuclei of both atoms. Thus both atoms posses lower energy and achieve higher stability Stability of atoms is also explained on the basis of electronic configuration of inert gasses. All the gasses are highly stable. Nobel gasses (inert gasses) have either two or eight electron in their outer most shell. This explains that atoms are highly stable when they either have eight or two electrons in their outer most shell. Octet rule: - The presence of eight electrons in outer most shell Duet rule: - The presence of two electrons in outer most shell TYPE OF BOND a) Ionic bond: -

Ionic bond is formed when both bonding atoms achieve octet or duet by loosing or accepting electron e.g. MgO (Magnesium Oxide)

Magnesium oxide is formed when two electrons from magnesium is transferred to oxygen. Thus both atoms achieve their octet. b) Covalent bond: - The bond is formed when the atomic orbital overlap each other. The two overlapped atomic orbital form a single orbital called – Bond Orbital which contains electrons from both atoms. Thus electrons in bond orbital is shared by both atoms forming the bond. Sharing of electrons follow octet and duet rule with few exception Covalent bond needs minimum two electrons to form a bond, one from each of the atom forming the bond. However the two electrons can also come from same atom

Single bond: - A bond is called single bond when one pair of electron is shared between bonded atoms. Double bond: - Double bond needs two pairs of electron to be shared. Triple bond: - For formation of triple bond three pairs of electron are required Coordination : It is the bond formed when the two electrons come from the same Atom bond

Sigma bond : ( σ – bond )

The σ – bond is formed by head on overlap of atomic orbital along a line drawn between the nuclei of the two atom. The σ - bond is formed by haring of one pair of electron.

Pi – bond (∏ - bond )

Pi – bond is formed in addition to σ – bond. After the formation of σ – bond when orbital of the bonding overlap side ways a pi – bond is formed. Carbon can form maximum two pi – bond First side way overlap of orbital forms one pi – bond and second side way overlapping of orbital form two pi – bond. A sigma bond forms a single bond. A pi – bond forms double bond one sigma and one pi – bond and a triple bond is formed when one sigma and two pi – bonds are formed.

Formulae: - Organic chemistry needs in depth knowledge of various Compounds, their physical and chemical properties, In order to study all these properties it is essential to know constituent of the compound

To study these compounds a system of formulae have been developed. There are various types of formulae and each serves a different purpose. Type of Formulae;Molecular formula;- It shows exact number of atoms of various elements present in the compound. Ethyl alcohol is represented as C2H5OH it means two carbon atoms, five hydrogen atoms attached to carbon and one atom of each oxygen and hydrogen attach to carbon Empirical formula:- It represent simplest ratio of number of atoms present in a Compound. Benzene (molecular formula C6H6 ) and Glucose( molecular formula C6H12O6 ) has empirical formula CH and respectively Structural formulae;- These are very important tools used by the organic chemists. It is like infrared glasses which allows to see through the darkness inside a molecule. It has been established that chemical and physical

behavior of a compound is determined by molecular structure. It shows connectivity among various atoms. Dash formula: -

The formulae not only shows exact number of atoms present in a molecule but also how they are connected with each other. e.g. ethyl alcohol and diethyl both have the same molecular formula C2H6O but different structure

Dash formula does not show three dimensional structures Dash and wedge formula :

This formula shows three dimensional structures as well as exact number of atom present in a molecule.

Connectivity is shown by these ways. A dash, a solid wedge and a dashed wedge. Atom attached to solid wedge represents that it is coming out of paper, while a dashed wedge shows atom going into the paper and simple dash means atoms are in the plane of paper.

Lewis dot Structure:

Condensed Formula: -

The structure shows how electrons of each atom are shared to complete either their octet or duet of the unshared electrons are shown attached to the particular atom. The electrons completing octet or duet are shown encircled.

The formula shows molecular formula in expanded form. Propyle alcohol has molecular formula C3H7OH while condensed formula is CH3CH2CH2OH. It does not show any bond and it also does not follow any bond order, e.g. in CH2Cl2 chlorine shown in the formula do not bond to hydrogen but bonded to carbon.

Bond line formula :

The formula does not show any carbon and hydrogen atom attached to carbon. Each line represents a bond. Carbon is present at each bending, line intersection, and the end of the line. Hydrogen attach to carbon is calculated by subtracting number of bond from valency of carbon

The other atoms or group of atom are shown at the location wherever these are present.

Fisher projections formula :

The carbon atom at the intersection of vertical and horizontal line is not shown. In Fisher projections the vertical lines are out of the plane of paper and horizontal lines are in the plane of paper.

Newman projection formula :

The formula is used only for the molecules which have two or more than two carbons. Molecules are viewed in such a way that the first two carbons are in line and one is exactly behind the other. Intersecting lines at a point represent one carbon while a large circle represents another. All sigma bonds are shown at the end of lines extended redially outwards

Ball and stick formula :

The model shows 3 – D effect of the molecule. Another important aspect of the model is that atoms, bond length and bond angles are made to correct scale and dimension. It means a large atom is shown larger than a smaller atom as well as longer bond length

shown larger than a smaller bond length.

All the structural representations have their usefulness and importance. Writing of Lewis dot structure Lewis dot structures are very important in organic chemistry. It shows the sharing of electron between bonded atom as well as how non bonded electrons are placed. It shows only valence electrons are arranged around the constituent atom in such a way that all atoms complete their octet and hydrogen atom its duet. The method and procedure of developing Lewis dot structure is explained by using a molecule of sulfur trioxide SO3 Step ( 1)

Calculate total number of valence electron of all the atoms present in the molecule

(2)

Determine the central atom - The central atom should be either of the element with least electronegetivty or of the element with single atom in the molecule. In the example the element with single atom is sulfur

(3)

Arrange sulfur in the centre with three oxygen atoms around it.

( 4)

Each oxygen atom is bonded to sulfur atom by one pair of electrons. Arrange three pairs around sulfur

( 5)

Arrange non bonded pairs of oxygen around oxygen to complete valence electron of oxygen.

(6)

The valence electrons of oxygen are complete. However valency of sulfur is incomplete.

(7)

Arrange one pair from any oxygen atom around sulfur to complete valency of sulfur

As a result of these steps every atom completes its octet and Lewis dot structure of sulfur trioxide is complete.

In formation of Lewis structure it is irrelevant that which electron comes from which atom. The only important fact is that hydrogen should complete its duet and the other atoms their octet. Valance: - It is the number of electrons of an atom which form bond. Valance of some of the element forming organic compound are

Formal charge: It is the difference in number of valence electron in bonded form and number of electron in valence shell of the unbounded atom. It is not the charge of ion but the charge on each atom in the molecule However sum of formal charges of all atoms is formal charge of the molecule.

Index of hydrogen deficiency: It is very useful in obtaining structural information about an unknown molecule from its molecular formula.. It is defined as number of pairs of Hydrogen atoms that must be subtracted from the molecular formula of corresponding alkane to give the molecular formula of

the compound under consideration.

For this purpose halogen is counted as hydrogen, Oxygen is not considered and nitrogen is counted as half hydrogen. Index of hydrogen deficiency for saturated hydrocarbon is always zero

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