ORGANIC CHEMISTRY - Resonance

The phenomenon of resonance is said to occur if for a molecule, we can write two or more than two Lewis structures which differ in the position of electrons and not in the relative position of atoms. The various Lewis structures are called canonical structures or the resonance structures. The real structure of the molecule is not represented by any one of the canonical structures but is a resonance hybrid of the various resonance structures. The various resonance structures are separated by a double headed arrow. This arrow should be clearly distinguished from that used for an equilibrium.
Resonance hybrid is always more stable than any of its canonical structures. Thus, the actual molecule of benzene is more stable than either of two Kekule structures. The magnitude of stability conferred for a molecule as a result of resonance is expressed in terms of resonance energy or delocalization energy. It is defined as the difference in internal energy of the resonance hybrid and that of the most stable canonical form.

Heat of hydrogenation :- Heat of hydrogenation is the amounts of heat evolved when one mole of an unsaturated compound is hydrogenrated.

Halogenation :- The halogenation of benzene and some others arenes is carried out at low temperatures, in the absence of sun light and in presence of Lewis acid such as anhydrous ferric or aluminium chloride as catalyst.

CHEMISTRY - Organic Compound

The simplest organic compounds containing only carbon and hydrogen are called hydrocarbons. They are considered to be parent organic compounds while all the organic compounds are thought to have been derived from them by replacement of one or more of their hydrogen atoms by other atoms or group's of atoms. As the study of ore chemistry advanced, the number of organic compounds became very large. As a result, it became rather inconvenient to study the chemical behavior of these compounds individually. Therefore, in order to simplify the systematize the study of organic chemistry, all the known organic compounds have been broadly divided into two categories depending upon the nature of their carbon skeleton. These are :-

1. Acyclic or open chain compounds.
2. Cyclic or closed chain compounds.

1. Acyclic or Open chain compounds :- These compounds contain open chains of carbon atoms in there molecules. The carbon chains may be either straight chains or branched chains.
Open chain compounds are also called alphabetic compounds since the earlier compounds of this class were obtain either from animal or vegetable fats.

2. Cyclic or closed chain or ring compounds :- These compounds contain one or more closed chain or rings of atoms in their molecules. Depending upon the constitution of the ring, these are further divided into the following two categories :-

* Homocyclic or Carbocyclic compounds :- These compounds contain rings which are made up of only one kind of atoms, I.e carbon atoms. These are further divided into the following two sub-classes.

(a) Acyclic compounds :- Carbocyclic compounds which are resemble alphabetic compounds in most of their properties are called alicyclic compounds.

(b) Aromatic compounds :- These are special type of cyclic unsaturated compounds. The name aromatic for this class of cyclic unsaturated compounds has been derived from the Greek word aroma meaning fragrant smell simple since most of the compounds discovered earlier had pleasant smell.

* Heterocyclic Compounds :- Cyclic compounds containing one or more heteroatoms in their ring are called heterocyclic compounds.
The heteroatoms commonly found in these compounds are oxygen , nitrogen and sulphur but occasionally phosphorus, boron, silicon and some metal atoms like tin, selenium etc. may also be present. Depending upon the chemical behavior , they are further classified into the following two categories :-

(a) Alicyclic Heterocyclic Compounds :-  Aliphatic cyclic compounds containing one or more heteroatoms in their ring are called alicyclic heterocyclic compounds.

(b) Aromatic heterocyclic compounds :- Aromatic cyclic compounds containing one or more heteroatoms in their molecule are called aromatic heterocyclic compounds.

CHEMISTRY - Polar and Non-Polar Covalent Bond

1. Non-polar Covalent Bonds - If two similar atoms come close to each other and form a bond by sharing their electrons, the shared electrons are equally attracted by the atoms as the electro negativity of the atoms is same. In other words, the shared electrons are not displaced towards any one atom. Hence, no poles are developed. This leads to the formation of completely non-polar bond. For example, if two hydrogen atoms form a bond, the electron pair will lie exactly in the middle between the two atoms and the resultant is non-polar. The important characteristics of a non-polar bond is that the electron cloud is completely systemic and there is no charge separation at all. 

2. Polar Covalent Bonds - When two dis- similar atoms, having different electro-negativities combine together to form a covalent bond, the shared pair of electrons does not lie at equal distances from the of both the bonded atoms but shifts towards the atom having greater electro negativity. Since the more electro negativity atom attracts the electrons more strongly, the distribution of electrons gets distorted, i.e. The electron cloud is displaced more towards the more electro negative atom. As a result, one end of the molecule Having more electro negative atom becomes slightly negatively charged whilethe other end acquires slightly positive charge. Thus, positive and negative roles are developed and this type of bond is called polar covalent bond. Lets consider the case of hydrogen chloride molecule. Chlorine is more electro negative than hydrogen. So the force of attraction, on the shared pair, exerted by chlorine is more than that by hydrogen. Hence, it will be shifted towards chlorine, i.e. Electron density of the electron cloud is more concentrated on chlorine side than on the hydrogen side. Molecules having polar bond are called polar molecules. In case of electro negativity difference between the two atoms bonded by a covalent bond is high, an electron can be completly free.

ORGANIC CHEMISTRY - Organic Reactions

We know that organic reaction takes place when reactant react with the intermediate and form a product this reaction is known as an organic reaction. Thus an organic reaction represented as :

Substrate + Reagent - Intermediate(s) - Product(s)

Types of organic reactions are :-

1. Substitution Reaction :- The reaction which involves the direct replacement or substitution of an atom or a group in a molecule by another atom or group is referred to as substitution reaction. In such reactions, bonds are broken and formed at the same carbon which is considered to be the reactive site of the substrate .

2. Addition Reaction :- The reaction which involves the addition of two reactive molecules to give a single molecule, as a product is known as Addition reaction. In such reactions, a triple bond may be converted into a double bond or single bond, and a double bond is usually converted into a single bond and the molecule acquires more saturation.

3. Elimination Reaction :- An elimination reaction involves the loss of two atoms or groups from the same or adjacent atoms of substance to form a multiple bond. It is depending upon the relative position of the atoms or groups eliminated .

4. Rearrangement Reactions :- Reactions which involves the migration of an atom or a group from one atom to another within the molecule are called Rearrangement reactions.

5. Molecular Reactions :- The reaction in which the covalent bond undergoes reorganization either intramolecularly or intermolecularly through cyclic transition states, under the influence of heat or light are called molecular reaction or pericyclic reactions. These reactions are characterized by a high degree of stereospecificity.

ORGANIC CHEMISTRY - Mechanism of Organic Reactions

We know that during a chemical reaction , bond between the reactant molecule break and new bonds  between the product molecules are formed. A chemical reaction may take place in one step or in more than one step through the formation of reactive intermediate. The complete description of how a particular reaction take place is called mechanism of the reaction.

* Homolytic Bond Fission :- In homolytic bond fission , each atom involved in covalent bond formation retains one electron from the shared pair of electrons, as a result neutral atoms or groups of atoms are formed called free redicals.

* Heterolytic Bond Fission :- In heterolytic bond fission , both bonding electrons are retained by one of the atoms , as a result of pair of ions are formed.

ORGANIC CHEMISTRY - Concept of Isomerism

The compound which differ in their chemical and physical properties, but have the same molecular formula are known as isomers and the phenomenon is known as Isomerism. Example of this are ethyl alcohol and dimethyl ether have the same molecular formula and have distinctly different compounds.

<b>Isomerism are classified into following :-</b>

<b>1. Structural Isomerism :- </b> Isomers which differ in the arrangement of atoms within the molecule are known as structural isomers and the phenomenon are known as Structural Isomerism.

Structural Isomerism further classified as :-

* Chain Isomerism :- Isomers which differ in the arrangement of corbon atom with in tthe molecule are known as Chain isomers and the phenomenon is known as Chain Isomerism.

* Position Isomerism :- Isomers which differ in the position of attached multiple bond or groupare known as position isomers and the phenomenon are known as Position Isomerism.

* Functional Isomerism :- Isomers which differ in the nature of the functional groups, associated with them are known as functional isomers and the phenomenon is known as Functional Isomerism.

* Metamerism :- Isomers which differ in the number of alkyl groups attached on either side of the functional group in the molecule are known as metamers and the phenomenon is known as Metamerism.

* Tautomerism :- Special type of isomers which exist in dynamic equilibirium with each other are known as tautomers and the phenomenon are known as Tautomerism.

* Ring-Chain Isomerism :- Isomers having open chain and cyclic structure are known as ring chain isomersand the phenomenon are known as Ring-Chain Isomerism.

<b>Stereo Isomerism :-</b> Ismers having the same structural formula but differ only in the spatial arrangement of the constituent atoms or groups are known as stereo isomers and the phenomenon are known as Stereo Isomerism.

Stereo Isomerism are further classified as :-

* Configurational Stereoisomerism :- The stereo isomers which interconvert with difficulty and thus are isolable under normal conditions are known as Configurational isomer and the phenomenon are known as Configurational Stereoisomerism.

* Conformational Stereoisomerism:- The stereo isomers which are easily interconvertible into each other at room temprature through rotations about single bonds are known as conformational isomers and phenomenon are known as Conformational Isomerism.

ORGANIC CHEMISTRY :- Hydrogen Bonding

Hydrogen bonding involves electron cloud transfer from hydrogen atom to the neighbouring electronegative atom and occurs in any system containing a proton donor group (X-H; OH, COOH, CONH2 ) and a proton acceptor provided the s-orbital of the proton can effectively overlap the p or pi orbital of the acceptor group. The strength of the hydrogen bond is at a maximum when the proton donor group and axis of lone pair orbital are collinear and is inversely proportional to the distance between X and Y.

When hydrogen is bonded to one of the highly electronegative atoms such as florine, oxygen, or nitrogen the shared pair of electrons of the covalent bond moves towards the more electronegative atom and the bond becomes considerable polar with the hydrogen atom carrying a partial positive charge and more electronegative atom carrying a partial negative charge.
When such molecule come closure , dipole-dipole attraction weakly binds the molecules together through hydrogen and is known as <b>hydrogen bonding</b>

<b>Classification of Hydrogen Bonding:-</b>
<b>1. Intermolecular Hydrogen Bonding :- </b> Intermolecular hydrogen bonding resulting in association of two or more molecule of the same or different compounds is known as intermolecular hydrogen bonding. Example of this is , the hydrogen bonding in water, alcohols and carboxylic acid is intermolecular .

<b>2. Intramolecular Hydrogen Bonding :-</b> Intramolecular hydrogen bond is formed between two atoms of the same molecule. Result of a five or six membered ring known as chelate ring, is formed.
Intramolecular hydrogen bonding occurs in o-Hydroxycorbonyl compounds, salicylic. And also in o-dihydroxybenzene and o-nitrophenol etc.

ORGANIC CHEMISTRY - Structure and Bonding

The main feature of organic chemistry is the covalent bond. The covalent bond, formed as a result of sharing of electrons between two atoms, is generally called a <b>Chemical Bond</b>. Lewis first presented the concept of covalent bond in organic chemistry. These were subsequently classified into polar and non-polar bonds, depending upon the electonegativitiesof the bonded atoms. A knowledge of the nature and strength of bonds is essential for understanding the structural concept of organic molecules. Before studying the synthesis and determination of structure of organic compounds, the study, both chemical and kinetic, of organic reactions.

<b>Various terms used in Structure and Bonding :-</b>

<b>Bond Length :-</b> The interatomic or internuclear distance in a covalent molecule is known as bond length. Bond length are generally affected by the hybridization, electronegativity, steric effect or delocalization.

<b>Bond Angles :-</b> An angle between adjacent bonds in polyatomic molecule is called bond angle. The shape of the molecule are dependent on the bond angles.

<b>Bond Energy :-</b> Bond energy is the average bond dissociation energy of particular type of bonds in multiatomic molecule. It is simply denoted by 'E'. Bond energy is measure the strength of a bond, greater the bond energy, stronger is the bond.

<b>Localized Bond :-</b> If the electron forming a covalent bond remain confined to the space between the two bonded atom only, such electrons are called localized electron and the bond formed by these electrons are known as localized bond.

<b>Delocalized Bond :-</b> If the electron forming a covalent bond are not confined to the space between the two bonded atoms, the electrons are called delocalized electrons and bond formed by these electrons are known as the delocalized bond.

<b>Resonance Energy :-</b> The concept of resonance has been developed by analogy to quantum and wave mechanics and applied to organic structural representations. The difference in energy between the resonance hybrid structure and the most stable chemical structure, is known as a resonance energy.