Substitution Reactions

The reactions in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or group of atoms are called substitution reaction. For example,

These reactions can be of two types:

Nucleophilic Substitution

In this type of substitution, atom or group of atoms in the molecule is replaced by a nucleophile. These can be either S_N1 (substitution, nucleophilic, unimolecular) or S_N2 (substitution, nucleophilic, bimolecular) type.

In the S_N1 reaction only one molecule is involved which dissociates spontaneously to generate a carbocation intermediate. The carbocation reacts with the added nuclophile in a fast step to yield the product.

Intermediate

In the S_N2 reaction the nucleophile attacks the substrate from a position 180^o away from the leaving group and that the reaction takes place in a single step without intermediates.

Transition state

Electrophilic Substitution

In this type of substitution, an electrophile attacks the carbanion of the substrate substituting one of the hydrogens or other groups.

Addition Reactions

Reactions in which two molecules react to form a single product are called addition reactions. For example,

Depending upon the type of attacking reagent, the substitution or addition reactions are also classified as free radical, electrophilic or nucleophilic substitution or addition reactions. For example, if the attacking reagent is nucleophile in addition reaction, the reaction is called nucleophilic addition.

Elimination Reactions

The reactions in which a small molecule is removed from adjacent carbon atoms resulting in the formation of additional (multiple) bond between them are called elimination reactions. For example,

Rearrangement Reactions

In these reactions the migration of atoms or groups of atoms takes place from one position to another within the molecule under suitable conditions.

For example,

Today, chemists consider organic compounds to be those containing carbon and one or more other elements, most often hydrogen, oxygen, nitrogen, sulfur, or the halogens, but sometimes others as well. Organic chemistry is defined as the chemistry of carbon and its compounds.

The Uniqueness of Carbon

There are more carbon compounds than there are compounds of all other elements combined. Plastics, foods, textiles, and many other common substances contain carbon. With oxygen and a metallic element, carbon forms many important carbonates, such as calcium carbonate (limestone) and sodium carbonate (soda). Certain active metals react with it to make industrially important carbides, such as silicon carbide, an abrasive known as carborundum, and tungsten carbide, an extremely hard substance used for rock drills and metalworking tools.

The great number of carbon compounds is possible because of the ability of carbon to form strong covalent bonds to each other while also holding the atoms of other nonmetals strongly. Carbon atoms have the special property to bond with each other to form chains, ring, spheres, and tubes. Chains of carbon atoms can be thousands of atoms long, as in polyethylene.

Polyethylene chain:

H H H H H H H H H H H
| | | | | | | | | | |
H-C-C-C-C-C-C-C-C-C-C-C-etc.
| | | | | | | | | | |
H H H H H H H H H H H

Structural Isomers

Isomers are classified as structural isomers, which have the same number of atoms of each element in them and the same atomic weight but differ in the arrangement of atoms in the molecule. For example, there ware two compounds with the molecular formula C2H6O. One is ethanol (also called ethyl alcohol), CH3CH2OH, a colorless liquid alcohol; the other is dimethyl ether, CH3OCH3, a colorless gaseous ether. Among their different properties, ethanol has a boiling point of 78.5°C and a freezing point of -117°C; dimethyl ether has a boiling point of -25°C and a freezing point of -138°C. Ethanol and dimethyl ether are isomers because they differ in the way the atoms are joined together in their molecules.