Alipathic Aromatic Alkens Alkenes and Alkynes Hydrocarbons for std 8 to 12

Cyclihexene

Introduction 

Special property of Carbon
Hydrocarbons
Classification 

Review
Carbon, hydrogen and oxygen are some of the non-metals.
The valency of carbon is four and the valency of hydrogen is one.
Carbon produces a number of covalent compounds by establishing covalent bonds with elements like hydrogen, oxygen, sulphur etc.

INTRODUCTION
You have studied the role of carbon in the origin and evolution of life on earth. Carbohydrates, proteins, lipids and vitamins are classes of a few organic compounds. These compounds are the essential components of food. The branch of chemistry which studies the copounds of carbon is broadly called organic chemistry. One of the important group of organic compounds that are useful to us is hydracarbons.

SPECIAL PROPERTY OF CARBON
Electronic configuration of carbon is 1s2 2s2 2p2

Electronic Configuration of Carbon

There are two unpaired electrons in 2p level. On excitation one electron from 2s level is promoted to 2p level. Thus there will be four unpaired electrons. Hence carbon is tertavalent. 

Carbon Tetravalent.

Carbon atoms share electrons, among themselves, and atoms of other elements leading to the formation of organic molecules with covalent bonds. The covalent bonds are fairly strong. Carbon develops links with carbon atoms forming a chain. This phenomenon, called catenation. Which is predominent in carbon. 

This occurs in one of the following three ways.

Catenation - Straight chain

Catenation - Branched chain

Catenation - Closed chain

HYDROCARBONS
Hydrocarbons are the organic compounds consisting of carbon and hydrogen, only. Petroleum fuels are basically hydrocarbons. Hydrocarbons are used as raw materials for the manufacture of plastics, detergents, dyes and many more polymers. 

Methane is the simplest hydrocarbon. Molecular formula of methane is CH4. The chemical bonding of CH4 is shown in figure. 

Organic Compound Contains Carbon and Hydrogen

The molecular formula does not explain the arrangement of atoms in a molecule. The representation of arrangement of atoms and bonds present in molecule is known as its structural formula. A bond “----” indicates a pair of shared electrons between two atoms. 

Double bonds Between Carbon Atoms

Observe the bonds formed between the carbon atoms. There are single bonds double bonds or triple bonds. Double bonds between two consecutive carbon atoms, indicates two pairs of shared electrons. The presence of triple bond indicates sharing of three pairs of electrons between two consecutive carbon atoms. 

ALIPHATIC AND AROMATIC HYDROCARBONS
Two important groups of hydrocarbons are Aliphatic and Aromatic hydrocarbons.
Aliphatic hydrocarbons : 

are those which are derived out of fat. Aliphatic hydrocarbons are further classified as alkanes, alkenes and alkynes.

"Alkanes" Contains Single Bond 

The above mentioned are compounds containing single bonds between carbon atoms. They are called Alkanes. They are also known as paraffins.
Observe the relationship between the number of carbon atoms and number of hydrogen atoms in each compound. Number of hydrogen atoms are two more than twice the number of carbon atoms in each compound. Therefore this class of compound is represented by a general molecular formula CnH2n+2’ where “n” stands for the number of carbon atoms.

Alkenes - Hydrocarbon Double Bond 

The above mentioned hydrocarbons are called alkenes. The hydrocarbon in which a double bond exists between two successive carbon atoms somewhere in the chain is termed as an Alkene. The general molecular formula of alkenes is CnH2n’. In these compounds two hydrogen atoms are less when compared with the corresponding saturated hydrocarbons (alkanes). 

Alkynes Triple bond between two carbon Atoms

The above mentioned hydrocarbons are called alkynes. Each molecule of alkynes contains a triple bond between two carbon atoms. In case where triple bond exists, we find that the number of hydrogen atoms are further decreased. There are two hydrogen atoms less than that of corresponding alkenes, hence these hydrocarbons re represented by the general molecular formula CnH2n-2’
Alkenes and alkynes are together generalised as unsaturated hydrocarbons.

Alicyclic saturated hydrocarbons :
Allcyclic hydrocarbons are also aliphatic in nature. Study the following compounds.

Alicyclic saturated hydrocarbons

The hydrocarbon in which carbon chain forms a ring structure is called a closed chain hydrocarbon. Such hydrocarbons are also called carbocyclic hydrocarbons. The general molecular formula of these bydrocarbons is CnH2n’

Aromatic hydrocarbons 

are a big class of hydrocarbons. Most of the members of this group have characteristic odour. Hence, they are called aromatic compounds.
Benzene is the first member of aromatic hydrocarbons. Molecular formula of benzene is C6H6. in benzene six carbon atoms are linked to each other by alternate single and double bonds to form a ring. Its structural formula is as follows: 

Benzene structural formula

Toluene is another important aromatic hydrocarbon. Molecular formula of toluene is C7H8. in this molecule a CH3 group is attached to a benzene ring. 

Structural formula of toluene (C6H5 CH3) 

Stuctural formula of Toluene

Both benzene and toluene are good organic solvents. Benzene is used to prepare dyes, medicines., polymers and perfumes.
Toluene is used in the manufacture of explsives like T.N.T. (Trinitrotoluene). 

Sturucture of Hydrocarbons

Do you want to know more ?
Benzene was first isolated from the gas obtained from the whale fat oil in 1825 by Michael Faraday. A molecule of benzene contains six atoms of carbon and six atoms of hydrogen. In 1865 Kekule proposed the benzene structure.

ISOMERISM
In some aliphatic hydrocarbons, a branch may exist in the carbon chain, Such compounds are called branched chain hydrocarbons. Let us consider the example of butane. Molecular formula of butane is C4H10. what could be its structural formula? Butane can be represented by the following structures. 

Isomerism - structure of Normal butane - Iso butane

Both are structures of butane. However, they differ in structural arrangement of carbon atoms. Compounds having same molecular formula with different structural formula are known as isomers. This phenomenon is called Isomerism.

Characteristics :
The isomers of a compound:
have the same molecular formula.
have different structures and
differ in their physical and chemical properties.