The natural source of hydrocarbons is petroleum or crude oil. Hydrocarbons can also be found in gaseous form in coalmines, in marshy areas and in biogas. Hydrocarbons are the simplest forms of organic compounds. They are classified into two main types: Aliphatic and Aromatic hydrocarbons. Aliphatic hydrocarbons are further classified as Saturated and Unsaturated hydrocarbons. Aromatic hydrocarbons are hydrocarbons that contain benzene rings or similar structural features. Benzene ring consists of a ring of six-carbon atoms with alternating single and double bonds. Aliphatic hydrocarbons are all hydrocarbons that do not contain benzene rings but are straight chained. The simplest Hydrocarbon is methane. Aliphatic hydrocarbons can be further classified to saturated and unsaturated hydrocarbons.
The alkenes are known as saturated hydrocarbons. As we already saw that Carbon is a tetravalent compound - each carbon atom is bonded by four single covalent bonds to four other atoms. Hence, the name saturated hydrocarbon. The simplest hydrocarbon consists of one carbon atom to which four hydrogen atoms are bonded. What we get is methane. You can represent the structure of methane by its molecular formula, CH4, which gives the number of kind of atoms in the molecule, or by its structural formula, which shows how the atoms are bonded to one another:
Covalent bonds have a directional property, that is, covalent bonds are directed in a particular way in space. The four valences of carbon in methane are directed towards the apices of a regular tetrahedron with carbon in the center. The various carbon-hydrogen bonds in methane make an angle of 109°28' with each other. Let us see what happens if one of the carbon-hydrogen bonds in a methane molecule is replaced by a carbon-carbon bond (C-C). The two carbon atoms link together by sharing a pair of electrons and forming a single covalent bond. Each carbon atom can now form covalent bonds with three hydrogen atoms as follows:
The above compound is called ethane. Proceeding in the same manner, we get a series of compounds called alkanes. Although complete structural formulas are helpful, we will not always draw all of the individual C H and C C. Structures of organic compounds are often given by condensed structural formulas or line formulas.
Condensed formulas of the first four alkenes are C H4 (Methane), CH3 CH3 (Ethane), C H3CH2CH3 (Propane) and CH2CH2CH3 (Butane). When line formulas become long, as in the case of octane, they are sometimes structured by grouping the CH2 units: C H3 (CH2)6 CH3
It may be noted that the tetrahedral arrangement of atoms bonded to carbon in all three molecules. Because of this, any continuous chain of three or more carbon atoms through a saturated hydrocarbon has a crooked path. Furthermore, there is free rotation around single carbon-carbon bonds. The shape of the molecule varies. At one moment, the molecule can curl up so the end carbons are close to each other, and later the molecule is stretched out with the end carbons far apart. The straight lines we draw on paper are not true representations of carbon chains.
If you examine the molecular and structural formulas of methane, ethane, and propane, you will find a pattern. Each additional carbon atom is accompanied by two more hydrogen atoms. The alkane with four carbon atoms is butane, C4H10; five carbon atoms yield pentane, C5H12, and so forth, with each additional step extending the chain by a -CH2- structural unit.
A series of compounds in which each member differs from the members before and after it by the same structural unit is called a homologous series. A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by CH2 group. The alkane series may be represented by the general formula CnH2n+2, where n is the number of carbon atoms in the molecule. With this formula, you can produce the molecular formula for any member of the series. For octane the alkane with 8 carbon atoms, n = 8. The number of hydrogen atoms is 2(8) + 2 = 18. The formula of octane is therefore C8H18.
The formulas and names of the first ten alkanes are shown in the following table.
MolecularFormula Name Number of Carbon atoms Prefix Melting Point(C) BoilingPoint(C) PhysicalState
Each alkane is named by combining a prefix and a suffix. The prefix indicates the number of carbons in the chain. The first ten prefixes used in this nomenclature system appear in the fourth column of the table given above. The suffix identifying an alkane is -ane. Thu, the mane of methane comes from combining the prefix meth-, indicating one carbon atom, with the suffix -ane indicating an alkane.
ISOMERISM IN THE ALKANE SERIES: Not all alkanes have their carbons bonded in a continuous chain; some have branches. The smallest alkane in which this is possible is butane, which has two possible structures:
Unbranched structure Branched structure
The difference in the structures of butane is shown above. The compound on the left is called normal butane, or n-butane, in which the four carbons are in a single chain. In isobutene, the structure at the right, there are only three carbons in the chain, with the fourth carbon branching off at the middle carbon of the three. Both compounds have the same molecular formula, C4H10. Two compounds having the same molecular formula but different structures are