HYDROCARBONS

             1. Introduction

 ‘Hydrocarbon’ means compounds of carbon and hydrogen only. Hydrocarbons play a key role in our daily life. You must be familiar with the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the 

abbreviated form of l iq u i e d petroleum gas whereas CNG stands for compressed natural gas. Another term ‘LNG’ ( l iq u i e d  natural gas) is also in news these days. This is also a fuel and is obtained by l i q u if i cation of natural gas. 

Petrol, diesel and kerosene oil are obtained by the fractional distillation of petroleum found under the earth’s crust.  Coal gas is obtained by the destructive distillation of coal. Natural gas is found in upper strata during drilling of oil wells. The gas after compression is known as compressed natural gas. LPG is used as a domestic fuel with the least pollution. Kerosene oil is also used as a domestic fuel but it causes some pollution. Automobiles need fuels like petrol, diesel and CNG. Petrol and CNG operated automobiles cause less pollution. All these fuels contain mixture of  hydrocarbons, which are sources of energy. Hydrocarbons are also used for the manufacture of polymers like polythene, poly propene, polystyrene etc. Higher hydro carbons are used as solvents for paints. They are also used as the starting materials for manufacture of many dyes and drugs. Thus, you can well understand the importance of hydrocarbons in your daily life. In this unit, you will learn more about hydrocarbons.

                  2. Classification

Hydrocarbons are of different types. Depending upon the types of carbon-carbon bonds present, they can be classified into three main categories– (i) saturated 

(i i) unsaturated

(i i i) aromatic 

hydrocarbons. Saturated hydrocarbons

contain carbon-carbon and carbon-hydrogen

single bonds. If different carbon atoms are

joined together to form open chain of carbon

atoms with single bonds, they are termed as

alkanes. On the other hand, if carbon atoms 

form a closed chain or a ring, they are termed

as cycloalkanes. Unsaturated hydrocarbons

contain carbon-carbon multiple bonds –

double bonds, triple bonds or both. Aromatic

hydrocarbons are a special type of cyclic

compounds. You can construct a large number

of models of such molecules of both types

(open chain and close chain) keeping in mind

that carbon is tetravalent and hydrogen is

monovalent. For making models of alkanes,

you can use toothpicks for bonds and plasticine balls for atoms. For alkenes, alky n e s 

and aromatic hydrocarbons, spring models can

be constructed.

             3. ALKANES

As already mentioned, alkanes are saturated

open chain hydrocarbons containing carbon - carbon single bonds. Methane (C H 4) is the first member of this family. Methane is a gas found in coal mines and marshy places. If  you replace one hydrogen atom of methane by  carbon and join the required number of  hydrogens to satisfy the tetra valence of the other carbon atom, what do you get? You get C 2 H 6. This hydrocarbon with molecular  formula C 2 H 6 is known as ethane. Thus you can consider C 2 H 6 as derived from C H 4 by replacing one hydrogen atom by -C H3 group. Go on constructing alkanes by doing this  theoretical exercise i.e., replacing hydrogen 

atom by –C H3 group. The next molecules will

be C 3 H 8, C4 H10 …

These hydrocarbons are inert under normal conditions as they do not react with acids, bases and other reagents. Hence, they were earlier known as para f fins (latin : par um, little; a f f i n i s, affinity). 

             

              STRUCTURE OF METHANE (C H 4) 

In alkanes, tetrahedra are joined together

in which C-C and C-H bond lengths are

154 pm and 112 pm respectively. You have already read that C–C and C–H σ bonds 

are formed by head-on overlapping of s p ³

hybrid orbitals of carbon and 1s orbitals of

hydrogen atoms.

1. Nomenclature and Isomerism

You have already read about nomenclature

of different classes of organic compounds in

Unit 12. Nomenclature and isomerism in

alkanes can further be understood with the

help of a few more examples. Common names

are given in parenthesis. First three alkanes

– methane, ethane and propane have only

one structure but higher alkanes can have

more than one structure. Let us write

structures for C4 H10. Four carbon atoms of

C4 H10 can be joined either in a continuous

chain or with a branched chain in the

following two ways :

Structures I and I I possess same

molecular formula but differ in their boiling

points and other properties. Similarly

structures I I I, IV and V possess the same

molecular formula but have different

properties. Structures I and I I are isomers of

butane, whereas structures I I I, IV and V are

isomers of pentane. Since difference in

properties is due to difference in their

structures, they are known as structural

isomers. It is also clear that structures I and

I I I have continuous chain of carbon atoms but

structures I I, IV and V have a branched chain.

Such structural isomers which differ in chain

of carbon atoms are known as chain isomers.

Thus, you have seen that C4 H10 and C5 H1 2

have two and three chain isomers respectively.