Hydrocarbons

Hydrocarbons are the important sources of energy.

After studying this unit, you will be able to

The term ‘hydrocarbon’ is self-explanatory which 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 liquified petroleum gas whereas CNG stands for compressed natural gas. Another term ‘LNG’ (liquified natural gas) is also in news these days. This is also a fuel and is obtained by liquifaction 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, polypropene, polystyrene etc. Higher hydrocarbons 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.

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

(ii) unsaturated and (iii) 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 as you have already studied in Unit 8. 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, alkynes and aromatic hydrocarbons, spring models can be constructed.

of the general formula for alkane family or homologous series? If we examine the formula of different alkanes we find that the general formula for alkanes is \ce{C_{n}H_{2n+2}}. It represents any particular homologue when n is given appropriate value. Can you recall the structure of methane? According to VSEPR theory (Unit 4), methane has a tetrahedral structure (Fig. 9.1), in which carbon atom lies at the centre and the four hydrogen atoms lie at the four corners of a regular tetrahedron. All H-C-H bond angles are of $109.5^{\circ }$.

In alkanes, tetrahedra are joined together in which C-C and C-H bond lengths are 154 pm and 112 pm respectively (Unit 8). You have already read that C–C and C–H $\sigma$ bonds are formed by head-on overlapping of $s{p}^3$ hybrid orbitals of carbon and 1s orbitals of hydrogen atoms.

As already mentioned, alkanes are saturated open chain hydrocarbons containing carbon - carbon single bonds. Methane (\ce{CH4}) 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 tetravalence of the other carbon atom, what do you get? You get \ce{C2H6}. This hydrocarbon with molecular formula \ce{C2H6} is known as ethane. Thus you can consider \ce{C2H6} as derived from \ce{CH4} by replacing one hydrogen atom by \ce{-CH3} group. Go on constructing alkanes by doing this theoretical exercise i.e., replacing hydrogen atom by \ce{-CH3} group. The next molecules will be \ce{C3H8}, \ce{C4H10} …

You have already read about nomenclature of different classes of organic compounds in Unit 8. 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 \ce{C4H10}. Four carbon atoms of \ce{C4H10} can be joined either in a continuous chain or with a branched chain in the following two ways :

H H H

replace any H by \ce{-CH3} H—C—C—H or \ce{C2H6}

H—C—H

H H H These hydrocarbons are inert under normal conditions as they do not react with acids, bases and other reagents. Hence, they were earlier known as paraffins (latin : parum, little; affinis, affinity). Can you think

Butane (n- butane), (b.p. 273 K)

isomers. It is also clear that structures I and III have continuous chain of carbon atoms but structures II, 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 \ce{C4H10} and \ce{C5H12} have two and three chain isomers respectively.

2-Methylpropane (isobutane) (b.p.261 K)

In how many ways, you can join five carbon atoms and twelve hydrogen atoms of \ce{C5H12}? They can be arranged in three ways as shown in structures III–V

Problem 9.1 Write structures of different chain isomers of alkanes corresponding to the molecular formula \ce{C6H14}. Also write their IUPAC names.

Solution \ce{CH3-CH2-CH2-CH2-CH2-CH3} (i) n-Hexane

Pentane (n-pentane) (b.p. 309 K)

2-Methylpentane

3-Methylpentane

2-Methylbutane (isopentane) (b.p. 301 K)

2,3-Dimethylbutane

2,2 - Dimethylbutane

2,2-Dimethylpropane (neopentane) (b.p. 282.5 K)

Structures I and II possess same molecular formula but differ in their boiling points and other properties. Similarly structures III, IV and V possess the same molecular formula but have different properties. Structures I and II are isomers of butane, whereas structures III, IV and V are isomers of pentane. Since difference in properties is due to difference in their structures, they are known as structural

structures I to V ? If you go on constructing structures for higher alkanes, you will be getting still larger number of isomers. \ce{C6H14} has got five isomers and \ce{C7H16} has nine. As many as 75 isomers are possible for \ce{C10H22}. In structures II, IV and V, you observed that \ce{-CH3} group is attached to carbon atom numbered as 2. You will come across groups like \ce{-CH3}, \ce{-C2H5}, \ce{-C3H7} etc. attached to carbon atoms in alkanes or other classes of

compounds. These groups or substituents are known as alkyl groups as they are derived from alkanes by removal of one hydrogen atom. General formula for alkyl groups is \ce{C_{n}H_{2n+1}} (Unit 8). Let us recall the general rules for nomenclature already discussed in Unit 8. Nomenclature of substituted alkanes can further be understood by considering the following problem:

Problem 9.2

Write structures of different isomeric alkyl groups corresponding to the molecular formula \ce{C5H11}. Write IUPAC names of alcohols obtained by attachment of \ce{-OH} groups at different carbons of the chain.

Solution Structures of – \ce{C5H11} group Corresponding alcohols Name of alcohol

(i) \ce{CH3-CH2-CH2-CH2-CH2-CH3-CH2-CH2-CH2-CH2-OH} Pentan-1-ol

(ii) \ce{CH3-CH-CH2-CH2-CH3} \ce{CH3-CH-CH2-CH2-CH3} Pentan-2-ol | | OH

(iii) \ce{CH3-CH2-CH-CH2-CH3} \ce{CH3-CH2-CH-CH2-CH3} Pentan-3-ol | | OH

\ce{CH3} 3-Methyl- \ce{CH3} | | butan-1-ol (iv) \ce{CH3-CH-CH2-CH2-CH3-CH-CH2-CH2-OH}

\ce{CH3} 2-Methyl- \ce{CH3} | | butan-1-ol (v) \ce{CH3-CH2-CH-CH2-CH3-CH2-CH-CH2-OH}

\ce{CH3} 2-Methyl- \ce{CH3} | | butan-2-ol (vi) \ce{CH3-C-CH2-CH3} \ce{CH3-C-CH2-CH3} | | OH

\ce{CH3} \ce{CH3} 2,2-Dimethyl- | | propan-1-ol (vii) \ce{CH3-C-CH2-CH3-C-CH2OH} | | \ce{CH3} \ce{CH3}

\ce{CH3} \ce{CH3} OH 3-Methyl- | | | | butan-2-ol (viii) \ce{CH3-CH-CH-CH3} \ce{CH3-CH-CH-CH3}

Remarks

Structure and IUPAC Name

Lowest sum and alphabetical arrangement

\ce{^{1}CH3-^{2}CH-^{3}CH2-^{4}CH-^{5}CH2-^{6}CH3} (a) (4 – Ethyl – 2 – methylhexane

Lowest sum and alphabetical arrangement

\ce{^{8}CH3-^{7}CH2-^{6}CH2-^{5}CH-^{4}CH-^{3}C-^{2}CH2-^{1}CH3} (b)

(3,3-Diethyl-5-isopropyl-4-methyloctane)

sec is not considered while arranging alphabetically; isopropyl is taken as one word

\ce{^{1}CH3-^{2}CH2-^{3}CH2-^{4}CH-^{5}CH-^{6}CH2-^{7}CH2-^{8}CH2-^{9}CH2-^{10}CH3} (c)

5-sec– Butyl-4-isopropyldecane

\ce{^{1}CH3-^{2}CH2-^{3}CH2-^{4}CH2-^{5}CH-^{6}CH2-^{7}CH2-^{8}CH2-^{9}CH3} (d)

Further numbering to the substituents of the side chain

5-(2,2– Dimethylpropyl) nonane \ce{^{1}CH3-^{2}CH2-^{3}CH-^{4}CH2-^{5}CH-^{6}CH2-^{7}CH3} (e)

Alphabetical priority order

3-ethyl-5-methylheptane

important to write the correct structure from the given IUPAC name. To do this, first of all, the longest chain of carbon atoms corresponding to the parent alkane is written. Then after numbering it, the substituents are attached to the correct carbon atoms and finally valence of each carbon atom is satisfied by putting the correct number of hydrogen atoms. This can be clarified by writing the structure of 3-ethyl-2,2-dimethylpentane in the following steps :

Problem 9.3 Write IUPAC names of the following compounds : (i) \ce{(CH3)3C-CH2-C(CH3)3} (ii) \ce{(CH3)2C(C2H5)2} (iii) tetra – tert-butylmethane

Solution (i) 2, 2, 4, 4-Tetramethylpentane (ii) 3, 3-Dimethylpentane (iii) 3,3-Di-tert-butyl -2, 2, 4, 4 - tetramethylpentane

i) Draw the chain of five carbon atoms: \ce{C-C-C-C-C}

ii) Give number to carbon atoms: 1– C 2– C 3– C 4– C 5 C

If it is important to write the correct IUPAC name for a given structure, it is equally

iii) Attach ethyl group at carbon 3 and two

Longest chain is of six carbon atoms and not that of five. Hence, correct name is 3-Methylhexane.

methyl groups at carbon 2

7 6 5 4 3 2 1 (ii) \ce{CH3-CH2-CH-CH2-CH-CH2-CH3}

iv) Satisfy the valence of each carbon atom