Chemistry Classification of hydrocarbons, Nomenclature and isomerism of hydrocarbons

Topics to be covered

`=>` General introduction
`=>` Classification of hydrocarbons
`=>` Alkanes
`=>` Nomenclature and isomerism
`=>` Alkyl groups
`=>` How to write structural formulae of compounds


The term ‘hydrocarbon’ stands for compounds of carbon and hydrogen only.


• The terms `color{red}("‘LPG’")` and `‘color{red}("CNG’")` used as fuels. `color{red}("LPG ")` is the abbreviated form of liquified petroleum gas whereas `color{red}("CNG")` stands for compressed natural gas. Another term `color{red}("‘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.

• `color{red}("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 `color{red}("CNG")`. Petrol and `color{red}("CNG")` operated automobiles cause less pollution.

• 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.

`color{red}("All these fuels contain mixture of hydrocarbons, which are sources of energy.")`


`color{red}("𝐒𝐚𝐭𝐮𝐫𝐚𝐭𝐞𝐝 𝐡𝐲𝐝𝐫𝐨𝐜𝐚𝐫𝐛𝐨𝐧𝐬:")` Saturated hydrocarbons contain carbon-carbon and carbon-hydrogen single bonds.

`color{red}("𝐔𝐧𝐬𝐚𝐭𝐮𝐫𝐚𝐭𝐞𝐝 𝐡𝐲𝐝𝐫𝐨𝐜𝐚𝐫𝐛𝐨𝐧𝐬 :")` Unsaturated hydrocarbons contain carbon-carbon multiple bonds – double bonds, triple bonds or both.

`color{red}("𝐀𝐫𝐨𝐦𝐚𝐭𝐢𝐜 𝐡𝐲𝐝𝐫𝐨𝐜𝐚𝐫𝐛𝐨𝐧𝐬 :")` Aromatic hydrocarbons are a special type of cyclic compounds.


The general formula for alkanes is `color{red}(C_n H_(2n+2))`, where n stands for number of carbon atoms and `color{red}((2n+2))` for number of hydrogen atoms in the molecule.

• Alkanes are saturated open chain hydrocarbons containing carbon - carbon single bonds.

• First member of this family is methane-found in coal mines and marshy place.

• Replacing one hydrogen of methane gives ethane `color{red}(C_2H_6)`.

• In the same way we can obtain the other The next molecules will be `color{red}(C_3H_8, C_4H_(10))` …
`color{red}(H - underset(underset(H)(|))overset(overset(H)(|))C -H overset("replace any" H "by" - CH_3)→ H - underset(underset(H)(|)) overset(overset(H)(|))C - underset(underset(H)(|)) overset( overset(H)(|))C - H)`

These hydrocarbons are inert as under normal conditions because they do not react with acids, bases and
other reagents. Hence, they were earlier known as paraffins (`color{red}("𝑙𝑎𝑡𝑖𝑛 : 𝑝𝑎𝑟𝑢𝑚, little; 𝑎𝑓𝑓𝑖𝑛𝑖𝑠,")`, affinity).

`color{green}("STRUCTURE OF METHANE:")`

According to VSEPR theory, methane has a tetrahedral structure (Fig. 13.1) which is multiplanar, in which carbon atom lies at the centre and the four hydrogen atoms lie at the four corners of a regular tetrahedron. All `color{red}(H-C-H)` bond angles are of `color{red}(109.5^°)`.

In alkanes, tetrahedra are joined together in which `color{red}(C-C)` and `color{red}(C-H)` bond lengths are 154 pm and 112 pm respectively. `color{red}(C–C)` and `color{red}(C– H sigma)` bonds are formed by head-on overlapping of `color{red}(sp^3)` hybrid orbitals of carbon and `1s` orbitals of hydrogen atoms.

Nomenclature and Isomerism:

• First three alkanes – methane, ethane and propane have only one structure but higher alkanes can have more than one structure.

• For eg. four carbon atoms of `color{red}(C_4 H_(10))` can be joined either in a continuous chain or with a branched chain in the following two ways :

• Different arrangements of `color{red}(C_5 H_(12)):`

• 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 isomers.

• 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. `color{red}(C_4H_(10))` and `color{red}(C_5H_(12))` have two and three chain isomers respectively.

`color{red}("Based upon the number of carbon atoms attached to a carbon atom, the carbon atom")` `color{red}(" is termed as primary (1°), secondary ((2°)")`, `color{red}("tertiary (3°) or quaternary (4°)")`.

• Carbon atom attached to no other carbon atom as in methane or to only one carbon atom as in ethane is called primary carbon atom. Terminal carbon atoms are always primary.

• Carbon atom attached to two carbon atoms is known as secondary.

• Tertiary carbon is attached to three carbon atoms.

• Neo or quaternary carbon is attached to four carbon atoms.
Q 3274367256

Write structures of different chain isomers of alkanes corresponding to the molecular formula `C_6H_(14)`. Also write their IUPAC



Groups like `color{red}(–CH_3, –C_2H_5, –C_3H_7)` 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 `color{red}(C_nH_(2n+1))`.

Q 3284367257

Write structures of different isomeric alkyl groups corresponding to the molecular formula `C_5H_(11)`. Write IUPAC names of alcohols obtained by attachment of `–OH` groups at different carbons of the chain.


Q 3204367258

Write IUPAC names of the following compounds :

(i) `(CH_3)_3 C CH_2C(CH_3)_3`
(ii) `(CH_3)_2 C(C_2H_5)_2`
(iii) tetra – tert-butylmethane


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


• 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

• 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 :

i) Draw the chain of five carbon atoms:

`color{red}(C – C – C – C – C)`

ii) Give number to carbon atoms:

`color{red}(C^1– C^2– C^3– C^4– C^5)`

iii) Attach ethyl group at carbon 3 and two methyl groups at carbon 2

`color{red}(C^(1) - text()^(2) underset(underset(CH_3)(|))overset(overset(CH_3)(|))C - text()^(3) underset(underset(C_2H_5)(|))C - text()^(4)C - text()^(5)C)`

iv) Satisfy the valence of each carbon atom by putting requisite number of hydrogen atoms :

`color{red}(CH_3 - underset(underset(CH_3)(|)) overset(overset(CH_3)(|))C - underset(underset(C_2H_5)(|))CH - CH_2 -CH_3)`

Q 3214367259

Write structural formulas of the following compounds :
(i) 3, 4, 4, 5–Tetramethylheptane
(ii) 2,5-Dimethyhexane


(I) `CH_3 - CH_2 -underset(underset(CH_3)(|))CH - underset(underset(CH_3)(|))overset(overset(CH_3)(|))C- underset(underset(CH_3)(|))CH-CH- CH_3`

(ii) `CH_3 - overset(overset(CH_3)(|))CH- CH_2 - CH_2 - overset(overset(CH_3)(|))CH-CH_3`
Q 3214467350

Write structures for each of the following compounds. Why are the given names incorrect? Write correct IUPAC names.
(i) 2-Ethylpentane
(ii) 5-Ethyl – 3-methylheptane


(I) `CH_3 -underset(underset(C_2H_5)(|))CH-CH_2 - CH_2-CH_3`

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

`text()^(7)CH_3 - text()^(6)CH_2 - text()^(5)underset(underset(CH_3)(|))CH - text()^(4)CH_2 - text()^(3)underset(underset(C_2H_5)(|))CH- text()^(2)CH_2 - text()^(1) CH_3`

Numbering is to be started from the end which gives lower number to ethyl group.
Hence, correct name is 3-ethyl-5- methylheptane.