loader image

Hydrocarbons

Definition of Hydrocarbons

Hydrocarbons are the simplest organic compounds. They are formed only by two types of atoms which are carbon and hydrogen. The question here is: “How many different compounds do you think two elements can form ?” There is thousands of hydrocarbons. The simplest hydrocarbon is methane which is the main component of natural gas, it is formed by only one carbon related to four hydrogen. The molecular formula of methane is CH4 .

In a hydrocarbon, carbon atoms can bond together by single, double, and triple bonds giving two main types of hydrocarbons: saturated and unsaturated.

Saturated hydrocarbons contain one single bond between carbon atoms and they are called Alkanes , but unsaturated hydrocarbons contain double or triple bonds between carbon atoms and they are called Alkenes (double bonds) and alkynes (triple bonds) .

Hydrocarbons

Alkanes

Hydrocarbons

Alkanes are the saturated hydrocarbons that have only single bonds between carbon atoms. Methane is the smallest one.

Number of CarbonName Molecular formula
1Methane CH4
2Ethane C2H6
3Propane C3H8
4Butane C4H10
5Pentane C5H12
6 Hexane C6H14
7Heptane C7H16
8Octane C8H18
9Nonane C9H20
10Decane C10H22

For alkanes, the general relation between the number of Carbons and the number of Hydrogen can be expressed as Cn H2n+2 n : Number of carbon 

Example : if n= 11 2n + 2= 22+2 = 24

This alkane contains 11 atoms of carbon and 24 Hydrogen Molecular formula : C11H24

Straight chain alkanes contain carbon atoms that are bonded to each other in a single straight line. There is another form of alkanes which are branched chain alkanes, they contain a straight chain called “parent chain” and branches called “substituent group”.

Hydrocarbons

Substituents are alkyl groups, the first four alkyl are mentioned in this table:

Methyl -CH3
Ethyl -CH2CH3
Propyl -CH2CH2CH3
Butyl -CH2CH2CH2CH3

Naming Branched chain alkanes

A step by step strategy is used to name the branched chain alkanes :

Step 1: Count the number of carbon atoms in the longest continuous chain.

Step 2: Number each carbon in the parent chain, starting with the carbon closest to the substituent group.  This gives all the substituent groups the lowest position numbers possible.

Step 3: Name each alkyl group substituent.

Step 4: If the same alkyl group appears more than once as a branch on the parent structure, use a prefix (di- , tri-, tetra- , and so on) before its name to indicate how many times it appears.

Number of the same alkyl group Prefix
2di-
3tri-
4tetra-
5penta-
6hexa-

Step 5: When different alkyl groups are attached to the same parent chain, place their names in alphabetical order.

Step 6: Write the entire name, using hyphens to separate numbers from words and commas to separate numbers.

Many examples are shown in the pictures below

Hydrocarbons
Hydrocarbons
Hydrocarbons

Chemical properties of alkane

Alkanes have low reactivity because they are nonpolar and have no charge, and because they have strong single bonds between carbon atoms.

Alkenes

Hydrocarbons

Alkenes are unsaturated hydrocarbons that contain at least one double covalent bond between carbon atoms. The simplest alkene has two carbon atoms with a double bound between them and they are bounded to 4 hydrogen atoms to give the molecule ethene (C2H4).

Number of CarbonName Molecular formula Structural formula
2Ethene C2 H4CH2 = CH2
3Propene C3H6CH2 = CH – CH3
41-ButeneC4H8CH2 = CH – CH2 – CH3
42-Butene C4H8CH3 – CH = CH – CH3

For alkenes, the relation between the number of Carbons and Hydrogen can be expressed as Cn H2n

n : Number of carbon 

Example : if n=11 2n= 22

This alkene contains 11 atoms of carbon and 22 Hydrogen. The molecular formula : C11H22

Naming alkenes

Alkenes are named in much the same way as alkanes. But Alkenes end in –ene. And when four or more carbon atoms are present, the location of the double bond is specified by a number before the name of the parent chain.

When naming branched-chain alkenes, follow the same rules as for alkanes, with two exceptions: First the parent chain is always the chain that contains double bond, whether it is the longest or not and the second is that the first carbon should be the one closer to the double bond.

The picture below represent some examples

Hydrocarbons

Alkynes

Hydrocarbons

Alkynes are unsaturated hydrocarbons that contain at least one triple covalent bond between carbon atoms. The simplest alkene has two carbon atoms triple bounded to each other and bounded to 2 hydrogen atoms to give the molecule ethyne (C2 H2).  

For alkynes, the relation between the number of Carbons and Hydrogen can be expressed as CnH2n-2

n : Number of carbon 

Example : if n=11 2n – 2 = 22 – 2 = 20

This alkane contains 11 atoms of carbon and 20 Hydrogen. the molecular formula is: C11H20

Naming Alkynes

Straight-chain and branched-chain alkynes are named in the same way as alkenes, except the ending is –yne.