Introduction to alkane
Alkanes are saturated hydrocarbon consist of carbon and hydrogen only without any functional group. The general formula of alkane is CnH2n+2. Example: methane (CH4), ethane (C2H6), propane (C3H8) etc.
Classification of alkane
There are three types of alkanes:
- Linear alkanes
- Branched alkanes
- Cyclic alkanes
1. Linear alkanes
Linear alkanes are alkanes where carbon atoms are linked together like a straight chain or a snake. The general formula of linear alkanes is CnH2n+2.
Here n is the number of -CH2– units in the linear alkane.
2. Branched alkanes
Branched alkanes are alkanes where carbon chain is split off making one or more branch. The general formula of branched alkanes is CnH2n+2, n>2. As for example:
3. Cyclic alkanes
Cyclic alkanes are alkanes where two carbons in any carbon chain are linked together to form a chain. The general formula of cyclic alkanes are CnH2n, n>3. As for example:
Alkanes with more that three carbons can stay in different structural isomers. This phenomena is known as isomerism.
- C1: methane
- C2: ethane
- C3: propane
- C4: 2 isomers: n-butane and isobutane
- C5: 3 isomers: pentane, isopentane, and neopentane
- C6: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane
The suffix -ane is used to name an alkane.
Sometime prefix n- is used to name a linear alkane to differentiate them from their branched analogues.
|Name||Formula||No. of Carbon||No. of Hydrogen|
In IUPAC system nomenclature of brached alkanes can be done in few steps:
- identify the longest continuous chain
- name the chain as back bone using the linear alkane formula with suffix -ane
- name the each branch changing the suffix -ane to -yl
- number the chain in such a way that the branch gets the lowest possible number
- now write the number or the position of the branch first, followed by the name of the branch and at last the name of the back bone or the straight chain
- if more than one same branches exist, write as di-, tri- etc. as a prefix.
- if more than one different branches exist, write the branches name alphabetically.
As for example:
To distinguish cyclic alkanes from other two alkanes the name of this type of alkanes are prefixed with cyclo-. As for example:
For branched cyclic alkanes, the position and name of the branch can be attached as suffix. As for example the name of following cyclic alkane is 1-chlorocyclohexane.
Physical properties of alkanes
- Alkanes are colorless.
- Alkanes with lower molecular weight are gases. As molecular weight increases the alkanes stays as liquid or solid.
- The boiling point of alkanes increases with the increase of the number of carbon atoms in the molecule.