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Alkane

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:

  1. Linear alkanes
  2. Branched alkanes
  3. 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:

Isomerism

Alkanes with more that three carbons can stay in different structural isomers. This phenomena is known as isomerism.

Such as:

  • 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

Nomenclature

The suffix -ane is used to name an alkane.

Linear alkanes

Sometime prefix n- is used to name a linear alkane to differentiate them from their branched analogues.

NameFormulaNo. of CarbonNo. of Hydrogen
MethaneCH414
EthaneC2H626
PropaneC3H838
ButaneC4H10410
PentaneC5H12512
HexaneC6H14614

Branched alkanes

In IUPAC system nomenclature of brached alkanes can be done in few steps:

  1. identify the longest continuous chain
  2. name the chain as back bone using the linear alkane formula with suffix -ane
  3. name the each branch changing the suffix -ane to -yl
  4. number the chain in such a way that the branch gets the lowest possible number
  5. 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
  6. if more than one same branches exist, write as di-, tri- etc. as a prefix.
  7. if more than one different branches exist, write the branches name alphabetically.

As for example:

Cyclic alkanes:

To distinguish cyclic alkanes from other two alkanes the name of this type of alkanes are prefixed with cyclo-. As for example:

AlkaneFormulaCycloalkaneFormula
PropaneC3H8CyclopropaneC3H6
ButaneC4H10CyclobutaneC4H8
PentaneC5H12CyclopentaneC5H10
HexaneC6H14CyclohexaneC6H12

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.
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