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Homologous seriesNature's chemistry

The alkanes, alkenes and cycloalkanes are examples of homologous series. A homologous series is a group of chemicals which have similar chemical properties and can be represented by a general formula.

Part of ChemistryNature's chemistry

Nature's chemistry

Hydrocarbons and homologous series

A homologous series is a family of hydrocarbons with similar chemical properties who share the same general formula.

We will look at three hydrocarbon series: alkanes, alkenes and the cycloalkanes. Hydrocarbons are compounds that contain only hydrogen and carbon.

Alkanes

The first homologous series is the alkanes. Their names all end in 鈥ane.

The alkanes have many uses:

  • methane 鈥 (natural gas) cooking, heating
  • propane 鈥 used in gas cylinders for BBQ etc
  • octane 鈥 used in petrol for cars

The general formula of the alkanes is \({C_n}{H_{2n + 2}}\). They are insoluble in water.

The first four alkanes. Methane (CH4) is a carbon atom bonded to four individual hydrogen atoms. Ethane (C2H6) has two carbons bonded to each other. The remaining three bonds on each carbon connect to a single hydrogen. Propane (C3H8) has a carbon with three hydrogens joined to a carbon with two hydrogens joined to a carbon with three hydrogens (CH3CH2CH3). Butane (C4H10) has a carbon with three hydrogens joined to a carbon with two hydrogens joined to a carbon with two hydrogens joined to a carbon with three hydrogens (CH3CH2CH2CH3).

The names, molecular formula and the structural formula of the first eight alkanes must be learned. Using a mnemonic can make learning the names easier.

MethaneCH4Monsters
EthaneC2H6Eat
PropaneC3H8Pupils
ButaneC4H10But
PentaneC5H12Prefer
HexaneC6H14Hairy
HeptaneC7H16Haggis
OctaneC8H18Occasionally
Methane
CH4
Monsters
Ethane
C2H6
Eat
Propane
C3H8
Pupils
Butane
C4H10
But
Pentane
C5H12
Prefer
Hexane
C6H14
Hairy
Heptane
C7H16
Haggis
Octane
C8H18
Occasionally

General formula, functional group and properties of alkanes.

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