Homologous Series: Alkanes

A homologous series is a group of compounds that:

  • have the same general formula

  • differ by CH in molecular formulae from neighbouring compounds

  • show a gradual variation in physical properties

  • have similar chemical properties

  

Alkanes are one example of a homologous series. They:

  • have the general formula of CH₂ₙ₊₂

    • this means that for every 1 carbon atom in an alkane, there are two times the amount of hydrogens... plus another two

  • differ by CH₂ in molecular formulae from neighbouring compounds

    • increasing the carbon chain length by 1 carbon atom, also increases the number of hydrogens by 2

  • show a gradual variation in physical properties

    • boiling points increase with increased carbon chain length

    • viscosity increases with increased carbon chain length

  • have similar chemical properties

54 alkanes-01.png

Because alkanes only contain single bonds between atoms, we describe them as saturated compounds. Think of when a sponge cannot hold any more water, it is "full". A saturated molecule can't fit any more single bonds into it. Alkanes can be considered "full".

Homologous Series: Alkenes

Alkenes are one example of a homologous series. They:

  • have the general formula of CH₂ₙ

    • this means that for every 1 carbon atom in an alkene, there are two times the amount of hydrogens

  • differ by CH₂ in molecular formulae from neighbouring compounds

    • increasing the carbon chain length by 1 carbon atom, also increases the number of hydrogens by 2

  • show a gradual variation in physical properties

    • boiling points increase with increased carbon chain length

    • viscosity increases with increased carbon chain length

  • have similar chemical properties

75 alkenes-01.png

Because alkenes contain at least one double bond between carbon atoms, we describe them as unsaturated compounds. Think of when a sponge cannot hold any more water, it is "full". A saturated molecule can't fit any more single bonds into it. Alkenes can fit more single bonds in (once the double bonds are broken).

  

Alkenes can have more than one carbon-carbon double bond, however you will only ever be expected to draw an alkene with one of these bonds. Also, when drawing only one double bond, for longer chain molecules this could be in several different places. For ethene and propene, the double bond can only go in one place, however for any chains longer than 3 carbons - this can get a bit more complicated. When the chemical formula is the same, but the structure is different, we call these isomers. You need to know the structures of all the isomers of butene that can form due to the double bond being in one of two places.

Reactions of Alkenes

The functional group (double carbon bond, C=C) allows alkenes to undergo addition reactions. The reaction is an 'addition' reaction because one molecule combines with another molecule, forming one larger molecule and no other products.

  

The test for alkenes makes use of the addition reaction, and involves adding bromine water to a sample of hydrocarbon. If an alkane is present the solution stays orange-brown, but an alkene will turn the solution colourless. This is because the bromine is added across the carbon-carbon double bond.

76 alkene reactions-01.png

Complete combustion

The complete combustion of alkanes and alkenes involves the oxidation of both the carbon and hydrogen atoms in hydrocarbons, producing carbon dioxide and water. For example:

ethane + oxygen → carbon dioxide + water

2CH(g) + 7O(g) → 4CO(g) + 6HO(l)

ethene + oxygen → carbon dioxide + water

CH(g) + 3O(g) → 2CO(g) + 2HO(l)

 
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