Homologous Series and Hydrocarbons

 

A homologous series is a group of molecules with similar chemical properties, chemical formula and physical properties. The groups included in National 5 Chemistry are alkanes, alkenes, cycloalkanes, alcohols and carboxylic acids.

A Hydrocarbon is a molecule containing Hydrogen and Carbon.

The names of the different molecules within groups of hydrocarbons depends on the number of carbons in the molecule as well as the functional groups present.

The first part of the molecules name depends on the number of carbon atoms in the molecule. 1=Meth, 2=Eth, 3=Prop, 4=But, 5=Pent, 6=Hex, 7=Hept, 8=Oct.

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Alkanes

Alkanes are a homologous series that has the most simple structural formula, containing carbon and hydrogen, with single bonds.

The names of alkanes all end in -ane.

Alkanes are used for fuels.

The chemical formula is CnH2n+2

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Branched Alkanes

You need to be able to draw and name branched alkanes.

You name the branches from the closest side, as seen in the given examples.

Alkenes

Alkenes are similar to alkanes in structure but have an added carbon-to-carbon double bond. The carbon-to-carbon double bond is the functional group of the alkenes.

Alkenes are UNSATURATED. Unsaturated means that there is a carbon-to-carbon double bond present within the molecule. Those without carbon-to-carbon double bonds are called saturated. Unsaturated molecules can undergo addition reactions.

The Chemical formula of alkenes is CnH2n

You name alkenes by the side that the double bond is closest to.

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Cycloalkanes

Cycloalkanes are a homologous series with the structural formula in the diagram. The main things you need to be aware of with cycloalkanes are the structural formula, chemical formula and that they are ISOMERS of ALKENES.

Isomers

Isomers are chemicals with the same chemical formula with different structural formulas.

Cycloalkanes and and alkenes with the same number of carbons are isomers.

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Addition Reactions

Addition reactions are when a molecule with a double bond adds onto an unsaturated molecule by adding onto their double bond.

A test commonly asked about in exams is “how to test to see if a molecule is unsaturated”. This can be done using bromine solution, because if bromine solution decolourises, that means that a substance is unsaturated.

Key Points!

  • Homologous Series

    A homologous series is a group of molecules with similar chemical properties, chemical formula and physical properties.

    The first part of the molecules name depends on the number of carbon atoms in the molecule. 1=Meth, 2=Eth, 3=Prop, 4=But, 5=Pent, 6=Hex, 7=Hept, 8=Oct.

  • Alkanes

    The names of alkanes all end in -ane.

    Alkanes are used for fuels.

    The chemical formula is CnH2n+2

  • Alkenes

    Alkenes are similar to alkanes in structure but have an added carbon-to-carbon double bond.

    Alkenes are UNSATURATED. Unsaturated means that there is a carbon-to-carbon double bond present within the molecule.

    Unsaturated molecules can undergo addition reactions.

    The Chemical formula of alkenes is CnH2n

  • Isomers

    Isomers are chemicals with the same chemical formula with different structural formulas.

    Cycloalkanes and and alkenes with the same number of carbons are isomers.

  • Addition Reactions

    Addition reactions are when a molecule with a double bond adds onto an unsaturated molecule by adding onto their double bond.

    A test commonly asked about in exams is “how to test to see if a molecule is unsaturated”. This can be done using bromine solution, because if bromine solution decolourises, that means that a substance is unsaturated.

Any Questions?