Alkanes - Introduction (Edexcel International A Level (IAL) Chemistry): Revision Note

Alkanes - Introduction

• Hydrocarbons are compounds containing hydrogen and carbon only

• Some of the hydrocarbon families that you are expected to know include: alkanes, alkenes and arenes

  • Alkanes and alkenes can be described as aliphatic

  • Arenes can be described as aromatic

• Alkanes have the general molecular formula C_nH_2n+2 

• They contain only single bonds and are said to be saturated

• Alkanes are named using the nomenclature rule alk + ane

  • The alk portion of the name depends on the number of carbons

    • 1 carbon = meth

    • 2 carbons = eth

    • 3 carbons = prop

    • 4 carbons = but

    • 5 carbons = pent

    • After 5 carbons, the naming of alkanes matches the names of the polygons in Maths

  •  The ane portion of the name suggests single bonds between the carbon atoms

The first six alkanes

• The first six members of the alkanes are:

• Methane

  • Molecular formula: CH₄

• Ethane

  • Molecular formula: C₂H₆

• Propane

  • Molecular formula: C₃H₈

• Butane

  • Molecular formula: C₄H₁₀

• Pentane

  • Molecular formula: C₅H₁₂

• Hexane

  • Molecular formula: C₆H₁₄

Types of alkanes

• Alkanes can be linear, branched or cyclic 

  • The key points are that there are no functional groups and only single bonds between the carbon atoms

• Cycloalkanes have the general formula C_nH_2n

  • This general formula is the same as the alkene general formula but cycloalkanes are still saturated

  • Cycloalkanes are named using the nomenclature rule cyclo + alk + ane

    • The cyclo portion of the name shows that there is a ring structure

    • As with alkanes, the alk portion of the name depends on the number of carbons

    •  The ane portion of the name suggests single bonds between the carbon atoms

Alkanes are compounds made up of carbon and hydrogen atoms only and contain no functional group

Alkanes - Reactions

• The unreactive nature of alkanes can be explained by two factors

  • The high bond enthalpies of the C-C and C-H bonds

  • The very low polarity of the sigma (σ) bonds present

Bond enthalpy 

• The C-C bond has a mean bond enthalpy of about 347 kJ mol^-1 and the C-H bond has a mean bond enthalpy of about 413 kJ mol^-1

• The C-H bond is stronger as the bond length is less than the C-C bond

  • This is because the hydrogen atom only consists of one shell, so the distance between the nuclei is shorter 

  • Creating greater force of attraction to the nuclei and the pair of electrons between them

Polarity

• The electronegativities of carbon and hydrogen are very similar, therefore the bonds will have very low polarity 

Ethane is an example of an alkane that lacks polarity due to almost similar electronegativities of the carbon and hydrogen atoms

Alkanes are combusted (burnt) on a large scale for their use as fuels

Complete combustion

• When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place

  • This means that all carbon and hydrogen will be oxidised to carbon dioxide and water respectively

alkane + oxygen $\stackrel{\mathbf{complete}\mathbf{combustion}}{\mathbf{\to }}$ carbon dioxide + water

• For example, the complete combustion of octane to carbon dioxide and water

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

Incomplete combustion

• When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised

• Some carbon is only partially oxidised to form carbon monoxide

alkane + oxygen $\stackrel{\mathbf{incomplete}\mathbf{combustion}}{\mathbf{\to }}$ carbon monoxide + water

• For example, the incomplete combustion of octane to form carbon monoxide

2C₈H₁₈ + 17O₂ → 16CO + 18H₂O

• Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present

• With a reduced supply of oxygen, carbon will be produced in the form of soot:

alkane + oxygen $\stackrel{\mathbf{incomplete}\mathbf{combustion}}{\mathbf{\to }}$ carbon + water

• For example, the incomplete combustion of octane to form carbon

2C₈H₁₈ + 9O₂ → 16C + 18H₂O

Free-radical substitution

• Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine / bromine)

• Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur

• The free-radical substitution reaction consists of three steps:

  • In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals

  • These radicals create further radicals in a chain reaction called the propagation step

  • The reaction is terminated when two radicals collide with each other in a termination step

The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place