3a. The ALKANE series of saturated hydrocarbons

• Alkanes are a group of hydrocarbon molecules in which all the carbon and hydrogen atoms are only joined by single covalent bonds (e.g. C-H or C-C).

  • Note that the name ends in ...ane eg methane, ethane, propane, butane etc.

• Carbon forms four bonds (C-C or C-H) and hydrogen just one bond (C-H) in alkane hydrocarbon molecules.

  • Check this out with the structural/displayed formula in the table below.

• Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2 where n is the number of carbon atoms in the molecule, so ..

  • when n = 1 you get CH₄, n = 2 gives C₂H₆, n = 3 gives C₃H₈ and then C₄H₁₀, C₅H₁₂, C₆H₁₄, C₇H₁₆, C₈H₁₈ etc.

  • As with naming all organic molecule series, eth.. means 2 carbon atoms in the chain, prop... means 3 and but.. means 4 etc. After that the name is directly derived from the number of carbon atoms in the chain eg pentane, hexane, heptane, octane etc. - match with the formulae quoted above.

• Alkanes are known as saturated molecules because other atoms cannot add to them.

  • In other words, the carbon atoms are bonded to as many other atoms as they can.

  • Compare alkanes with unsaturated alkenes - unsaturated with a double C=C bond.

• Physical properties: The first four in the series are shown below and are all colourless smelly highly flammable gases. The larger alkanes are colourless liquids and the biggest members of the series are white waxy solids.

They are not very reactive unless burned!

The principal source of alkane hydrocarbons is crude oil - see section 2. Fractional distillation of crude oil & uses of fractions

(1) is the molecular formula: a summary of the totals of each atom of each element in one molecule.

(2) is a 'shorthand' version of the full structural formula (3).

(3) is called the structural formula or displayed formula: it shows how all the atoms are linked by covalent bonds (the dashes —).

(4) is a '3D' representation of the structural formula (3) showing the spatial arrangement of the atoms.

(5) is a simple 'molecular diagram' showing the atoms as spheres joined together

(main molecule in natural gas)

(1)   (2)   (3)    (5)

(4)      

(1)   (2)

(3)       (4)    (5)

in bottled gas

(1)   (2)

(3)    (5)

in bottled gas

C₅H₁₂

C₆H₁₄

C₇H₁₆

C₈H₁₈

C₉H₂₀

C₁₀H₂₂

  C₁₁H₂₄

C₁₂H₂₆

Advanced Level Notes on the Structure and Naming of Alkanes

 

3b. The reactions of Alkanes

The complete combustion of hydrocarbons (excess air)

Introduction via coal combustion

• The fuel coal consists mainly of carbon, which, if burned/ignited in excess air, combusts to form carbon dioxide.

  • carbon + oxygen ==> carbon dioxide

  • C_(s) + O_2(g) ==> CO_2(g)

  • This is what you expect to happen in an open domestic coal fire.

• If not enough air/oxygen is available, coal will only 'half' burn to form the deadly odourless, colourless and toxic gas carbon monoxide.

  • 2C_(s) + O_2(g) ==> 2CO_(g)

  • This can happen if organic material, coal or peat is smouldering underground and is obviously a dangerous situation.

  • If it was formed in a domestic coal fire it will quite happily burn with a pale blue flame to form the 'safe' combustion product carbon dioxide gas.

  • 2CO_(s) + O_2(g) ==> 2CO_2(g)

• The diagram shows how to detect the products of hydrocarbon combustion e.g. burning candle wax.

• When hydrocarbons are burned in air a fast exothermic reaction occurs releasing heat and forming carbon dioxide and water - their formation is an oxidation reaction.

• It is an oxidation reaction because of oxygen atom gain by the carbon and hydrogen atoms of the hydrocarbon molecules.

• The carbon dioxide is chemically detected with limewater - with which it forms a white precipitate (milky appearance) of calcium carbonate.

• The water is chemically detected either by

  • (i) anhydrous white copper sulphate turning blue

  • or

  • (ii) dried blue cobalt chloride paper turning pink.

• A physical test for water is to measure its boiling point (should be 100^oC).

When a hydrocarbon molecule (reactant) burns in an excess of air-oxygen their are only two products of the reaction. The carbon atoms are oxidised on combining with oxygen to form carbon dioxide molecules, and the hydrogen atoms are oxidised to water molecules ('hydrogen oxide'). This section ignores the combustion of the pollutant sulphur.

general word equation: hydrocarbon + oxygen ==> carbon dioxide + water

word equations e.g. methane + oxygen ==> carbon dioxide + water

and the corresponding symbol equation is

CH_4(g) + 2O_2(g) ==> CO_2(g) + 2H₂O_(l)

Note that one CO₂ for every C, and one H₂O for every two H's in the hydrocarbon molecule.

In terms of displayed formula the equation would be written as ...

... in which every individual atom is shown and how it is bonded ('connected') with other atoms in the molecule. All the dashes represent the covalent bonds between the atoms in the molecules.

Another example is the combustion of propane ...

propane + oxygen ==> carbon dioxide + water

C₃H_8(g) + 5O_2(g) ==> 3CO_2(g) + 4H₂O_(l)

and in terms of displayed formula and balancing numbers ...

and the above diagrams show how the atoms have rearranged themselves in the reaction after the reactant bonds are broken (C-H, O=O and C-C in ethane etc. below)) and the new bonds formed in the products (C=O and O-H). Note the number of atoms of each element must be the same on each side of the equation (1C, 4H's and 4 O's, Law of Conservation of mass) and the products are different substances with different properties compared to the reactants. See Elements, Compounds and Mixtures page for more on writing and balancing equations

for ethane the more awkward symbol equation is ...

2C₂H_6(g) + 7O_2(g) ==> 4CO_2(g) + 6H₂O_(l)

and for pentane the symbol equations is ...

C₅H_12(l) + 8O_2(g) ==> 5CO_2(g) + 6H₂O_(l)

More on incomplete combustion is described on the pollution page section 4a