Alkanes and alkenes
Francine Taylor-Campbell, Contributor

Here are some important points to note:

• Carbon forms a large number of compounds due to its ability to bond with itself, forming single, double and triple bonds.
• Each organic compound belongs to a homologous series in which the members have a general formula, show similar chemical properties, contain the same functional group and show a gradual change in physical properties along the series.
• Hydrocarbons burn in oxygen to produce carbon dioxide, water and heat.
• Alkanes are saturated compounds and contain single C-C bonds. They undergo substitution reactions. They have the general formula C_nH_2n+2
• Alkenes are unsaturated compounds and contain double C=C bonds. They undergo addition reactions. They have the general formula C_nH_2n.
• Cracking is the process of breaking down large alkane molecules into smaller alkane and alkene molecules.
• Alkenes can be differentiated from alkanes as they rapidly decolourise bromine water and acidified potassium permanganate solution.
• Isomerism describes the existence of compounds with the same molecular formula, but different structural formula.
• Alkanes and alkenes are obtained from natural gas and crude oil (petroleum) which are mixtures of hydrocarbons. Natural gas contains CH₄, C₂H₆, C₃H₈ and C₄H₁₀ and petroleum contains liquid hydrocarbons with gaseous hydrocarbons dissolved in the liquid.
• Substitution occurs when atoms in the molecule (alkane) are replaced by other atoms: CH₄ + Br₂ == CH₃Br + HBr
• Addition occurs when atoms or radicles are added to the molecule to form a single compound: C₂H₄ + Br₂ == C₂H₄Br₂

Additional reactions of alkenes

1. Hydrogenation - addition of hydrogen; using a nickel or platinum catalyst at 200°C to produce an alkane.

C₂H₄ + H₂ = C₂H₆

2. Hydration is the addition of water. In the presence of phosphoric acid on silica at 300°C and 60 atm, alcohols are formed.

C₂H₄ + H₂O === C₂H₅OH (ethanol)

3. Addition of hydrogen halides (HCl, HBr, HI)

C₂H₄ + HCl = C₂H₅Cl (chloroethane)

4. When ethane is heated to 200°C under pressure with a catalyst

it polymerises to form a white solid, polythene or polyethene.

nC₂H₄ = (C₂H₄)n

Let us now attempt the following question.

Question 1

1. Catalytic cracking of a hydrocarbon, A, which contains three carbon atoms, produces two new hydrocarbons, B and C. B is an alkene and A is an alkane.

(i) Explain the term catalytic cracking.

(ii) Write structures for the hydrocarbons A, B and C.

(iii) Describe how it is possible to obtain ethanol starting with Compound B.

Include in your answer: the reagents to be used, the reaction conditions and a balanced chemical equation for the reaction.

Answers

1. (i) Catalytic cracking is the process of breaking up larger molecules (alkanes) into smaller ones (alkanes and alkenes) in the presence of a catalyst (nickel).

(ii) The hydrocarbon is C₃H₈.

Comment: Remember cracking is usually carried out on alkanes.

C₃H₈ === CH₄ + C₂H₄

A - C₃H₈

B - C₂H₄

C - CH₄

(iii) Ethanol can be obtained by hydration of B in the presence of phosphoric acid (on silica) catalyst at 300°C and 60 atm.

C₂H₄ + H₂O == C₂H₅OH.

Christopher Chin (left), welding and fabrication teacher at Demhan Town High School, work with his students as they manufacture a stove at the school, recently. - Rudolph Brown/Chief Photographer

Francine Taylor-Campbell is an independent contributor.