Download Alkanes and alkenes

Alkanes and alkenes
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 CnH2n+2
Alkenes are unsaturated compounds and contain double C=C bonds. They undergo addition
reactions. They have the general formula CnH2n.
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.