Section IB 1

The compound C₂H₄ can be used as a starting material for the preparation of many substances.

a) name the compound C₂H₄ and draw its structural formula.

C₂H₄ is ethene.

b) In the reaction scheme below state the type of reaction and identify the reagent needed for each reaction.

C₂H₄ CH₃CH₂OH CH₃COOH

ans (b)

Step 1: C₂H₄ CH₃CH₂OH can be carried out by passing the ethene with steam over a suitable catalyst, such as phosphoric acid supported on alumina.

Step 2: CH₃CH₂OH CH₃COOH is an oxidation reaction using potassium dichromate (VI) in acidic solution under reflux conditions.

c) C₂H₄ can be converted into one of the following compounds in a single step reaction C₂H₃Cl or C₂H₄Cl_1.

Draw the structural formula for each of these compounds and identify the compound that can be formed directly from C₂H₄.

ans (c)

Structural formulae:


chloroethene	1,2-dichloroethane

Compound II, dichloroethane can be formed directly from ethene by reaction with chlorine.

CH₂=CH₂ + Cl₂ CH₂Cl-CH₂Cl

d) one of the compounds in c) has an isomer. Draw the structural formula of the isomer and explain why it cannot be formed directly from C₂H₄.

ans (d)

The compound 1,2-dichloroethane has an isomer, 1,1-dichloroethane which cannot be made from ethene.

1,1-dichloroethane

The reason is that the chlorine molecule must add to the ethene across the double bond, with one chlorine atom going to each of the two carbons.

e) C₂H₄ can also react to form a polymer. Name this type of polymer and draw the structural formula of a section of this polymer consisting of three repeating units.

ans (e)

Ethene undergoes addition polymerisation. The product is called an addition polymer.

CH₂=CH₂ -(-CH₂-CH₂-)-

Three repeating units:

-(-CH₂-CH₂-)-(-CH₂-CH₂-)-(-CH₂-CH₂-)-

f) Polymers can also be formed in a different type of reaction. Identify this type of reaction and name two different types of such polymers.

ans (f)

The other form of polymerisation is condensation polymerisation.

Two different kinds of condensation polymers: polyester and polyamides

g) The polymer with the repeating unit -(-NH-CH(CH₃)-CO-)- exists as optical isomers.

(i) State a test for optical isomers
(ii) Identify the chiral centre in the repeating unit
(iii) Draw the two enantiomeric forms of the repeating unit.

ans (g)

(i) Optical isomers rotate the plane of polarised light.

(ii) the chiral (asymmetric) centre (carbon atom) has four different groups attached, -(-NH-CH(CH₃)-CO-)-

(iii)