Module 4: Alkenes

Question 1

Define the term ‘geometrical isomerism’

Answer 1

A compound which has the same structural formula but a different arrangement in space due to restricted rotation around the C=C bond

Question 2

State why geometrical isomers have restricted rotation around the C=C bond

Answer 2

The Pi bond above and below the plane stops the rotation

Question 3

Explain how covalent bonds are formed

Answer 3

• A 2p2 orbital from each carbon atom overlaps to form a single C-C bond
• The resulting bond is called a sigma bond
• Two 2p orbitals also overlap to form a second bond called a Pi bond
• This gives rise to the planar arrangement around C=C bonds

Question 4

Why are alkenes very reactive?

Answer 4

They are electron rich

Question 5

Explain what is meant by the term ‘cis-trans isomerism’

Answer 5

If both the carbon atoms on the double bond contain the same group, cis-trans isomerism can occur, however each carbon atom must have two different groups

Question 6

Explain why alkenes are more reactive than alkanes

Answer 6

The Pi bond above and below the plane in an alkene is weaker than the sigma bond in an alkane

Question 7

What is an addition reaction?

Answer 7

A reaction in which two molecules react together to make a saturated product
E.g.
| | | |
C=C + A—B —> — C—C —
| | | |

Question 8

State the conditions needed for hydrogenation of alkenes

Answer 8

-Nickel catalyst
-150 oC
-H2 gas

Question 9

State the product formed during a halogenation reaction between ethene and chlorine

Answer 9

1,2-dichloroethane

Question 10

Describe how to test for alkenes

Answer 10

-Use bromine water (an orange liquid)
-Alkenes= change from orange to colourless
-Alkanes= does not have any effect so stays orange
(Alkanes and alkenes are both colourless liquids)

Question 11

Define the term ‘hydration of alkenes’

Answer 11

Alkenes will react with steam, H2O in the presences of phosphoric acid (H3PO4) to produce alcohols

Question 12

State the products of a reaction between propene and steam

Answer 12

Propan-1-ol and propan-2-ol

Question 13

Define the term ‘electrophile’

Answer 13

Electron pair acceptor

Question 14

What is heterolytic fission?

Answer 14

-The breaking of a covalent bond with both of the bonded electrons going to each atom forming a cation (+ion) and an anion (-ion)
-Electrons have a negative charge so will be attracted to the cation (+ion)

Question 15

When does electrophilic addition occur?

Answer 15

Only occurs if there is unsymmetrical hydrocarbons

Question 16

What is a primary carbocation?

Answer 16

-When a carbon atom is only bonded to one other carbon atom
-This makes it less stable
-Minor product

Question 17

What is a secondary carbocation?

Answer 17

-When a carbon atom is bonded to two other carbon atoms
-More stable
-Major product

Question 18

What is a tertiary carbocation?

Answer 18

-When a carbon atom is bonded to three other carbon atoms
-More stable
-Major product

Question 19

Define the term ‘polymerisation’

Answer 19

A process in which small molecules called monomers join together to form large molecules consisting of repeating units

Question 20

Define the term ‘addition polymerisation’

Answer 20

When all of the atoms in the monomer are used to form the polymer

Question 21

Explain how scientists process waste polymers

Answer 21

-Combustion for energy production (produces heat which is turned into steam to generate electricity)
-Recycle plastics by breaking them down and reusing them as starting materials
-Biodegradable bottles make from plants which dissolve in water/ air
-Photodegradable so energy from the sun breaks down the waste