4.1.3: Alkenes

Question 1

Explain why E/Z isomerism is shown in some alkenes

Answer 1

• Can’t rotate around a c=c double bond
• Each C of the C=C double bond must have different groups attached (can draw diagram)

Question 2

Suggest why poly(ethenol) can dissolve in water

Answer 2

Lots of OH groups present which can hydrogen bond with water

Question 3

What would be observed in a reaction where an alkene is reacted with Br2

Answer 3

Orange to colourless colour change

Question 4

Give the reagents and conditions to turn an alkene into an alcohol

Answer 4

Steam and acid catalyst

Question 5

What does a repeat unit of an addition polymer look like ?

Answer 5

Brackets with extended arms and subscript n on RHS ONLY

Question 6

Give an advantage and 2 disadvantages of using combustion to dispose of waste chlorine polymers

Answer 6

• Energy can be used to produce electricity
• Can form HCl and CO

Question 7

Describe 3 ways waste polymers can be disposed of sustainably

Answer 7

• Recycling
• Used as organic feedstock
• Combustion for use as energy production

Question 8

What forms when an alkene reacts with water (hydration) ?

Answer 8

An alcohol forms

Question 9

What are conditions required to turn an alkene into an alcohol (hydration)

Answer 9

Steam and acid catalyst
- 60atm
- 300 degrees
- H3PO4

Question 10

What will the structure of a monomer used to produce a polymer look like

Answer 10

An alkene

Question 11

How would sodium hydrogencarbonate remove the toxic gas HCl ?

Answer 11

Neutralise HCl

Question 12

How is HCl removed from waste gasses ?

Answer 12

Passed through an alkali to neutralise it

Question 13

Give 2 advantages of using naturally sourced polymers

Answer 13

• Reduces dependency on finite resources
• Biodegradable

Question 14

How can a H+ catalyst be identified in a reaction

Answer 14

It is not used up

Question 15

Explain why cis and trans compounds react with Br2 to produce the same structural isomer

Answer 15

Product is saturated and single bonds allow rotation

Question 16

How are alkenes used in the manufacture of margerine ?

Answer 16

H2 and Ni catalyst which makes unsaturated vegetable oil

Question 17

Describe 3 features of alkenes

Answer 17

Unsaturated hydrocarbons with C=C double bonds containing pi bonds

Question 18

How do sigma bonds form ?

Answer 18

The head on overlap of s orbitals between the bonding atoms

Question 19

How do pi bonds form ?

Answer 19

The sideways overlap of p orbitals above and below the bonding atoms

Question 20

Explain the shape and bond angle form around each carbon in the C=C double bond ?

Answer 20

Trigonal planar 120 as the bond pairs have equal repulsion

Question 21

Define stereoisomerism

Answer 21

Compounds with the same structural formula but with a different arrangement of atoms in space

Question 22

What is a cis isomer and what letter represents it ?

Answer 22

Z - when higher priority groups are on same plane

Question 23

What is a trans isomer and what letter represents it ?

Answer 23

E - when higher priority groups are on opposite planes

Question 24

Are pi or sigma bonds weaker and why?

Answer 24

Pi has lower bond enthalpy as electrons density is spread above and below nuclei meaning weaker electrostatic attraction

Question 25

Explain reactivity of alkenes

Answer 25

High reactivity due to pi bond which increases electron density so can more easily polarise electrophiles

Question 26

By what mechanism do alkenes react ?

Answer 26

Electrophilic addition

Question 27

Define electrophile

Answer 27

An electron pair acceptor

Question 28

What happens when alkenes are reacted with hydrogen halides ?

Answer 28

Form a haloalkane

Question 29

What happens when alkenes are reacted with hydrogen and what are the conditions ?

Answer 29

Form an alkane
- Ni catalyst
- 150 degrees

Question 30

Which of the reaction pathways from an alkene is not electrophilic addition ?

Answer 30

Hydrogenation (H2 + Nickel catalyst)

Question 31

What happens when alkenes are reacted with steam and acid catalyst?
Name a suitable catalyst

Answer 31

Form an alcohol + H+

Question 32

What happens when alkenes are reacted with halogens ?

Answer 32

Dihaloalkanes form

Question 33

What can happen when unsymmetrical alkenes form haloalkanes ?

Answer 33

Multiple products form from carbocation intermediates

Question 34

When does a major product form ?

Answer 34

From the secondary carbocation (more stable) which has 2 or more alkyl groups bonded to the C+

Question 35

Give 3 benefits of the development of biodegradable and photodegradable polymers

Answer 35

• Conserve finite resources
• Leave no toxic residues
• Renewable so less plastic waste disposed of

Question 36

Why does hydrogen react more readily with alkenes than alkanes ?

Answer 36

The bond enthalpy of C-C sigma bonds are higher than pi bonds