Organic 4: Alkenes

Question 1

Why are alkenes more reactive than alkanes?

Answer 1

High electron density of the C=C double bond

Question 2

Describe the structure of ethene.

Answer 2

Planar
120 degree bond angle

Question 3

What is restricted rotation?

Answer 3

Double bonds cannot rotate due to the presence of the pi orbital in the double bond, which creates a region of high electron density that must be broken for rotation to occur

Question 4

What are the CIP rules?

Answer 4

Compare the groups attached to each carbon on either side of a double bond
The highest priority group will be the group with the highest Mr
If the highest priority groups are on the same side of the double bond, it is a Z-isomer
If they are on opposite sides, it is an E-isomer

Question 5

Describe the physical properties of alkenes.

Answer 5

Similar to alkanes
Alkenes are insoluble in water
Higher bond enthalpy than alkanes

Question 6

Give the equation for a complete combustion reaction of ethene.

Answer 6

Ethene + Oxygen –> Carbon Dioxide + Water

Question 7

Explain the mechanism for producing a halogenoalkane from an alkene.

Answer 7

1) The delta positive hydrogen on H-Br is attracted to the double bond of the alkene due to its high electron density
2) A pair of electrons from the C=C bond moves to the hydrogen, forming a bond and creating a carbocation
3) The electrons in H-Br move towards the more electronegative Br, breaking the H-Br bond leaving a Br- ion
4) The Br- ion is then attracted to the C+ on the carbocation, forming a bond with one of its pairs of electrons

Question 8

Describe the mechanism for producing a dihalogenoalkane from an alkene.

Answer 8

Electrophilic addition

Reagents: Aqueous halogen solution, or chlorine gas
Conditions: Room temperature
Products: Dihalogenoalkanes

Question 9

Describe the mechanism for producing a halogenoalkane from an alkene.

Answer 9

Electrophilic addition

Reagents: Hydrogen halide
Conditions: Room temperature
Product: Halogenoalkane

Question 10

Why is the major product formed from a secondary or tertiary carbocation?

Answer 10

Because alkyl groups have a positive inductive effect, which stabilises the positive charge of the carbocation

Question 11

Describe the mechanism of forming an alkylhydrogensulfate from an alkene.

Answer 11

Electrophilic addition

Reagents: Concentrated sulfuric acid
Conditions: Room temperature
Products: Alkylhydrogensulfate

*Water can then be added to produce an alcohol and sulfuric acid

Question 12

Describe the mechanism for producing an alcohol from an alkene using water.

Answer 12

Electrophilic addition
(Direct industrial hydration)

Reagents: Steam
Conditions: Acid catalyst (e.g. H3PO4)
High temperature, high pressure
Products: Alcohols

Question 13

What is a polymer?

Answer 13

A large molecule built up from monomers

Question 14

What is an addition polymer?

Answer 14

A polymer made from alkene monomers based on ethene

Question 15

What happens when monomers polymerise?

Answer 15

The double bond opens up, and monomers bond together to form a backbone of carbon atoms

Question 16

Name the polymer formed by phenylethene.

Answer 16

Polyphenylethene

Question 17

Why are addition polymers unreactive?

Answer 17

Because they form long, saturated alkane molecules with strong non-polar bonds

Question 18

Why are addition polymers not biodegradable?

Answer 18

They cannot be attacked by biological agents like enzymes due to their strong non-polar bonds which makes them very unreactive

Question 19

Describe mechanical recycling.

Answer 19

Separating plastics to be washed and sorted, ground into small pellets, and remoulded into new items

Question 20

Describe feedstock recycling.

Answer 20

Plastics are heated to a temperature that will break the polymer bonds and produce monomers, which can then be used to make new polymers

Question 21

How do you draw a repeating unit of an alkene?

Answer 21

Question 22

How do you test for alkenes?

Answer 22

Bromine water
Will undergo an electrophilic addition reaction
Positive result: Orange-brown –> colourless

Question 23

What is polychloroethene?

Answer 23

Waterproof polymer
Electrical insulator
Doesn’t react with acids

Question 24

What state is polychloroethene (PVC) in its pure form?

Answer 24

A rigid plastic
Due to strong intermolecular bonding between polymer chains (i.e. dipole-dipole)
Which prevents them from moving over each other

Question 25

What is polychloroethene used for?

Answer 25

uPVC window frame coverings
gutterings

*can be made more flexible by adding plasticiser to weaken intermolecular forces, so it can be used for insulation of electrical wires and waterproof clothing