[3.3.4] Alkenes

Question 1

What are alkenes?

Answer 1

• Alkenes are unsaturated hydrocarbons.

Question 2

What is the general formula for an alkene?

Answer 2

CnH2n

Question 3

Describe the bonding in alkenes.

Answer 3

• Alkenes contain a carbon-carbon double bond somewhere in their structure.
• The arrangement of bonds around the C=C is planar and has the bond angle 120.
• C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond.
• π bonds are exposed and have high electron density.
• They are therefore vulnerable to attack by species which ‘like’ electrons; these species are called electrophiles.

Question 4

What type of isomerism do alkenes exhibit? How does it arise?

Answer 4

• Alkenes exhibit a type of isomerism called E-Z stereoisomerism.
• This arises when:
  
  • There is a restricted rotation around the C=C double bond.
  • There are two different groups/atoms attached to both ends of the double bond.

Question 5

How do you name E-Z stereoisomers?

Answer 5

1. First determine the priority groups on both sides of the double bond.
   
   • The priority group is the atom with the bigger atomic number.
2. If the priority groups are on the same side of the double bond, it is labelled Z from the german zusamenn (The Zame Zide).
   
   • e.g. Z-1,2-dichloroethene.
3. If the priority groups are on opposite sides of the double bond, it is labelled E from the German entgegen (The Epposite Side).
   
   • e.g. E-1,2-dichloroethene.

Question 6

Draw and name the two E-Z stereoisomers of but-2-ene.

Answer 6

Question 7

Define electrophile.

Answer 7

An electron pair acceptor.

Question 8

What is an addition reaction?

Answer 8

A reaction where two molecules react together to produce one.

Question 9

Why do alkenes undergo addition reactions?

Answer 9

• The double bonds in alkenes are areas with high electron density.
• This attracts electrophiles (electron pair acceptors) and alkenes undergo addition reactions as a result.

Question 10

What is the change in functional group, role of reagent, conditions and mechanism when bromine reacts with alkenes?

Answer 10

CHANGE IN FUNCTIONAL GROUP

• Alkene -> dihalogenoalkane

ROLE OF REAGENT

• Electrophile, Brδ⁺.

CONDITIONS

• Room temperature (not in UV light).

MECHANISM

• Electrophilic addition.

Question 11

Draw and state the mechanism when bromine reacts with ethene.

Name the product and explain what happens in this mechanism.

Answer 11

MECHANISM: Electrophilic addition.
PRODUCT: 1,2-dibromoethane.

MECHANISM EXPLANATION

• As the Br₂ molecule approaches the alkene, the pi-bond electrons repel the electron pair in the Br-Br bond.
• This induces a dipole and Br₂ becomes polar and electrophilic (δ⁺).
• The intermediate formed, which has a positive charge on a carbon atom, is called a carbocation.

Question 12

What is the change in functional group, role of reagent, conditions and mechanism when hydrogen bromide reacts with alkenes?

Answer 12

CHANGE IN FUNCTIONAL GROUP

• Alkene -> halogenoalkane

ROLE OF REAGENT

• Electrophile, Hδ⁺.

CONDITIONS

• Room temperature.

MECHANISM

• Electrophilic addition.

Question 13

Draw and state the mechanism when hydrogen bromide reacts with but-2-ene.

Name the product and explain what happens in this mechanism.

Answer 13

MECHANISM: Electrophilic addition.
PRODUCT: 2-bromobutane.

MECHANISM EXPLANATION

• HBr is a polar molecule because Br is more electronegative than H.
• The Hδ⁺ is attracted to the electron-rich pi bond.

Question 14

Draw the mechanisms for the two products formed from the reaction between hydrogen bromide and but-1-ene. State whether each product is a minor or major product.

Explain why two products are formed and what determines whether they are a major or minor product.

Answer 14

1. 2-bromobutane is the major product and 1-bromobutane is the minor product.
2. This reaction leads to two products (a major and a minor product) because the alkene is unsymmetrical.
3. In the electrophilic addition of alkenes, the major product is formed via the more stable carbocation intermediate.
   
   • A carbocation intermediate is a carbon atom with a positive charge.
4. In this case, 2-bromobutane is a secondary carbocation, whereas, 1-bromobutane is a primary carbocation. Secondary carbocations are more stable than primary carbocations.
   
   • This is because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion.

Question 15

Describe the order of stability for carbocations.

Answer 15

• Tertiary > secondary > primary.
  
  • What determines whether a carbocation intermediate is tertiary, secondary or primary is by how many methyl groups are surrounding it.
    
    • Tertiary carbocation intermediates are surrounded by 3 methyl groups.
    • Secondary carbocation intermediates are surrounded by 2 methyl groups.
    • Primary carbocation intermediates are surrounded by 1 methyl group.

Question 16

In exam answers, how would you go about identifying and explaining the major and minor products produced in an electrophilic addition reaction of an alkene?

Answer 16

1. Draw out both carbocations and identify them as primary, secondary and tertiary.
2. State which is the more stable carbocation, this will be the major product.
   
   • In order of stability: tertiary > secondary > primary.
3. State that the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron-releasing and reduce the charge on the ion.
4. If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved.

Question 17

There are TWO stages when sulfuric acid reacts with alkenes.

In the FIRST stage, what is the change in the functional group, the role of the reagent, the conditions & the mechanism?

In the SECOND stage, what is the change in the functional group, the reagent, the conditions & the type of reaction?

Answer 17

STAGE 1

1. CHANGE IN FUNCTIONAL GROUP
   • Alkene -> alkyl hydrogensulfate
2. ROLE OF REAGENT
   • Electrophile, H₂SO₄.
3. CONDITIONS
   • Room temperature.
4. MECHANISM
   • Electrophilic addition.

STAGE 2

1. CHANGE IN FUNCTIONAL GROUP
   • Alkyl hydrogensulfate-> alcohol
2. REAGENT
   • Water.
3. CONDITIONS
   • Warm mixture.
4. MECHANISM
   • Hydrolysis.

Question 18

Draw and state the mechanism when sulfuric acid reacts with propene.

Name the product.

Answer 18

MECHANISM: Electrophilic addition.
PRODUCT: Propan-2-ol.

(The overall role of sulfuric acid is that of a catalyst as it is regenerated)

(With unsymmetrical alkenes a minor and major product can also be formed similar to the electrophilic addition reaction involving hydrogen bromide)

Question 19

How would you test for the presence of an alkene?

Answer 19

• Bromine water decolourises in the presence of a double bond.
• This can be used as a test for the presence of a double bond in a molecule.

Question 20

How are addition polymers formed?

Answer 20

They are formed from alkenes in addition polymerisation.

Question 21

Why are addition polymers unreactive?

Answer 21

They are unreactive due to the strong C-C and C-H bonds.

Question 22

Identify the repeating unit from a section of this poly(alkene).

Answer 22

Question 23

Draw the repeating unit for a polymer formed from the monomer ethene.

Name the addition polymer.

Answer 23

Question 24

Draw the structure of the monomer that forms poly(propene).

Name the monomer.

Answer 24

Question 25

Draw the polymer repeating unit for but-2-ene. Begin by drawing out the monomer with groups of atoms arranged around the double bond, then draw the repeating unit.

Answer 25

Question 26

What are the typical uses of poly(chloroethene), PVC, and describe how its properties can be modified using a plasticiser.

Answer 26

- PVC is apolymer that is **waterproof**, an **electrical insulator** and **doesn't react with acids**. - In its pure form it is a **rigid plastic** due to the **strong intermolecular forces between polymer chains preventing them from moving over each other**. - In this **un-plasticised** form, it is used to make **uPVC window frame coverings** and **guttering**. - If a plasticiser is added, the intermolecular forces are **weakened** which allows the chains to move more easily, resulting in **more flexibility** in the polymer. - In this form, PVC is used to make **insulation on electrical wires** and **waterproof clothing**.