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AQA A-Level Organic Chemistry > [3.3.4] Alkenes > Flashcards

[3.3.4] Alkenes Flashcards

Structure, Bonding & Reactivity, Addition Reactions of Alkenes and Addition Polymers

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1
Q

What are alkenes?

A
  • Alkenes are unsaturated hydrocarbons.
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2
Q

What is the general formula for an alkene?

A

CnH2n

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3
Q

Describe the bonding in alkenes.

A
  • 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 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.
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4
Q

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

A
  • 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.
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5
Q

How do you name E-Z stereoisomers?

A
  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.
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6
Q

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

A
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7
Q

Define electrophile.

A

An electron pair acceptor.

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8
Q

What is an addition reaction?

A

A reaction where two molecules react together to produce one.

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9
Q

Why do alkenes undergo addition reactions?

A
  • 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.
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10
Q

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

A

CHANGE IN FUNCTIONAL GROUP

  • Alkene -> dihalogenoalkane

ROLE OF REAGENT

  • Electrophile, Brδ⁺.

CONDITIONS

  • Room temperature (not in UV light).

MECHANISM

  • Electrophilic addition.
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11
Q

Draw and state the mechanism when bromine reacts with ethene.

Name the product and explain what happens in this mechanism.

A

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.
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12
Q

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

A

CHANGE IN FUNCTIONAL GROUP

  • Alkene -> halogenoalkane

ROLE OF REAGENT

  • Electrophile, Hδ⁺.

CONDITIONS

  • Room temperature.

MECHANISM

  • Electrophilic addition.
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13
Q

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

Name the product and explain what happens in this mechanism.

A

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.
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14
Q

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.

A
  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.
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15
Q

Describe the order of stability for carbocations.

A
  • 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.
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16
Q

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?

A
  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.
17
Q

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?

A

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.
18
Q

Draw and state the mechanism when sulfuric acid reacts with but-1-ene.

Name the product.

A

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)

19
Q

How would you test for the presence of an alkene?

A
  • 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.
20
Q

How are addition polymers formed?

A

They are formed from alkenes in addition polymerisation.

21
Q

Why are addition polymers unreactive?

A

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

22
Q

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

A
23
Q

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

Name the addition polymer.

A
24
Q

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

Name the monomer.

A
25
Q

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.

A
26
Q

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

A
  • 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.

AQA A-Level Organic Chemistry (6 decks)

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