AQA A level Chemistry 3.3.4 Alkenes

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36 Terms

1

Why are alkenes more reactive than alkanes? (2)

- Alkenes are more reactive because of the C=C double bond

- Which has a high electron density

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2

What is an electrophile in the context of alkene reactions? (2)

- Electron-deficient species that accepts a pair of electrons

- Contains an atom with a partial positive charge (delta+)

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3

What happens to the double bond (C=C) during an electrophilic addition reaction? (3)

1. The double bond breaks

2. The electrons are donated to the electrophile

3. Forming a single bond and saturated (C-C) products

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4

How does the high electron density of the C=C bond contribute to the reactivity of alkenes? (1)

Makes the C=C bond readily attacked by electrophiles

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5

What is the general equation for electrophilic addition reactions in alkenes? (1)

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6

Draw the general mechanism for electrophilic addition reactions in alkenes? (3)

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7

What is the test for alkenes using bromine water? (2)

- Add bromine water to the alkene.

- The red-brown colour will decolourise to show a carbon-carbon double bond

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8

Write the equation for the reaction of ethene with bromine to form 1,2-dibromoethane. (1)

C2H4 + Br2 → C2H4Br2

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9

Draw the mechanism for the electrophilic addition of bromine to ethene. (4)

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10

Describe the stages of the electrophilic addition mechanism of bromine to ethene. (4)

1. The electron-rich area of the double bond induces a dipole on the bromine molecule.

2. Electrons from the C=C move to δ+ Br, forming a carbon-bromine bond and releasing Br⁻.

3. The other carbon in the double bond becomes a carbocation.

4. The Br⁻ ion acts as a nucleophile, attacking the carbocation to form the final product

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11

Draw the mechanism for the electrophilic addition of hydrogen bromide to ethene. (4)

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12

Draw the mechanism for the electrophilic addition of sulphuric acid to ethene. (4)

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13

What is the product of hydrolysis of ethyl hydrogensulfate with water? (1)

Ethanol (CH3CH2OH)

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14

Write the equation for the hydrolysis of ethyl hydrogensulfate with water. (1)

CH3CH2OSO3H + H2O → CH3CH2OH + H2SO4

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15

What is the overall equation for the reaction of an alkene with concentrated H2SO4 followed by hydrolysis? (1)

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16

Why does sulfuric acid act as a catalyst in the overall reaction? (1)

Sulfuric acid is not used up in the reaction

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17

What happens when alkenes react with electrophiles in an unsymmetrical environment? (1)

They form two products: a major product and a minor product

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18

How is the major product in the electrophilic addition of unsymmetrical alkenes determined? (1)

From the more stable carbocation intermediate

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19

What are the three types of carbocations in increasing order of stability? (3)

Primary (1°) < Secondary (2°) < Tertiary (3°)

<p>Primary (1°) &lt; Secondary (2°) &lt; Tertiary (3°)</p>
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20

What makes tertiary carbocations more stable than primary carbocations? (2)

- Presence of more alkyl groups stabilizes the carbocation.

- Positive inductive effect of alkyl groups helps stabilise the positive charge

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21

What is the positive inductive effect in carbocations? (1)

It is the electron-releasing property of alkyl groups that stabilises the positive charge on the carbocation

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22

Why do unsymmetrical alkenes give rise to different products during electrophilic addition? (2)

- Different carbocation intermediates form during the reaction

- Leading to products in unequal amounts

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23

Which product forms in higher amounts during electrophilic addition to unsymmetrical alkenes? (1)

The product derived from the more stable carbocation forms in higher amounts

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24

Draw the mechanism for the electrophilic addition of HBr to an unsymmetrical alkene, showing the formation of both major and minor products. (6)

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25

What is addition polymerisation? (3)

- A reaction where small molecules (monomers, alkenes) join together

- To form a very large molecule called a polymer (poly(alkene))

- Which is unreactive

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26

What is the polymer formed when ethene molecules join together? (2)

- The polymer formed is poly(ethene)

- Which is saturated (C-C bonds)

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27

Draw the structure of the repeating unit of poly(ethene). (3)

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28

What is poly(chloroethene) commonly known as? (1)

PVC

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29

Why does pure poly(chloroethene) tend to be hard and rigid? (1)

It has permanent dipole forces between chains due to the δ⁻ Cl and δ⁺ C

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30

What is the purpose of plasticisers in PVC? (2)

- Added to reduce the effectiveness of intermolecular forces

- Making the plastic more flexible

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31

What are the most common plasticisers used for PVC? (1)

Phthalates (esters of phthalic acid)

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32

List one use of unplasticised PVC. (3)

Doors, windows, or guttering

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33

List one use of plasticised PVC. (3)

Cable insulation, imitation leather, or inflatable products

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34

Why are most plastics not biodegradable? (1)

They are not broken down by microbes quickly or at all

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35

How can poly(ethene) be recycled? (1)

It can be separated, washed, melted, and remoulded into new products

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36

How can poly(propene) be recycled? (1)

It can be heated to break polymer bonds, producing monomers that can be used to make new plastics.

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