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Chemistry Alevel - Module 4 > 4.1.3 - Alkenes > Flashcards

4.1.3 - Alkenes Flashcards

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1
Q

What is the general formula of alkenes?

A

CnH2n

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2
Q

What type of hydrocarbon are alkenes?

A

Unsaturated hydrocarbons — they contain a carbon–carbon double bond.

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3
Q

Why are numbers used in naming some alkenes?

A

To indicate the position of the double bond when positional isomers are possible.

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4
Q

What types of bonds make up a carbon–carbon double bond in alkenes?

A

One sigma (σ) bond and one pi (π) bond.

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5
Q

What kind of species attack the π bond in alkenes?

A

Electrophiles — they are attracted to areas of high electron density.

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6
Q

Why is the π bond reactive?

A

It has high electron density and is exposed above and below the plane of the molecule.

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7
Q

How is the sigma bond in a C=C formed?

A

By overlap of two sp² orbitals from each carbon atom.

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8
Q

How is the π bond formed in alkenes?

A

By sideways overlap of unhybridized p orbitals on each carbon, forming a π bond above and below the molecule’s plane.

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9
Q

Can rotation occur around a C=C double bond?

A

No — rotation is restricted due to the π bond.

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10
Q

Which bond is weaker: sigma or pi?

A

The π bond is weaker than the σ bond.

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11
Q

What is the shape and bond angle around the C=C double bond?

A

Trigonal Planar with a bond angle of 120°.

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12
Q

What is stereoisomerism?

A

Isomers with the same structural formula but a different spatial arrangement of atoms.

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13
Q

Why do alkenes exhibit E/Z stereoisomerism?

A

Because there is restricted rotation around the double bond.

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14
Q

What conditions are required for E/Z isomerism to exist?

A
  1. Restricted rotation around the double bond.
  2. Two different groups attached to each carbon in the double bond.
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15
Q

What is the CIP rule for assigning priority?

A

Compare atomic numbers of atoms directly bonded to each C of the double bond; higher atomic number = higher priority.

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16
Q

What does ‘Z’ mean in stereoisomerism?

A

Zusammen = same side — priority groups are on the same side of the double bond.

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17
Q

What does ‘E’ mean in stereoisomerism?

A

Entgegen = opposite sides — priority groups are on opposite sides of the double bond.

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18
Q

What is cis-trans isomerism?

A

A special case of E/Z isomerism where the two substituent groups are the same.

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19
Q

What is the boiling point of Z-1,2-dichloroethene?

A

60°C — it is polar due to C–Cl bonds on the same side.

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20
Q

What intermolecular forces are present in Z-1,2-dichloroethene?

A

London forces and permanent dipole–dipole attractions.

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21
Q

What is the boiling point of E-1,2-dichloroethene?

A

48°C — it is non-polar because dipoles cancel out.

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22
Q

What intermolecular forces are present in E-1,2-dichloroethene?

A

Only London forces.

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23
Q

Why are alkenes more reactive than alkanes?

A

Because the π bond has lower bond enthalpy and is more accessible to reactants.

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24
Q

What is an addition reaction?

A

A reaction where two molecules react together to form one product.

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25
What is the functional group change in hydrogenation of alkenes?
Alkene → Alkane
26
What reagent is needed in hydrogenation of alkenes?
Hydrogen gas (H₂)
27
What are the conditions for the hydrogenation of alkenes?
Nickel catalyst and heat
28
What type of reaction is the hydrogenation of alkenes?
Addition (also classified as a reduction).
29
What is an electrophile?
An electron pair acceptor — attracted to high electron density areas like C=C.
30
Why do alkenes undergo electrophilic addition?
Because their π bond is electron-rich and attracts electrophiles.
31
What is the functional group change in halogenation?
Alkene → Dihaloalkane
32
What reagent is needed for halogenation?
Bromine or chlorine.
33
What are the conditions need for halogenation?
Room temperature (no UV light)
34
What is the mechanism for the halogenation of alkenes?
Electrophilic addition.
35
What type of reagent is involved in halogenation of alkenes?
An electrophile (Br δ⁺– Br δ⁻)
36
What type of bond fission occurs in the halogenation of alkenes?
Heterolytic fission.
37
What is the functional group change of the reaction of hydrogen bromide with alkenes?
Alkene → Haloalkane
38
What reagents can be used in the reaction of hydrogen bromide with alkenes?
HBr or HCl
39
What are the conditions used in the reaction of hydrogen bromide with alkenes?
Room temperature
40
What is the mechanism used in the reaction of hydrogen bromide with alkenes?
Electrophilic addition
41
What is the electrophile in the reaction of hydrogen bromide with alkenes?
The H atom in HBr or HCl, as it is slightly positive (δ⁺).
42
Why is HBr a polar molecule?
Because bromine is more electronegative than hydrogen, making Hδ⁺ and Brδ⁻.
43
In electrophilic addition, which atom in HBr attacks the alkene?
The hydrogen (Hδ⁺) attacks the electron-rich π bond.
44
What happens when an unsymmetrical alkene reacts with HBr?
Two isomeric products can form: a major (90%) and a minor product (10%).
45
What does Markovnikov’s Rule state?
In most cases, the hydrogen adds to the carbon with more hydrogens, and the halogen adds to the carbon with fewer hydrogens.
46
Why is one product favoured in unsymmetrical addition?
Because it proceeds via a more stable carbocation intermediate.
47
What is the order of stability for carbocations?
Tertiary > Secondary > Primary
48
Why are tertiary carbocations more stable?
Because alkyl (methyl) groups are electron-releasing and help stabilise the positive charge.
49
How do methyl groups stabilise a carbocation?
They reduce the charge density on the positive carbon by donating electron density.
50
What is hydration in organic chemistry?
The addition of water (H₂O) across a double bond to form an alcohol.
51
What is the functional group change in hydration of alkenes?
Alkene → Alcohol
52
What reagent is needed for the hydration of alkenes?
Steam (H₂O in gas form)
53
What catalyst is used for the industrial hydration of alkenes?
Phosphoric acid (H₃PO₄)
54
What are the essential conditions for alkene hydration?
High temperature (300–600°C), high pressure (70 atm), phosphoric acid catalyst.
55
What type of reaction is alkene hydration?
Addition reaction
56
Why is hydration of alkenes used industrially?
It allows direct conversion of alkenes to alcohols in one step.
57
What are addition polymers?
Polymers formed from alkene monomers by addition reactions.
58
What is the name of the process by which addition polymers are formed?
Addition polymerisation
59
What type of monomers form addition polymers?
Alkenes (or molecules containing C=C double bonds).
60
Why are poly(alkenes) generally unreactive?
Because they contain strong carbon–carbon (C–C) and carbon–hydrogen (C–H) single bonds.
61
What are addition polymers?
Polymers formed when alkene monomers join together in an addition reaction.
62
What is the process of forming addition polymers called?
Addition polymerisation.
63
What are the properties of poly(alkenes)?
They are generally unreactive due to strong carbon–carbon and carbon–hydrogen bonds.
64
What is one major industrial use of alkenes?
Catalytic hydrogenation of unsaturated vegetable oils to make margarine.
65
What catalyst is used in hydrogenation of vegetable oils?
Nickel (Ni).
66
What happens to the vegetable oil during hydrogenation?
The C=C double bonds are converted into C–C single bonds, making the oil more saturated.
67
Why does hydrogenation of oils increase their melting point?
Because saturated molecules pack closer together, making the substance harder and more solid.
68
Are vegetable oils saturated or unsaturated?
They are usually polyunsaturated (contain multiple C=C double bonds).
69
What are the main methods of dealing with waste polymers? (4)
1. Separation and recycling 2. Feedstock for cracking 3. Combustion for energy 4. Use of biodegradable and alternative polymers
70
How does separation and recycling work?
Polymers are sorted by type, then melted and remoulded into new products.
70
What is feedstock recycling?
Using waste polymers as raw materials for cracking to produce new monomers and chemicals.
71
How is combustion used to deal with waste polymers?
Polymers are incinerated to release energy, which can generate electricity.
72
What is a major concern with burning halogenated plastics like PVC?
They can produce toxic acidic gases such as hydrogen chloride (HCl).
72
How can chemists reduce the environmental damage from burning PVC?
By removing or neutralising hydrogen chloride fumes during combustion.
72
What types of alternative polymers are being developed?
Biodegradable polymers, compostable polymers, soluble polymers, and photodegradable polymers.
72
What is the usage of poly(propene)?
Carpet; Ropes
72
What are biodegradable polymers made from?
Renewable biological sources such as isoprene (natural rubber), maize, and starch.
72
Why are biodegradable polymers useful?
They break down naturally in the environment, reducing plastic pollution.
72
What is the usage of poly(ethene)?
Plastic Bags; Plastic Bottles
73
What is the usage of poly(phenylethene)  Polystyrene?
Packaging materials
74
What is the usage of poly(chloroethene)?
Pipes; Door Frames
75
What is the usage of Super glue?
Super glue
75
What is the usage of poly(tetrafluoroethene)  PTFE/Teflon?
Non-stick kitchenware
76
What is the usage of PMMA/Perspex?
Plastic Window
77
What are the ways in which plastics can be disposed? (6)
1. Landfill 2. Combustion 3. Electricity generation 4. Reuse 5. Recycle 6. Organic Feedstock
77
What are the disadvantages of recycling? (4)
1. Plastics must be sorted into different types 2. Expensive 3. Labour intensive 4. Requires high technology
78
How do photodegradable polymers break down?
They are broken down chemically using energy with wavelengths similar to light. Once the break down begins it is not possible to stop the process.
79
Explain what happens in organic feedstock.
Plastics are separated and broken down into small organic molecules through a series of reaction. The molecules can then be used produce plastics and in other industries.
80
Give a disadvantage of photodegradable polymers.
May not be exposed to sufficient light.

Chemistry Alevel - Module 4 (5 decks)

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