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AQA A Level Organic Chemistry > 4. Alkenes > Flashcards

4. Alkenes Flashcards

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1
Q

Are alkenes saturated or unsaturated?

A

Unsaturated

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2
Q

Do alkenes contain a double bond?

A

Yes

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3
Q

What is the arrangement of atoms around the carbon-carbon double bond in alkenes?

A

Trigonal planar

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4
Q

What is the bond angle of atoms around the carbon-carbon double bond in alkenes?

A

120°

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5
Q

What do the p orbitals on each C in the double bond overlap to form?

A

Two different types of bond; σ or π

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6
Q

What are the two ways that alkenes can react?

A
  • by free radical addition

* by electrophilic addition

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7
Q

How is a pi bond formed?

A

Due to the overlap of p-orbitals

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8
Q

What gives alkenes chemical properties that are not seen in alkanes?

A

The C-C double bond

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9
Q

What is an addition reaction?

A

The combination of two or more molecules to form a single molecule as a result of breaking the double bond

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10
Q

Which type of reaction is faster; addition or substitution?

A

Addition

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11
Q

Why are addition reactions generally faster than substitution?

A

Only weak π bonds are broken in addition, rather than strong σ bonds

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12
Q

What is an electrophile?

A

A species that can accept a pair of electrons from a species with high electron density

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13
Q

What is the area of high electron density in an alkene?

A

The σ-bond and π-bond

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14
Q

Why can σ and π bonds attract electrophiles?

A

They are areas of high electron density

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15
Q

In alkenes, after an electrophile is attracted to the σ or π bond, what happens?

A

The electrophile undergoes heterolytic fission

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16
Q

What is heterolytic fission?

A

Breaking of a covalent bond which results in both electrons going to the same atom

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17
Q

Why are alkenes much more reactive than alkanes?

A

They can undergo addition, and are able to react with electrophiles as well as free radicals

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18
Q

What is likely to happen in an alkene, considering that the bond is non-polar?

A

Homolytic fission may be undergone instead of heterolytic

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19
Q

What are symmetrical alkenes?

A

Where both carbon atoms in the double bond are attached to the same groups

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20
Q

What is it called when an atom has both carbon atoms in the double bond attached to the same groups?

A

Symmetrical

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21
Q

What are unsymmetrical alkenes?

A

Alkenes with the two carbons in the double bond not attached to the same groups

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22
Q

What are possible electrophiles in addition reactions with symmetrical alkenes?

A
  • halogens (e.g. Br-Br)
  • hydrogen halides (e.g. H-Br)
  • H2SO4
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23
Q

What can symmetrical alkenes react with to undergo an addition reaction?

A
  • halogens (e.g. Br-Br)
  • hydrogen halides (e.g. H-Br)
  • cold concentrated H2SO4
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24
Q

What are the stages in an addition reaction of a symmetrical alkene?

A
  • delta +ve atom in electrophile attracted to electron rich C=C
  • double bond breaks and pair of electrons allow one of the C atoms to bond to the delta +ve atom
  • as a result second carbon in C=C is left electron deficient and a carbocation is formed
  • rest of electrophile molecule is electron rich with a lone pair and -ve charge - this is attracted to +ve C atom and lone pair used to form a bond (cancelling both charges)
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25
Q

How can bromine react in an addition reaction with a symmetrical alkene, even though it is non-polar?

A

In the presence of alkenes, electrons move to one side of molecule - acquires temporary dipole

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26
Q

What is formed when Br₂ acts with a symmetrical alkene?

A

A halogenoalkane

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27
Q

During the reaction with Br₂ and a symmetrical alkene, what is the colour change?

A

Orange to colourless

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28
Q

Why is bromine water a good test for unsaturation?

A

When Br₂ reacts with a symmetrical alkene, the double bond must break to react - showing that the alkene is unsaturated

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29
Q

What conditions does the addition reaction between a symmetrical alkene and bromine water take place under?

A

Takes place readily at room temperature - fairly quickly

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30
Q

Why does hydrogen bromide undergo addition with symmetrical alkenes?

A

The H in the H-Br bond has a positive dipole and is attacked by the pair of electrons on the C=C bond, which undergoes heterolytic fission

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31
Q

What conditions does the addition reaction between a symmetrical alkene and hydrogen bromide take place under?

A

Takes place readily at room temperature - fairly quickly

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32
Q

What are the two steps in the addition reaction of alkenes and sulphuric acid?

A
  1. Alkenes undergo electrophillic addition with cold concentrated sulphuric acid
  2. If the mixture is warmed and then water added, the H₂SO₄ group will be replaced by an OH group and alcohol formed
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33
Q

What is formed when ethene undergoes electrophillic addition with cold sulphuric acid?

A

Ethyl hydrogen sulphate

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34
Q

What is formed when ethyl hydrogen sulphate is warmed and water added?

A

Ethanol and sulphuric acid

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35
Q

What reaction happens when ethyl hydrogen sulphate is added to water and warmed?

A

Hydrolysis

36
Q

What is hydrolysis?

A

Using water to break covalent bonds

37
Q

What is the overall equation for the addition reaction and hydrolysis, involving ethene and sulphuric acid?

A

CH₂CH₂ + H₂O → CH₃CH₂OH

38
Q

What does the sulphuric acid in the addition reaction and hydrolysis with alkenes act as?

A

A catalyst

39
Q

What is the addition reaction and then hydrolysis of alkenes used for in the laboratory?

A

Useful way of converting alkenes into alcohols

40
Q

What happens if unsymmetrical alkenes react with unsymmetrical electrophiles?

A

The atoms in the electrophile can bond to either carbon in the double bond producing a different carbocation - this results in two possible outcomes

41
Q

What are some examples of unsymmetrical electrophiles?

A
  • H-X

* H₂SO₄

42
Q

What two products could be formed from the reaction of propene with hydrogen bromide?

A
  • 1-bromopropane

* 2-bromopropane

43
Q

There are two products formed from the reaction of propene with hydrogen bromide. Are they formed in equal quantities?

A

No

44
Q

When an unsymmetrical alkene reacts with an unsymmetrical electrophile, what determines the likelihood of each product being formed?

A

The stability of the carbocation intermediate

45
Q

When will a carbocation be more stable?

A

When there are more alkyl groups attached to the positively charged carbon

46
Q

Why are carbocations more stable when there are more alkyl groups attached to the positively charged carbon?

A

Because alkyl groups tend to release electrons, known as the electron releasing effect

47
Q

What is the electron releasing effect?

A

When carbocations are made more stable when the positively charged carbon is attached to more alkyl groups due to alkyl groups releasing electrons

48
Q

What happens when a carbocation is more stable, in terms of how likely it is to be produced?

A

More stable the carbocation = more likely it is to be produced

49
Q

What is the major product?

A

The product that is more likely to be produced

50
Q

What is the major product formed from?

A

The more stable carbocation from the two possible intermediates

51
Q

What is the minor product formed from?

A

The less stable carbocation

52
Q

What is the order of stability of carbocations?

A

1° < 2° < 3°

53
Q

What can the electron releasing effect sometimes be known as?

A

The inductive effect

54
Q

What will be the major product formed in the reaction between propene and hydrogen bromide?

A

2-bromopropane

55
Q

What will be the minor product formed in the reaction between propene and hydrogen bromide?

A

1-bromopropane

56
Q

Why do more alkyl groups stabilise carbocations?

A

Alkyl groups contain a greater electron density. This density is attracted towards and reduces the charge on the C atom

57
Q

What can addition polymers also be known as?

A

Polyalkenes

58
Q

What are addition polymers formed from?

A

Alkenes and substituted alkenes

59
Q

What are substituted alkenes?

A

Alkenes where the hydrogens have been replaced by other atoms or groups

60
Q

What are addition polymers made up of?

A

Small molecules containing a double bond

61
Q

How is an addition polymer formed?

A
  • double bond is broken in many monomers, under pressure, forming the repeating unit
  • repeating units join until a large number of monomers have joined together
62
Q

What are the molecules called that have had their double bond broken, before they form monomers?

A

Repeating units

63
Q

How is a polymer represented?

A

The repeating unit, in brackets, with the small n

64
Q

Examples of addition polymers?

A

DNA, silk, rubber

65
Q

How is the repeating unit written?

A

Without brackets or n - just what is inside the brackets

66
Q

How are polymers named?

A

Adding poly as a prefix to the name of the monomer

67
Q

What is the polymer produced when ethene is the monomer?

A

Poly(ethene)

68
Q

Are addition polymers saturated?

A

Yes

69
Q

Why are addition polymers unreactive?

A

They are saturated

70
Q

What do the intermolecular forces between polymer chains depend on?

A

The atoms within the polymer and how they are arranged

71
Q

Are polymer chains polar or non-polar?

A

Non-polar

72
Q

What intermolecular force are polymer chains held together by?

A

Van der Waal’s forces (unless electronegative atoms like C/F are present)

73
Q

When will polymer chains not be held together by Van der Waal’s forces?

A

When there are electronegative atoms like C and F present

74
Q

Properties of high density poly(ethene)?

A
  • fewer branches
  • stronger IMFs
  • stronger polymers, more rigid
75
Q

What might HDPE be used to produce?

A

Plastic bottles, bottle caps, water pipes

76
Q

Properties of low density poly(ethene)?

A
  • more branched chains
  • weaker IMFs
  • weaker polymers, more flexible
77
Q

What might LDPE be used to produce?

A

Plastic bags, plastic wraps, cable insulation

78
Q

Which has stronger intermolecular forces; HDPE or LDPE?

A

HDPE as there is less branching

79
Q

What is poly(chloroethene) commonly known as?

A

PVC - polyvinyl chloride

80
Q

Description of polyvinyl chloride?

A

Hard and brittle

81
Q

Bonding in polyvinyl chloride?

A

Permanent dipole-dipole forces between molecules due to electronegativity of Cl in C-Cl bonds

82
Q

What might polyvinyl chloride be used to produce?

A

Window frames, drain pipes

83
Q

Has the knowledge and understanding of the production and properties of polymers changed over time?

A

Yes

84
Q

What can be added to polymers to change their properties?

A

Plasticisers

85
Q

What do plasticisers do?

A

Force chains of polymers further apart, allowing them to slide over each other, making them m`ore flexible

86
Q

For a symmetrical alkene to undergo electrophilic addition with a halogen, how must the halogen be beforehand?

A

The halogen molecule dissolved in water or in an organic solvent

87
Q

Conditions for addition polymerisation?

A

High pressure

AQA A Level Organic Chemistry (16 decks)

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