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Chemistry > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

What is nucleophilic substitution?

A

Halogenoalkanes are susceptible to attack by nucleophiles

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2
Q

What is a nucleophile?

A

Electron pair donor

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3
Q

Nucleophiles can be…

A

Charged ions or neutral molecules

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4
Q

Why are halogenoalkanes susceptible to attack by nucleophiles

A

-because the electron pair on the nucleophiles is attracted to the δ+ carbon

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5
Q

Why do halgenoalkanes have a δ+ carbon?

A

Because the halogen is more electronegative - making the C - X bond polar

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6
Q

How does nucleophilic substitution work?

A

-lone pair on the nucleophile is able to form a bond w the carbon atom
-carbon can only maintain four bonds, the C-X bond breaks heterolytically
- the halogen atom is kicked out as a halide ion (known as the leaving group)

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7
Q

Wat is the definition of heterolytically?

A

-both electrons go to the halogen

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8
Q

Why aren’t hydrogen atoms kicked out instead during nucleophilic substitution?

A

-halogens are better leaving groups as they are more electronegative (accepting electrons more readily)

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9
Q

What kind of bond is formed between the carbon atom and the nucleophile?

A

Dative

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10
Q

Substitution is when one atom in a molecule is…

A

Replaced for another

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11
Q

In the case of halgenoalkanes the ……….. atom is replaced

A

Halogen

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12
Q

What is the general mechanism for nucleophilic substitution?

A
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13
Q

What is the nucleophile and conditions in nucleophilic substitution using NaOH/KOH reagent

A

Nucleophile: :OH-
Conditions: warm, aqueous

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14
Q

In NaOH or KOH everything dissolves because…

A

There is a 50/50 of aqueous/organic so everything dissolves

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15
Q

What is the nucleophile and conditions when using the reagent KCN for nucleophilic substitution?

A

Nucleophile: :CN-
Conditions: warm ethanolic

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16
Q

Why is there no :OH- substitution when using KCN as a reagent?

A

It is completely organic

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17
Q

What is the nucleophile and conditions when using NH3 as a reagent?

A

Nucleophile: :NH3
Conditions: excess ammonia

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18
Q

When using NH3 as a reagent ………..is set up

A

-equilibrium
-excess NH3 favours substitution

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19
Q

Why are nucleophilic substitution reactions particularly useful in synthesis?

A

-it increases the length of the carbon chain

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20
Q

Why is excess ammonia needed in nucleophilic substitution using ammonia as a reagent?

A

-extra deprotonation step
-prevent polysubstitution

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21
Q

What is the outline of the nucleophilic substitution using ammonia

A
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22
Q

The product of nucleophilic substitution using NH3 is a nucleophile itself, therefore…

A

It could go on to attack another halogenoalkane molecule
(Polysubstitution)

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23
Q

What is hydrolysis?

A

-splitting a molecule up using water (or aqueous hydroxide ions)

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24
Q

The mechanism for the reaction between water and a halogenoalkane is…

A

Nucleophilic substitution, with an additional deprotonation step

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25
Q

What are two factors we need to consider when making a prediction about the reactivity of the C-X bond

A

-size of bonds/ bond enthalpy
-difference in electronegativity

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26
Q

Are shorter C-X bonds more reactive or less reactive?

A

Less reactive as they are stronger

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27
Q

Are more polar C-X bonds more reactive or less reactive?

A

More reactive
-the delta positive carbon is more susceptible to attack

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28
Q

How does the reactivity of the C-X change down the group?

A

Increases

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29
Q

Why does reactivity of the C-X bond increase down the group?

A

-bond enthalpy is smaller down the group
-atomic radius increases - bond length increases
-weaker ESFA between shared e- and nuclei

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30
Q

What is more important in determining the rate of hydrolysis, polarity or bond length?

A

Bond length

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31
Q

What happens when a halogenoalkane reacts with NaOH/KOH under hot and aqueous conditions?

A

Elimination reaction

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32
Q

When does the :OH- ion act as a base?

A

During elimination reactions

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33
Q

What is a base?

A

A proton acceptor

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34
Q

What does the :OH- attack in elimination reactions?

A

-a proton (a hydrogen atom) from a carbon atom adjacent to the carbon atom with the halogen attached

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35
Q

What is formed in elimination reactions?

A

-alkenes
-water
-halide ion

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36
Q

What is the outline of the mechanism for the reaction between 2-bromopropane and NaOH in hot and ethanolic conditions?

A
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37
Q

When is elimination more likely than nucleophilic substitution?

A

-When the C δ+ is blocked by alkyl groups
-the :OH- is more likely to act as a base as the H atoms are more accessible
-tertiary halogenalkanes

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38
Q

When is nucleophilic substitution more likely to happen than elimination?

A

-when there is easy access to the C δ+
-OH:- is more likely to act as a nucleophile
-primary halogenalkanes

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39
Q

Stratospheric ozone plays an ……………………. In preventing …………………………. From reaching the earth’s surface

A

-essential role
-harmful uv radiation

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40
Q

How is ozone formed?

A

-O2 -> 2O* + 2O2 -> 2O3
-3O2 -> 2O3

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41
Q

What is the equation for the depletion of ozone?

A

2O3 -> 2O2 + 2O*
2O* -> O2

2O3 -> 3O2

42
Q

What are CFCs?

A

-chlorofluorocarbons
-halogenoalkane so containing chlorine and fluorine and no hydrogen
-quite unreactive and can remain in the lower atmosphere for a long time

43
Q

What happens when CFCs reach the stratosphere?

A

-high frequency UV light causes the C-Cl bonds to break by homolytic fission
-this release chlorine radicals

CCl3F -> *CCl2F + *Cl

44
Q

How do chlorine radicals breakdown O3 into O2?

A

1) O3 + Cl* -> O2 + ClO*
2) ClO* + O3 -> Cl* + 2O2

2O3 -> 3O2

45
Q

How can we tell that the chlorine radical acts as a catalyst in the breakdown of ozone?

A

-it does not appear in the overall equation

46
Q

What does the break down of ozone cause?

A

-a hole in the earths ozone layer which protects the planet from harmful UV rays from the sun

47
Q

Why would a bromofluorocabron to produce bromo radicals rather than for a chlorofluorocarbon to release chlorine radicals?

A

C-Br bond is longer and more reactive as it has weaker ESFA

48
Q

Why are alkenes more reactive than alkanes?

A

Alkenes contain C=C which means they are more reactive

49
Q

What is the shape and bond angle for alkenes?

A

-Trigonal planar
-120 digress

50
Q

Why do alkenes have low boiling points?

A

-weak VDW forces of attraction
-simple molecular

51
Q

How does boiling point vary with chain length?

A

Increases

52
Q

How does solubility vary with chain length?

A

-increases (organic solvents only)

53
Q

What is an electrophile?

A

Electron pair acceptor

54
Q

Why are alkenes susceptible to attack by an electrohpile?

A

-they have a c=c
-very electron dense so are susceptible to attack

55
Q

Why are the double bonds of alkenes readily attacked by electrophiles?

A

-the C=C double bond in alkenes is an area of high electron density

56
Q

What happens to 2 out of the 4 electrons in the double bond when the double bond is attacked by electrophiles?

A

-they can be used to form 2 new covalent bonds
-this is called an addition reaction
-hence the alkenes undergo electrophilic addition reactions

57
Q

What is the reaction mechanism for the homologous series of alkenes?

A

Electrophilic addition

58
Q

Name the 4 steps of electrophilic addition.

A

1- dative bond forms between carbon and δ+ H
2- heterolytic fission
3-carbocaition formed
4- bromide ion bonds (dative) w/ carbon cation

59
Q

Why is Br-Br not polar?

A

No difference in e-

60
Q

What happens when Br-Br approaches the electron rich double bond?

A

-the δ- of the double bond induces a dipole in the Br2 molecule
-electrons in the Br-Br bond are repelled to the bromine atom furthest from the double bond resulting in δ+ on the bromine atom closest to the double bond

61
Q

The electrophilic addition between alkenes and halogens forms…

A

Dihalognealkanes

62
Q

What does H2SO4 contain?

A

-two acidic hydrogen atoms which are both bonded to an oxygen

63
Q

What does electrophilic addition reaction between alkenes and sulphuric acids form?

A

-alkylhydrogensulphates

64
Q

When the double bond is not in the middle of the carbon chain…

A

There are two possible products of electrophilic addition

65
Q

What type of carbocation is this molecule and why?

A

-secondary
-two r groups stabilising the C+

66
Q

What type of carbocation is this molecule?

A

-primary
-only one R group stabilising the C+

67
Q

Electrophilic addition proceeds via the most….

A

Stable carbocation intermediate

68
Q

Why are carbocations more stable when there are more alkyl groups attached

A

-because alkyl groups are electron inducing

69
Q

What does it mean when something is electron inducing?

A

-it pushes electron density away from themselves slightly towards the positive charge
-stabilising the positive charge

70
Q

Something that is electron inducing has a…

A

Positive inducing effect

71
Q

What is markovnikovs rule?

A

-cheat method of working out the major product of a reaction (not a scientific explanation)
-states that in an electrophilic addition reaction to asymmetrical alkenes, always add the hydrogen atom from the electrophile to the carbon atom with the most hydrogens already attached

72
Q

What are addition polymers formed from?

A

-alkenes
-alkenes are the monomers

73
Q

What is a monomer?

A

-small molecules used to make very large molecules called polymers

74
Q

What happens when the monomers polymerise?

A

The double bond opens up and the monomers bond together.

75
Q

What are the physical properties of a polymer determined from?

A

The strength of the IMF between the polymer chains

-this in term depends on the structure of the polymer chains, as well as the type of atoms in the polymer

76
Q

What is a use of polyethene?

A

-carrier bags

77
Q

What is a use of polypropene?

A

-yoghurt containers

78
Q

What is a use of PTFE (Teflon)?

A

Waterproof fabric

79
Q

What is the strongest type of IMF force between polyethene chains?

A

VDW

80
Q

What is the strongest type of IMF force between polypropene chains?

A

VDW

81
Q

What is the strongest type of IMF force between Teflon chains?

A

VDW

82
Q

What is the melting temperature for polyethene?

A

110

83
Q

What is the melting point for polypropene?

A

160

84
Q

What is the melting point for Teflon?

A

327

85
Q

How are ethane molecules polymerised?

A

-by a free radical mechanism

86
Q

How can ethene’s polymerisation be controlled?

A

-using a Ziegler-natta catalyst making much more regular and linear polymers

87
Q

How is low density polyethene synthesised?

A

-polymerisation of ethene at high pressure and high temperature via a free radical mechanism

88
Q

What is the structure of LDPE

A

Branched

89
Q

What are the properties of LDPE

A

-semirigid
-tough
-waterproof

90
Q

What are uses of LDPE

A

-plastic bags
-sheeting
-electrical cables

91
Q

How are high density polyethenes synthesised?

A

-temperature and pressure just a little over room conditions and a Ziegler natta catalyst

92
Q

How can the properties of polymers be changed?

A

By adding plasticisers

93
Q

What are plasticisers and how do they work?

A

-plasticisers are small molecules that fit between polymer chains forcing them apart and allowing them to slide across each other, making them more flexible

94
Q

What do plasticisers do to the IMF between polymer chains?

A

-makes them weaker

95
Q

Why are polyalkenes inert?

A

They contain strong non-polar C-C and C-H bonds
-they are not attacked by acids bases or nucleophiles

96
Q

What is mechanical recycling?

A

-plastics are seperated into different types, washed and ground into pellets.
-these can then be melted and remoulded

97
Q

What are the disadvantedges of mechanical recycling?

A

-requires energy
-produces toxic gases

98
Q

What is feedstock recycling?

A

-plastics are heated to a temperature that breaks the polymer bonds and produces new monomers which are used to make new plastics

99
Q

What are the issues with feedstock recycling?

A

-energy required
-produces less toxic gases

100
Q

Plastics can only be melted and remoulded…

A

A limited number of times because each time the plastics properties are degraded

101
Q

What is poly(vinyl chloride) made of?

A

H2-C=C-HCL

102
Q

What is poly(vinyl chloride) used for?

A

Sewage pipes
Aprons

Chemistry (13 decks)

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