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A-Level Organic Chemistry AQA (Paper 2) > Alkenes And Addition Polymers > Flashcards

Alkenes And Addition Polymers Flashcards

1
Q

What are alkenes?

A

•Unsaturated hydrocarbons- they have one or more carbon-carbon double bond.

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2
Q

Are alkenes more reactive than alkanes?

A

•Yes.

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3
Q

Why are alkenes more reactive than alkanes?

A

•The double bond makes them more reactive due to the high concentration of electrons (high electron density) between the two carbon atoms.

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4
Q

What is ethene a starting material for? (3)

A
  • Polymers e.g. polyethene, PVC, polystyrene and terylene fabric.
  • Antifreeze.
  • Paints.
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5
Q

How are alkenes produced?

A

•In large quantities when crude oil is thermally cracked.

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6
Q

What is the general formula of alkenes?

A

•CnH2n.

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7
Q

How do you name alkenes? (2)

A
  • The same way alkanes are names but use ‘ene’ ending.

* Use a number to show where the double bond is.

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8
Q

What kind of shape are alkenes?

A

•Planar (flat)- the double bond prevents the rotation.

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9
Q

Draw the shape of ethene. (2)

A

•Planar molecule.
•Each bond angle is roughly 120 degrees.
(See textbook page 216)

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10
Q

Why can’t double bonds rotate? (3)

A

•As well as a normal C-C single bond, there is a p-orbital (which contains a single electron) on each carbon.
•These two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond (π-orbital, its presence means the bond cannot rotate).
•Also known as restricted rotation.
(See textbook page 217)

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11
Q

Alkenes with more than three carbons form… (3)

A
  • Chain isomers.
  • Position isomers.
  • Geometrical isomers.
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12
Q

What are position isomers?

A

•Isomers with the double bond in different positions, between a pair of adjacent carbon atoms in different positions in the carbon chain.

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13
Q

Give an example of a position isomer for C4H8. (Draw diagram)

A

•But-2-ene and but-1-ene.

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14
Q

What is geometrical isomers? (3)

A
  • A form of stereoisomerism.
  • The two stereoisomers have the same structural formula but the bonds are arranged differently in space.
  • It occurs only around the C=C double bonds.
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15
Q

Give an example of geometrical isomerism for C4H8. (Draw diagram)

A

•Z-but-2-ene and E-but-2-ene.

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16
Q

What are the physical properties of alkenes? (3)

A
  • The double bond doesn’t greatly affect properties such as boiling and melting points.
  • vdW forces are the only IMF between alkene molecules- the more carbon atoms, the higher the melting and boiling points.
  • Insoluble in water.
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17
Q

How does the double bond affect the reactivity of alkenes?

A

•It increases the bond enthalpy (consists of sigma (σ) and pi bonds (π))- the C=C forms an electron-rich area in the molecule, which can easily be attacked by positively charged reagents (electrophiles).

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18
Q

What are electrophiles?

A

•Electron pair acceptors.

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19
Q

Give an example of an electrophile.

A

•H^+ ion.

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20
Q

What reactions do alkenes undergo? (2)

A
  • Electrophilic additions.

* Combustion.

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21
Q

Explain why the π part of the bond is weaker than the σ part.

A

•The electron density in the σ^- orbital is connected between the nuclei and holds them together better than the electron density of π-orbital, which is above and below the plane of the molecule.

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22
Q

Draw/write an equation for the combustion reaction of ethene.

A

(See textbook page 220)

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23
Q

Why are alkenes not usually used as fuels?

A

•Their reactivity makes them very useful for other purposes.

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24
Q

Why are reactions of alkenes typically electrophilic addition reactions? (2)

A
  • The four electrons in the C=C make it a centre of high electron density.
  • Electrophiles are attracted to it and can form a bond by using two of the four electrons in the C=C (of the four electrons, the two that are in a π-bond.
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25
Q

What is the mechanism for electrophilic addition? (4)

A
  • 1). The electrophile is attracted to the double bond.
  • 2). Electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged ion of have a positively charged area.
  • 3). A positive ion (a carbocation) is formed.
  • 4). A negatively charged ion forms a bond with the carbocation.
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26
Q

How do alkenes react with hydrogen halides?

A

•E.g. HCl, HBr and HI add across the double bond to form a halogenoalkane.

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27
Q

Draw and explain the mechanism of the electrophilic addition of ethene with hydrogen bromide. (7)

A

•Bromine is more electronegative than hydrogen, so the hydrogen bromide molecule is polar, H^𝛿+—Br^𝛿-.
•The electrophile is the H^𝛿+ of the H^𝛿+—Br^𝛿-.
•The H^𝛿+ of HBr is attracted to the C=C bond because of the double bond’s high electron density.
•One of the pairs of electrons from the C=C forms a bond with the H^𝛿+ to form a positive ion (called a carbocation), whilst at the same time the electrons in the H^𝛿+—Br^𝛿- bond are drawn towards the Br^𝛿-.
•The bond in hydrogen bromide breaks heterolytically.
•Both electrons from the shared pair of in the bond fo to the bromine atom because it is more electronegative than hydrogen leaving a Br^- ion.
•The Br^- ion attaches to the positively charged carbon of the carbocation forming a bond with one of its electron pairs.
(See textbook page 220)

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28
Q

What happens during electrophilic addition when there are asymmetrical alkenes?

A

•When the double bond is not exactly in the middle of the chain there are two possible products- the bromine of the hydrogen bromide could bond to either of the carbon atoms of the double bond.

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29
Q

Draw the mechanism of the electrophilic addition of propene with hydrogen bromide.

A

•The products are 2-bromopropane (major) and 1-bromoproane (minor).

30
Q

Why do you get major and minor products from electrophilic addition reactions? (4)

A
  • Alkyl groups e.g. -CH3, -C2H5 have a tendency to release electrons, the push electrons away from each other (positive inductive effect), represented by an arrow along their bonds to show the direction of the release.
  • This electron releasing effect tends to stabilise the positive charge of the intermediate carbocation.
  • The more alkyl groups there are attached to the positively charged carbon atom, the more stable the carbocation is.
  • More product is produced from the more stable carbocation.
31
Q

What is 1° carbocation?

A

•1 carbon directly attached to the positively charged carbon.

32
Q

What is 2° carbocation?

A

•2 carbons directly attached to the positively charged carbon.

33
Q

What is 3° carbocation?

A

•3 carbons directly attached to the positively charged carbon.

34
Q

How do alkenes react with halogens? (3)

A
  • They react rapidly with chlorine gas or with solutions of bromine and iodine in an organic solvent to give dihalogenoalkanes.
  • The halogen atoms add across the double bond.
  • Halogen molecules act as the electrophiles.
35
Q

Draw and explain the mechanism of the electrophilic addition of ethene with bromine. (6)

A

•A bromine (or other halogen) molecule is likely to have an instantaneous dipole, Br^𝛿+—Br^𝛿-(an instant late the dipole could be reversed Br^𝛿-—Br^𝛿+.
•The 𝛿+ end of this dipole is attracted to the electron-rich double in the alkene- the bromine molecule has become an electrophile.
•The electrons in the double bond are attracted to the Br𝛿+, they repel the electrons in the Br—Br bond and this strengthens the dipole of the bromine molecule.
•Two of the electrons from the double bond form a bond with the Br𝛿+ and the other bromine atom becomes a Br^- ion.
•This leaves a carbocation, in which the carbon atom that is not bonded to the bromine has the positive charge.
•The Br^- ion now forma a bond with the carbocation.
(See textbook page 222)

36
Q

What are the main steps of electrophilic addition? (2)

A
  • 1). Formation of the carbocation by electrophilic addition.
  • 2). Rapid reaction with a negative ion.
37
Q

How do you test for alkenes/unsaturation (carbon-carbon double bonds)? (3)

A
  • Add a few drops of bromine water/solution (reddish-brown) are added to the alkene.
  • Positive result: the solution is decolourised because the products are colourless.
  • Basically electrophilic addition between alkene and bromine.
38
Q

What are the conditions for the reaction of an alkene with sulfuric acid? (2)

A
  • Concentrated sulfuric acid adds across the double bond.

* Occurs at room temperature, sometimes described as ‘cold’ room temperature.

39
Q

What type of reaction is it when an alkene reacts with concentrated sulfuric acid?

A

•Exothermic.

40
Q

Draw the structure of sulfuric acid.

A

•H-O-S(two carbonyl groups)-O-H

41
Q

Draw and explain the mechanism of the electrophilic addition of ethene with concentrated sulfuric acid. (4)

A

•The electrophile is partially positively charged hydrogen atom in the sulfuric acid molecule.
•This shown as H^𝛿+-O^𝛿–SO3H.
•The carbocation which forms then reacts rapidly with the negatively charged hydrogensulfate ion.
•Asymmetrical alkenes have major and minor products.
(See textbook page 223)

42
Q

What happens when water is added to the product formed when an alkene and concentrated sulfuric acid react together?

A

•An alcohol is formed and sulfuric acid is reformed.

43
Q

How does water react with alkenes? (2)

A
  • Water adds on across the double bond in alkenes.
  • This reaction is used to make alcohols industrially and is carried out with steam, a suitable temperature and pressure, using a strong acid catalyst e.g. phosphoric acid (H3PO4).
44
Q

Draw and write an equation and mechanism to show the electrophilic addition reaction between ethene and water.

A

CH2=CH2 (g) + H2O (g) —> CH3CH2OH (g)

See exercise book for mechanism

45
Q

What are polymers?

A

•A molecule of a repeated chain of monomers.

46
Q

What are addition polymers? (3)

A
  • Made from a monomer or monomers with a carbon-carbon double bond (alkenes).
  • The monomers are based on ethene.
  • Made with no other product.
47
Q

What is the general formula of the monomer?

A

•Two hydrogens bonded to a carbon, double bonded to a carbon bonded to a hydrogen and alkyl group ‘R’.

48
Q

What are condensation polymers?

A

•Polymers and another smaller molecule produced (usually, but not always water).

49
Q

What happens when monomers polymerise?

A

•The double bond opens and the monomers bond together to form a back boned of carbon atoms.

50
Q

How do you represent when monomers polymerise?

A

(See card)

51
Q

What is the addition polymer called formed from the monomer ethene its common trade name and its use? (3)

A
  • Poly(ethene).
  • Polythene.
  • Carrier bags, washing up bowls.
52
Q

What is the addition polymer called formed from the monomer propene, its common trade name and its use? (3)

A
  • Poly(propene).
  • Polypropylene.
  • Yogurt containers, car bumpers.
53
Q

What is the addition polymer called formed from the monomer chloroethene, its common trade name and its use? (3)

A
  • Poly(chloroethene).
  • PVC (polyvinyl chloride).
  • Aprons, vinyl records, drain pipes.
54
Q

What is the addition polymer called formed from the monomer propenenitrile, its common trade name and its use?

A
  • Poly(propenenitrile).
  • Acrylic.
  • Clothing fabrics.
55
Q

What is the addition polymer called formed from the monomer phenylethene its common trade name and its use? (3)

A
  • Poly(phenylethene).
  • Polystyrene.
  • Packing materials, electrical insulation.
56
Q

What are plasticisers? (2)

A
  • Small molecules that get in between the polymer chains to allow them to slide more easily past one another and makes the polymer more flexible.
  • This is how PVC is made more rigid for drainpipes and flexible enough for plastic aprons.
57
Q

What are polyalkenes? (3)

A
  • A long backbone saturated alkane molecule.
  • Alkanes have strong non-polar C-C and C-H bonds making them unreactive molecules.
  • They are not biodegradable.
58
Q

What does it mean to be biodegradable?

A

•When substances are attacked by biological agents like enzymes to be broken down.

59
Q

What is low-density polythene? (4)

A
  • Made by polymerising ethene at high pressure and high temperature via a free-radical mechanism.
  • This produces a polymer with a certain amount of chain branching.
  • This is a consequence of the random nature of free-radical reactions.
  • The branches do not pack together particularly well and the product is quite flexible, stretches well and has a fairly low density.
60
Q

What do the properties of low-density polythenes make them suitable for? (3)

A
  • Packaging (plastic bags).
  • Sheeting.
  • Insulation for electrical cables.
61
Q

What is high-density polythene? (4)

A
  • Made at temperature and pressures a little greater than normal room conditions.
  • Uses a Ziegler-Natta catalyst.
  • This results in a polymer with much less chain branching (around one branch for every 200 carbons on the main chain).
  • The chains can pack well together, making the densitys of the plastic greater and its melting point high.
62
Q

What do the properties of high-density polythenes make them suitable for? (2)

A
  • Milk crates.

* Buckets and bottles, for which low density polythene would be insufficiently rigid.

63
Q

What are the solutions to pollution by plastics? (2)

A
  • Mechanical recycling.

* Feedstock recycling.

64
Q

What is mechanical recycling? (2)

A
  • Different types of plastics are separated.

* Plastics are washed and once they are sorted the may be ground up into small pellets.

65
Q

What can the small pellets of plastics melted and remoulded into?

A

•Recycled soft drink bottle made from PET (polyethylene terephthalate) are used to make fleece clothes.

66
Q

What is feedstock recycling? (2)

A
  • Plastics are heated to a temperature that will break the polymer bonds and produce monomers.
  • Monomers are used to make new plastics.
67
Q

What problem can be posed with feedstock recycling, using poly(propene) as an example? (2)

A
  • Poly(propene) is a thermoplastic and it will soften when heated to be melted and reused.
  • This can only be done a limited number of times because at each heating, some chains break and become shorter, which degrades the plastic’s properties.
68
Q

Explain why poly(tetrafluoroethane) would be unreactive.

A

•It is saturated (only has single bonds in the carbon chain) and its main chain is non-polar.

69
Q

Explain why bromine, a non-polar molecule, is able to react with an alkene. (2)

A
  • The electrons in bromine are repelled by the high electron density of the double bond in the alkene.
  • ∴The slightly positive bromine atom acts as an electrophile.
70
Q

Which stereoisomer of of but-2ene is less polar?

A

•E-but-2-ene, the C=C bond can not rotate and each carbon in the double bond has two different groups attached.

A-Level Organic Chemistry AQA (Paper 2) (17 decks)

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