3.3.4- Alkenes (PAPER 2)

Question 1

What are alkenes?

Answer 1

homologous series of hydrocarbons with general formula C2H2N.

contain a C=C double bond= UNSATURATED.

alkenes with 4+ C atoms display positional isomerism

Question 2

What are the physical properties of alkenes?

Answer 2

Boiling point increases with increasing molecular mass: more atoms= greater vdw’s, greater intermolecular forces= more energy to separate molecule.

Greater branching= lower boiling point as molecules cannot pack as tightly together.

Non-polar so insoluble in water, miscible with most organic solvents.

Question 3

What effect does the double bond/ pi bond have on alkenes?

Answer 3

The pi bond prevents rotation about the sigma bond, giving rise to E/Z isomerism.

double bond makes alkenes more reactive than alkanes as it is an area of high electron density.

Question 4

What is E/Z isomerism present in alkenes?

Answer 4

A type of stereoisomerism- when compounds have same structural formula but bonds arranged differently in space.

E= groups on opposite sides(L+R) of double bond
Z= groups on same side of double bond (ZAME ZIDE)

Question 5

How to determine whether E or Z isomers?

Answer 5

1) give atomic number to each atom attached directly to C atom at end of double bond.

2) those with highest atomic numbers have priority- if they are the same use next bonded atom.

3) if priority atoms on same side= Z isomers, if priority atoms on different sides= E isomers.

Question 6

How to test for unsaturation/ double bond?

Answer 6

Alkene shaken with bromine water solution.

Colour change from orange to colourless when unsaturated compounds present.

dibromoalkane formed is colourless.

Question 7

What is an electrophile?

Answer 7

A lone pair acceptor.

Question 8

What occurs during electrophilic addition?

Answer 8

The electron pair in the pi bond is donated to the electrophile to form a new single sigma bond.

Question 9

What are the 3 electrophiles for electrophilic addition of alkenes?

Answer 9

Br2, HBr, H2SO4

Question 10

Draw the reaction between ethene and bromine to form 1,2-dibromoethane.

Answer 10

Br-Br under alkene, induced dipole created with d+Br close to alkene and d-Br far

curly arrow from double bond to d+Br, and from Br-Br bond to one Br

double bond breaks and one Br joins to C, other bromine ion displaced, carbocation formed.

curly arrow from :Br- to carbocation forming molecule with 2Br

Question 11

Draw the reaction between ethene and HBr.

Answer 11

HBr has permanent dipole- +H and -Br

curly arrow from double bond to +H, curly arrow from H-Br bond to Br

H bonds to C with the most H, carbocation formed.

curly arrow from :Br- to carbocation forming haloalkane.

Question 12

What happens when hydrogen halide reacts with unsymmetrical alkene?

Answer 12

2 products can form as hydrogen can bond to 2 different C

Question 13

How to know which product is the major product?

Answer 13

The product which has the most alkyl groups attached to the carbocation.

tertiary carbocation> secondary carbocation> primary carbocation (1 alkyl group)

Question 14

Draw the reaction between propene and HBr and which is major product.

Answer 14

curly arrow from C=C to H, curly arrow from H-Br bond to Br.

H can bond to either C forming 2 diff carbocations, curly arrow from Br to 2 diff carbocations.

Major product= 2-bromopropane (2 alkyl groups compared to 1)

Question 15

Draw the structure of sulfuric acid.

Answer 15

S in middle

2 single bonds to oxygen with H on each end

2 double bonds to oxygen

Question 16

What occurs when an alkene reacts with concentrated sulfuric acid?

Answer 16

Hydrogensulfates are formed, this occurs at 0.C

Question 17

Draw the reaction between H2SO4 and ethene.

Answer 17

O-H bond on sulfuric acid has d+ H and d-O, curly arrow from C=C to d+ H, curly arrow from O-H bond to O.

C=C bond breaks, H joins to C, 2nd C becomes a carbocation. Curly arrow from :O- left to carbocation.

ethylhydrogensulfate formed with O from HSO4 attached to previous carbocation.

Question 18

Why are major and minor products formed in different amounts?

Answer 18

The major product is formed by a (primary/secondary/tertiary) carbocation intermediate whereas the minor product is formed by a (primary/secondary/tertiary) carbocation. One carbocation is more stable.

Question 19

What is addition polymerisation?

Answer 19

the formation of long chain molecules from lots of small molecules joining together.

Question 20

Describe addition polymers

Answer 20

The monomers are alkenes, the polymerisation requires high temperatures and pressure, as well as a catalyst.

They are alkanes even though they are made from alkenes, contain non-polar strong C-C and C-H bonds, making it difficult to break them so addition polymers are unreactive.

Question 21

What happens when alkenes form a polymer?

Answer 21

The double bond in the alkene opens up and joins one monomer to another.

Polymer is named by using poly (monomer)

eg. polymer made from ethene is called poly(ethene)

Question 22

How to identify/ draw repeating unit?

Answer 22

Take 2 adjacent C atoms from main chain and draw them, with the atoms above and below.

Square brackets around, covalent bond/line from each C atom.

SMALL NUMBER TO THE RIGHT- eg if polymer has 14C, the small number would be 7

Question 23

How to identify/ draw the monomer used to make the polymer?

Answer 23

Take 2 adjacent C atoms from main chain and draw them, with atoms above and below.

Replace the single bond with a double bond.

Question 24

What is poly(chloroethene) used for?

Answer 24

Making pipes

Question 25

What is poly(chloroethene)/PVC?

Answer 25

Rigid polymer used to make plastic pipes.

Chemical called a plasticiser can be added to make flexible PVC used for flooring and insulating electrical cables.

Question 26

How does a plasticiser alter the structure of PVC?

Answer 26

Polymer chains in PVC are attracted to eachother by intermolecular forces- permanent dipole dipole and VDW’s.

Plasticiser is small molecule that fits between polymer chains causing them to move further apart, weakening the intermolecular forces between them so polymer chains move over eachother.

Question 27

What is poly(propene) used for?

Answer 27

Making carpets

Question 28

What is poly(phenylethene) used for? =POLYSTYRENE

Answer 28

Making foam food containers and drink cups.

Question 29

What are environmental issues with addition polymers?

Answer 29

They are non-biodegradable: not broken down by microorganisms so pollute the environment.

Involve the use of crude oil which is a non-renewable resource: crude oil needs to be transported+ refined requiring energy.

Occupy a lot of space in landfills.

Question 30

How to reduce environmental effects of addition polymers?

Answer 30

Combusting waste polymers for energy instead of sending to landfill- can release harmful chemicals.

Polymer waste can be sorted into different polymers and recycled.

Feedstock recycling by converting waste polymers back to simpler hydrocarbons which can be cracked: polymers do not need sorting.

Producing new biodegradable polymers- not addition polymers.