4.1.3 Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons containing a C=C bond, comprising a π bond and a σ bond

Question 2

What is a π bond?

Answer 2

A sideways overlap of adjacent p-orbitals above and below the bonding atoms
Relatively weak compared to σ bonds

Question 3

Why is there not free rotation in alkenes?

Answer 3

Restricted rotation of π bond

Question 4

What is the shape around each carbon atom?

Answer 4

Trigonal planar shape
120°
Around each C=C
Electrons repel each other and arrange themselves in a position where they are as far apart from each other as possible

Question 5

Why do alkenes attract electrophiles?

Answer 5

A double bond is electron rich as it has 4 electrons situated between the 2 bonded carbons
Therefore alkenes attract electrophiles

Question 6

What is an electrophile?

Answer 6

Attracted to electrons (alkenes)
An electron pair acceptor
E+
Cl+
H-Cl

Question 7

Hydrogenation reaction
Draw diagram (no mechanism)

Answer 7

150°C and nickel catalyst
See notes
Ethene + H2 -> ethane

Question 8

Halogenation reaction
Draw mechanism

Answer 8

Ethene + Br2 -> dibromoethane
Mechanism: electrophilic addition
Forms a carbocation
Used to indicate presence of alkene- solution turns from orange to colourless

Question 9

Hydration reaction
Draw mechanism

Answer 9

H3PO4
65atm
300°C
Primary and secondary carbocation
See notes

Question 10

What is the stability of carbocations?

Answer 10

3°>2°>1°
Tertiary, secondary, primary

Question 11

Hydrohalogenation
Draw mechanism

Answer 11

See notes

Question 12

What bonds are broken and formed in addition polymers?

Answer 12

π bonds in alkenes are broken and new σ bonds are formed with other alkenes to make a long chain hydrocarbon which can have substitution on the chains

Question 13

Draw monomer and repeating unit of Ethene

Answer 13

See notes

Question 14

Draw monomer and repeating unit of propene

Answer 14

See notes
Double check - difficult one

Question 15

Draw monomer and repeating unit of vinyl chloride

Answer 15

See notes
Other names: ethenylchloride and chloroethene

Question 16

Draw monomer and repeating unit of tetrafluoroethene

Answer 16

See notes
poly(tetrafluoroethene)
PTFE - non-stick saucepans

Question 17

Draw monomer and repeating unit of ethenylbenzene

Answer 17

Styrene
See notes

Question 18

Draw 2 units of but-1-ene

Answer 18

See notes

Question 19

Draw equation for polymer formation from but-2-ene

Answer 19

See notes

Question 20

How can waste polymers be combusted to produce energy?

Answer 20

• have high calorific value and can ve burnt in power stations
• chemical energy transferred can be used to drive turbines and generate electricity

Question 21

How can toxic waste products be removed when formed from combustion?

Answer 21

• HCl formed from combustion of halogenated plastics (e.g. PVC)
• gas scrubbers (a base neutralises an acidic gas)
• HCl neutralised by CaO

Question 22

How can waste polymers be used as an organic feedstock?

Answer 22

• once sorted into different types, a series of chemical reactions can break plastic polymers into small organic molecules
• recovered chemicals can be used in other industrial reactions

Question 23

How do biodegradable polymers work?

Answer 23

• biodegradable polymer e.g. starch is mixed with addition polymer
• over time starch breaks down making polymer chains smaller and material biodegrades

Question 24

How do photodegradable polymers work?

Answer 24

• photodegradable plastics break down chemically using energy wavelengths similar to light
• either:
• addition polymer with bonds that are weakened by absorption of light
• have an additive that is affected by light that weakens bonds in the polymer
• once exposed to light it begins to degrade and it isn’t possible to stop the process