Alkenes 4.1.3

Question 1

What is the general formula for alleges?

Answer 1

CnH2n

Question 2

How does the double bond affect alleges reactivity, why?

Answer 2

It makes them more reactive, the carbon carbon double bond is a region of high electron density

Question 3

What two distinct bonds make up the double bond in alkenes? Define each bond.

Answer 3

Sigma bond - overlap of orbitals between C atoms

Pie bond - sideways overlap of p-orbitals, adjacent p-orbitals above and below bonding (4 atoms)

Question 4

What bond angle is present around carbon carbon double bonds?

Answer 4

120• , only 3 bonding regions.

Question 5

Define a structural isomer.

Answer 5

The same molecular formula but a different structural formula.

Question 6

Define a stereoisomer

Answer 6

Molecules with the same structural formula but a different arrangement in space.

Question 7

What are the two thins that are needed for a stereoisomer to form? What are the two types of stereoisomers and how do they differ?

Answer 7

• Carbon carbon double bond
• Different groups attached to each carbon in the double bond

E - even, the heaviest groups are on opposite sides
Z - same, the heaviest groups are on the same side

Question 8

How do we name alkenes and when must numbering be introduced?

Answer 8

___ene

Once you get to but. Eg but-2-ene

Question 9

Why are alkenes more reactive than alkenes?

Answer 9

Because of the relatively low bond Enthalpy of the pie bond.

Question 10

Write a word equation for the hydrogenation reaction. What are the necessary reagents and conditions for this reaction?

Answer 10

Alkene + H2 ➡️ Alkane
•Hydrogen
•Nickel Catalyst (finely divided)
•High Temp.

Question 11

When is hydrogenation used in the industry?

Answer 11

Manufacturing of margarine.

Question 12

Write an equation for halogenation. What are the necessary reagents and conditions for this reaction?

Answer 12

Alkene + X2 ➡️ Haloalkane
•Cl2, Br2, I2
•Room temp
•Non-polar solvent, eg. CCl4

Question 13

If you added bromine water to a halogenation reaction what colour change would be seen?

Answer 13

Orange/brown to colourless. As the alkene turns to a haloalkane.

Question 14

During halogenation reactions where do the halogens go in the alkene?

Answer 14

On either side of the double bond.

Question 15

Write an equation for the addition of hydrogen halides. What are the reagents and the conditions for this reaction?

Answer 15

Alkene + HX ➡️ Haloalkane (mono)

• HCl or HBr or HI
• Room Temperature
• Non-polar solvent e.g CCl4

Question 16

How does reactivity of the hydrogen halides change from HF to HI? Why?

Answer 16

It increases. Bond strength decreases as they are easier to break therefore they are more reactive.

Question 17

How are minor and major products formed?

Answer 17

When unsymmetrical alkenes react with hydrogen halides or other non symmetrical molecules.

Question 18

Write a general word equation for hydration. What are the conditions and the reagents?

Answer 18

Alkene + water ➡️ Alcohol

•Steam (H2O)
600K (Temp>100•)
•60 atm
Phosphoric acid catalyst (H3PO4)

(2 products if an unsymmetrical alkene.

Question 19

What is the main mechanism that we study for alkenes called?

Answer 19

Electrophilic addition

Question 20

Define an electrophile

Answer 20

An electron pair acceptor.

Question 21

What is a carbocation?

Answer 21

A positively charged ion formed during electrophilic addition it is an intermediate molecule.

Question 22

What is Markovinikovs rule?

Answer 22

When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogen a attached to it already.

Question 23

How do you decide where to add the H when you react the alkene with compound HX?

Answer 23

Add it to the carbon which has the most H atoms directly attached to it.

Question 24

Define a polymer

Answer 24

Polymers are long chained molecules made by the joining of many small chained molecules called monomers.

Question 25

What happens to the bonds in alkenes during addition polymerisation?

Answer 25

A saturated chain is produced as a pie bond is broken and two new sigma bonds are made.

Question 26

How many types of monomers can addition polymers be made out of?

Answer 26

ONE TYPE OF MONOMER ONLY!!

Question 27

Draw a polymerisation equation for polyethene.

Answer 27

Look in pictures for the answer

Question 28

What must you remember when writing out polymer equation and sub units?

Answer 28

Only include n if it is an equation!!

Question 29

Draw a repeat unit for tetrafluoroethene (CF2CF2)

Answer 29

Look at pictures😊

Question 30

What is the main problem with addition polymers?

Answer 30

They are generally non-biodegradable so they need to be recycled or reused.

Question 31

What is thermal recycling?

Answer 31

The polymers are melted down and then used as feedstock for petroleum products.

Question 32

What is mechanical recycling?

Answer 32

The polymers are processed and used as feedstock for other plastic products.

Question 33

Give 2 advantages for recycling.

Answer 33

• Reduces the amount of waste sent to landfill
• Reduces energy consumption
• Reduces pollution
• Reduces global warming
• Conserves natural resources
• Promotes sustainable use of resources

Question 34

Give 2 disadvantages of recycling.

Answer 34

• Not always cost effective
• High initial cost
• Not widespread on a large scale
• Recycling sites may not be safe and hygienic.

Question 35

What are the three other main alternatives to recycling?

Answer 35

• Incineration - generates energy but it requires high temperatures and releases carbon dioxide
• Biodegradable and photodegradable plastics - these are being modified and produced
• Re-use!

Question 36

Define an alkene.

Answer 36

An unsaturated hydrocarbon with a C=C double bond.