3.4 Alkenes

Question 1

What are the features of Alkenes?

Answer 1

• Unsaturated
• High electron density around double bond
• Cycloalkenes have 2 less hydrogen

Question 2

What is an electrophile?

Answer 2

Electron pair acceptor,attracted to electron dense double bond

Question 3

What is the test for Alkenes?

Answer 3

Bromine water turns from brown/orange to colourless

Question 4

Why do Alkenes display stereoisomers?

Answer 4

Do not rotate on double bond

Question 5

What is the mechanism of Alkene reactions with Hydrogen Halides and halogens?

Answer 5

Electrophilic addition

Question 6

Why does the H in a hydrogen halide attract the double bond?

Answer 6

The halide is more electronegative than the hydrogen forming a polar bond between the two where hydrogen has a delta positive charge

Question 7

What happens in electrophilic addition reactions?

Answer 7

Double bond is broken due to attraction to delta positive atom and the delta positive atom joins the molecule. Lone pair left on delta negative reaction is attracted to the positive carbon cation.

Question 8

What do electrophilic addition reactions form?

Answer 8

Alkanes

Question 9

What happens in electrophilic addition reactions with halogens?

Answer 9

Halogen molecules act as electrophiles
This is due to induced dipole forces

Question 10

What happens with asymmetrical alkenes during electrophilic addition?

Answer 10

When double bond is not exactly in the middle there are several possible products

Question 11

What is the positive alkyl inductive effect?

Answer 11

• Some reactions produce one product more than another
• Alkyl groups can release electrons which stabilise positive charge of carbocation
• the more alkyl groups attached to carbocation the more stable it is.
• e.g. Tertiary Carbocation more stable than secondary carbocation

Question 12

What is Markovnikovs rule?

Answer 12

That the H atom adds to the carbon with most H atoms (Halogen goes to C with fewest)

Question 13

What are the benefits of using Ethene and steam over Ethene + H2SO4?

Answer 13

-Seperation of products is easier
- 1 step not 2
- No waste product (high atom economy)

Question 14

What is the process of electrophilic addition when making alcohols from an alkene?

Answer 14

1. Electron dense double bond is attracted to delta positive hydrogen
2. OSO3H has a lone pair on the oxygen which is attracted to the positive carbocation
3. Hydrogen sulfate made
4. Hydrogen sulfate is hydrolysed forming alcohol+water

Question 15

What is the overall equation for making alcohol from alkene with Ethene + H2SO4?

Answer 15

CH2=CH2 + H2O —-> CH5CH2OH

Question 16

What is the purpose of H2SO4 in the Ethene fermentation reaction?

Answer 16

Catalyst because it isn’t used up in the reaction

Question 17

What are the conditions for Ethene and steam reaction?

Answer 17

CH2=CH2+ H2O <> CHeCH2OH

Question 18

What happens in addition polymerisation?

Answer 18

n(monomers) —-> (polymers)n
No other products

Question 19

What is the down side of condensation polymerisation?

Answer 19

Small by-product is formed

Question 20

How to draw a polymer?

Answer 20

1. Displayed formula with just the double bond and everything else just written in (like with geometric isomers)
2. A single c-c bond instead of double
3. Trailing bonds to left and right
4. Brackets around it and n
5. Name = poly (monomer)

Question 21

What are the properties of a polymer?

Answer 21

• Unreactive due to being saturated (and mostly non polar)
• IMF affects properties 1.longer chains = higher bp due to greater vdw 2. Less branching so chains pack closer together

Question 22

What is PVC?

Answer 22

Polychloroethene

Question 23

What are the properties of PVC?

Answer 23

• Hard but brittle material (e.g. drainpipes)

Question 24

What are the purpose of plasticisers?

Answer 24

Added to make polymer flexible
Weaken imf by getting between chains and pushing them apart
Called plasticised ——-