M4 Chapter 13 - Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons that contain at least one carbon to carbon double bond (C=C) made up of a pi bond and a sigma bond.

Question 2

What is the general formula of alkenes?

Answer 2

CnH2n

Question 3

How is a pi bond formed?

Answer 3

Electrons in the adjacent p orbitals overlap above and below the carbon atoms. They can only be made after a sigma bond is formed.

Question 4

What bond restricts the rotation of carbon atoms?

Answer 4

Pi bond.

Question 5

What is the angle and shape of a double bond?

Answer 5

·Trigonal Planar (3 regions of electron density around each of the carbon atoms)
·120 degrees due to same repulsion from eachother

Question 6

Are they more or less reactive than alkanes? Why?

Answer 6

More reactive due to high electron density of double bond and that the pi bond is slightly easier to break.

Question 7

What type of intermolecular forces do alkenes have?

Answer 7

Only London forces due to the non-polar nature of the bonds.

Question 8

Are alkenes soluble in water?

Answer 8

No due to non-polar bonds.

Question 9

What are stereoisomers?

Answer 9

Isomers with the same structural formula but different arrangement of atoms in space.

Question 10

What are the two types of isomers that can be formed using alkenes? Why?

Answer 10

E/Z Isomerism = Due to restricted rotation (due to pi bonds above and below Carbon plane)
Cis - Trans Isomerism = If two of the same substituents are attached to each carbon.

Question 11

What is E/Z Isomerism?

Answer 11

Stereoisomerism around double bonds where the rotation around the double bond is restricted.

Question 12

What are the two prerequisites for E/Z Isomerism?

Answer 12

C=C double bond
Different groups attached to each Carbon of the double bond

Question 13

What is Cis-Trans Isomerism?

Answer 13

A special type of E/Z Isomerism where there is a C=C present but one of the groups attached to each Carbon atom is the same.

Question 14

Which isomer in cis-trans corresponds to the E/Z isomer?

Answer 14

Cis = Z
Trans = E

Question 15

What is Cahn-Ingold-Prelog?

Answer 15

A system of naming cis-trans isomers.

Question 16

What do the Cahn-Ingold-Prelog rules state?

Answer 16

• If the groups of higher priority are on the same side of the double bond, the compound is a Z-isomer.
• If the groups of higher priority are diagonally placed across the double bond then the compound is the E isomer.

Question 17

What is an electrophile?

Answer 17

A species that are electron pair acceptors.

Question 18

Why are alkenes more reactive than alkanes?

Answer 18

Due to the presence of pi bond.

Question 19

What reactions can alkenes undergo?

Answer 19

Electrophillic Addition:
- Addition of Hydrogen Halides (HBr)
- Addition of Halogens (Br2)
- Addition of Steam (H2O)
- Hydrogenation (H2)

Question 20

Alkene Addition (HBr):
What is the reactant, condition, product and reaction type?

Answer 20

Reactants: Alkene and HBr
Conditions: Hydrogen Bromide gas or concentrated hydrobromic acid.
Products: Bromoalkane (Minor/Major Products)
Reaction Type: Electrophillic Addition

Question 21

Alkene Addition (Br2):
What is the reactant, condition, product and reaction type?

Answer 21

Reactants: Alkene and Halogen (Br2)
Conditions: Bromine Liquid/Bromine Water
Products: Dibromoalkane
Reaction Type: Electrophillic Addition/Bromination

Question 22

Alkene Addition (Steam/H2O):
What is the reactants, conditions, products and reaction type?

Answer 22

Reactants: Alkene and Steam (H2O(g))
Conditions: Heat and Acid Catalyst (H3PO4)
Products: Alcohol
Reaction Type: Electrophillic Addition or Acid Catalysed Hydration

Question 23

Alkene Hydrogenation (H2):
What are the reactants, conditions, products and reaction type?

Answer 23

Reactants: Alkene and H2
Conditions: 150C and Nickel Catalyst
Products: Alkane
Reaction Type: Addition or Hydrogenation

Question 24

What is the most stable type of carbocation intermediate? Why?

Answer 24

Alkyl groups have a positive inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms. Ie: A tertiary carbocation.

Question 25

Major products will be formed from which kinds of carbocations?

Answer 25

Tertiary (or the most stable available)

Question 26

What conditions are needed for the electrophillic addition of H2O to an alkene? What is this type of reaction called?

Answer 26

Steam in the presence of acid catalyst (H3PO4) Reaction is called Hydration or just electrophillic addition

Question 27

What are the products of the hydration reaction?

Answer 27

An alcohol

Question 28

Draw the mechanism for the addition of water to ethene.

Answer 28

refer to goodnotes...

Question 29

What conditions are needed for the electrophillic addition of a hydrogen halide to an alkene?

Answer 29

Hydrogen halide gases must be room temperature.

Question 30

Draw the mechanism for the reaction of HBr and ethene.

Answer 30

Refer to goodnotes.

Question 31

What is the reaction called when a halogen is added to alkene?

Answer 31

Halogenation

Question 32

How does a molecule with a non-polar bond react as if it is an electrophile?

Answer 32

C=C double bond with a high electron density induced a temporary dipole in the halogen molecule delta + making it attracted to the C=C.

Question 33

Draw a mechanism for the reaction between bromine and ethene.

Answer 33

Refer to goodnotes g.

Question 34

How can an alkene be converted to an alkane? What is the reaction called and what are the required conditions?

Answer 34

Alkene + Hydrogen --> Alkane Hydrogenation Conditions = 150C and Nickel Catalyst

Question 35

How can we test if a compound is unsaturated? (With C=C)?

Answer 35

When adding Bromine Water (orange) dropwise to a sample of an alkene, the bromine is added across the double bond of the alkene. Hence removing the bromine molecules. Therefore, if an alkene is present then Orange -- > Colourless.

Question 36

What is Markownikoff's rule?

Answer 36

It mentions minor and major products. Where the more stable carbocation intermediate will lead to the major product and the lesser stable carbocation intermediate will lead to the minor product.

Question 37

What is an addition polymer?

Answer 37

Many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule.

Question 38

What are monomers? What form do they usually take?

Answer 38

Molecules which combine to form a polymer. Usually have a C=C bond which breaks to leave a repeating pattern.

Question 39

Draw how you would represent the polymerisation of ethene.

Answer 39

H H ( H H ) | | ( | | ) H2C = CH2 → -- ( CH2–CH2 ) -- | | ( | | ) H H ( H H ) n

Question 40

What are the ways in which plastics can be disposed? (6)

Answer 40

- Landfill - Combustion - Electricity Generation - Reuse - Recycle - Organic Feedstock

Question 41

What are the disadvantages of recycling?

Answer 41

Plastics must be sorted into different types Expensive Labour intensive Requires high technology

Question 42

How does photodegradable polymers break down?

Answer 42

They are broken down chemically using energy with wavelengths similar to light. Once the break down begins, it is not possible to stop the process.

Question 43

Explain what happens in organic feedstock.

Answer 43

Plastics are separated and broken down into small organic molecules through a series of reactions. The molecules can then be used to produce plastics and in other industries.

Question 44

Give a disadvantage of photodegradable polymers.

Answer 44

May not be exposed to sufficient light