4.1.3 Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons that contain at least one C=C bond made up of a pi bond and a sigma bond

Question 2

What is the general formula of alkenes?

Answer 2

Cn H2n

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

120

Question 6

Are alkenes more or less reactive than alkanes?

Why?

Answer 6

More reactive

This is due to high electron density if double bond and the fact that the pi bond is slightly easier to break

Question 7

What intermolecular forces of attraction do alkenes have?

Answer 7

Only london forces due to non-polar bonds

Question 8

Are they soluble in water?

Why?

Answer 8

No

Non polar bonds

Question 9

Write an equation for the complete combustion of pent-2-ene

Answer 9

CH3CH=CHCH2CH3 + 1.5 O2 –> 5CO2 + 5H2O

Question 10

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

Answer 10

EZ isomers
due to restricted rotation

Cis-trans isomers
if two of the same substituent are attached to each carbon

Question 11

What is an electrophile?

Answer 11

Species that are electron pair acceptors

Question 12

What is the most stable type of carbocation intermediate?

Why?

Answer 12

A tertiary carbocation

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

Question 13

Major products will be formed from which kinds of carbocations?

Answer 13

Tertiary (or the most stable available)

Question 14

What conditions are needed for the electrophilic addition of H2O to an alkene?

What is this type of reaction called?

Answer 14

Steam in the presence of an acid catalyst
Usually phosphoric acid

Reaction is called hydration

Question 15

What are the product(s) of the hydration reaction?

Answer 15

An alcohol

Question 16

Draw a mechanism for the addition of water to ethene

Answer 16

mechanism

Question 17

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

Answer 17

Hydrogen halide gases must be at room temperature

Question 18

Draw a mechanism for the reaction of HBr and ethene

Answer 18

mechanism

Question 19

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

Answer 19

Halogenation

Question 20

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

Answer 20

C=C double bond with high electron density induces a temporary dipole in the halogen molecule

Delta positive atom is attracted to the double bond

Question 21

Draw a mechanism for the reaction between bromine and ethene

Answer 21

mechanism

Question 22

How can an alkene be converted into an alkane?

What is the reaction called?

What are the required conditions?

Answer 22

Alkene + hydrogen = alkane

Hydrogenation

Conditions: 150 degrees, nickel catalyst

Question 23

What is an addition polymer?

Answer 23

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

Question 24

What are monomers?

What form do they usually take?

Answer 24

Molecules which combine to form a polymer

Usually have a C=C bond which breaks to leave a repeating pattern

Question 25

Draw how you would represent the polymerisation of ethene

Answer 25

drawing

Question 26

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

Answer 26

Landfill
Combustion
Electricity generation
Reuse
Recycle
Organic feedstock

Question 27

What are the disadvantages of recycling?

Answer 27

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

Question 28

How do photodegradable polymers break down?

Answer 28

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 29

Explain what happens in organic feedstock

Answer 29

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 30

Give a disadvantage of photodegradable polymers

Answer 30

May not be exposed to sufficient light