Alkenes

Question 1

What are Alkenes?

Answer 1

They are unsaturated hydrocarbons containing 1 or multiple C=C bonds, which contains a pi-bond

Question 2

What are pi-bonds?

Answer 2

They are a sideways overlap of adjacent p-orbitals above and below the bonding C atoms and a sigma bond
- restricted rotation around it

Question 3

What is a sigma bond?

Answer 3

An overlap of orbitals directly between bonding atoms

Question 4

What is the shape and bond angle around each C in the C=C of alkenes?

Answer 4

• Trigonal planar
• 120º

Question 5

What are stereoisomers?

Answer 5

Compounds with the same structural formula but different arrangement in space

Question 6

What is E/Z isomerism?

Answer 6

• Needs restricted rotation around C=C and 2 different groups attached to each C atom of the C=C group
• They have restricted rotation due to the pi-bonds electron density above and below the plane of the sigma bond

Question 7

What is Cis-trans isomerism?

Answer 7

When 2 of the substituent groups attached to each carbon atom of the C=C group are the same

Question 8

What are the CIP rules?

Answer 8

• Attached groups are prioritised by atomic number
• If 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 on opposite sides of the double bond, the compound is a E isomer

Question 9

What are the properties of alkenes?

Answer 9

-Low bond enthalpy
- Pi-bond position

Question 10

How does low bond enthalpy effect an alkene?

Answer 10

Alkenes contain C=C double bonds which consist of a pi-bond and a sigma bond. The pi-bond has a lower bond enthalpy than the sigma bond therefore, it breaks more readily allowing alkenes to react more easily

Question 11

How does the pi-bonds position effect an alkene?

Answer 11

There is concentrated electron density above and below the sigma bond and the pi-bonds are more exposed meaning it is more open to electrophile attacks

Question 12

How can alkanes be formed from alkenes?

Answer 12

Hydrogenation

Question 13

What is the process of hydrogenation?

Answer 13

• Mix the alkene with hydrogen
• Add a nickel catalyst at 423K
• Addition reaction occurs to form an alkane
• One mole of hydrogen is required per double bond

Question 14

How can you test for unsaturation in a carbon chain?

Answer 14

Bromination

Question 15

What is the process of bromination?

Answer 15

• Alkenes react with Cl or Br at room temperature
• Add bromine water drowse to a sample
• Bromine adds across the double bond
• Orange to colourless= C=C bond
• Unsaturated carbon chains decolourise bromine water

Question 16

How haloalkanes form from alkenes?

Answer 16

• They react with gaseous hydrogen halides at room temperature
• If the alkene is a gas, 2 gases are mixed
• If the alkene is a liquid, the hydrogen halide is bubbled through it
• It can also react with concentrated HCl or Hbr
• Unsymmetrical alkenes react with hydrogen halides to give 2 possible products

Question 17

How can alcohols be formed from alkenes?

Answer 17

Hydration

Question 18

What is the process of hydration?

Answer 18

• React alkenes with steam, H20
• Occurs with phosphoric acid catalyst
• Steams adds across the double bond
• There are two possible products

Question 19

What happens in electrophilic addition of a hydrogen halide?

Question 20

What happens in electrophilic addition of a halogen?

Question 21

What is Markownikoff’s rule?

Answer 21

• 2 isomeric products formed if unsymmetrical alkenes react with hydrogen halides
• Hydrogen of a hydrogen halide attaches to a carbon atom with more hydrogen atoms and less carbon atoms
• Electrophilic addition forms 2 possible carbocations either primary or secondary
• Primary carbocations have a positive charge on a carbon atom at the end of the chain
• Secondary carbocations have a positive charge on a carbon atom with the 2 carbon chains attached

Question 22

What are polymers?

Answer 22

Large molecules formed from many monomers

Question 23

What is an electrophile?

Answer 23

A species that are electron pair acceptors

Question 24

What is an addition polymer?

Answer 24

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

Question 25

What are the ways in which plastics can be disposed?

Answer 25

• Landfill
• Combustion
• Electricity generation
• Reuse
• Recycle
• Organic feedstock

Question 26

What are the disadvantages of recycling?

Answer 26

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

Question 27

How do photodegradable polymers break down?

Answer 27

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 28

What happens in organic feedstock?

Answer 28

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

Question 29

What is a disadvantage of photodegradable polymers?

Answer 29

May not be exposed to sufficient light