6.3 Alkenes

Question 1

What is a 𝜋 bond?

Answer 1

Overlap of p orbitals sideways above and below the 𝜎 bond.

Question 2

What are alkenes?

Answer 2

Unsaturated hydrocarbons that contain at least one C=C bond made up of a 𝜋 bond and a 𝜎 bond

Question 3

What bond restricts the rotation of carbon atoms?

Answer 3

𝜋 bond

Question 4

What is the angle and shape of a double bond?

Answer 4

Trigonal planar
120°

Question 5

What is the qualitative test for alkanes?

Answer 5

This tests for C=C double bond using bromine water. If the alkane is present bromine water decolourises.

Question 6

Are they more or less reactive than alkanes? Why?

Answer 6

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

Question 7

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

Answer 7

E/Z isomers - due to the restricted rotation
Cis-trans isomers - if two of the same substituents are attached to each carbon

Question 8

What is an electrophile?

Answer 8

Species that are electron pair acceptors

Question 9

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

Answer 9

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 10

Major products will be formed from which kinds of carbocations?

Answer 10

Tertiary (or the most stable available)

Question 11

What conditions are needed for the electrophilic addition of H 2
O to an alkene? What is this type of reaction called?

Answer 11

Steam in the presence of an acid catalyst, usually phosphoric acid
Reaction is called hydration

Question 12

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

Answer 12

An alcohol

Question 13

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

Answer 13

Hydrogen halide gases must be at room temperature

Question 14

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

Answer 14

Halogenation

Question 15

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

Answer 15

C=C double bond with a high electron density induces a temporary dipole in the halogen molecule → δ+ atom attracted to double bond

Question 16

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

Answer 16

Alkene + hydrogen = Alkane
Hydrogenation
Conditions → 150°C, nickel catalyst

Question 17

What is an addition polymer?

Answer 17

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

Question 18

What are monomers? What form do they usually take?

Answer 18

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

Question 19

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

Answer 19

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

Question 20

What are the disadvantages of recycling?

Answer 20

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

Question 21

How can chemists limit the problems caused by polymer disposal?

Answer 21

• Developing biodegradable polymers
• Removing toxic waste gases caused by incineration of plastics

Question 22

Explain what happens in organic feedstock

Answer 22

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