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 𝜋 bond and a 𝜎 bond

Question 2

What is the general formula of alkenes?

Answer 2

CnH2n

Question 3

How is a 𝜋 bond formed?

Answer 3

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

Question 4

What bond restricts the rotation of carbon atoms?

Answer 4

𝜋 bond

Question 5

What is the angle and shape of a double bond?

Answer 5

Trigonal planar

120°

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 intermolecular forces of attraction do they 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 + 7½O2 → 5CO2 + 5H2O

Question 10

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

Answer 10

E/Z isomers - due to the restricted rotation

Cis-trans isomers - if two of the same substituents 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

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

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

Answer 16

Hydrogen halide gases must be at room temperature

Question 17

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

Answer 17

Halogenation

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

What is an addition polymer?

Answer 20

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

Question 21

What are monomers? What form do they usually take?

Answer 21

Molecules which combine to form a polymer

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

Question 22

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

Answer 22

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

Question 23

What are the disadvantages of recycling?

Answer 23

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

Question 24

How does photodegradable polymers break down?

Answer 24

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 25

Explain what happens in organic feedstock

Answer 25

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 26

Give a disadvantage of photodegradable polymers

Answer 26

May not be exposed to sufficient light