3) Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons containing a C=C bond comprising a π-bond and a σ-bond.

Question 2

What is a π-bond?

Answer 2

Sideways overlap of adjacent p-orbitals above and below the bonding C atoms.

Question 3

What is the shape and angle around a double bond?

Answer 3

• There are three regions of electron density around each carbon atom.
• The three repel as far as possible, so the bond angle created is 120º.
• All of the atoms are in the same plane.

Question 4

Explain what is meant by the term stereoisomers.

Answer 4

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

Question 5

What 2 conditions must a molecule satisfy in order to have E/Z isomerism?

Answer 5

• A C=C double bond.

- Different groups attached to each carbon atom of the double bond.

Question 6

What are the Cahn-Ingold-Prelog priority rules to identify the E and Z stereoisomers?

Answer 6

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

Question 7

Why are alkenes more reactive?

Answer 7

• Because of the presence of the π-bond.

- π-bond is weaker than a σ-bond, therefore, is broken more readily.

Question 8

What 4 things do alkenes have addition reactions with?

Answer 8

• Hydrogen.
• Halogens.
• Hydrogen Halides.
• Steam, in the presence of an acid catalyst.

Question 9

What is made when alkenes react with Hydrogen? Give a suitable catalyst.

Answer 9

• Alkanes are made.

- Nickel catalyst.

Question 10

What is made when alkenes react with halogens?

Answer 10

Dihaloalkanes.

Question 11

What is the test for a C=C double bond?

Answer 11

Any compound containing a C=C double bond will turn bromine water orange to colourless.

Question 12

What do alkenes react with hydrogen halides to form?

Answer 12

Haloalkanes.

Question 13

What is made when alkenes react with steam? Give a suitable catalyst.

Answer 13

• Alcohols are formed.

- H3PO4 catalyst, phosphoric acid.

Question 14

Use the ‘Alkenes’ card to test the knowledge of the hydrogenation of propene.

Answer 14

Rate knowledge 1-5.

Question 15

Define what is meant by an electrophile.

Answer 15

An electron pair acceptor.

Question 16

What are curly arrows used to show in reaction mechanisms?

Answer 16

The movement of a pair of electrons.

Question 17

Use the ‘Alkenes’ card to test the knowledge of electrophilic addition.

Answer 17

Rate knowledge 1-5.

Question 18

State the type of bond fission involved in the electrophilic addition.

Answer 18

Heterolytic fission.

Question 19

Describe three ways in which chemists can reduce this environmental damage caused by the disposal of polymers.

Answer 19

• Develop biodegradable polymers.
• Develop ways of making polymers from plant-based substances.
• Designing processes with high atom economy.

Question 20

Once polymers have been used, they become waste. Outline two ways that waste polymers are processed usefully, rather than just dumped in landfill sites.

Answer 20

• Separation into types and recycling.

- Combustion for energy generation.

Question 21

How does the structure of trans-hex-3-ene differ from that of cis-hex-3-ene?

Answer 21

H’s are diagonal to each other in the trans.

Question 22

Explain why both cis and trans hex-3-ene react with Br2 to produce the same structural isomer.

Answer 22

The product is saturated hence there is no restricted rotation.

Question 23

State two difficulties in the disposal of polymers like poly(propene).

Answer 23

• Non-biodegradable.

- Produces toxic fumes when burnt.

Question 24

State the problem associated with the combustion of polymers such as PVC.

Answer 24

Toxic fumes are produced.