13 Alkenes

Question 1

What is an unsaturated hydrocarbon? (1 mark)

Answer 1

A hydrocarbon with at least one double or triple bond, or ring in it’s chain.

Question 2

What is a pi bond? (3 marks)

Answer 2

The overlap of two p-orbitals.
The pi-electron density is concentrated above and below the line joining the nuclei of the bonding atoms.

Question 3

Can pi bonds rotate, why can/can’t they? (2 marks)

Answer 3

No, it locks the 2 carbon atoms in position.

Question 4

What is the shape around each carbon atom in an Alkene, why? (3 marks)

Answer 4

Trigonal planar.
Three regions of electron density, repel equally (120 degrees).

Question 5

What is the general formula for an Alkene? (1 mark)

Answer 5

Cn H2n

Question 6

Why are alkenes more reactive than alkanes? (2 marks )

Answer 6

The double bond is an area of high electron density.
The pi bond is weaker.

Question 7

What are dienes? (1 mark)

Answer 7

Hydrocarbons that contain two C=C bonds.

Question 8

Compare and explain the boiling points of an alkene to it’s corresponding alkane. (4 marks)

Answer 8

The alkene had a boiling point that is a few number of degrees lower than the corresponding alkane.
Each alkene has 2 fewer electrons than the alkane.
So the induced dipole-dipole interactions are slightly weaker.
Less energy is needed to overcome interactions.
Therefore, lower boiling point.

Question 9

What is sp^2 hybridisation? (2 marks)

Answer 9

The mixing of two electrons from a p orbital and one from an s orbital.
This creates three new hybrid orbitals at the same every level.

Question 10

Why is a pi bond not as strong as a sigma bond? (1 mark)

Answer 10

Because they are sideways, rather than forward.

Question 11

What must a compound have to have E/Z isomers? (2 marks)

Answer 11

C=C double bond
Different groups attached to each carbon atom in the double bond.

Question 12

What is cis-trans isomerism? (3 marks)

Answer 12

Special case of E/Z isomerism
C=C bond
Same groups stretched to each carbon atom in the double bond.

Question 13

Explain the Cahn-Ingold-Prelong rules. (2 marks)

Answer 13

The higher the atomic number, of the element directly attached to the C, the higher the priority.

Question 14

What is stereoisomerism? (2 marks)

Answer 14

Same molecular and structural formula
Different spatial arrangement

Question 15

Why do Z isomers have a higher boiling point than E isomers? (2 marks)

Answer 15

Z isomers are polar molecules
This means there are permanent dipole-dipole and induced dipole-dipole forces.
E isomers only have induced.

Question 16

Why do E isomers have a higher melting point than Z isomers? (2 marks)

Answer 16

E isomers have a linear shape
So the pack together better than the Z isomers
Therefore have stronger intermolecular forces when solid.

Question 17

What is the hydrogenation of an alkene? (2 marks)

Answer 17

Mixed with hydrogen
Passed over a nickel catalyst
Forms an alkane

Question 18

What is the halogenating of an alkene? (2 marks)

Answer 18

Mixed with Cl or Br
At room temperature
Forms a haloalkane

Question 19

Describe the reaction between an alkene and hydrogen halide. (2 marks)

Answer 19

Mixed with a hydrogen halide
At room temperature
Forms a haloalkane

Question 20

What is the hydration of an alkene?

Answer 20

React with steam
Phosphoric acid catalyst
Forms an alcohol

Question 21

What is an electrophile? (2 marks)

Answer 21

Attracted to regions of negative charge (that can accept a paid of electrons).
Donate electrons.

Question 22

What mechanism are the addition reactions of alkenes? (1 mark)

Answer 22

Electrophilic addition reactions

Question 23

What is the chemical test for the presence of C=C bonds? (1 mark)

Answer 23

Aqueous bromine
Orange —> colourless

Question 24

What do electrophilic addition reactions of unsymmetrical alkenes result in? (2 marks)

Answer 24

Carbocation intermediates —> possibility of two different products

Question 25

What does Markovnikov’s rule state, explain? (4 marks)

Answer 25

The stability of carbocation intermediates depend on the number of alkyl groups attached. Alkyl groups are electron donating groups. Electron donating groups decrease the size of the positive charge on the carbon. So the carbocation becomes more stable.

Question 26

How are long saturated chains (addition polymers) formed from alkenes? (2 marks)

Answer 26

Polymerisation- repeated addition reactions

Question 27

In what conditions is industrial polymerisation conducted in? (2 marks)

Answer 27

High pressure High temperature

Question 28

What is a repeat unit, and what mustn’t you forget about them? (3 marks)

Answer 28

The part that repeats in the polymer over and over again. Always in square brackets Put an n after the bracket (shows that there is a large number of repeats)

Question 29

What mechanism is the polymerisation of alkenes? (1 mark)

Answer 29

Free radical addition

Question 30

How can chemists modify the strength of intermolecular forces between polymer chains? (1 mark)

Answer 30

By including different substituent groups

Question 31

What does including plasticisers do to a polymer, why? (3 marks)

Answer 31

Lower the melting point and tensile strength. Increase flexibility. The plasticisers are relatively small so they can penetrate between the polymer chains (reducing the strength of the intermolecular forces between them)

Question 32

What does increasing the chain length of a polymer chain do to a polymer? (2 marks)

Answer 32

Increase melting point and tensile strength Decrease flexibility

Question 33

What does branching do to polymers? (2 marks)

Answer 33

Lower density, melting point, tensile strength and rigidity because it stops the molecules lining up regularly.

Question 34

Give two ways that waste polymers can be reused? (2 marks)

Answer 34

Melted and remoulded Cracked and provide organic feeds

Question 35

What is an E isomer? (1 mark)

Answer 35

The priority groups are diagonal.

Question 36

What is a Z isomer? (1 mark)

Answer 36

The priority groups are on the same side.

Question 37

What is a cis isomer?

Answer 37

The same groups are on the same side (make a C shape)

Question 38

What is a trans isomer? (1 mark)

Answer 38

The same groups are on diagonal sides.