Alkenes

Question 1

Describe the nature of a double bond including electrons.

Answer 1

3 of carbons available electrons are used to make sigma bonds (2 C-H and 1 C-C) the 4th electron in a P orbital is used to make a Pi bond between 2 carbons.

Question 2

What are the differences between a Pi and sigma bond?

Answer 2

A Pi bond: sideways overlap of the p orbitals which prevents rotation of atoms around the C=C. Also has a lower bond enthalpy therefore is weaker so breaks more easily so in reactions it’s the Pi bond that breaks.

Question 3

Why are alkenes more reactive than alkanes?

Answer 3

The Pi bond electrons are above and below the sigma bond and there are more exposed to attack. Also Pi bond is weaker due to lower bond enthalpy.

Question 4

What shape do alkenes have and why?

Answer 4

A trigonometry planar around the C=C due to electron pair repulsion between 3 bonding regions of shared electrons.

Question 5

What is a stereoisomer?

Answer 5

Molecules with the same molecular formula and the same structural formula but a different arrangements of atoms in space.

Question 6

Why does E/Z isomerism occur?

Answer 6

Due to no free rotation around the C=C bond and 2 different groups are attached to each C atom of the C=C.

Question 7

Describe an E isomer and a Z isomer:

Answer 7

E- the high priority groups are on opposite sides

Z- the high priority groups are on the same side

Question 8

What is cis and trans isomerism?

Answer 8

A special case of E/Z isomerism when each c of the C=C has a H attached.

Question 9

Describe a Cis isomer and a Trans isomer:

Answer 9

Cis: Hyrdogens are on the same side
Trans: hydrogens are on opposite sides

Question 10

How do you work out priority and what happens if the 1st atom of each group on 1 C is the same?

Answer 10

You look for the highest atomic number and that is the higher priority
You go along each group until a difference in elements is found

Question 11

What happens in hydrogenation of alkenes, what is formed and what are the conditions?

Answer 11

C=C bond breaks and 2 Hydrogens are added- becomes saturated
Forms an alkane
Conditions: Nickel catalyst and 150 degrees

Question 12

What happens during halogenation of an alkene what is formed and what are the conditions and what type of reaction is it?

Answer 12

The C=C bond breaks open adding X2
Forms a haloalkane (dihaloalkane)
Conditions RTP
Type: electrophilic addition

Question 13

Describe the mechanism for the halogenation of an alkene:

Answer 13

Temp induced dipole across X2 including the partial charges
Curly arrow from C=C to partially positive X
Curly arrow from X2 bond to partially negative X
Carbocation
Curly arrow from negative X ion with lone pair to carbocation
Haloalkane produced

Question 14

What happens during adding of a hydrogen halide to an alkene what is formed and what are the conditions and what type of reaction is it?

Answer 14

C=C breaks open and H and X are added to the alkane chain
Haloalkane formed
Conditions= RTP
Type: electrophilic addition

Question 15

Describe the mechanism of the adding of a hydrogen halide to an alkene:

Answer 15

Temp induced dipole across H-X including the partial charges
Curly arrow from C=C to partially positive H
Curly arrow from H-X bond to partially negative X
Carbocation
Curly arrow from negative X ion with lone pair to carbocation
Haloalkane produced

Question 16

Describe the hydration of an alkene, what is formed and what conditions are needed?

Answer 16

C=C bond breaks open and a H and an OH join the alkane chain
Forms an alcohol
Conditions: steam and a conc H3PO4 or H2SO4 acid catalyst

Question 17

What is an unsymmetrical alkene?

Answer 17

One that does not have the same 2 groups attached to each C of the C=C bond

Question 18

What happens if you add HX to an unsymmetrical alkene and what isomerism is this?

Answer 18

There are 2 possible products

Structural positional isomerism

Question 19

How do you predict which isomer is the major?

Answer 19

You look at the stability of the carbocation:
Primary carbocation (1 R group) = least stable
Secondary carbocation (2 R group) bit more stable
Tertiary carbocation (3 R groups) most stable
The X is more likely to go to the most stable
The H is more likely to go to the least stable (with more hydrogens)

Question 20

Why is a carbocation more stable with more R groups?

Answer 20

The R groups push electrons towards the C+ spreading the charge over the groups therefore the more R groups= more stable

Question 21

What is Markownikoff’s Rule?

Answer 21

When a hydrogenhalide is added to an unsymmetrical alkene the H from the HX attaches the carbon of the C=C bond that has the greatest number of H atoms directly attached

Question 22

What is addition polymerisation?

Answer 22

A reaction where many alkene monomers join together to make a long chain polymer

Question 23

Why are polymers hard to dispose of?

Answer 23

They are non biodegradable

Question 24

What are 3 ways polymers can be disposed of?

Answer 24

1. Recycling
2. Feedstock recycling
3. Incineration for energy

Question 25

Describe the process of recycling and advantages and disadvantages:

Answer 25

1. Separated
2. Washed
3. Melted
4. Remoulded
   Pros- saves landfill space
   - decreases demand for fossil fuel
   Cons- collection requires fuel

Question 26

Describe incineration of polymers and possible problems:

Answer 26

Waste polymers are burned to generate energy

Cons- halogenated polymers release harmful gases eg HCl when burnt

Question 27

What is feedstock recycling?

Answer 27

When polymers are cracked into synthesis gas, mixture of CO and H2 this can be used as fuel or chemical feedstock to make other organic products

Question 28

What are bio polymers?

Answer 28

They are made from plant materials e.g maize which will biodegrade due to bacterial activity minimising the environmental impact
They are compostable as they break down in similar time to naturally compostable materials

Question 29

What are photodegradable polymers?

Answer 29

Polymers that become brittle when exposed to sunlight for long periods
UV light is absorbed by the C=O bond causing it to vibrate and break splitting chain into smaller products which bacteria convert to CO2 and H2O