Alkenes Flashcards
Describe the nature of a double bond including electrons.
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.
What are the differences between a Pi and sigma bond?
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.
Why are alkenes more reactive than alkanes?
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.
What shape do alkenes have and why?
A trigonometry planar around the C=C due to electron pair repulsion between 3 bonding regions of shared electrons.
What is a stereoisomer?
Molecules with the same molecular formula and the same structural formula but a different arrangements of atoms in space.
Why does E/Z isomerism occur?
Due to no free rotation around the C=C bond and 2 different groups are attached to each C atom of the C=C.
Describe an E isomer and a Z isomer:
E- the high priority groups are on opposite sides
Z- the high priority groups are on the same side
What is cis and trans isomerism?
A special case of E/Z isomerism when each c of the C=C has a H attached.
Describe a Cis isomer and a Trans isomer:
Cis: Hyrdogens are on the same side
Trans: hydrogens are on opposite sides
How do you work out priority and what happens if the 1st atom of each group on 1 C is the same?
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
What happens in hydrogenation of alkenes, what is formed and what are the conditions?
C=C bond breaks and 2 Hydrogens are added- becomes saturated
Forms an alkane
Conditions: Nickel catalyst and 150 degrees
What happens during halogenation of an alkene what is formed and what are the conditions and what type of reaction is it?
The C=C bond breaks open adding X2
Forms a haloalkane (dihaloalkane)
Conditions RTP
Type: electrophilic addition
Describe the mechanism for the halogenation of an alkene:
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
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?
C=C breaks open and H and X are added to the alkane chain
Haloalkane formed
Conditions= RTP
Type: electrophilic addition
Describe the mechanism of the adding of a hydrogen halide to an alkene:
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
Describe the hydration of an alkene, what is formed and what conditions are needed?
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
What is an unsymmetrical alkene?
One that does not have the same 2 groups attached to each C of the C=C bond
What happens if you add HX to an unsymmetrical alkene and what isomerism is this?
There are 2 possible products
Structural positional isomerism
How do you predict which isomer is the major?
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)
Why is a carbocation more stable with more R groups?
The R groups push electrons towards the C+ spreading the charge over the groups therefore the more R groups= more stable
What is Markownikoff’s Rule?
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
What is addition polymerisation?
A reaction where many alkene monomers join together to make a long chain polymer
Why are polymers hard to dispose of?
They are non biodegradable
What are 3 ways polymers can be disposed of?
- Recycling
- Feedstock recycling
- Incineration for energy
Describe the process of recycling and advantages and disadvantages:
- Separated
- Washed
- Melted
- Remoulded
Pros- saves landfill space
- decreases demand for fossil fuel
Cons- collection requires fuel
Describe incineration of polymers and possible problems:
Waste polymers are burned to generate energy
Cons- halogenated polymers release harmful gases eg HCl when burnt
What is feedstock recycling?
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
What are bio polymers?
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
What are photodegradable polymers?
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
