Chapter 13 Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons which contain carbon and hydrogen atoms only, joined together with at least one C-C double bond.
What is the general formula of aliphatic alkenes that contain only one Carbon double bond?
CnH2n
However, both Cyclic alkenes and and alkenes containing more than one double bond do not conform to the same general formula.
What is the polarity of alkenes?
Non-polar, alkenes will dissolve in non-polar solvents only.
What is the shape and bond angle around the double bond of an alkene?
Trigonal Planar, 120 degrees.
This is because there are three bonding pairs of electrons and no lone pairs.
Electron pairs repel, bond pairs repel equally and lone pairs repel more.
All atoms are in the same plane
How are the Carbon atoms in a double bond of an alkene bonded together?
For each Carbon atom of the double bond, three electrons out of the four outer bonding electrons are used to form three sigma bonds- one electron to the other Carbon atom of the double bond and the two other electrons to two other atoms (either Carbon or Hydrogen)
However, the last electron forms a π bond.
What is a π bond?
The sideways overlap of adjacent p-orbitals above and below the carbon atoms.
What is a σ bond?
The direct overlap of orbitals between bonding atoms.
How many π bonds are there in a double bond?
One π bond
It is the electron density which is concentrated above and below the plane of both Carbon atoms.
What does the π bond prevent the Carbon atoms in the double bond from doing?
The π bond prevents the two carbon atoms from rotating around the double bond.
What is stereoisomerism?
Stereoisomers have the same structural formula but different spatial arrangement of atoms or groups.
When can E/Z stereoisomerism be shown?
Alkenes can show E/Z isomerism when there are different atoms/ groups of atoms bonded to each carbon atom of the Carbon to Carbon double bond.
Why can E/Z isomerism in alkenes be shown?
Different atoms/ groups are fixed in different spatial arrangements due to the lack of rotation of the C=C
What is cis trans isomerism?
Special case of E/Z isomerism.
Molecules must still have a C=C and each carbon in the double bond must be attached to two different groups/atoms, however cis trans isomers will have an identical group across the double bond.
Eg - The Y group is identical across the double bond, however each Carbon atom is still attached to two different group/ atoms- X and Y or Z and Y
X Y
\ /
C====C
/ \
Y Z
What is a cis isomer?
The identical group is bonded on the same side of the molecule.
What is a trans isomer?
The identical group is bonded on opposite sides of the molecule.
When can cis trans isomerism only be used?
There must be an identical group attached across the double bond
Why are alkenes more reactive than alkanes?
Due to the presence of the TT bond which has a lower bond enthalpy compared to the sigma bond, therefore it is weaker as less energy is required to overcome.
What can alkenes undergo addition reactions with?
Hydrogen
Halogens
Hydrogen Halides
Steam
During the hydrogenation of alkenes what is required for the reaction?
Presence of a nickel catalyst
Up to 150 degrees
What is the test for alkenes?
Orange Bromine water turns colourless in the presence of a double bond.
When can two possible products be formed in an addition reaction?
If an unsymmetrical alkenes reacts with an unsymmetrical compound two possible products can be formed.
During the hydration of alkenes what is required for the reaction?
The alkene reacts with steam in the presence of a Phosphoric acid catalyst at a temperature greater than 100 degrees
What is the mechanism of alkenes?
Electrophilic addition
Explain the Electrophilic addition mechanism between but-2-ene and Hydrogen Bromide.
1- Bromine is more electronegative than hydrogen, therefore Hydrogen bromide is polar and contains a slightly positive hydrogen atom bonded to a slightly negative bromine atom.
2- The electron pair in the TT bond is attracted to the partially positive hydrogen atom which causes the double bond to break
3- A bond then forms between the hydrogen atom of the H—Br molecule and a Carbon atom that was part of the double bond
4- The H—Br bond then breaks by heterolytic fission, so the electron pair moves to the Bromine atom
5- A Bromide ion, Br-, and a carboncation is formed.
6- The Br- ion reacts with the Carboncation to form the addition product
What is a carboncation?
Contains a positively charged Carbon atom
Explain the Electrophilic addition mechanism between propene and bromine.
1-Bromine is non-polar, however when it approaches an alkene, like propene, the TT electrons interact with the electrons in the Br—Br
2- This interaction causes polarisation of the Br—Br bond, inducing a dipole of slightly positively charged Bromine atom one end and a slightly negative Bromine atom the other end.
3- The electron pair of the TT bond is attracted to the slightly positive Bromine atom causing the double bond to break
4- A bond forms between one of the Carbon atoms and a Br atom
5- The Br—Br bond breaks by heterolytic fission and the electron pair moves to the slightly negatively charged Br atom
6- Br- ion and a Carboncation is formed
7- Br- ion reacts with a Carboncation to form the addition product of the reaction
What is the trend in the stability of carbocations?
Primary Carbocation- least stable
Secondary Carbocation
Tertiary Carbocation - more stable
What is the major product?
The major product is the most stable carbocation
How do Carbocations become more stable?
Carbocation stability is linked to the electron donating ability of alkyl groups. Each alkyl group donates and pushes electrons towards the positive Carbocation. Therefore, the more alkyl groups there are attached to the positive Carbocation, the more the charge is spread out making the ion more stable.
What is a Primary Carbocation?
Has one alkyl group attached to the positive Carbocation
What is a Secondary Carbocation?
Has two alkyl groups attached to the positive Carbocation
What is a Tertiary Carbocation?
Has three alkyl groups attached to the positive Carbocation
What are polymers?
Large molecules formed of multiple repeating units of smaller molecules known as monomers.
What is addition polymerisation?
Formation of a long chain molecules from lots of small molecules joining together with no other products.
How is poly(ethene) formed?
Addition polymerisation of a large number of ethene monomers at high temperature and pressure.
How is poly(chloroethene) formed?
Addition polymerisation of a large number of chloroethene monomers at high temperature and pressure.
How do you identify monomers from polymer chains?
Identify the repeating unit in the polymer chain which represents the original monomer from which the polymer was made.
How does recycling polymers reduce its environmental impact?
Conserving fossil fuels and decreasing the amount of waste going to landfill.
How are polymers recycled?
Firstly, the discarded polymers have to be sorted by type.
Then, the polymers are chopped into flakes, washed, dried and melted.
Finally, the recycled polymer is cut into pellets and used by manufacturers to make new products.
Why is the disposal of PVC hazardous?
PVC is hazardous due to the high chlorine content within the polymer. This means that it cannot be dumped at landfill, it also cannot be burnt as it releases hydrogen chloride, a corrosive gas into the atmosphere.
What is PVC recycling?
Solvents are used to dissolve the polymer, high grade PVC is then recovered by precipitation from the solvent. The solvent can then be reused.
How can waste polymers be used?
Used as fuel.
Waste polymers can be incinerated to produce heat, generating steam, which in turn drives a turbine producing electricity.
This has been used to produce electricity for the National Grid.
What is feedstock recycling?
The chemical and thermal process that can reclaim monomers, gases or oils from waste polymers. These products can then be used a raw materials for the production of new polymers.
What is the advantage of using feedstock recycling?
It is able to handle unsorted and unwashed polymers.
What are bioplastics?
Renewable products made from plant starch, cellulose, plant oils and proteins.
What are the advantages of using biodegradable polymers?
Biodegradable polymers can be broken down by microorganisms into water, carbon dioxide and biological compounds.
Compostable polymers degrade and leave no toxic residues.
This protects the environment as well as conserving oil reserves.
What are photodegradable polymers?
Polymers which contain bonds that are weakened through the absorption of uv light which starts the degradation.
What is an electrophile?
Electron pair acceptor
What is heterolytic fission?
When a covalent bond breaks, one of the bonded atoms takes both of the electrons from the bond forming an anion and a cation.
The atom that takes both electrons becomes a negative ion (anion)
The atom that does not take the electrons becomes a positive ion (cation)
Eg) H3C—Cl —-> H3C+ + Cl:-
: = pair of electrons
Define the term unsaturated
Compound with one or more double or triple bonds
How do molecules act as Electrophiles?
A molecule will approach the double bond, the high electron density of the double bond repels electrons in the approaching molecule, inducing a dipole. The slightly positive end of the molecule can then accept a pair of electrons.
What is a monomer?
A small molecule that combines with other monomers to form a polymer
What is a repeat unit?
An arrangement of atoms that occurs in a polymer structure over and over again
What should you do first when working out the polymer made via addition polymerisation?
Draw the monomer to represent the structure of an ethene molecule.
What are the Cahn- Ingold-Prelog priority rules?
Look at the 2 atoms bonded directly to each C atom of the C=C
Highest atomic number= highest priority
What is meant by geometric isomer?
Referring to the spatial arrangement of atoms/ groups.
E/Z isomerism or cis trans isomerism
What is the general formula for cyclic alkenes?
CnH2n-2
State two general problems in the disposal of polymers
Cannot biodegrade and release toxic fumes when burnt
H OH H OH
| | | |
—C—C—C—C—
| | | |
H H H H
Portion of a polymer chain
Suggest why this polymer is soluble in water, but polybut-1- ene is insoluble in water.
Because the polymer can interact with the water molecules to form Hydrogen bonds with the water molecules due to the alcohol functional group.
Halogenated plastics such as PVC can produce toxic waste gases if combusted. One of these waste gases can be removed by a reaction with sodium hydrogencarbonate.
Name this waste gas
Hydrogen chloride
Give an example of another type of degradable polymer (other than biodegradable) and state what makes this polymer break down.
Photodegradable polymers degrade as they absorb UV light (radiation) which breaks the covalent bonds between atoms
Biodegradable polymers are often made from plant material and break down through microbial action to produce carbon dioxide and water. Manufactures claim these types of polymers to be carbon neutral.
Explain how these manufactures claim this and suggest why producing these polymers is actually unlikely to be carbon neutral overall.
Plants took in Carbon dioxide when they were alive using it for photosynthesis, therefore they should be carbon neutral as when they degrade they return the same levels of Carbon dioxide out. However, the polymers are unlikely to be carbon neutral due to the overall carbon footprint involving transport and manufacture which will produce more CO2 throughout its entire lifecycle.
