Chemistry: Unit F322: Chains, Energy and Resources
Chemistry: Unit F322: Chains, Energy and Resources
Alkenes
Alkenes are unsaturated hydrocarbons. They contain one or more carbon-carbon double bonds.
← Alkenes have a trigonal planar shape.
← The bond angle is 120
Reactions of alkenes
Alkenes typically take part in addition reactions. They are more reactive than alkanes because they are unsaturated – they contain a double bond. A molecule can be added across the double bond causing the pi bond to be broken, forming one saturated compound.
1. Hydrogenation – the addition of a H2 molecule to make an alkane.
Alkenes are reduced by hydrogen gas:
This process of hydrogenation is important in the manufacture of margarine. Unsaturated vegetable oils are converted to solid fats by hydrogenation. This is known as hardening of vegetable oils.
2. Halogenation – reaction with halogens to form a dihalogenoalkane.
Alkenes react with bromine, chlorine and iodine by the chemical mechanism of electrophillic addition.
This reaction is used as a test for saturation; if the reagent contains a double bond, bromine water will decolourise under standard conditions.
3. Reaction with hydrogen halides to form halogenoalkanes.
Alkenes react readily with hydrogen halides in an addition reaction by mechanism of electrophillic addition.
Hydrogen bromide is not usually stored in laboratories. In order to make it, concentrated H2SO4 is reacted with KBr in the presence of the alkene in situ.
4. Hydration reaction with steam to produce alcohol
Alkenes react with steam to produce alcohol. This is a major industrial process for the manufacture of ethanol in the UK.
Ethanol can be produced by fermentation, but this hydration reaction has a higher atom economy as it is an addition reaction. It is used in industry in paint production and for chemical feedstock.
Mechanism of Electrophillic Addition
Polymerisation – making addition polymers from alkenes
Addition polymers are formed when alkene molecules join together to form long chain molecules. The monomer is the alkene molecule from which the polymer is made. The monomer becomes a repeat unit when its double bond has broken.
Problems with Polymers
-----------------------
They have the general formula CnH2n
Pi bonds are formed when two P orbitals overlap laterally, creating an area of high electron density above and below the plane of the flat alkene molecule. The pi bonds do not contribute to the shape of the molecule. There is restricted rotation about the double bond created.
Sigma bonds are formed by the overlap of two S orbitals, creating a single covalent bond. There are three pairs of sigma electrons around each carbon, meaning that the shape around each one trigonal planar.
Isomerism
Structural
Stereoisomerism
o Position – where the functional group is in a different place in the chain
o Chain – where there are braches off the main carbon chain
“Molecules which have the same molecular formulae but different structural formulae.”
“Molecules which have the same molecular and structural formulae but different arrangement of atoms in space”
E/Z isomerism exists in molecules where there is a double bond and when each carbon atom in the double bond is attached to two different groups.
Z (cis) 1,2 dichloroethene – groups are both below the plane of the molecule
E (trans) 1,2 dichloroethene – groups are opposite each other
Position Isomers of Pentene
Chain Isomers of Pentene
2 methyl but-2-ene
Ethene + H2 ( Ethane
Conditions Necessary:
Finely divided Nickel catalyst
High temperature
High pressure
Propene + Br2 ( 1,2dibromopropane
Cyclohexene ( 1,2-dichlorocyclohexane
Ethene + HBr ( Bromoethane
Conditions for Hydration:
Phosphoric Acid (H3PO4) catalyst
High temperature
High pressure
Ethene + H2O ( Ethanol
H - Br
+
(
The electrons of the HBr bond are repelled by the electrons of the Àbond of the alkene. This introduces a dipole and the HBr bond breaks by Heterolytic fission.
À bo are repelled by the electrons of the πbond of the alkene. This introduces a dipole and the HBr bond breaks by Heterolytic fission.
π bond breaks
δ+ end of the HBr (H) is electrophillic – it accepts a pair of electrons from the πbond and forms a dative covalent bond with the carbon atom
The carbocation intermediate is very reactive – one of its carbons has a free electron, and is therefore +ve. The negative Br- bromide ion is attracted to this and forms a covalent bond.
Compound formed (this could be a halogenoalkane, or a dihalogenoalkane)
Monomer: Ethene
Repeat Unit
Polymer: Polyethene
The double bond of the alkene opens out to link all of the monomers together with single bonds. The electrons in the π bonds form new σ bonds between monomer units.
Conditions: High pressure, heat, Ziegler-Natta catalyst
Polymers contain non polar bonds, such as C-H bonds, or strong bonds, like C-F bonds. This makes them unreactive, and so they are non-biodegradable. They cannot be broken down by organisms in the natural environment. They exist in the environment for a very long time, and so contribute to overflowing landfill sites.
What is the issue?
What can chemists do to resolve it?
Chemists are developing new polymers which are biodegradable and compostable. They can be derived from raw materials such as starch, maize, cellulose and lactic acid.
Polymers must be recycled as they are not biodegradable. However, recycling is made very hard because plastics are difficult to separate.
Polymers have identification codes (numbers 1-7) and are separated into these groups using optical scanning techniques, to make sorting quicker and less labour intensive. They are then processed, chopped, washed and the impurities are then removed, before being melted and remoulded.
Polymers cannot be incinerated because toxic fumes are released – PVC is particularly hazardous because Hydrogen chloride and PCBs (polychlorinatedbiphenyls) are formed.
Burning polymers under controlled conditions produces heat energy. Plastics are turned into briquettes which are then burned to produce electricity. Polymers can also be cracked to give new alkenes, like synthesis gas. This can be used as a chemical feedstock for conversion into useful products or as a fuel in oil refineries.
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