Alkenes and Polymers
Alkenes are hydrocarbons that contain a double bond between two adjacent carbon atoms in the molecule. Alkenes are unsaturated as they contain less than the maximum number of single bonds.
Examples of alkenes
Name Formula Structure
The general formula of an alkene is: C H
Isomers of alkenes
Isomers are molecules with the same molecular formula but with different structures. There are two types of isomerism associated with alkenes, structural isomerism and geometric isomerism.
Structural isomers are alkenes with the same molecular formula, but with the double bond in different positions in the chain. For example, but-1-ene and but-2-ene are structural isomers:
Geometric isomers are alkenes with similar structural formula, but differ in the arrangement of atoms or groups of atoms around the double bond. For example, cis-but-2-ene and trans-but-2-ene:
The geometric isomers are two different molecules because the carbon-carbon double bond (C=C) is rigid and does not allow rotation around it.
Long-chain alkanes can be broken into smaller molecules by passing them over a hot catalyst (e.g. pumice or alumina). This process is called cracking and results in an alkene molecule and a shorter-chain alkane molecule:
Long-chain alkane Shorter-chain alkane + alkene
Cracking is used in industry to convert less valuable fractions from the distillation of oil (e.g. fuel oil) into more valuable fractions such as gasoline. For example, decane can be cracked at 500°C over alumina to give ethene and octane:
In the laboratory, cracking can be performed using hot pieces of broken porcelain pot.
Reforming of alkanes
After long-chain alkanes have been converted to the more valuable shorter -chain alkanes required as fuel for the internal combustion engine, they are treated further. It has been discovered that branched-chain alkanes burn more smoothly in car engines than straight-chain alkanes and are therefore more useful as fuels. Straight-chain alkanes are converted into branched chain alkanes by the process of reforming:
Reforming is carried out by passing lower-grade gasoline vapour over a platinum catalyst at a high pressure and temperature. The process is often called platforming.
Reactivity of the alkenes
Alkenes contain a reactive double bond. Other molecules can react with the ‘spare’ bond in addition reactions to form new molecules. Examples of addition reactions are:
Reaction of an alkene with hydrogen to form an alkane
Reaction of an alkene with water to form an alcohol- a ‘hydration’ reaction (see later)
Reaction of an alkene with bromine to form a dibromoalkane
The chemical test for an alkene
Alkenes decolourise orange bromine water:
Hardening of oils
Oils- e.g. sunflower oil- are liquid alkenes that contain a long hydrocarbon chain. Many oils contain lots of double bonds in their structure: they are ‘polyunsaturated’. By treating them with hydrogen and platinum catalyst, the double bonds can be converted to single bonds- i.e. they can be saturated. The change results in a more flexible molecular structure which raises the melting point of the molecule as more interactions can take place between the long hydrocarbon chains. The resulting solid (a fat) is called margarine and is hard enough to be spread on bread.
If the pressure of the hydrogen gas is carefully controlled, some of the double bonds will remain and the margarine will also be polyunsaturated. Polyunsaturated fats are healthier to eat.
Industrial preparation of ethanol
Ethanol is prepared industrially by the direct hydration of ethene at 300°C and 60bar using a phosphoric acid (H3PO4) catalyst. Obviously, the water required for the reaction is present as steam under these conditions. Ethene is itself obtained from crude oil via the cracking of alkane molecules (see below).
The preparation of ethanol by the direct hydration of ethene contrasts with the preparation of ethanol from a carbohydrate (e.g. sucrose) by fermentation.
Advantages of direct hydration compared to fermentation:
Disadvantages of direct hydration compared with fermentation:
Uses of ethanol
Ethanol is used as a fuel (e.g. spirit burners) and as a solvent (e.g. in perfumes, mouthwash, deodorant). Industrial methylated spirit contains ethanol with added methanol and other compounds to make it unfit to drink.
Two or more alkene molecules can react with each other if heated with a catalyst to form long chains or addition polymers.
Definition of an addtion polymer:
The process by which a polymer is made is called addition polymerisation. The molecules that add together to form an addition polymer are called monomers.
An example of a polymerisation is ethene reacting at high pressure and temperature in the presence of a catalyst to form poly(ethene). Poly(ethene) is more commonly known as polythene:
Poly(ethene) is tough, easily moulded and is an excellent electrical insulator. It is durable, it does not corrode and it is not affected by the weather. It therefore has many uses including the manufacture of shopping bags and washing-up bowls and also as insulation for telephone cables.
Since the monomer units add together to form the polymer, the process is described as addition polymerisation. Poly(ethene) is an example of an addition polymer. Other examples are:
Poly(chloroethene) is an addition polymer also known as polyvinylchloride (PVC). It is stronger and harder than poly(ethene). it is used extensively for gas and water pipes. The monomer is chloroethene.
Poly(tetrafluoroethene) is an addition polymer also known as PTFE and Teflon. It has unusual properties: it withstands very high temperatures and it forms very slippery surfaces. It is therefore used for non-stick frying pans. The monomer is tetrafluoroethene.
Poly(methyl 2-methylpropenoate) is an addition polymer commonly known as Perspex. It is transparent and is often used as a substitute for glass. The monomer is methyl 2-methylpropenoate- usually known as methyl methacrylate.
Poly(propene) is an addition polymer that is sometimes known as polypropylene. It is tougher than poly(ethene) and is used to make ropes and for packaging. Its high melting point enables it to resist boiling water, so it is used to make containers for food.
A plastic is a substance that can be moulded by application of heat and pressure. The word is usually applied to man-made polymers such as poly(ethene) which contain up to 50,000 monomer units per molecule. Plastics are usually divided into two types: thermo-softening plastics (or thermoplastics) and thermo-setting plastics.
Thermo-softening plastics (thermoplastics)
Definition of a thermo-softening plastic:
Examples of thermo-softening plastics are poly(ethene), poly(chloroethene) and polystyrene. They can be easily moulded by blowing or injecting the molten plastic into a mould. They are easily recycled.
Definition of a thermo-setting plastic:
An example of a thermo-setting plastic is Bakelite. It can be heated and moulded only once- usually by compression moulding.
The difference in thermal properties of the two types of plastic lies in the molecular structure of their polymer chains. Thermo-setting plastics have cross-linked polymer chains which are bonded to each other. The chains are held firmly in place and no softening takes place on heating. Thermo-softening plastics do not have cross-linked polymer chains. On heating the polymer chains flow over each other , the plastic softens and it can be moulded into a new shape.
Disposal of plastics
In the past thirty years, plastics have been increasingly used in place of metal, glass and natural fibres such as cotton and wool. However, they now contribute significantly to household waste and their disposal can be a problem. in the past, they were simply buried in land-fill sites such as old quarries. Now, these sites are nearly all full so other methods of disposal are proving more satisfactory.
Many plastics can be burned and schemes have been developed to use the energy generated for heating purposes. Incineration has to be efficient to minimise the formation of toxic gases such as hydrogen cyanide (HCN), carbon monoxide (CO) and nitrogen oxides (NOx).
Some thermo-softening plastics are easily recycled and many local councils now collect waste plastic for this purpose. For instance, large quantities of black plastic bags and sheeting are produced for re-sale.
One solution to the problem of waste disposal has been the development of bio-degradable plastics which are broken down by bacteria. Several other types of environmental-friendly plastics are also in production, including those that degrade in sunlight and those that dissolve in water.
Space for Additional Notes on Alkenes and Polymers
A Review of the Alkenes and Polymers Topic
Each topic in the IVth Form Chemistry course is divided into a series of 'learning targets'. These are listed below for work on Alkenes and Polymers. For each 'learning target' you should make an assessment of how you are progressing, in terms of increasing your knowledge and developing a clear understanding of the principles. You should assess your progress on a 1 - 3 scale as follows:
1= I feel confident about this aspect of the work and I am encountering few problems.
2= I am making reasonable progress, but I have encountered a few difficulties and feel that I need to go over these particular areas again.
3= I am finding this aspect of the topic difficult.
Remember, be fair to yourself - be honest!!
a) Understand that covalent bonding involves the sharing of electron pairs and can be
illustrated by dot & cross diagrams
b) Know and understand that alkenes are unsaturated hydrocarbons
c) Understand how to write the formulae and draw the structures of ethene and
d) Understand that alkenes are produced, together with smaller alkanes, from the
cracking of larger alkanes
e) Know that ethene is mainly produced by cracking the naphtha fraction from crude
f) Understand why alkenes undergo addition reactions, using the decolourisation of
bromine and the hydration of ethene as examples
g) Understand that vegetable oils can be hardened by catalytic hydrogenation to form
h) Know a simple chemical test for distinguishing alkenes from alkanes
i) Understand the advantages and disadvantages of the two ways of producing
ethanol: fermentation and direct hydration of ethene
j) Understand addition polymerisation and give example
k) Understand the difference between thermosoftening and thermosetting plastics in
terms of their molecular structure and relate this to their properties
l) Understand the relationship between properties of plastics and their uses
Ideally, all of your responses will be ‘1’. However, this is rarely the case first time through! If you have written a ‘2’ anywhere, you may wish to read through your notes again or look at the relevant page in your text book. If you have written ‘3’ as a response to any of the questions, see me for further help.