From your syllabus
(d) describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene and propene (including the Markovnikov addition of asymmetric electrophiles to propene):
(i) *addition of hydrogen, steam, hydrogen halides and halogens
(ii) *oxidation by cold, dilute, acidified manganate(VII) ions to form the diol
(iii) oxidation by hot, concentrated, acidified manganate
(VII) ions leading to the rupture of the carbon-to-carbon double bond in order to determine the position of alkene linkages in larger molecules
(iv) polymerisation (see also Section 10.8) (e) *describe the mechanism of electrophilic addition in alkenes, using bromine/ethene and hydrogen bromide/propene as examples
Akenes document in word
Alkenes are hydrocarbons with single bonds between 2 consecutive C atoms
Physical Properties of Alkenes
- They have the General Formula: CnH2n with all C–C bonds. Example: C2H4 – Ethene.
- The simplest alkenes are gases at room temperature, then liquids, finally solids, due to increased molecular mass. This decrease in volatility is due to increasing Van der Waal’s forces
- They have typical covalent, physical properties (i.e. almost insoluble in water, soluble in organic solvents)
- The boiling point of each alkene is just a bit lower but very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and most of butenes are gases at room temperature
- The C= C bond each carbon atom is sp2 hybridized with 120o bond angles and trigonal planar structure.
- These orbitals then form a sigma (σ) bond and a Pi (π) bond with each hydrogen or carbon. These bonds cannot rotate.
All two pictures above correspond to the molecule of ethene (C2H4)
Geometric (cis-trans) isomerism.
Alkanes do not allow rotation around the double bond. So if there are two different isomers for the same compound. The ones in which the alkyl groups in the same side of the double arrow are called cis isomers, the ones with the alkyl groups in the opposite side of the double bond are called trans isomers.
- Cis and Trans isomers can also be represented by skeletal formula.
The electrons in the π (pi) bond are delocalized (they belong in the electron cloud at the top and underneath the carbon carbon σ (sigma) bond, so they are more susceptible to the attack of electrophiles.
They can react under certain conditions in the following reactions:
- Addition of
- Hydrogen to form Alkanes(hydrogenation, cis and trans fats)
- Steam to form Alcohols
- Electrophilic addition of
- Halogen (Chlorine, Bromine, etc)
- Hydrogen Halide
- Sulphuric acid.
- Oxidation with cold dilute KMnO4
- Oxidation with hot concentrated KMnO4
- Addition of
A) Addition of Hydrogen:
1) Hydrogenation of Alkenes:
We will work with ethene but the same reaction occurs with any alkene. Ethene reacts with hydrogen using a nickel catalyst at around 150°C. The product of this reaction is ethane.
Ni/150° to 200°
CH2=CH2 + H2 CH3CH3
You need to know what saturated and unsaturated means. which are the conditions of the reaction and which is the catalyst.
This reaction is barely used unless we are hydrogenating another substance, like vegetable oil to produce margarine. Not all margarines are made by hydrogenation of the double bonds in fat acids. When an oil is hydrogenated and loses all the double bonds, it is SATURATED. It cannot accept more hydrogens in the molecule. If the molecule has one double bond is MONOUNSATURATED, and if it has several double bonds is called POLYUNSATURATED.
When the hydrogenation is PARTIAL we will have a PARTIAL HYDROGENATED FAT. Having still some double bonds, leads to two types of fats, CIS and TRANS.
Trans fats are harmful to the organism. We cannot process them and they increase the levels of BAD cholesterol in blood. Be careful when you eat something. Read the labels and be sure you do not eat TRANS FATS.
HOW CATALYSTS WORK
2) Making margarine:
Margarine is made by adding hydrogen to the carbon-carbon double bonds in animal or vegetable fats and oils. These are called Hydrogenated fats
Not all margarine is made by hydrogenation.
Animals and vegetable oils have similar molecules. If it is liquid at room temperature, they are called oils. If they are solid, we call them FAT.
The higher the number of carbon-carbon double bonds, the lower the melting point, because their Van der Waals forces are weaker.
If there aren’t any carbon-carbon double bonds, the substance is said to be saturated.
Here you have an example of a saturated fat structure, what we normally call triglyceride or fat molecule
A typical mono-unsaturated oil might be:
If there are two or more carbon-carbon double bonds in each chain, then it is said to be polyunsaturated.
You can raise the melting point of the oil by hydrogenating it in the presence of a nickel catalyst. Conditions are carefully controlled so that some, but not necessarily all, of the carbon-carbon double bonds are hydrogenated.
This produces a “partially hydrogenated oil” or “partially hydrogenated fat”.
You need to hydrogenate enough of the bonds to give the final texture you want. However, there are possible health benefits in eating mono-unsaturated or polyunsaturated fats or oils rather than saturated ones – so you wouldn’t want to remove all the carbon-carbon double bonds.
The flow diagram below shows the complete hydrogenation of a typical mono-unsaturated oil.
The downside of hydrogenation as a means of hardening fats and oils
There are some probable health risks from eating hydrogenated fats or oils. Consumers are becoming more aware of this, and manufacturers are increasingly finding alternative ways of converting oils into spreadable solids.
One of the problems arises from the hydrogenation process.
The double bonds in unsaturated fats and oils tend to have the groups around them arranged in the “cis” form.
The relatively high temperatures used in the hydrogenation process tend to flip some of the carbon-carbon double bonds into the “trans” form. If these particular bonds aren’t hydrogenated during the process, they will still be present in the final margarine in molecules of trans fats.
The consumption of trans fats has been shown to increase cholesterol levels (particularly of the more harmful LDL form) – leading to an increased risk of heart disease.
Any process which tends to increase the amount of trans fat in the diet is best avoided. Read food labels, and avoid any food which contains (or is cooked in) hydrogenated oil or hydrogenated fat.
3) Manufacturing ethanol
Ethanol is manufactured by reacting ethene with steam. The reaction is reversible. The process require the following conditions: 300 ºC and 60-70 atm pressure
Only 5% of the ethene is converted into ethanol at each pass through the reactor. By removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to achieve an overall 95% conversion.
Manufacturing other alcohols
If you start from an unsymmetrical alkene like propene, you have to be careful to think about which way around the water adds across the carbon-carbon double bond.
Markovnikov’s Rule says that when you add a molecule HX across a carbon-carbon double bond, the hydrogen joins to the carbon atom which already has the more hydrogen atoms attached to it.
Thinking of water as H-OH, the hydrogen will add to the carbon with the more hydrogens already attached. That means that in the propene case, you will get propan-2-ol rather than propan-1-ol.
The conditions used during manufacture vary from alcohol to alcohol. The only conditions you will need for UK A level purposes are those for making ethanol.
i) Ethene + fluorine
Although you do not need to know this reaction for your exam, here it goes!
Fluorine reacts with ethene violently (explosive reaction) producing Carbon and HF only.
CH2=CH2 + 2 F2 2C + 4 HF
ii) Ethene + Chlorine, Bromine or Iodine
The three reactions are the same. If you know one, you know them all.
The rates of reaction are different, Chlorine reacts faster and Iodine is the slowest one. This is because the bond energy
Rates of reaction of halogens with the double bond. The rates decreases down the group so Chlorine reacts faster than Bromine. The reaction with Iodine is really slow. All of them work in the same way, so if you know one, you know them all.
All of them react breaking the double bond and placing one atom bonded to a Carbon.
CH2=CH2 + Cl2 CH2ClCH2ClThis reaction mechanism can be seen in the power point presentation about MECHANISMS
Testing ethene with bromine waterBromine water is used to test the double bond. By mixing an alkene with bromine water (orange-brown-redish) the solution becomes colorless.
Alkenes decolorize bromine water.
In the reaction with Bromine ater, the expected product is 1,2-dibromoethane.
BUT…The major product isn’t 1,2-dibromoethane as expected. The water also gets involved in the reaction, and most of the product is 2-bromoethanol and Hydrogen bromide is produced.
B) Electrophilic addition of Hydrogen Halides.
Addition to a symmetrical alkene:All alkenes react with the hydrogen halides. A hydrogen atom joins to one of the carbon atoms originally in the double bond, and a halogen atom to the other.
For example, with ethene and hydrogen chloride, you get chloroethane:
With but-2-ene you get 2-chlorobutane:
Addition to unsymmetrical alkenes
How do the hydrogen and the halogen add across the double bond?
When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.
If HCl adds to an alkene that is assimetric, like propene, there is only one main product.
In this case, the hydrogen becomes attached to the CH2 group, because it has more hydrogens than the CH group
B) Electrophilic addition of sulphuric acid
If sulphuric acid adds to an unsymmetrical alkene like propene, there are two possible ways it could add. You could end up with one of two products depending on which carbon atom the hydrogen attaches itself to.
However, in practice, there is only one major product.
This is in line with Markovnikov’s Rule which says:
- When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.
BUT THIS DOES NOT END HERE!……….WITH THIS, WE CAN MAKE ALCOHOLS!!!
Ethene is passed into concentrated sulphuric acid to make ethyl hydrogensulphate (as above). The product is diluted with water and then distilled.
The water reacts with the ethyl hydrogensulphate to produce ethanol which distils off.
More complicated alkyl hydrogensulphates react with water in exactly the same way. For example:
Notice that the position of the -OH group is determined by where the HSO4 group was attached. You get propan-2-ol rather than propan-1-ol because of the way the sulphuric acid originally added across the double bond in propene.
Using these reactions
These reactions were originally used as a way of manufacturing alcohols from alkenes in the petrochemical industry. These days, alcohols like ethanol or propan-2-ol tend to be manufactured by direct hydration of the alkene because it is cheaper and easier.
Oxidation of alkenes with cold dilute potassium manganate(VII) solution
Alkenes react with potassium manganate(VII) solution in the cold. The colour change depends on whether the potassium manganate(VII) is used under acidic or alkaline conditions.
If the potassium manganate(VII) solution is acidified with dilute sulphuric acid, the purple solution becomes colourless.
If the potassium manganate(VII) solution is made slightly alkaline (often by adding sodium carbonate solution), the purple solution first becomes dark green and then produces a dark brown precipitate.
Chemistry of the reaction
We’ll look at the reaction with ethene. Other alkenes react in just the same way.
Manganate(VII) ions are a strong oxidising agent, and in the first instance oxidise ethene to ethane-1,2-diol (old name: ethylene glycol).
Looking at the equation purely from the point of view of the organic reaction:
The full equation depends on the conditions.
Under acidic conditions, the manganate(VII) ions are reduced to manganese(II) ions.
Under alkaline conditions, the manganate(VII) ions are first reduced to green manganate(VI) ions . . .
. . . and then further to dark brown solid manganese(IV) oxide (manganese dioxide).
POLYMERIZATIONAlkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers. In general
lots of monomer molecules → a polymer molecule
The animation shows how several chloroethene monomers can join end-to-end to make poly(chloroethene), which is also called PVC.
Alkenes can act as monomers because they have a double bond:
- Ethene can polymerise to form poly(ethene), which is also called polythene.
- Propene can polymerise to form poly(propene), which is also called polypropylene.
Examples of polymers and their uses
polymerusepolyetheneplastic bags and bottlespolypropenecrates and ropespolychloroethenewater pipes and insulation on electricity cables
Polymers have properties that depend on the chemicals they are made from, and the conditions in which they are made. Modern polymers have many uses, including:
- waterproof coatings
- fillings for teeth
- dressings for cuts
- hydrogels for making soft contact lenses and disposable nappy liners
- shape memory polymers for shrink-wrap packaging
Polymer molecules can have branches coming off them, which change the properties of the polymer.
Comparison of two types of poly(ethene)
|LDPE – low-density poly(ethene)||HDPE – high-density poly(ethene)|
|Branches on polymer molecules||many||few|
|Maximum useable temperature||85ºC||120ºC|
Plasticisers are substances that let the polymer molecules slide over each other more easily. This makes the polymer softer and more flexible. For example, poly(chloroethene) or PVC is a hard polymer. Unplasticised PVC, usually called uPVC, is used to make pipes and window frames. PVC with plasticisers is soft and flexible. It is used for floor coverings, raincoats and car dashboards.
Poly(ethenol) is a polymer that dissolves in water to make slime. The viscosity of the slime can be changed to make it thick or runny by varying the amount of water.
A large molecule produced when small molecules join together. These small molecules are called monomers.
There are two types of polymers: natural and synthetic
The ones we find in nature: cellulose, starch, proteins nails, hair, bones, muscles, etc.
The ones that are man-made. Nylon, polystyrene (Styrofoam), polyesters, polyamides, PVC etc.
It is the process of producing a polymer. There are two types of polymerisation
All the atoms in the monomer are used to form the polymer
Many molecules of Ethene Polyethene (in presence of a catalyst and heat & pressure)
All monomers join up forming a small molecules that are released in the process. Most of the time the molecule is water or a hydrogen halide. (Hydrogen chloride for example)
Many molecules of Aminoacid protein + water
The equation shows the original monomer and the repeating unit in the polymer
- Formation of Polyethene
- Formation of Polypropene
- Formation of PVC
- Formation of TEFLON
Important: You should recognize the main functional organic groups used in polymerization
These functional groups can be in both sides of the molecules or combined.
Note: the green boxes between the functional groups are generic branches. We do not concentrate in this. We only look at the reaction within the functional groups and how the polimerisation takes place
In the condensation Polimerisation, different monomers get together and one of the following molecules will be produced (H2O or HCl)
We will study only 4 types of condensation polymers:
- Polypeptides, example: Proteins
- Polysaccharides, example: Starch
- Polyesters, example: Terylene
- Polyamides , example: Nylon
The monomers for the condensation polymers we will study are as follows:
- When 2 aminoacids combine,they form a dipeptide.
- If three aminoacids combine, we have a tripeptide.
- With more of 30 aminoacids, we have a special polypeptides we call them proteins.
- chains can be lined up with each other
- the C=O and N-H bonds are polar due to a difference in electronegativity
- hydrogen bonding exists between chains. Many are Soluble in water.
- Starch is made of diols that get together to form the polymer
- Polyesters are formed by diols and dioic acids forming ester groups in each bond.
Nylon or Polyamides are groups of diamines and dioic acids. Both together form amide bonds