AS - Alkenes
Alkenes are hydrocarbons with double bonds between 2 consecutive Carbon atoms
Alkanes are saturated compounds. there is no possibility of adding more atoms to the molecule.
The double bond permits the addition of new atoms to the double bond, so they undergo addition reactions where the double bond is broken and new atoms enter the molecule.
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
Structure of the –C=C– bond
- 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.
- Both pictures above correspond to the molecule of ethene (C2H4)
- The Pi bond prevents rotation around the c-c bond so the molecule has a “FIXED” structure. That’s why these molecules may present stereoisomers (cis-trans)
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:
A) Addition Reactions of
- Hydrogen to form Alkanes(hydrogenation, cis and trans fats)
- Steam to form Alcohols
- Electrophilic addition of
- Halogen (Chlorine, Bromine, etc)
- Hydrogen Halide
- Sulfuric acid.
B) Oxidation Reactions
1) with cold dilute KMnO4
2) with hot concentrated KMnO4
Adding Hydrogen to the double bond
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
For this reaction or similar you should remember
- what saturated means
- conditions of the reaction (temperature and pressure if specified)
This reaction is used to hydrogenate 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
The process of hydrogenation is made using a solid catalyst of Nickel. The step by step is shown below:
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
Adding steam to the double bond: Preparing ethanol
Ethanol is manufactured by reacting ethene with steam. The reaction is reversible.
conditions: 300 ºC and 60-70 atm pressure
Removing the ethanol as it forms shifts the equilibrium towards the formation of more alcohol.
You need to be careful where the -OH and where the -H of the water will add to the double bond in the case that the molecule is not symmetric.
The rule specifies that when a molecule “HX” is added to the double bond, the hydrogen in that molecule will add to the carbon atom with already more hydrogens, so, if we separate the water molecule in H-OH, the hydrogen will attach to the carbon with more hydrogens.
That’s why in the case of propene, you will get propan-2-ol rather than propan-1-ol.
Adding Halogens to the double bond (Mechanism Required)
Adding a halogen to the double bond ends in a dihalide.
- The reaction works the same with Bromine and Iodine.
- The reaction with F2 is different (and it is not required).
- The rate of reaction is faster for Chlorine (think about the reactivity of halogens) and the slowest is Iodine.
- The halogens will be placed at each side of the double bond, attached to different carbons)
Testing ethene with bromine water (Mechanism required)
Adding Hydrogen Halides to the double bond (Mechanism required)
The same as in the addition of Steam, the hydrogen will be added to the carbon in the double bond that already has more hydrogens.
Following the Markovnikov’s rule, the halogen will be added to the carbon with less hydrogens and the hydrogen will be added to the carbon with the more hydrogens already.
Adding Sulfuric Acid to the double bond (Mechanism required)
If sulphuric acid adds to an asymmetrical alkene, like propene, the same mechanism applies to it. One of the hydrogens in the acid will attach to the carbon with more hydrogens and the Hydrogen sulfate left behind will bond with the carbon with less hydrogens. The name is alkyl hydrogensulfate
This is in line with Markovnikov’s Rule, again…
Oxidation of alkenes with cold dilute potassium manganate(VII) solution
“The oxidation process is losing electrons or gaining oxygen, in this case, the oxygen gained is represented with [O], meaning, oxygen is added to the compound that comes from the oxidizing agent”
Alkenes react with cold diluted potassium permanganate -manganate(VII)- to produce a “diol” .
The resulting color is different under acidic or alkaline conditions.
- In acidic medium the purple solution becomes colorless.
- In basic medium the solution first becomes dark green and then produces a dark brown precipitate. The dark green solution is because of the presence of the MnO42-
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 generallots of monomer molecules → a polymer moleculeThe 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
|Maximum useable temperature
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 bond