2.5 REACTIONS OF ALKANES AND ALKENES & POLYMERISATION

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Hi guys! Another topic on organic chemistry here for you. I keep uploading these organic chemistry topics because many people find it most difficult. I hope that everything I show you will somehow decrease your struggle with this topic.

In this unit, we're going to learn the reactions of alkanes and alkenes. Be prepared, there are a lot of new definitions in this unit, so get a piece of paper and write them all down. You can stick them somewhere where you walk every day: your bedroom, bathroom, etc. This way you'll read them automatically all the time.

We're starting with:

Alkanes

What do you know about them? 1. Single carbon bond C-C --> every alkane consists of hydrogen and carbon atoms only. All of the bonds between the carbons are single. By this, you know that it is an alkane. 2. Saturated hydrocarbon --> all C-C single bond are saturated, which means that you can't add anything to it. It has no more space for another atom. 3. End with -ane --> nothing more to say about it, just all alkanes end with -ane like butane, ethane, etc. 4. Usually unreactive --> they are unreactive because as we said before, they are saturated (they are not interested in reacting with other atoms). Furthermore, they also consist of hydrogen and carbon atoms only, so the molecule is non-polar and only has weak Van der Waals forces. 5. The general formula CnH2n+2 --> this is how we write the formula for alkanes. Whatever, the number of carbon atoms you have you need to multiply it by 2 and add 2. Let's take ethane as an example. We have 2 carbons, therefore we do (2x2)+2 which is 6. As you can see the example on the picture above, ethane has 6 hydrogens.

Reactions of alkanes:

1.Cracking:

• Cracking is used when smaller molecules are needed. Smaller molecules are more useful therefore cracking is used a lot.

• They mostly produce methane, ethane and propane because they are the simplest molecules.

This is the type of equations that you need to be able to write for cracking:

e.g. Cracking of dodecane to produce ethene and a straight-chain alkane. As you can see in the picture above, you were asked to produce ethene, so you can straight away write that down. The next part of the equation is just what's left from the dodecane.

2.Combustion:

There are two types of combustions:

1. Complete combustion- this is the good combustion (although carbon dioxide is produced, it is safer for us than incomplete combustion). During this type of combustion, alkane reacts with oxygen and carbon dioxide is produced together with water.

e.g. I included an example in the picture above using methane. As you can see oxygen is added and then carbon dioxide and water are produced. After you put that in an equation, you need to balance it.

2. Incomplete combustion- this happens when the insufficient amount of oxygen is supplied. Carbon monoxide is produced which is toxic (because it prevents the haemoglobin from carrying oxygen around the body) and even worse than producing carbon dioxide. This is a reaction we are trying to avoid. It also produces less energy than complete combustion.

e.g. I again included an example for you. After you done putting every molecule in the equation, balance it and you'll be done.

3. Halogenation:

It is sometimes called photochlorination because we use chlorine in this reaction. There are three steps in this type of reaction. We will go through each one and give definitions of different words as we go. There is also a mechanism for this reaction which you need to know, off by heart, you just need to remember it. This is a mechanism for methane: Stage 1 - Initiation (the reaction that starts the process). The process starts by UV light breaking a chlorine molecule into free radicals (species with an unpaired electron) by homolytic bond fission (when a bond breaks and each atom gets one electron, they are shared equally).

Looking at the photo above, you can see that we have a chlorine molecule and it splits into two chlorine radicals (radical always has a little dot next to it). Stage 2 - Propagation (chain reaction, it is continuous). In this stage, radical is used as one of the reactants and then a radical is produced. This is why it is called a chain reaction. The underlined parts in the equation (on the picture above) always stay the same, and the other parts depend on what molecule we are using to start with. Stage 3 - Termination (ending the process). This happens when two radicals meet and then the reaction is completed. Sometimes, in the exam, you are asked to write an overall equation for halogenation. I included this in the picture above.

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The next are:

Alkenes:

What do you know about them? 1.Carbon double bond C=C --> in alkenes you have at least one C=C bond. There are still only hydrogens and carbons. So, if an organic compound has a C=C bond, it means that's an alkene. 2. Unsaturated molecule --> an unsaturated hydrocarbon can open its C=C bond and accept other atoms. When it does that, it will no longer be an alkene but alkane. 3.End with -ene --> all alkenes end with -ene. Such as butene, methene, etc. 4.Very reactive --> alkenes are very reactive. The reason for this is the C=C double bond which as we said can open and accept other atoms. As alkanes, they also only have carbons and hydrogens and weak Van der Waals forces. 5. The general formula CnH2n --> this is generally how you write the molecular formula for alkenes. Depending on how many carbons you have, in alkenes, you need to double it to get the number of hydrogens.

Structure of alkenes:

In alkenes, the double bond gives an area of high electron density and therefore, alkenes are open to attack by electrophile (electron-deficient species, that accept a lone pair of electrons). But this is something we are going to come back to when we revise reactions of alkenes. Now, let's focus on the structure of alkenes. The drawings on the picture above show exactly what I mean. One p orbital electron on each carbon atom is not used in bonding and these two (because we have two carbons bonding) 2p orbital electrons overlap sideways, above and below the molecule, creating a pi bond. The pi orbitals formed by the overlap, make the molecule reactive. Molecules, which need electrons (electrophiles) will get attracted to this bond.

Reactions of alkenes:

1. Testing for C=C double bond:

• Using bromine (electrophile) --> orange colour of bromine water is decolourised if a C=C double bond is present. I included a written reaction for you in the picture above.

• Using potassium manganate (oxidising agent) --> purple colour of potassium manganate is decolourised if a C=C double bond is present. Again, the reaction is on the picture above.

2. Hydrogenation:

• This reaction is self-explanatory, during this reaction we add hydrogen to an alkene. The double bond opens up and accept two hydrogen atoms making an alkane. The catalyst and conditions for this reaction are nickel, 70atm and 200 degrees Celcius.

3.Electrophilic addition:

In this type of reaction, reagents combine to give one molecule.

Here is a mechanism for electrophilic addition of HBr to an ethene (look at the picture above).

• First of all, HBr undergo heterolytic bond fission ( a bond breaks and the electrons go towards one of the atoms). Bromine is more polar therefore it receives the electrons and now hydrogen is slightly positive. We talked about the alkenes being very reactive and having electronegative C=C bond. Therefore, C=C attract that slightly positive hydrogen towards itself.

• The next step is that we have a hydrogen attached to the alkene and now we just need the bromine. It is completely negative by now and is attached to the molecule as well.

• Lastly, you no longer have an alkene but a halogenoalkane.

The mechanism would work exactly the same if we had Br2. One bromine would become lightly positive and the other slightly negative.

A different situation is seen with propene. This time, we also follow the mechanism but we can put bromine either on the outside carbon or on the carbon inside. They are called major and minor products as you can see above.

The reason why the first reaction is called a major reaction is because the second carbocation is more stable than the first one because the positive charge can be more spread out.

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Polymerisation

Polymerisation - it is the joining of many monomers, to make a large polymer molecule.

Monomers - a small molecule that can be made into a polymer.

Alkenes are used for this process because they undergo addition polymerisation. This means its double bond opens up and joins the monomers. Here is an example for you (see the picture above).

e.g. using ethene

• You have an ethene molecule (remember to put a little n next to it - this shows the number of molecules). The double bond opens up ready to join with another monomer.

• It is no longer called an ethene but a poly-ethene. Again keep in mind that little 'n' that you need to put next to the polymer.

• In blue, I also showed you how a section of a polymer of ethene looks like and the repeat unit (this is a part that extends the polyethene). We know that ethene has two carbons, so the repeat unit has to have 2 carbons.

There are a couple of polymers and their uses that you need to know and recognise. Looking at the photo above you can see a little table with the 4 you need to understand. I included how their alkene looks like, how its polymer looks like and name of the polymer and of course their use.

1. Ethene - We already talked about it. The polymer of ethene is called poly-ethene. When you draw it in the exam, put a little 'n' next to it. It is used in plastic bags.

2. Propene - It has one more carbon than ethene and therefore CH3 is a branch because we can only have two carbons with a double bond in between. The polymer of propene is called poly-propene and it is used in food containers.

3. Phenylethene - In this molecule, one of the hydrogens is substituted with a carbon ring. The name of this polymer is poly(phenylethene) and it is used in insulators.

4. Chloroethene - This is our last one, one carbon is substituted with a chlorine molecule, as the name suggests. The polymer is called poly(chloroethene). It is used in water pipes and waterproof clothing.

You can also see how the polymer looks like because I included drawings of them. The thing that keeps on happening, is the double bond which opens up. Therefore, alkanes couldn't be used for polymerisation because they don't have that double bond, they only have a single bond.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!