O-Level Chemistry | 25 | Macromolecules [1/2]

now this is the last chapter of organic chemistry so in this last chapter we're going to look at another class of compounds of organic compounds called macromolecules note that I mentioned that is a class of compound is not a family of compounds right so what exactly are macromolecules we can actually reduce from the name right macro macro here means large is the opposite of micro alright so macro means large what are molecules molecules are covalent compounds or covalent substances with simple molecular structure right so essentially macromolecules are covalent compounds but with a very very large

size okay so some of your me at this point in time think about giant molecular structure alright so we have learned that covalent compounds have two types of structures you can have simple molecular you can have giant molecular structures all right like your a diamond which looks like this where you have a giant structure and all the atoms are connected by strong covalent bonds okay for macromolecule is not yet a giant molecule by a giant molecule so some of your meeting that o macromolecule arm is equal to giant molecular structure answer is no okay macro

molecule is somewhere in between simple molecules and giant molecules right so some macromolecules they actually they are more they resemble simple molecules more all right so the properties resembles that of simple molecules just that they are much much larger than your conventional simple molecule but it's not yet a giant structure so it doesn't have the properties of a giant molecule like sin or diamond or graphite now so at this point in time actually we have seen an example of a macro molecule I don't know whether you are able to think of one all right a

giant a large molecule but not one with a giant molecular structure okay we have actually learned in Chapter of alkenes where alkenes can undergo addition reactions and alkenes can be added to another alkene yes by this point you should know what I'm driving at I'm talking about polymers okay when many alkene molecules come together they form a poly alkene that is called polymer so as mentioned an example of a macro molecule is a polymer alright and how do we get a polymer we get many small molecules for example alkenes you get many small molecules combining

together to form a giant molecule now the small molecules that forms your polymers they are caught monomers and the process by which monomers come together to form a polymer is called polymerization so essentially in this chapter of macromolecules we are look going to look at polymers the different kinds of polymers and how they are formed so there are two kinds of polymers natural polymers which are those that occurs naturally some of some examples will be your proteins your carbohydrate your fat all this unnatural polymers okay however in the syllabus we are no longer looking at

natural polymers if you look at some very very old 10 year series or all worksheets some there are some questions that involve natural polymers but those are no longer in the syllabus so right now we only focus on synthetic polymers synthetic polymers there are some examples would be poly it in you and desire your plastic bags your nylon which is used to make cloth and we're going to look at a few more examples later now essentially there are two types of polymers again dependent depending on how they are form so we have addition polymers which

are formed from addition polymerization and we have condensation polymers which are formed from condensation polymerization alright so very importantly you have to know from the start there are two kinds of polymers there are two kinds of polymerization and we're going to look at one at a time we are going to look at addition polymerization in this lesson and then in the next lesson we're going to look at condensation polymerization now by the name addition polymerization we have come across the name addition in chemistry before it's under the topic of alkenes we learned that alkenes contain

a carbon-carbon double bond alright and therefore it is coordinated li unsaturated and we can add some more atoms to it so therefore alkenes can undergo addition reactions so for addition polymerization the monomers that we are looking at essentially alkenes and the we are the conditions for polymerization to occur you need high temperature high pressure and a catalyst the specific temperature and pressure you don't have to know because it varies with the type of monomer that you are using with the type of alkene they are using so now I'm going to illustrate an example of addition

polymerization how it in how many it in molecules can come together to form poly tip okay so the monomers for poly teen is actually many eighteen molecules this is the structure of eighteen okay so what happens when many eating molecules come together is this they will add to one another and if you can recall in when we learn about addition reactions the first step to an addition reaction is to break one of the bond in the double bond after breaking one of the bonds in the double bond he will look something like this okay now

you realize that each carbon atom in the structure that I've just drawn has only three bonds what does it mean it means that it's not stable it can form one more bond all right so each carbon across the double bond or in the functional group can actually form one more but so what do they bond with they bond with the carbon that's next to them okay and so on all right so essentially when you're probably when you're eating and it goes polymerization it forms actually a very large alkane okay this large alkane is cut your

poly tip now how do we name a polymer it's very simple the name of a polymer will simply use poly bracket the name of your monomer alright so if your monomer over here is a tip then the polymer form is called polythene if your monomer is for example butene then the polymer that's form is called poly Beauty all right so whatever the name of the monomer the name of the polymer is just poly and the name of the monomer in bracket now there are many ways in which we can represent a polymer so I'm going

to show you in this particular slide so as mentioned I'm going to use the example of quality the monomer in poly it in his beam okay we have seen just now that the polymer is a very very large alkene so how do we represent the polymer because it's impossible to draw everything out right and it extends indefinitely on the right hand on the left of the structure so we can represent a polymer by something called a repeating unit why is a repeating unit a repeating unit is the path of a polymer that repeats itself okay

so if you can recall the poly it in will look something like this okay I'm not going to draw in the hydrogen's but the part that repeats is actually the the part of your monomer okay so if I were to take this part that I've just drawn up and I repeat to the right and to the left okay what essentially I get is the structure of the polymer so again the repeating you need or the repeat unit is the partner repeats itself that duplicates itself in the polymer right essentially we can get the repeat unit

from the monomer all we need to do is to break one of the bond in the double bond and then we add a bond to the left and to the right of the carbon that will give us the repeating unit now to represent the polymer remember we mentioned that the repeat unit is the part of the polymer that repeats itself so the entire polymer is just n times of the repeating unit now your n can vary can vary from 10,000 to 30,000 so we don't exactly know our what is the number and for each polymer

it can vary from one to another right but we can represent a polymer by using a square bracket and then putting an N n stands for an integer it represents a number it represents how many times the repeat unit is being duplicated to form the polymer okay now in some questions you may also be asked to represent a section of the polymer that means they don't want the whole polymer they just want a part of the polymer for that the convention is to draw three times of the repeating unit okay so I have this is

one repeat unit another repeat unit and the turtle bit unit so this would be a section of polythene okay so very importantly in a question you need to look at what is ask whether you're asked to draw a repeat unit of the polymer the whole polymer or a section of the polymer now there are essentially two kinds of questions that you will get for this chapter the first type is that you are given the monomer and you're asked to draw the structure of the polymer the second type is that they give you the structure of

the polymer and you are asked to find out what is the monomer that gives the polymer okay so we're going to look at each kind of question one at a time now in this question you are asked to name the polymer form when this molecule polymerizes and draw the structure of the repeating unit of the polymer all right this question may seem easy but when I get students to do in class about half of them will get it wrong right why is because there is a trick that has yet to be taught all right so

let us go through this question together if you want please pause at this point and try it yourself and later you compare to the answer given so we have this monomer over here how do we name the monomer it's very simple we can see this special feature in the molecule so it tells us that it is an alkene right and there are three carbon atoms so this monomer is essentially four pin okay and the name of the polymer form as mentioned you just need to add a poly in front and poly propene so getting the

name of the polymer is easy now how do we draw the structure of the polymer now the trick to drawing the structure of an addition polymer is that we have to rearrange the monomer ok so first thing we need to rearrange the monomer in to the format of a hitch I get something that looks like this okay so this is carbon number one this is carbon number two so I'm going to label it on the monomer given as well right so now let's look at carbon number one what is fun on carbon number one there

are two hydrogen's now what is form it was found in carbon on carbon number two carbon number two is born one that to one hydrogen it's bonded to a ch3 group okay so once you have rearranged your monomer into a hitch form all you need to do is to duplicate the monomer in the exact same form all right and then we apply the same rules as before what are the rules we have to first erase or break one of the bond in the carbon-carbon double bond and then we join all the monomers together so this

is a session of the polypropylene that is formed from the polymerization of protein okay the question asks for repeating you need so the repeating unit is just like that is that unique that repeats itself so this is your repeating you need okay this is your repeat unit and this represents a section of polymer alright so again when if you if you need to draw the structure of an addition polymer the trick is to rearrange your monomer in the form of a hitch okay so draw out your functional group your carbon-carbon double bond and then look

at what is present on each carbon atom by rearranging a monomer in this form you stack them side by side and then you enjoy them together to get the polymer that is the trick to drawing the structure of an addition polymer okay now this is the next type of question that you again so in this question you are given you are given the structure of the polymer you're supposed to draw the structure of your monomer now there's also a trick to solving such questions to solve to get the structure of the monomer from a section

of the polymer the key is to look keep the repeating unit you look at the structure which is the part that's repeating itself and hopefully you can see that this is your repeat unit okay so how do you get your monomer back from your repeat unit is this so again this is your repeat you need to get back your monomer you do the reverse what do I mean by reverse in order to get a polymer remember we had to break one of the bond in the carbon-carbon double bond and then we add bonds to each

carbon on the side so now the reverse would mean that we draw out the repeat unit first okay so there's the repeat unit and then now we do the reverse we go back we erase the bonds at the site and then we add back the bond are across the two carbons so this is the monomer for the polymer that's shown in the question ok so again to get back your monomer from your polymer you locate the repeating unit erase the bonds at the site and add back the double bond to the two carbons now many

students even after learning the rule were still get the wrong answer okay why because students will tend to do something like this okay so this is the answer that is commonly given how is this different from the one that I've just drawn and the answer is this even though the one in RIT is more aesthetically pleasing where each group is placed exactly in the middle all right of the carbon but this is not correct okay why because this carbon atom is bonded to another carbon atom so when you put a ch3 group in the middle

it doesn't show or it's not very accurate it may even imply that a carbon is bonded to a hydrogen alright so bear in mind when you are drawing structures of alkenes especially here the carbon atom must be bonded to another carbon atom all right so the line must go straight to a carbon atom before you write out the remaining of the group okay so please bear in mind now some examples of addition polymer they are actually quite prevalent around us your plastic bag they are polythene your styrofoam foot container is polystyrene your water pipes are

made of PVC polyvinyl all right and your frying pan is made from Teflon which is PTFE okay so you will be good for you to remember some examples of addition polymers