[Music] hello and welcome to this video a revision video on aq8 alkenes and my name is Chris Harris I'm from Allawi two tents calm and basically we're just going to go through the alkenes topic and give you a quick overview of this topic to make sure that you're getting the right information and to make sure that you've covered everything that is on the specification in the alkenes topic like I said and these powerpoints that I'm using on here they are available to be purchased if you just click on the link below the video in the description box and you'll be able to get them there and so you can use them for vision do whatever you want with them really but they'll quite handy addition to your other revision material that you may have as well okay so like I say this is linked to the specification these are the specification points and for aq8 okay right so let's have a look first so we're looking at the introduction to alkenes and so basically these are unsaturated hydrocarbons so this means that they obviously contain double bonds and they have the general formula cnh2n and now this general formula V only applies if you've got one double bond in the molecule and this is what we're assuming with all leave all of these alkenes in this topic okay so they are hydrocarbons and just like alkanes and they contain hydrogen carbon only they're unsaturated they have one double covalent bond and they undergo addition reactions which you'll see later on so here's some examples this is ethan on the left there which is ch2 ch2 this is the structural formula for it this is the displayed formula and and this one which is butyl 1 3 dying this one's got two alkenes two double bonds and so again here's the structural formula for it make sure you can identify the alkyne as a displayed and structural as well and the double bonds they have a high electron density loads of electrons here this makes them pretty reactive and you also get cyclo alkenes as well and these have two fewer hydrogen's and they're straight chain counterparts so obviously these have but the main reason is because the whole thing loops back on itself and on so we'll be using up space where the would have been hydrogens and straight chain to join onto another carbon so this is cyclo pentene c5 h8 so just watch out for them as well okay so like I say these things undergo addition reactions and alkenes are attacked by electrophiles because they have double bonds okay so the double bond like we said it has a loads and loads of electrons haven't attacked by electrophile and basically the electrophile adds to the molecule this one we'll call it addition reactions okay so the electrophile is an electron pair acceptor so remember we looked at and nucleophiles as well you need to know about them and nucleophiles have them lone pair of electrons well these are just kinda like the opposite so these accept the electron pair and basically they're deficient in electrons themselves and they are attracted to that double bond so examples of electrophiles are things with a positive charge like no.2 plus h plus etc so these have a deficiency in electrons or you can use polar molecules with a delta positive so things like hbr h2 so4 etc and we're going to look at them examples and very soon so all electrophilic addition reactions we use the curly Erol obviously to show mechanisms it always starts from the double bond very tempting to draw it the other way around but it must often double bond because that's where the electrons are and Curly ours always show where the electrons are going from and where they go into the most electrons are obviously in the double bond and so so this is obviously a type of addition reaction and this is the electrophile the e+ and just put the letter e to symbolize an electrophile however the electrophiles could be Delta positive as well as I said before okay and basically this is accepting electrons from the double bond and we're adding onto it so you'll see this a lot okay so let's look at the first mechanism because quite a few in this one and we're going to add bromine to it and basically this is a test for an alkene so if we add bromine water to an alkene it should decolorize and this is basically a sign of an alkene so adding bromine water to an alkene it causes color change from brown orange because it's a kind of bromine to colorless because the product that we produce is color so didn't actually have a color so that's right decolorize it and basically your bromine is the electrophile and this adds to the alkene and it forms a dye bromo alkene i'd like to say this is colorless so let's have a look at the mechanism here so you can see here that the BR to this is a little bit tricky little bit the BR two normally isn't polarized because they're both as electronegative as each other however when the bromine molecule approaches an area where there's loads of electrons like a double bond the electrons shift to the opposite side of the molecule because they're repelling each other and just temporarily when this is near the arkeen we get this dipole that's being created you can see that the bromine on this side has got a delta positive because closest to the double bond so the electrons that were here and now been shoved over to the left hand side of the molecule so that's quite important so we don't create this kind of induced dipole and and we have obviously this electron pair is in the double bond we've got loads of them there this is attracted to the dance positive and what we then do is then break the bromine bond so let's have a look at the arrows so curly I will going from the double bond to the bromine with the Delta positive on and then because obviously this is forming a bond with the bromine this bond has to break so the electrons in that single bond go into the bromine which you can see on there okay so what we form is a carbo cation intermediate okay intermediates are basically just molecules which are formed and in the middle of a full reaction and this is an example of one we call it carbo cation because we've got a carbon with a positive charge cations a positive carbo for carbon so it's a carbo cation and and what we have there's the bromine that we valid on notice the double bond is now gone because you've broken it and but we have the single bond remaining and the bromine is now attached onto the side there and we then have obviously our BR - that was left over with a lone pair obviously this is going to go and attack the Delta band at the positive carbon calf line like this and obviously then you're going to form your colorless want to dye bromo ethane that's made and you can see what we've done we start with an alkene and we produce our colorless diaper mo and and dr die bromo eath a molecule that could be can be another alkene didn't have to be thin but it Dean but there's your product that is colorless okay let's look at a slightly easier example very similar mechanism in fact the mechanism is the same and so alkenes reacts with hydrogen halides to form halogen or alkanes okay so let's have a look so we're going to use hbr is an example but you could use other halogens as well but HBR follows the same mechanism as the addition of a halogen like we've seen before and it can apply to other halogen here lines like this ooh so here it is here now the difference with this one is that it's polarized so there's no temporary dipole being created as a permanent dipole because you've got Delta positive Delta negative and the obvious we got the electron pair in here in the double bond and that's going to go in an attack that Delta positive hydrogen which is then going to break the bromine you can see it's just the same mechanism as before okay just using different molecule now obviously what we're doing here now is we're adding our hydrogen there's the hydrogen that we've just added on there it is double bonds broken again carbo cation forms and there's our bromide ion that's left behind which is there and then this basically go in and attack that Delta that's positive carbo cation not a delta positive and we're going to form bromo ethane okay now notice this is instead of dive-bomb oh this is just like just single boroughs of mala bromo mainly because the other atom in this molecule was hydrogen that's been added on to the end there okay so as you it's a little bit tricky when you've got a symmetrical molecules will kind of show that later on okay so what we'll show it now then right so we've got reactant hydrogen halide these are unsymmetrical alkenes so when we react these we produce two different products got to be really really careful this okay and this is a key area and you've just got to watch out for it okay so the amount of the two products is determined by the stability of the carbo cation intermediate remember we talked about that just before that was that middle part there okay so the more alkyl groups that are bonded to the carbon cation the more stable the intermediate is okay and you've got to think about this the middle bit and think about is it the middle stage of the of the full reaction and think well is it stable which one is more stable which one would form the most stable intermediate okay so this is because what we've got is alkyl groups remember these are things like ch3 ch2 ch2 parts to the molecule basically just alkyl groups it could be a nice Isle methyl doesn't really matter or anything these push electrons into the positive carbon caffeine this makes it more stable because it stabilizes the lead carbo cation and it means it's more likely it will form okay so you're looking for what's bonded to that carbon caffeine so let's have a look this one's a primary carbo cation okay the R group just represents an alkyl group again it could mean anything the red arrow just shows you where the electrons have been donated to remember these push electrons into the copper cation this is what we call a primary carbon cation because the carbon is attached to one alkyl group to roll-out primary and we've got secondary and then we've got tertiary so secondary has got two alkyl groups attached to the carbo cation this is called secondary and tertiary has got three alkyl groups now the one which is the most stable is this one because we have three alkyl groups all pushing electrons in to this area of to this carbon which has very few electrons because it's got this positive charge so this is going to be more stable so in terms of the reaction mechanism going via a tertiary our cation the products produced gone via tertiary carbyl cation are going to be more likely to form then go and via a primary carbo cation or secondary for that matter so really look out B or what type of carbo cation you produced the M the kind of CP if you can form a tertiary or a secondary the products via these are going to be more likely than viral primary so we'll have a look at this specific example so here we've got one here this is pro warning okay you can see it's an unsymmetrical alkene because obviously you can see the double bonds on one side of the molecule so this is pro pani and here's our hydrogen halide again okay it's got Delta positive Delta negative so what we're going to do is can do the same mechanism as what we did before okay so we'll do a bond goes into Delta positive breaks that bond the hydrogen adds onto the molecule there's the hydrogen there okay so it wasn't there before we've now added it now we've formed a carbo cation this is a primary carbo cation because we've only got one and east isle group effectively this whole bit is attached to the carbon the rest of the ma hydrogen so this is primary not very stable and it is a lone pair okay let's have a look at the other option okay so this is where the hydrogen now instead of adding there it adds there instead so there's the hydrogen now if you see the positive carbo cation is now in the middle of the of the molecule so now we have two methyl groups either side pushing electrons in this is more stable because this is a secondary carbo cation so basically the hydrogen can either add there or it can add there but the one which is going to give you the most stable intermediate is adding the hydrogen there and then you leaving your carbo cation there okay so that's what we've got so let's have a look at the first one bromine obviously adds on there we form one bromo propane we call this a minor product because we don't get much of it the intermedia isn't as stable as this version which this one you get to bromo propane which is a major product so you can see here that we've got two different products but you're going to get more of the to bromo propane because it's gone by a secondary carbon cation then one ball more propane which just IMing okay so let's look at another example so we looked at bromine we've looked at the hydrogen bromide now we're going to add another molecule this is sulfuric acid okay so alkenes they react with cold concentrated sulfuric acid and they form alkyl hydrogen sulfate okay so this is quite a big molecule so you got to be ready for this one right sulfuric acid is used as a catalyst in this reaction and this actually makes an alcohol from an alkene so and when you do the M alcohols topic you'll notice that there's a there's a specific reaction that you need to know which is going from an alkene to an alcohol and use sulfuric acid this mechanism is just showing you how the sulfuric acid acts as a catalyst okay this is quite quite detailed so make sure you kind of you know this effectively the back your hand suppose so this mechanism like to see it shows how it interacts with the alkene to form the intermediate and the intermediate is alkyl hydrogen sulfate so let's have a look so this is sulfuric acid this is the formula for it h2 so4 you've got to know that formula we've just put some polar charges on here on the oxygen and the hydrogen there's a lark in that we're going to use and we're going to use this to make an alcohol but we're going to use sulfuric acid as a catalyst so the mechanism is actually pretty much the same so it's not too bad okay so there we go there's the first step remember it goes from the double bond to the Delta positive and then the electrons move into the oxygen so if actually this hydrogen is now going to be bonded to this molecule okay so there it is that's the hydrogen we've just added on we have a carbo cation intermediate just as normal and then what we've got left is this sulfate or hydrogen sulfate group here hydrogen and then sulfate is so4 so 40 minus so we've got this negative charge because we've just nicked the hydrogen from the end of it lone pair on the oxygen and this is going to go onto here so we're going to have a bond or an hour all gone from here on to there so it is okay so it goes onto there and you form your Delta positive so and you can see that our intermediate is now formed this is called our alkyl hydrogen sulfate so this one is ether hydrogen sulfate because we've got two carbons and there's the hydrogen sulfate bit which is this bit at the bottom and so we're going to see obviously this is just our intermediate this is not an alcohol so we're going to see how we can take this product here it's intermediate and take it further to form your alcohol so let's have a look so the alkyl hydrogen sulfate they've just made in the previous step this has to be reformed the sulfuric acid sorry has to be reformed so we're going to take the alkyl hydrogen sulfate I'm basically going to add cold water to the efile hydrogen sulfate that we've used before and this will form ethanol we call this hydrolysis so it's really really important so we have a look on here there it is there's our alkyl or ether hydrogen sulfate we're going to add water to it there's no mechanisms are quite here we just form your ethanol there it is there okay and now obviously the Oh H has been added here we can get a no H from here but we also have a hydrogen that's left over from this water molecule and all the hydrogen does is just adds itself onto the oxygen when this breaks off and we form and sulfuric acid is remade and that's proof that this is a catalyst and we call this hydrolysis because we're basically the words and lysis means to break to break something up to cleave so hydro meaning water so all we're doing is were breaking a bond using water and you might see hydrolysis and quite a few times that's all it means so hydrolysis meaning breaking using water okay so here's the overall reaction look step one we took our alkyne reacted the sulfuric acid and we've produced our ethyl hydrogen sulfate which is this molecule here we then took the ether hydrogen sulfate we made we are added water to it so it's go to cold water and we form alcohol this is ethanol plus your sulfuric acid is reformed there's your catalyst okay so make sure you know how this works quite detailed and it actually overlaps of the alcohols topic as well so there it is sulfuric acid is reformed also and we using asymmetric alkenes we can also produce two products as well so just be aware of the types of products that we can produce just like with the showed you with the other one your minor in your major you can also get this here as well so just be just be wary of how this how this could could pan out obviously this one is a smash glanceable you can only get one product but you could get two with with an asymmetric Aki's okay alkenes are really useful for and plastics making plastics or types of polymers so alkenes are monomers okay so these are just single and repeat or these are just single units and we can join these monomers together to form polymers and it goes by an addition polymerization process so let's look at some examples of polymers and we have obviously made from monomer units these are things that make them up we have natural and synthetic on so natural polymers things like proteins and natural rubber and synthetics are polyethylene and polypropylene which obviously which we can make synthetically so let's have a look so we've got polymers of these been used for wild Charles Goodyear in particular and he discovered vulcanized rubber and basically all he did he added chemicals to a natural rubber and he made it harder wearing and which was ideal for tiles so I'm obviously we can still see Goodyear tires around em now but it all started from this this dis guy called Charles Goodyear I think he discovered by accident as well I think he was he was looking for it but I don't think he he purposefully actually designed the chemicals I think it was an accidental discovery but he did want to look for a stronger rubber and obviously in the last hundred years have been developed we've got plastics such as what bottles like poly thin nylon Teflon as this is the nonstick stuff you get and these polymers you revolutionize the way we live and we use plastics all the time and the likelihood is you're probably and sitting within and a few few meters of plastics you may even be aware in polymers and like synthetic polymers obviously we've got new polymers have been made all the time and obviously we've got things like electronic gadgetry touchscreen etc so these things have become an increasingly more used and obviously we need to look at ways in which it would be sustainable so we can recycle etcetera so let's have a look so you're to make your poly propene okay which is type plastic we've seen before we need a monomer propane and we need to add a few of these together and make polypropylene okay so there's propane okay I've drawn it just so we can see just so we pronounce the C double bond o basically what happens when we join these together the double bond opens up and we form our polymer now there's a few things to kind of notice here as well the N bit means that we've got a lot of repeat units and so that's quite important this is our repeat unit this bit here this is your alkene obviously being expanded and so the double bonds obviously not here anymore because it's broken and we now have these trailing bonds and these extend beyond the brackets so make sure when you're drawing your brackets that you draw your trailing bonds that extend beyond this okay so this double bond obviously opening up and this one's shown to repeat units you can see here these are two monomer units two monomers here and this is to repeat units notice we don't have the N here now because in the exam what they might ask you to do is just show you a specific number of repeat units this one's showing to put an N there suggest that you have loads them and that's not what they're after so they're after to repeat units in this case so yeah poly alkenes these ones here these are now saturated molecules normally nonpolar and because they're just full of carbons and hydrogens and so they're pretty stable and they're unreactive obviously this is a downside because this can now make it really difficult for them to degrade in landfill because they are so stable so they kind of make a victim of their own success I suppose and so it's important to be able to kind of recycle these or maybe reuse them or do something with them instead of putting them into landfill because that's not that's not brilliant it's not the ideal solution so let's have a look at some of these properties so we've got intermolecular forces and these govern the properties of polymers a bit like with simple molecular as well so most poly alkene chains are nonpolar okay so all they have is dis van der Waals forces between the chains remember the van der Waals forces are the weakest forces weakest intermolecular force okay so basically the longer the chain the closer they are they're there so the closer they can kind of pack together and and the more kind of surface air you've got for the van der Waals forces take a hold so basically the longer the chain the higher the melting point these chains which are shorter and have a lot of branching member branching means a compact together as closely they tend to be more flexible and they're weaker okay so they're not as strong as your longer chains which are more straight okay so yeah polymers with no or very little branching tend to beat em and long tend to be more rigid and strong as long as you can identify the properties of these and what it means in terms of the molecule and that's the main thing so you've got some player Keens and have halogens into a PVC which you might using drain pipes etc then and these obviously have a more stronger permanent dipole dipoles because you've got now you've got a polar part to it you've got the chlorine bit now which is creating a polar bonds and we can get forces stronger than van der Waals when we've got a dipole dipole so obviously these are going to make the plastic a little bit different and things like PVC is pretty useful in terms of outdoor wear so things like obviously like your drain pipes and guttering etc and it can it can weather storms you know it's it's really strong it's durable even when it's covered in water and wind and snow and frost and things it still generally lasts a reasonable length of time compared to other properties quite lightweight and it's cheap as well so it's useful things like covering and plasticizers right these are extra things which you can add to your your plastics and you can change the properties of them in plasticizers are a good way of doing this so what they do plasticizers make the polymers more flexible okay that's quite useful if you want to make things like and fabrics and for clothing or materials for things like bouncy castles etc you want it's more plastic so plasticizers slide between the polymer air chains and what they do is they push them apart so it makes them a little bit softer weakens the intermolecular forces between the chains and they can now slide over each other and you can bend the plastic a lot more a lot easier and which is can have useful properties for certain things so plasticizers normally like a say PVC you can add plasticizers to PVC and it can change the properties so let's have a look there's your chloro ethane ethene sorry there's a glory thing and then we've got loads of these ends we've got a number of them we join them together and with one Polly Chloe see okay so notice we still keep the Ian bit okay Polly just meaning more than one so we're joining them up and there is an example of polly chlorine okay now PVC and is made from long closely packed polymer chains like this and the hard but brittle and legacy reused in drainpipe serves pretty useful but it's used if you have a plasticizers same polymer exactly the same Polly we haven't changed polymer all done is added a plasticizer pushes the chains further apart makes it more flexible and this is ideal for things like I say clothing or electrical wiring seen see this is softer same plastic you wouldn't believe it and but it is it's the same plastic or we've done is added an additive to it and you can see they're clearly different properties to them so that's really useful for a chemist to be able to do that and that's it and that's the alkenes topic for AQA basically in a nutshell and so I hope that was pretty useful and there's two bits ready - this is your mechanisms are the most important things really they generally loads and loads of marks and this obviously the polymers bit as well because that's you use for alkenes is to make plastics and that's the whole point of it so yeah like say just finely em you can if you want to purchase these powerpoints you can use them for revision or supplement revision you obviously can if you click on the link in the description box and you'll be able to find them there but that's it bye-bye