[Music] hello my name is chris harris and i'm from alley chemistry and welcome to this video on hydrocarbons uh this is topic 14 for the cia that's the cambridge internationals course so if you're studying the cie um exam or the syllabus then this video is perfect for you and in fact there's a full range of videos for year one and year two chemistry all on alloy chemistry youtube channel just hit the subscribe button that'd be massively appreciated just to show your support um all these slides here these are actually just powerpoint slides and they are available to purchase if you want your own copies of them from my test shop they're great value for money and grit to kind of complement your revision material if you click on the link in the description box below you can get a hold of them there but obviously i'll kind of talk through the uh the main content of this okay so let's make a start um so obviously this is topic uh like say topic 14 and it's to do with um hydrocarbons so hydrocarbons probably the best place to start before talking about anything else is hydrocarbons obviously compounds which are made up of hydrogen and carbon basically which is quite a lot of molecules and obviously it fits within the organic chemistry topic um you'll find as well a lot with this particular cie topics um as the topics kind of blend into each other there's a lot of overlap you know they kind of rely on information from previous topics to know this one so make sure you're familiar with particularly the introduction to organic chemistry and as organic chemistry which is um the the previous topic which is um topic 13. um so many topics now um it's a topic 13 um to make sure that you understand the basics of what organic chemistry is and obviously the topics after this then kind of build up from that from that video as well from that topic so let's start with this first so this is an introduction to alkanes first so alkanes are cluster saturated hydrocarbons so what that means is um they have no double bonds basically there's no double bonds in them at all and they have the general form of cnh2n plus two now alkanes are hydrocarbons they obviously contain carbon and hydrogen only um they're saturated as i mentioned before and so there's no double bonds in there and they're actually bonded each carbon is bonded four times um and obviously produces um your alkanes as you can see there so you've got methane ethane propane as an example um you also get cycloalkanes as well um now that's basically they're still alkanes but you basically form it into a loop as you can see there so there's your kind of your cyclic compound so this is cyclopentane and all you do is you put the word cyclo in front of it and obviously how many carbons are in there that's five so that's pentane um now they have a slightly different general formula and that's cnh2n mainly because um two of the hydrogens are removed and to make it into a loop like this okay and cytokines like i say they're still saturated they don't have any double bonds in there at all so each carbon is bonded four times so it's still classed as a saturated compound all right so let's look at um where we can get these alkanes from um and kind of the properties of alkanes now alkanes are um well the vast majority of arcanes are actually found um naturally in crude oil so crude oil is obviously a natural gas for that matter so obviously this is what we can extract from the ground or from under the sea um and actually we can get the alkanes we can extract them from the ground and we can separate them out because oil has a mixture of different um hydrocarbons in there so each hydrocarbon has a different property so it's important that we can separate that out to get each individual um alkane or what we call fraction so like i say it's found in crude oil and there's a mixture of them and this is an example of a fractionating column so basically what happens is the crude oil is vaporized so once it's dug out the ground it's vaporized um in a furnace um and you can see it goes in here and it's heated to about 350 degrees celsius so crude oil has got a mixture of different hydrocarbons in there and then once it's been in there um it then enters the column so it goes in at the bottom here as you can see there and it rises through the trays so obviously it's it's very priced some of it will be liquid and some of it won't be but the longest hydrocarbons don't actually vaporize they kind of fall out of this as a sludge or as a liquid and kind of sink to the bottom of the column which is about 400 degrees celsius anything which is a gas will kind of rise upwards up through the column now the column has a temperature gradient so you can see it's getting cooler as we get to the top and as the vapor rises parts of the mixture condense at different temperatures okay so depending on when the um when the liquid condenses when the gas condenses back into a liquid and at what temperature it does that will depend on which tray it will sit at basically and the reasons for that is actually your longer chain hydrocarbons so the ones with say 15 to 19 carbon chains are um have a higher boiling point and so therefore they will condense first at 370 degrees celsius ones with shorter hydrocarbon chains and will actually remain um as it well will be a liquid a lot higher up so it'll condense at this point some of them won't condense at all even at room temperature and actually will still come out as a gas at the top um now obviously what we get this is quite useful because it means we can then um take off the different fractions these are called fractions that come off here so off here you've got 20 to 40 carbon uh carbon chains coming off here 15 to 19 11 to 15 5 to 12 and then one to four the shortest carbon chains come off at the top and your longest ones 40 plus kind of come off at the bottom here so what we're doing is we're taking a mixture of different hydrocarbons putting them through a fractionating column separating them out and we're getting different chain lengths so it's a bit like a big filtration system um so like say the gas will then come out at the top there and you'll notice we have different um uh different um products that are produced i suppose on the back of it so you've got natural gas which um sits at the top petrol kerosene diesel oil and fuel oil and then you've got um anything towards the bottom is normally things like asphalt so bitumen for example which is used to make tarmac um you might get wax paraffin wax etcetera coming out the bottom so let's have a look at some of these different uses and what we can actually do with them because it's quite a big industry the petrochemical industry so um obviously gas um comes off right at the top is used um in lpg that's liquefied petroleum gas and stove gas so if you have a barbecue if you have like a um you might have a gas cylinder that powers the barbecue and gas that comes into your home if you do have gas coming into your home so these are um fractions right that come right at the top of that column there and petrol is the next one that's pretty obvious that's just using cars kerosene is used in fuel jet fuel um so um has a slightly longer hydrocarbon chain a little bit more energy in it uh diesel oil is used in um trains normally ships so it's quite heavy oil um fuel oil there they go fuel oil is used in ships um so diesel is a bit thinner so a fuel oil is quite heavy so lots of energy in there obviously a ship can't be refueling all the time it's out to sea quite a lot you know quite a lot um and then obviously finally the the one that comes off right at the bottom is bitumen and that's used in roofing and um tarmac etcetera so all of these have come from crude oil and we separate them out to provide different uses and obviously that pretty much runs at the moment our day-to-day life we we rely heavily on fossil fuels to to do all these things so so you can see it's quite um quite useful uh uh useful product i suppose sometimes though some of them products it's a bit pot luck so you get your um you get your crude oil it's got loads of different hydrocarbons in there um and you might end up getting maybe let's go the extreme end let's see you get 80 percent of that fracture of that oil is actually tarmac or bitumen and the rest of the 20 percent is kind of the higher up fractions now the problem with that is um that might be fine if you've got a massive road building project and you need all that all that kind of tarmac to do that but in reality that doesn't really happen and obviously there's still a demand for fuel and petrol and things like that and natural gas so there's this technique whereby we can actually break down or cut bigger or longer hydrocarbon chains into smaller more useful ones to meet consumer demand and we call this process cracking so kraken is is basically as it suggests cracking is to crack or break a molecule apart into shorter more high demand and products so like i say obviously you've got fractional distillation is fine for separating it out but sometimes we want to try and produce products that serve a demand so your heavier fractions like um fuel oil and bitumen are in lower demand than your lighter fractions which is petrol which is likely to be in higher demand because we drive cars and lorries and trucks etcetera so we can take them heavier fractions and crack them and basically break the longer hydrocarbon chains into the shorter ones and here's an example so you've got um door deck in which is um a carbon with 12 a hydrocarbon with 12 carbons in there and we can break that down into two parts we can form deck in which is c10h22 so that's a shorter chain and we form an alkene as well which will look at alkenes later on um so that's c2h4 so there's two types of kraken you've got thermal and you've got catalytic kraken so obviously two are slightly different i'm going to look at each one in turn and look at the difference and why they're actually important so let's start with thermal cracking first so as the name suggests it's heat it's something to do with heat so high temperatures are needed for this type of kraken we need high pressures as well um so about 70 atmospheres of pressure so that's quite a lot of pressure and the products of thermal cracking are mainly alkenes okay so we don't really produce a lot of arcanes but we produce an awful lot of alkenes now alkenes are useful because we make plastics from it we make polymers from that and obviously propane is used to make polypropylene so um you know a really useful method but as you can see it's probably going to be quite expensive because you need a lot of heat and a lot of pressure so we have to make sure that if we are using this to make plastics then obviously you know it's worth it there's obviously a tendency to try and recycle plastics rather than you know make new plastics from raw materials because obviously these are finite resources but also because of the cost you know the cost of making new plastic is quite expensive as you can see so the other type of cracking is catalytic cracking now as this suggests we use catalysts here so high temperature and a slight pressure is used so the temperature is 450 degrees celsius um but a zeolite catalyst is used and that helps to lower the temperature needed now if you remember back in some of the previous topics where he looked at energetics and looking at using catalyst to reduce the energy required for um it obviously alters the pathway doesn't it for a reaction to happen it means less energy is required to get that going then catholic the crack you know catalysts helped to do that now normally the products of catalytic cracking are mainly what we call aromatic hydrocarbons and this is useful in fuels so um aromatic compounds are basically benzene rings now you won't see benzene ring too well you won't see it very you know quite quite rarely actually in year one but if you're doing the full a level and you're gonna do year two benzene you'll learn a lot more about benzene in year two so using a zeolite catalyst which is a bit like a um there's like a solid catalyst with loads of poles in there and that obviously lowers the temperature and the pressure so that lowers the cost it's more environmentally friendly and it speeds up the process however you're not producing alkenes which could be used to make plastics so it depends what you want from it as to which process is probably the best one to use okay so on the back of that so okay so we've got our alkanes we've extracted it from the you know from crude oil and we've cracked it to get the right ones we need to use it for something and obviously one of the users we'd seen before was the combustion of alkanes now we're going to look at reactions of complete combustion of alkanes to show what happens because alkanes are actually really useful for um obviously burning as a fuel and for heating homes and and workplaces but also for getting us around using you know fuel cars so um and they burn in oxygen completely so if you've got a plenty full supply of oxygen then they will produce two products which is carbon dioxide and water so alkanes obviously they're good they're good as fuels they produce large amounts of energy so they've got a good enthalpy change obviously they have to be changed quite high and obviously they're used to power vehicles and and factories and also for in terms of um for britain um we use um natural gas to generate electricity in britain although like i say there's a there's a tendency to move away from fossil fuels and use um renewables such as solar and and wooden farms so you know that will kind of um you know phase out over the next few years but as it stands that's what they're doing so an equation for complete combustion you will be expected to be able to write these equations so this is an example using butane as you can see here so you've got butane reacting with six and a half oxygen producing four lots of carbon dioxide and five lots of water you've got to be prepared to be able to balance these equations okay you always produce carbon dioxide and water when you have a complete combustion of alkanes now in reality obviously you don't always get that it's rare to get complete combustion um we obviously tried to strive to it but it is feasible to suggest actually that most fuels don't burn completely and there is some incomplete combustion related to it and this is when it gets a little bit more serious so when you've got alkanes burning with a limited supply of oxygen in other words there's not enough oxygen there then we produce a poisonous gas of carbon monoxide and we produce carbon which is soot and obviously this happens when you get incomplete combustion so let's have a look at a reaction here showing um showing obviously showing this so incomplete combustion of butane producing c or and or co2 so there's your butane reacts with oxygen to form carbon monoxide and water so carbon monoxide is poisonous but also it can produce a mixture of carbon monoxide and carbon dioxide and this just shows you an example of that equation it's really important again when you're writing these out make sure you balance the equations now in the question they will tell you if it's incomplete combustion which products have been produced obviously carbon monoxide is invisible it's odorless you can't smell it or taste it and but such you can so if they're talking about soot then it's just carbon if it's talking about soot and carbon monoxide then you need to include both on this side so it just depends what they're asking for but the principle is it's the fuel with the oxygen because you need oxygen there and then whatever products are produced whether whether it's incomplete or complete you will always produce water though okay so obviously like we said carbon monoxide is poisonous um effectively it binds to the hemoglobin in your blood it prevents the um red blood cells carrying oxygen around your around your um obviously around your body um and obviously you know that effectively leads to um you know your body shutting down it's it's toxic it's really serious and so that's why if you have a boiler normally a gas boiler you would normally have that in normally you'd have that in a garage somewhere or you would have it away from the house sometimes you might have a gas boiler in a bedroom for example ideally it's not brilliant so what you would do is had to have a carbon monoxide alarm near that so that if there was carbon monoxide leaking from the boiler because it was burning in completely then um you know it would wick anybody up who's in that bedroom so they're really important um we can remove carbon monoxide though by using a catalytic converter um and obviously um catalytic converters are fitted into cars um as you may have seen before um so that's assuming quite important mo we'll look at the impacts of this as well the burning fuels on the environment for this topic so the incomplete combustion of butane so this is producing soot and i've just put the reaction there just to kind of give you an idea of what it looks like and obviously sort isn't great either and it causes breathing problems obviously it's it's basically what you know what sort is it's black um and it clogs up um you know and clog up engines it can stick inside of engines etc um so it's not great so you can see here really if we are burning fuel we want to try and avoid or try and limit the amount of incomplete combustion that's occurring right so um let's look at kind of going back and saying right well how can we remove some of these um harmful gases and you've seen these kind of littered around these examples come in in and out of quite a few of the topics with the site with the cie syllabus so you might have seen this already um but you've got when you burn when you burn a few within a car you actually produce um um other gases as well so one of them is nitrogen monoxide so nitrogen monoxide is created by reacting nitrogen and oxygen in the air that naturally occurs in the air um it is um obviously it gets pushed through your engine the engine is obviously hot um the spark plugs in the engine as well um if it's petrol of course glow plugs if it's a diesel it doesn't matter it's the same thing and that basically forces nitrogen and oxygen that's in the air anyway to help burn the fuel to bond together and form nitrogen monoxide as you can see on there as well as carbon monoxide as we've seen before so nitrogen monoxide as we said before um the nitrogen monoxide is obviously harmful it will come onto that in a moment and where that where that fits in um but cars are fitted with catalytic converters and they react with the carbon monoxide and nitrogen monoxide um before it actually leaves the leaves the exhaust and calorie converters they use a rhodium platinum and palladium alloys it's a mixture of different alloys in there quite a precious metal um all of them are the very expensive metals and basically they help to convert carbon monoxide and nitrogen monoxide from burning hydrocarbons in a car to carbon dioxide and nitrogen and obviously this is fine to release in the atmosphere this isn't great because obviously it's a greenhouse gas but it's probably better than carbon monoxide or nitrogen monoxide leaking into the atmosphere okay so let's have a look at some of these then obviously when you burn a hydrocarbon you do get hydrocarbons which don't burn fully so ideally in a car or any type of engine you would burn all the fuel but in reality that doesn't happen and some of these unburnt hydrocarbons and new oxides of nitrogen as well that come out of the exhaust do contribute to photochemical smog at lower levels now ozone is a bit of a kind of creates a bit of a problem here so ozone occur occurs at the lowest level of the atmosphere so we call that the troposphere and this exists as um obviously a photochemical smog exists is when you've got sunlight you've got hydrocarbons and nitrogen dioxide and they mix to form that ozone and a lot of the hydrocarbons and nitrogen dioxide actually come from cars and factories obviously from burning them now the real problem with that is that when you get solid particles and ozone mixing so these are unburnt hydrocarbons then we create this photochemical smog and this one is an example of photochemical smog in shanghai um you can see it's very misty and obviously it's not very good you don't want to breathe that in because it's it's it's not good for your lungs since it damages your lungs isn't it so um now the the key thing here though is that um ozone in its own right is also toxic to humans we shouldn't be breathing in ozone either um and ozone should exist kind of well out of the kind of remits where we breathe um and unfortunately um it can be created through the pollution that we make um and obviously put into the atmosphere so you know we need to be aware of these and what what the impact of it is the other kind of environmental side is acid rain obviously when we burn fossil fuels and fuels naturally contain sulfur in these fuels and then when you burn the fuel you obviously burn the sulfur and sulfur reacts with the oxygen that's in the air that creates sulfur dioxide as well as your oxides of nitrogen and that then drifts up into the atmosphere reacts with the um obviously reacts with the water to create acid rain and you can see in the picture there acid rain causes a lot of damage to the wildlife obviously to plants um and to erosion of buildings and obviously aquatic life as well in the rivers and obviously like i said they do contain these impurities and they produce sulfur dioxide now that sulfur dioxide is acidic obviously it reacts with the water in the atmosphere and it produces sulfur sulfuric acid nitrates or oxides of nitrogen to this air they do react with water as well and they form nitric acid so you've got a couple of things going on here now um obviously your your nitrates we've seen how they've how they're created it's nitrogen and oxygen in the atmosphere when they come into contact with hot engines because of combustion then you form your oxides of nitrogen okay and they form nitric acid so there's a lot of different um environmental considerations to take it's really important that you're aware of the different elements of burning fuels and why we get them and what the impacts are on the environment as well so ways in which we can reduce um the levels of so2 and no oxides of nitrogen um normally um it's probably more prominent in manufacturing facilities where they may produce some of these harmful pollutants because they're burning fossil fuels is we use um flue gas scrubbing basically so scrubbing is basically where we use wet scrubbing um it's basically where you um you have your acidic gases going through a um say a flu for example so a chimney um and then what you do is you fire an alkali at the acidic gases and nuts designed to neutralize some like sulfur dioxide and your oxides of nitrate oxides of nitrogen as well and effectively the wet scrubbing the alkali is calcium carbonate or we can use calcium oxide and we dissolve that in water make a solution that's alkaline spray that onto the acidic gases and and the idea is that we can then use that potentially use the byproduct and and um you know sell it perhaps or use it for something else or dispose of it safely but we can use and you'll see later on in in the year one topic infrared spectroscopy and that can be used to monitor pollution levels by identifying bonds in a pollutant molecule and you'll come onto that a little bit later on like i say in a later topic i think that's topic 22 i believe so that's the um that's the um analysis topic the analysis bit and obviously we can use we can use infrared to measure how much pollutant is actually present in the atmosphere and that's how scientists know about the the levels of these pollutants in the atmosphere and what can be done and i know again in some areas of the uk you have congestion charging zones particularly in the capital and some of the cities as well um and it's really there to try and reduce the levels of these pollutants in hot spots so areas where there's high traffic okay so we're going to look at and you would have seen this before and you'll see this come up quite a few times again you'll probably see it in the halogens topic as well and so that um hilo alkanes topic um later on topic which i believe is topic 15. but it the this topic it comes in here basically because it's an alkane and alkanes and halogens kind of form the same type of um same type of kind of category so i'm going to briefly go through this um but effectively we can use an alkene and react it with a halogen and to form halogenoalkanes now there's a lot more on halogenoalkanes in like saying topic 15 so this is really looking at the alkane bit first so there's three stages to forming a halogenoalkane um and we use this through free radical reactions and the reaction itself has three stages you've got initiation propagation and termination so initiation this is where radicals um effectively are produced we use uv light and we use that to effectively break a bond homolytically and we produce two radicals on the back of it so we call this a photochemical reaction and once we've got the radicals the radicals will then react with non-radicals and we call that propagation stage so that's basically where the main reaction happens you get these chain reactions that keep on reacting and producing new products and then you obviously you obviously have a a phase where some of the reactions have to come to an end at some point and we call that a termination stage and this is when you have two radicals that were produced um in the initiation and say propagation stage that collide together and that ends the reaction so it terminates it so let's have a look at a specific example and here we're going to look at obviously making halogena alkenes so chloromethane we can make chloromethane using this reaction here so you've got methane reacting with chlorine and that'll form chloromethane and hydrochloric acid so the first step is the initiation step and this is when you've got sunlight or uv that breaks the cl bonds so that's in chlorine and this is a process called photodissociation so basically using light to dissociate to break a bond now because it's homolytic efficient basically the electron in the bond jumps one way and the other one jumps the other way so we get an equal split of the electrons then we form two radicals which are incredibly reactive now the propagation stage is actually where the chlorine radical here um is actually used to make a methyl radical which is ch3 dot and then that methyl radical reacts with more chlorine that hasn't been split um to make chloromethane and under the cl dot radical is produced and then that radical can then react further so let's have a look at the first example there's your chlorine radical there reacting with methane as a molecule and that'll produce your methyl radical with the dot there and hydrochloric acid so a new radical has been formed so effectively the hydrogen from here is now part of the chlorine and then you're left with the ch3 dot and that's ch3 dot this radical can then react with chlorine that hasn't been split and that will form your chloromethane which is there and a chlorine radical at the end and then that radical can then go back to react with more methane to form more methyl radicals and then that will form more chloromethane so this reaction will keep on going as long as you've got a radical and a non-radical that's the propagation stage there will come a point when this reaction will stop we call this termination and termination is where you've got two radicals that collide together and they make a stable non-radical molecule so for example classic example is here so you've got your methyl radical reacts with the chlorine radical and that will form chloro methane as you can see on there so um you get if there's loads of cl dots so there's loads of chlorine in here then you will get di tri and tetra the alkanes and so you get loads of them um and if not you mainly get this single substitution here so effectively we've substituted the hydrogen and methane for a chlorine if you have loads of chlorine knocking around there let's see if we've got large amounts of this compared to methane then you will get dichloromethane trichloromethane tetrachloromethane you'll probably get multiple substitutions if not if you've got a good amount of methane in here and not as much of this then you're more likely to just get this product here okay so let's have a look at the other side of it and this is alkenes so we've looked at arcanes and they're basically just saturated molecules they have no double bonds in there at all alkenes on the other hand are unsaturated hydrocarbons and they do have a double bond so um the general formula for an alkene with one double bond is cnh2n okay so alkenes these are hydrocarbons and they which means they do contain carbon and hydrogen and only just like an alkane and they're unsaturated it means they must have at least one double bond some alkenes can have more than one double bond and the types of reactions they undergo as are generally addition reactions because you can add extra molecules to that double bond and we'll look at some of the reactions later so let's have a look at the first one here for example you've got ethene as an example or this one you've got four carbons this is buta one three diene so basically you've got two alkenes here and they're on the first and third carbon here so it's diene and double bonds they have a really high electron density this makes them alkenes are pretty reactive certainly a lot more reactive than alkanes so with alkanes to get them to react you normally need uv light you need to create very reactive radicals with alkenes you don't need to go to that level of extreme reaction um cycloalkenes and so they have two height two fewer hydrogens than their straight chain versions so these are straight chain versions here obviously you you basically take taken two hydrogens away so this one is called cyclopentene as an example so very similar to what you had with psychoalkanes okay so um let's have a look at some of these reactions then so as we mentioned alkenes kind of undergo addition reactions and they're attacked by electrophiles okay so electrophiles are electron loving species or these are um species which will target areas where there's a high level of electron density or they've got a delta negative or negative charge um now alkenes have a good level of electrons because they've got a double bond and obviously this double bond makes it attractive to electrophiles now what electrophiles do is they will add to the molecule um but electrophiles are electron pair acceptors so remember nucleophiles are the opposite so they have to have a lone pair of electrons and they donate it to an atom when they're reacting um with electrophiles they accept electrons so they're deficient in electrons themselves and are naturally attracted to the double bond so an example an electrophile example are positive charged ions so you've got no2 plus for example h plus examples of electrophiles um you might have polar molecules um hbr can be an electrophile h2so4 can be electrophile so they don't have to have a proper positive charge they can have a delta positive charge here so obviously the hydrogens on here are delta positive so all electrophilic addition reactions um obviously they have a curly arrow and you will see more of these curly arrows as well right the way through organic chemistry so this is basically curly arrow is is um basically called a mechanism so mechanism is basically a diagram to show chemists what happens or to show people what happens in a reaction and an arrow represents the movement of electrons so arrows must go from where electrons are and where they're going to so in this case for an alkene the electrons are actually sitting in the double bond as you can see there so there's a high density of electrons and the arrow moves from the double bonds into the electrophile which is here and we're going to use that as a generic example which is e plus um and basically the electrons move into there so when you're drawn these mechanisms and we'll see more of these later on you'll see that's the reason why i've drawn it from there to there okay the general rule is we're not general you must draw arrows going from a place where electrons are and where they're going to and obviously all the electrons are in the double bond here there's a high density in there anyway okay so let's have a look at a some of these reactions of alkenes um now some of these can actually double up as tests for alkenes as well and you might have seen this one actually where you can test for alkenes by adding bromine water to it and it'll decolorize bromine water so what we're going to do here is look at the reaction what actually happens when we add bromine water to alkenes so um let's have a look here so decolorization of bromine water as i mentioned before so adding bromine water to an alkene causes a color change and this has gone from brown bromly orange to a colorless solution if an alkene is present so bromine is obviously brownie orange it's the electrophile so that's the electrophile and this adds to the alkene forming dibromoalkane and diver armor yep start again dibromo alkane is colorless get me words out right so let's have a look then so we've got an alkene here and this is ethene so and then we've got your br2 here this is your bromine molecule now bromine um has an induced dipole on it so um it's got a delta positive and delta negative again you would have seen this in the um bonding topic i think it's topic three i think um when you look at structure and bonding and we looked at van der waals forces to induce dipoles so this dipole in bromine bromine doesn't normally have any polarity at all but it will have a polarity when it comes close to another molecule which can kind of pull electrons or push electrons away from it and this dipole exists when it's near something like an alkene so the br2 is polarized okay so the double bond here is basically pushing the electrons that we're sitting here over onto this side of the molecule so that's basically a van der waals reaction but this dipole is created now the electron pair in the double bond here is attracted to the delta positive bromine and this forms a bond okay and it starts to break this double bond across so let's have a look there it is okay so electrons going from here on towards this uh delta positive bromine now this is the effect we start to form a bond with bromine which can only form one bond so this bond has to break okay so because obviously bromine can't have two bonds so it breaks that bond and basically sends it on its way so what we form is this carbo-cation intermediate so the bromine that's that was here is now on the molecule here we're now left in a position where this carbon doesn't have enough bonds because it's broken that double bond there so we have this positive charge that's attached to this carbon thankfully we've got this br minus that was unceremoniously kind of booted out of this kind of relationship here um and now it's floating around on its own with a lone pair of electrons and a minus charge you probably might spot this and think ah hang on you've got a positive and a negative charge here so these are going to attract aren't they you'll be absolutely right so these electrons from this br minus jump onto that positive charge because obviously there's a strong attraction there between them and then what you form is colorless and one two dibromo ethan and you can see obviously that reaction there so this is effectively brown this bitty brownie orange that's colorless and when we add that to that we form this product here which is colorless and that's how we can say it's basically a test for alkenes quite smarter right so um let's have a look at some other reactions so we're going to look at hydration of alkenes something we all should be doing in this hot weather um alkenes need water as well they don't really but um this reaction is obviously as it suggests joker decide is hydration it means adding water to something so basically we're just adding water to alkene and we're going to look at the reaction here so alcohols are produced when we hydrate an alkene it's quite useful so um we can use steam and we use an acid catalyst to get this reaction um going and we can make ethanol by reacting steam and ethene together with phosphoric acid catalyst and we have a temperature of 300 degrees celsius and a pressure of 60 atmosphere is needed now this would have been um quite a common um method of producing um say ethanol or methanol for example and actually this was obviously throughout the pandemic there was a mass production of alcohol gels um you know because alcohol is good at killing viruses and bacteria it's a good sterilizer and so this method is would have been one of the methods that would have been kicked into action by large chemical firms to produce mass-produced um alcohol gels and cleaning products and this is one of the reactions that would have had to have done so um alkenes there we are so we've got ethyne in this case reaction with water which is steam um and then what we can then do um is then obviously react them together with the um with the acid catalyst and we produce ethanol um it's a reversible reaction um with an initial yield of only five percent that's not pretty it's not very good is it so any unreacted alkene is recycled through and the overall yield can actually be 90 to 95 so we can basically just keep reacting it through so it's quite an efficient reaction so you can see why it's quite popular okay so let's have a look at another one and now this don't get this confused with um obviously that was hydration of an alkene this is hydrogenation of um of an alkene and hydrogenation is adding hydrogen okay so they come with all these kind of jazzy names for it so don't get it confused with hydration hydration and hydrogenation different things so hydrogenation we're adding hydrogen to a double bond now we're going to follow the same reaction mechanism of what we've seen before so we've got this delta positive delta negative um polarity in hydrogen now normally hydrogen doesn't have this polarity it's induced again so it's a van der waals force so this polarity only exists because this molecule here is coming close to this one which has loads of electrons in it and the electrons in this molecule that were evenly spread are now shoved to kind of one side of the molecule and it creates this dipole so what happens just like what we've seen before is the electrons in this double bond will actually go to the delta positive hydrogen in um in the h2 molecule so let's have a look there it is okay so this will break this double bond obviously hydrogens form a bond here so this bond will have to break as well so we basically break that one and then what we're left with is an intermediate now this intermediate is you know just like what we've seen before there's no difference there and look what we're left with we're left with a h minus this is a hydrogen with two electrons it's outer shell a delta positive carbon a carbocation and lo and behold this is definitely going to attack that because they've got it's just too much of an attractive force for them and then what we do is we form ethane so we're forming an alkane from alkene now this might be quite useful if you if you have surplus alkene you need to make a fuel again so it's another reaction of doing that and this is called hydrogenation of um alkenes okay so let's have a look at another reaction so again we're going to look at alkenes but we're going to add um um halogens to them to form a halogenoalkane so remember we looked at the reaction where he took an alkane to form a halogenoalkane so remember you had to use uv and incredibly reactive free radicals just to produce a halogenoalkane with alkenes you don't need that extreme kind of radical reaction to do that um because alkenes are more reactive than alkanes anyway they've got this double bond it's like a ready-made kind of reactive spot i suppose so here we're going to look at an example of hydrogen bromide so hbr and it's going to follow the same mechanism as the addition of a halogen it doesn't make any difference because they might ask you to do hbr a hydrogen halide or it might be just br2 or you know br2 cl2 etcetera so we'd seen obviously the bromine and the bromine reaction uh we've seen that already with alkenes this is using hbr so let's have a look so hbr the difference with hbr and just normal br so br2 um is that hbr is actually a permanent dipole so there's no induced polarity here so it's already polarized already so it's already kind of teed up ready to go and you've got the double bond loads of electrons in there the electrons are going to jump and move towards this delta positive hydrogen so you can see there's loads of familiarity here i know it looks so there's a lot of kind of reactions but they're all following the same pattern aren't they so it shouldn't be too bad and then the electrons jump from the bond the hbr bond onto the bromine and then we form this carbocation intermediate again just like what we've done before here we have the br minus and the c the carbocation the c plus they're obviously to attack each other because they're obviously very attracted to each other attack sounds so brutal doesn't it they like each other um and bromo ethane is formed and here we are there's your broadway thing that's made so you can see these mechanisms are very very similar we're just using different reagents but the the principle in the process is actually the same with all of them okay so this is where it gets a little bit more complicated here okay and we need to kind of have a brain switched on and it'll all be switched on already you're probably shouting it is but um you need to have your brain switched on for this bit because it gets a little bit weird at this point you get different kind of nuances and tweaks here so addition of hydrogen halides i've seen the reaction there now when you react hydrogen halides with an unsymmetrical alkene it produces two different products just when you thought you were getting it okay so so the amount of the two products is actually determined by the stability of the carbocation intermediate so remember when we show them reactions you have the c plus you have that intermediate in the middle now that can be stable or not very stable now the principle which governs the kind of stability of that carbocation is very much based on how many alkyl groups are bonded to that carbocation and the more alkyl groups you've got bonding to that the more stable that intermediate is going to be okay so let's have a look so alkyl groups are remember they're just like um alkanes so there might be ch3 groups there might be ch2 ch3 groups that's all an alkyl group is and what these do is these actually push electrons towards the positive carbocation and it helps to stabilize it so it's a bit like having um i don't know it's a bit like having a the carbocation is very unstable so it's a bit like a ship that's got holes in it and if you've got methyl groups they're like people who are on the ship helping to try and patch them holes up the more people you have the more stable the ship is likely to be because you can stop it from sinking so let's have a look at this in terms of a chemical um version so here we've got a primary carbocation so the carbocation is bonded to one methyl group and you can see it on there okay so the red arrow um basically denotes the um the kind of donation of electrons towards this carbocation which this is the unstable bit here and the obviously represents the alkyl group now you've got one here if you had a secondary carbocation you've obviously got two of these groups pushing electrons into this carbocation which is going to kind of create a more stable intermediate and a tertiary one is going to do that even more so this if you can form this type of intermediate that's the best outcome for a um a reaction to happen so anything which goes via this intermediate is going to be more likely to happen than by going via this intermediate okay um so obviously because i know we mentioned this before but in molecules they're incredibly lazy okay molecules always want to try and follow the lowest energy path possible and why not you know that makes it makes it all right doesn't it but so these molecules here obviously this is the most stable so that if you have an option of going by this way it's just gonna go by that way isn't it if that doesn't exist or that can't be formed then it will go for the next stable so let's have let's actually put some kind of specific examples to this okay so here we've got probe one in okay so this is um definitely an unsymmetrical alkene because the double bond is obviously on the right hand side of the molecule rather than on on the left so it's definitely there's no symmetry there so here's our hbr here delta positive delta negative on the bromine so we're going to use the same reaction mechanism as what we've seen before yeah so we're going to obviously go electrons move to the delta positive hydrogens that breaks that bond and we form this intermediate here okay now look at the carbocation here this carbon has only got one um carbon bonded to it so it's a primary carbocation so this is pushing electrons into that carbocation and that's it now if we go the other way okay and let's say actually instead of the hydrogen here adding on to that carbon there how about we add this hydrogen onto this carbon on that side okay and then what we do is we create the carbocation in the middle and now all of a sudden we've got two methyl groups pushing electrons into this carbon here that is a secondary carbocation and is more stable so if it's going to choose if this reaction is going to proceed either by this or this it's obviously going to choose this route isn't it because it's more stable so let's work out the reactions so obviously the electrons from here jump on there and it forms one bromo propane or the second reaction forms two bromo propane now this is a major product you're gonna get more of this being produced than you are of this one and this kind of leads into this um this rule called macarnikoff's rule and mccarnickoff basically came up with this principle where he said the major product i this one because you're gonna get more of this being produced um is when we add a hydrogen halide to an unsymmetrical alkene is where the hydrogen adds to the carbon with the most number of hydrogens already attached to it so let's go through that so here's your hydrogen here okay now it is gonna obviously the major product is formed if we add that hydrogen on this carbon because that's got two hydrogens already attached to it rather than that carbon which has only got one hydrogen so mccarnikoff's rule basically said that um the most stable version is where the hydrogen here adds on to a carbon that has the most number of hydrogens which is going to be this carbon here and that's exactly what's happened down here so that hydrogen has added itself onto the carbon with the most number of hydrogens and so therefore we form this major product of two bromopropane so that's actually called mccarnickoff's rule so there we are try and say that one quick you get all get here all sorts of exotic names of people don't you and different reaction types so yeah so basically any as soon as you see an unsymmetrical alkene and you're adding hbr hydrogen heli to it just think of this fella here all right okay so um let's have a look at some other reactions as well let's have a look at the oxidation of alkenes you see you didn't think you could do a lot with alkenes you can do a lot with it can't you um it's a bit like i like to see alkenes a bit like brown sauce you can kind of add loads of you can use brown sauce with loads of different things like bake and sandwiches sausage sandwich um i don't know a burger anything like that you know so you can use that it's quite versatile so what we're doing here is basically saying well what can you add brown sauce to so this is basically right well you've got alkenes what can we do with alkenes and you can do loads of different things of it so you just need to know them that's all um so alkenes they can be oxidized um obviously using acidified manganate ions so that's the agent that we use the oxidizing agent however the strength of the oxidizing agent whether it's concentrated or dilute and the temperature of the reaction matters okay this actually has a big um a big determining factor and you need to know both of them okay so we in this example we're going to use cold dilute acidified manganate ions okay so it's going to be a cold condition and we're using dilute acidified manganate ions okay so let's have a look so you've got your alkene okay we're going to react it with water okay and we're going to obviously have your oxidizing agent now when you do this when you use an acidified manganate that's cold we haven't heated up the oxidizing agent then we get um a diol okay so we get one two dihydroxy ethane so we form a diol here so effectively we're adding water and a catalyst to form your diel which is there so under these conditions there we are we produce two oh groups and we call them diol so just be aware of that okay so let's do the same but this time we're going to use hot concentrated acidified manganate ions instead it's the same reaction but let's see what we get now so you've got a hot concentrated oxidizing agent this is powerful enough to break that c double bond c in alkenes that's pretty powerful so we actually form a variety of different products just to kind of complicate things a little bit further however it is dependent on what groups um represent r1 r2 r3 and r4 in the diagram okay so you can have loads of different things attached to here so these are just generic groups that we can attach to it so the r groups could could be any alkyl group that should be b could be any alkyl group or a hydrogen for example so it could be a methane it could be a hydrogen for example um so and this formula essentially represents all possible alkenes so these are all the different alkenes that it could be it's a very generic molecule okay so the products of these reactions can actually help us to identify an unknown alkene which is quite uh which is quite useful so let's have a look at some of these reactions so the c the the c c double bond breaks and two molecules with a c double bond o group are formed and we call that a carbonyl group so you see in everything here aren't you so a c double bond o that's called a carbonyl group so here we are so we've got an alkene which is here so we've got r1 r3 and r2 and r4 we add our oxidizing agent and it will form this okay so you've got a carbonyl group and another carbonyl group here basically what we've done is we've split that bond in half and we've put an oxygen on on both sides of it now this is only if it's using hot concentrated okay magnetic ions so if r1 r2 r3 and r4 are alkyls so in other words hydrocarbon groups then we make a ketone okay and if one of them is a hydrogen then we form an aldehyde so here you might have r1 and r3 now if this if these are two alkyl groups here we form a ketone okay so that would call it ketone if one of them is a hydrogen the other one can mean i'll kill it doesn't matter then we form an aldehyde now you're going to see a lot more of these later on um in in a future topic beyond this and so you will kind of learn about this a little bit more and but you are expected to know these and what these are a little bit more detailed so you will see them later on so if an aldehyde is produced um this can be oxidized further to a carboxylic acid so if one of these is an aldehyde so let's say if we had a hydrogen and uh say a methyl group here so that would make it an aldehyde then we can oxidize that further to a carboxylic acid so we can produce another product as well so you can see you get a right mixed bag with this type of reaction here again you're going to see a lot more of this in in a later um a later topic as well let me try and find out which topic it is it's topic 18. so if you if you're interested go to topic 18 and that has a little bit more information and it's also topic 17 as well so 17 18. okay so if we have a situation though where both r groups are on um both r groups on the same side of the double bond for example r1 and r3 then we form a different product so here's the kind of here's the kind of molecule here so let's have a look at this red box so in this situation an aldehyde is formed which is methanol which is there however this is easily obviously oxidized further and we can form carbon dioxide and water so this is the situation where both r groups are on the same side of the double bond okay so from the same side so if these are basically identical and then we form this product here so you can see your two hydrogens there and then this molecule here can then be used to produce carbon dioxide and water so in other words if that and that is a hydrogen and not an alkyl group then we can produce this product and we can oxidize that and that will form carbon dioxide and mortar so clearly if we do have um an alkene with two hydrogens on this side and we put it through this reaction here and we get a gas produced then we know that on one of the carbons at least one of the carbons in the alkene has two hydrogens attached to it on the same side of that double bond if it didn't you would never produce carbon dioxide and water and that's how we can use this type of reaction to help identify what an alkene is okay what type of alkene we have so you can see it's quite tricky there's a lot of information here so the best thing to do is to practice of course okay so that's it then so that's the end of um this topic for hydrocarbons which is topic 14. um hopefully that was helpful hopefully it was useful like i said the full range of a level um topics for the cie is on hollywood chemistry youtube channel please hit the subscribe button to show your support that would be great um also these are available to purchase if you wish i've bundled them together into organic topics but you can buy the the full range for year one and year two it's great value for money you use it as part of your revision the link is in the description box below and from the test shop um right that's it then um i hope that was useful that's it bye bye