welcome to another episode of Terry's notes today we're going to look at alkanes and [Music] alkenes the general formula for an alken is CN 2 CN h2n + 2 alkenes are saturated compounds they have carbon to carbon single bonds present in them now the first member of this homologous Series has one carbon atom and if we use the general formula well we would not write the one so we put c h and it'll be 2 * 1 + 2 which is 4 so that's how we got C 4 and the displayed formula is written as this a carbon with four hydrogen atoms around it the next member is ethane ethane has two carbon atoms so formula will be C2 H if use the formula it'll be 2 * 2 which is 4 + 2 which is 6 so that is how we got C2 H6 therefore we have two carbon atoms and six hydrogen atoms propane has three carbon atoms so the formula will be c 3 H use the formula it will be 2 * 3 which is 6 plus two which gives us8 so that's how we got this and we have the displayed formula will be will look like this we have three carbon atoms and eight hydrogen atoms and the next member is buttine which has four carbon atoms this is written as C4 H if we use the formula it'll be 2 * 4 which is 4 2 8+ 2 which gives us 10 so that's how we got C4 h10 and therefore the displayed formula will look like this it has four carbon atoms and 10 hydrogen atoms so what you need to know is the general formula which is this you need to know that alkanes are saturated compounds and we need to be able to draw the display formula of the members of this homologous series we also need to know the reactions of alenes the first reaction is combustion alkanes burn in oxygen with a clean blue flame to produce carbon dioxide and water so for this example we have methane methane plus oxygen gives us carbon dioxide and water the second reaction we're going to look at is halogenation alkanes react with chlorine gas in the presence of UV light this is the key thing to remember here this reaction takes place in UV light to produce chlorinated alkanes the reaction is called a substitution reaction right remember I said alkenes undergo substitution reactions the hydrogen atoms in the alkan molecule are replaced with chlorine atoms and several different products could be produced because it is possible to replace all the hydrogen atoms in the alkan so if we look at one possible reaction we can have ch 4 plus cl2 and we can get C Tre CL plus HCL so what we've done is replace this hydrogen atom with one chlorine atom further substitution could take place in which case we can get so these are several possible reactions that can take place I'll just draw the structure so you'll see what is happening so if we replace all the carbon atoms in mean we get this last compound here which is tetr chloromethane right or ccl4 so the key thing to note here is that when you react an alkan with chlorine in the presence of UV light you can get various products and the reaction type is a substitution reaction you need to be able to give some examples of alkanes alenes are used as fuels when an alen is burnt in oxygen termal energy is produced right the reaction is exothermic butane and propane are used for cooking alkenes are also used as petrol for cars some alkanes are used in the manufacturer of other compounds methane for example is used to manufacture ammonia now let's look at a next homologus series called alkenes now the general formula for alken is cnh h2n and alkenes are unsaturated compounds they have carbon to carbon double bonds and alken undergo addition reactions so when we compare an alken to an alkan we notice that it's unsaturated it has carbon to carbon double bonds and it under goes addition reactions which is different to an alken so the first member of this Series has two carbon atoms because we need to have a double bond so the formula will be C2 H if we use the general formula it'll be 2 * by 2 which is four so the formula is C2 H4 and we draw two carbon atoms with a double bond and we end up with this displayed formula keep in mind that carbon can only form four bonds around itself so if you look at this carbon atom here we have one two three four carbon four Bond sorry the next member of the this group is has three carbon atoms so it'll be C3 H if we use the formula it'll be 3 * by 2 which is 6 so that's how we got this and this is the displayed formula the next one is but one in which is it has four carbon atoms so it'll be C4 H if we use the formula it'll be 4 by two which is 8 so we got this and this is a displayed formula now if you notice we have a double bond this is the double bond here we have a double bond here we have a double bond here all right so if we are given um a displayed formula we can identify if it is an alken or not now we look at the reactions of alkenes in the case of combustion when an alen is burnt in oxygen it burns with a smokey yellow flame to produce carbon dioxide and water now the flame is Smokey because of the higher proportion of carbon in the molecule and example if we have ethine and we burn it in oxygen we get carbon dioxide and water which is similar to the reaction of an alken next type of reaction we need to look at is hydrogenation alkenes react with hydrogen gas in the presence of a nickel Catalyst at a temperature of 200° CSUS you need to be able to recall the reaction conditions so it's a nickel catalyst and a temperature of 200° C so this example we have e again so etin plus hydrogen gives you ethine all right if we draw use the displayed formula we have C2 H4 plus H2 you have your Catalyst and high temperature of 200° C and we get ethane so this is one way to convert a alken into an alken so you have an alken here and you have an alken the next type of reaction is called halogenation alkenes undergo addition reactions with halogens now when we speak about halogens we are referring to chlorine bromine iodine essentially [Music] group seven elements that is what a halogen is so in this example we have etin again and it's reacting with bromine to give us one two di bromo eane this is a test used to identify an alen a bro mean solution is decolorized when we say decolorized the color changes from reddish brown to colorless and this type of reaction as we stated here is an addition reaction the next type of reaction we're going to look at is something called hydration alkenes react with steam using a phosphoric 5 acid Catalyst which is H3 po4 and a temperature of 300° c and a pressure of 60 atmospheres to produce an alcohol so again if we start off with e plus steam it will give us ethanol so you have eth molecule here reacting with H2O gas which is steam you need to have the phosphoric 5 acid Catalyst a temperature of 300° c and a pressure of 60 atmospheres and the product is ethanol and this is a this reaction is called hydration we also need to be familiar with the reaction with acidified potassium manganate 7 you need to know that pottassium manganate 7 is an oxidizing agent and this color of this is purple when acid ifed pottassium manganate 7 is added to an alken the purple solution rapidly decolorizes and when we say decolorizes it goes from purple to colorless this is another test for an alken an alken does not produce this reaction you need to be familiar with this equation so you have e plus H2O plus o this o in the square brackets represents the acidified potassium manganate 7 which is the oxidizing agent and the product we get is a diol which is Ethan one to diol meaning that we have two o groups being added now if we would have come here alkenes and alkenes if we look at the type of reactions they undergo an alken under go substitution reactions whereas an alken under goes addition reactions reaction with bromine in water now in the case of an alkan remember we said they have single bonds and they are ter so the alken is a saturated compound and the alken is unsaturated so the reaction with bromine and water there's no reaction with an alkan however we get a a reaction with an alen you get rapid decolorization of bromine the solution changes from reddish brown to colorless you need to be familiar with this because a question may ask what is observed and when we ask for an observation we need to specify a color change or what what do we see so it changes from reddish to Brown with an alken and and there's no reaction with an alken if we react with acidified pottassium manganate 7 there is no reaction with an alkine but in the case of an alken you get rapid decolorization of the acidified pottassium Mangin 7 it goes from purple to colorless so if we are asked what is the observation the observation will be the solution changes from purple to colorless isomerism isomers are compounds that have the same molecular formula but different structural formula what this means is that several compounds can have the same molecular formula but when you draw the display formula there are different forms so for example look at this compound C4 h10 it is butane we can draw it this way or we can draw it this way if we look at this we see that this compound has formula C4 h10 this one has formula also C4 h10 but if you look at the structures it is drawn differently this is a straight chained hydrocarbon and this one is a branch hydrocarbon so this one is called butane whereas this one is called two meyl propane have another example here we have the molecular formula C4 h8 and we see that they are three isomers so this one is called but 1 this is but 2 and this last one is called two metyl propene so they all have the same molecular formula but the structures are drawn differently and that brings us to the end of the homologos series alkanes and alkenes for