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Video transcript

let's start with an introduction to alkanes alkanes all have the general molecular formula of cnh2n plus 2 where n is the number of carbons that are in your molecule for example if you have one carbon in your molecule you plug one into your formula there and you would get c1 and you would get 1 times 2 plus 2 which is of course 4 so ch4 which is methane and we've seen the dot structure for methane many times right so carbon with four hydrogen's around it like that so methane is the simplest alkane what about two carbons right we'll plug it into your formula that would be c2 and that'd be 2 times 2 which is 4 plus 2 which is 6 so c2h6 is the is the molecular formula for a 2 carbon alkane which we call ethane so this would be the dot structure for ethane carbon bonded to another carbon with six hydrogen's around it like that so there are millions upon millions of organic compounds and you have to have some sort of logical way to name them so one carbon is methane 2 carbons is F Ain and these these names were determined by what's called the IU pact nomenclature system IU pact stands for International Union of pure and applied chemistry which was a bunch of chemists getting together and saying we're going to name all these molecules in a very systematic way so for the rest of this course we're going to focus on I you pack nomenclature for naming organic molecules and I've kind of summarized IU Pack nomenclature up in this little table here if you have one carbon your parent name is meth and if you're working with an alkane your ending is a name so therefore a a 1 carbon alkane is called methane as we have already seen a 2 carbon alkane the root is F and so that would be ethane and so of course your ending would change depending on what functional group you are working with but here we're just working with alkanes 3 carbons the root is pro so if I were to draw a 3 carbon alkane like that that would be the dot structure for propane for carbons is beaut so if I were to draw a four-carbon alkane like that that would be butane and of course five carbons would be pentane so I could go ahead and draw a pentane and and we could we could keep going here but I think you get the idea all right these are all straight chain alkanes meaning it's just one one line of carbons one carbon right after the other six carbons would be hex or hexane seven carbons HEPT or heptane eight octane nine nine ten is decane and we can see the rest of them here on deck four eleven carbons dodyk Tritech tetra Tech Penta deck and I costs four twenty four twenty so this this holds true not only for alkanes but for other functional groups so it's important to memorize all of these parents names here so so far we talked about straight chain alkanes what about if you get a branched chain alkane like this so you can see it's no longer just one carbon one after another in a straight chain you can see there's something coming off of that carbon chain so first first let's find the longest carbon chain so this is a skill that you have to develop when you're doing I you pack nomenclature so if I if I start over here I find the longest carbon chain and if I if I go like this you can you can see the carbon chain that I'm going for like that how many carbons are there in that chain well there was one right here and then two and then three and then four and then five so there are five carbons in this chain and when we go back up here to our I you pack nomenclature table we see that five carbons will have pent and it will be pentane so the parent name will be pentane for this example right so for this molecule you're going to name this pentane and what is coming off of my straight-chain alkane right I have something coming off of it right here I have a a one carbon a one carbon ch3 group branch of my pentane molecule this is called a substituent okay so a substituent is something coming off of your parent chain right it's a group or groups that is connected to your parent chain so a group connected to your parent chain so how are we going to how are we going to name this substituent well it's one carbon and this is what's called this is what's called an alkyl group so an alkyl group and notice it's not an alkane it's an alkyl so the ending for an alkyl group will be yl okay but we're still going to use our parent name to name alkyl group so if there's one carbon right I go back up here to my I you pack table here and I say well one carbon in organic chemistry has the parent name of meth and this is an alkyl group which has a yl ending so I have I have meth plus yl so this is called a methyl group which we've said several times already in these videos so we have a a methyl group attached to my straight chain to my parent chain here which is called pentane so there's a methyl group coming off of pentane in the second position we will get into details about IU pack nomenclature in the next video so for right now let's just let's just be able to identify substituents coming off of our parent chain and the first step when you're doing i you pack nomenclature is always to find the longest carbon chain possible so for this molecule it is a five carbon chain let's look at a more challenging example here are three dot structures for the exact same molecule and let's see if we can find some some carbon chains for this molecule here so let's let's find the longest the longest carbon chain we possibly can so I'm going to start with the top left one and I'm going to look at this portion of the molecule here and I'm going to try to find my longest carbon chain so that might be my first my first guess here so how many how many carbons in that chain well this would be 1 2 3 4 5 6 & 7 so if you were to name this as a parent chain this would be heptane since I have 7 carbons on it like that what about these substituents coming off of heptane well I have a 1 carbon alkyl group coming off of the second position so one carbon we've seen that would be called a methyl group I have a two carbon substituent coming off of the third carbon so let's go back up here and refresh our memory what would be the parent name for two carbons for organic chemistry all right so so two carbons would be F and since this is an alkyl group it would be called an ethyl group so we have an ethyl group coming off of carbon 3 so this is a methyl group right here and then this is an ethyl group so we have an a methyl group coming off of carbon 2 and we have an ethyl group coming off of carbon 3 and coming off of carbon 4 is yet another ethyl group like that let's let's say we chose a different way to find our longest carbon chain let's say we started down here so let's say we said oh that looks like that might be the longest carbon chain to me at first glance so let's see what we have let's see how many carbons we have if we said this is our longest carbon chain so let's let's number them let's call this carbon one let's call this carbon two three four five six and seven so once again this would be called heptane what sort of substituents do we have coming off this molecule we have a methyl group coming off of carbon 2 we have an ethyl group coming off of carbon 3 and we have another ethyl group coming off of carbon 4 so that's the exact same situation we had for them for the first example here so these are the same thing so it doesn't really matter which one of those you chose you with you would be naming it the exact same name let's compare those two to the molecule to the example down here again it's the same molecule but let's say you chose a different path let's say let's say you chose down here so you said oh this looks like it's the longest carbon chain to me so you go like this and you say alright that's my longest carbon chain how many carbons are in that well this would be one two three four five six and seven so what sort of substituents do we have in this molecule well coming off of carbon four we can see there is an ethyl group coming off of carbon three we can see this looks kind of complicated um it's not really a straight chain this is much more complex substituent which we'll get to naming in a future video so for this molecule we have a total of two substituents for the top molecule we have a neat we have example of three substituents so the question is which one of these will be the correct way to name my molecule according to IUPAC nomenclature so I have two chains of equal length both of these chains are seven carbons so how do I break that tie I pack rule state you choose the parent chain with the greatest number of substituents so the top one has three substituents the bottom example has two substituents so if you were to name this molecule using IUPAC nomenclature you would choose the top the top way of of naming it okay which again we will get to in more detail in the next few videos here so let's look at cyclo alkanes now alright so we've just done straight chain alkanes we looked at branched chain alkanes let's look at cyclo alkanes so this is a pretty funny dot structure here let's let's see how many carbons are in this triangle well of course there's one two and three carbons so if I were to to draw what this molecule looks like if I were to draw all the atoms involved well there'd be three carbons like that and to complete the octet around carbon there'd have to be two hydrogen's on each carbon like that so that's a cyclo alkane so it's its carbon forming rings now so the molecular formula for this molecule there be three carbons and a total of six hydrogen's and we can see that the pattern for a cyclo alkane would therefore have to be if you have n carbons you must have two n hydrogen's for a cyclo alkane this cyclo alkane has three carbons so we go back up here to our I you pack nomenclature table and we say that three carbons should be proq right and this is an alkane it be propane but it's a cyclo alkane so we would actually call this cyclopropane so this molecule is called cyclo propane like that the next molecule looks like a square how many how many carbons are in it - there are four so that would be butane but since it's a cyclic molecule it be called cyclo butane and let's do two more examples alright so the next one is a pentagon so that's five carbons so it'd be pentane so these cyclopentane so here we have cyclopentane and probably the most important cyclo alkane would of course be six carbons and six carbons would be cyclohexane so this guy down here would be named as cyclohexane so that's an introduction to alkanes and cycloalkanes the next two videos we'll get into much more detail about nomenclature