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Video transcript

so here are the rules for naming alkanes and cycloalkanes you first find the longest carbon chain and you name it so let's go ahead and do that so we can see that the longest carbon chain for this example has five carbons which we know is pentane the second step is to number the chain to give these substituents the lowest number possible so I could number this chain from the left or I could number it from the right if I number it from the left I give that substituent a number of two if I numbered it from the right that substituent have a number of four so numbering from the left for this example is the correct way to approach it identify and name your substituents is step three well I know that this is a methyl group and that methyl group is found on carbon two so I would write two methyl pentane and that would be the correct I you pack name for this dot structure I don't have to worry about step four because I have only one substituents so when we see multiple substituents we'll have to think about using the alphabet rule so two methyl pentane is the correct IUPAC name for this molecule and from that name you should be able to draw the dot structure let's do another one here so let's follow our steps find the longest carbon chain and name it so I have one two three four five six so this would be hexane for my longest carbon chain number to give your substituents the lowest number possible so once again I have a choice of numbering from the left or from the right in this case numbering from the left would give my substituents the lowest number possible what are my substituents well I have two methyl groups this time so I have to use a prefix I have two methyl groups so I'm going to call this dimethyl so Die methyl hexane and those methyl groups are coming off of carbon 2 so I have to write to to dimethyl hexane so when you have prefixes um you would use die for two you would use try for three you would use tetra for four you would use Penta for five and hexa for six alright so let's do another nomenclature example let's look at this molecule find the longest carbon chain so let's see that would be one two three four five six seven and name it so seven would be heptane so I have heptane here number two give your substituents the lowest number possible my options are to start from the left or to start from the right this time makes more sense to start from the right because that gives my first substituent a number of two which should be lower than if I started from the left so one two three four five six and seven so that is heptane identify your substituents and name them is is the third step so that's a methyl group coming off of carbon 2 and that is an ethyl group coming off of carbon four which one comes first the ethyl group or the methyl group step four says it's the alphabet right you arrange them alphabetically so e comes before M so you're going to put ethyl before methyl so you have a met and ethyl group coming off of carbon four so for ethyl and a methyl group coming off of carbon 2 so 2 methyl if I can squeeze it in here so I have 4 ethyl 2 methyl heptane is the official IU PAC name for this molecule what about something like this alright so this is a little bit more complicated let's let's look at how on my longest carbon chain and how many carbons are in my longest carbon chain right that would be 8 all right this is the exact same dot structure so both of these are going to be octane so octane number your carbon chain to give these substituents the lowest number possible well if I number from the left I would have my substituent with a number of two if I number from the right I would have a substituent with a number of two so it's not immediately obvious to me which would be the correct way to start from so let me go ahead and just number all of them one two three four five six seven eight and if I go this way one two three one two three four five six seven eight so let's go ahead and name both of these and let's what we get well what kind of substituents do I have on the left I have let's see how many methyl groups a total of five methyl groups so it'd be Penta methyl so Penta methyl octane and obviously they both would be Penta methyl Octane's it's the same dot structure here so Penta methyl octane where are those methyl groups on the left they are on two three three seven seven so two three three seven seven Penta methyl octane from on the right my methyl groups are on two two six six seven so let me go ahead and put that down two two six six seven Penta methyl octane so I want to give my substituents the lowest number possible but I've already seen that that first number right two versus two is a tie so so that doesn't quite work what I need to do is go to the next number so I have a three over here and I have a two over here and you always want to do the the lowest number possible so since two is a lower number than three this is actually the correct I you pack name for this molecule two two six six seven Penta methyl octane this is called the first point of difference rule you want to give your substituents the lowest number possible so if there's a tie with the first number you go to the second number and compare those numbers and if there's a tie with the second number you go on to the third number and so on so in this case the first point of difference came with the second number let's look at cycloalkanes so here i have a a cyclo alkane and you name it the same way you would name a straight-chain alkane you first find find the longest number of carbons in this case it's in a ring alright so we have a total of six carbons in a ring which we know is cyclo hexa ichael hexane so that's my parent name so cyclohexane is my parent name number the ring to give the substituents the lowest number possible well here's my here's my substituent I know that's an ethyl group I want to number my ring to give that substituent the lowest number possible so obviously that must be number one so which way do I go around my ring well it doesn't matter for this example because it'd be the exact same says i have only one substituent so so six carbons all the way around with an ethyl group coming off of carbon one so i could call this one ethyl cyclohexane or you could even leave off the one and just say ethyl cyclohexane it's implied that the ethyl group is coming off of carbon 1 here so that's how to name a cyclo alkane let's look at a more complicated cyclo alkane here so so now i have again i can see that it's cyclohexane that's my that's my base name here so this is going to be cyclohexane so i go ahead and put that down next i have to number to give my substituents the lowest number possible so so what at which one of these two carbons is going to be number one right I could make I could make this carbon number one or I could make this carbon number one well first point of difference rule if I make the top carbon number one that's going to give me two number ones versus if I make the if I make this carbon down here number one I have only one substituent coming off of carbon 1 so the first point of difference rule says this could not be carbon number 1 so carbon number 1 has to be this top carbon up here like that so how do i number my ring well I could go I could go this way I could go 1 2 3 4 5 6 or I could go the other way around I could say that's number 1 2 3 4 5 6 which way is correct well if I think about my substituents this would be an ethyl group coming off of carbon 3 whereas this would be an ethyl group coming off of carbon 5 so the one on the left is the correct way to number your ring because it gives your substituents the lowest number possible so now we're ready to go ahead and name it let's see look at all of my substituents and on this first example here I have two methyl groups coming off carbon ones that'd be dimethyl I have an ethyl group coming off of carbon 3 so when I think about the alphabet rule I'm going to put the ethyl group first so I have 3 ethyl 1 1 dimethyl cyclohexane as the official name now sometimes students get confused with the alphabet rule ah think about the parent name right think about M versus e e comes before M in the alphabet prefixes don't matter so don't think about alpha alphabetizing something with the D for da I think about ethyl versus methyl and that will give you three oh one one dimethyl cyclohexane four as the correct i you pack name for this molecule let's look at another example here alright so what do i do what do i do for this one well first thing identify the longest carbon chain in this case what kind of cyclo alkane is it well five carbons so it's going to be cyclopentane so cyclo pentene so they're both going to be cyclopentane here since it's the same molecule so cyclopentane well which one gets number one I could make I could make this a number one and this is number two or I could make this a number one and this is a number two so let's just go ahead and name them both and then let's see which one is the correct name so on the Left I have one ethyl to methyl cyclopentane and on the right I would have two ethyl one methyl cyclopentane and the question is which is the correct I you pack name first point of difference rule doesn't really work because I have I have one that's one that I have one that's two so there is no first point of difference for this so if the first point of difference rules fails then you go to the alphabet rule and you say to yourself alright so so which one is which one is going to get a number one is it is it the ethyl that's going to get a number one or is it the methyl that's going to get a number one if first point of difference rule fails you go to the alphabet rule so E comes before M so the ethyl group gets priority so ethyl is number one and therefore this is the correct I you pack name for the molecule like that all right so let's let's look at one more here so how do I name this guy so it's kind of funny-looking well let's let's see how many carbons are in my longest carbon chain so there's one two three four five six and seven so there is seven carbons in that chain how many carbons are there in the cyclo port portion well there's 1 2 & 3 so I have more carbons in my straight-chain alkane than I do in my and my cyclo alkane and therefore I'm going to name this as an alkane so there are 7 carbons so it's going to be heptane and I have a cyclo alkane as an alkyl group now so if this wasn't an alkyl group I'd call it cyclopropane I dropped the Ain ending because now an alkyl group so becomes cyclopropyl so I have a cyclopropane Oh purple group coming off of my heptane coming off of let's see which carbon is that let's go ahead and number it 1 2 3 4 5 6 7 so I have a cyclopropyl group coming off of carbon 4 so it's 4 cyclopropyl heptane so when you have more carbons in your chain than you do in your cyclo alkane you name it you name as a cyclo alkyl group and then your alkane let's look let's remind ourselves an example up here where we had where we had 6 carbons in our site in our cyclo alkane only 2 carbons in our alkyl in that case you name it as an alkyl cyclohexane and if there's a tie let's say if there's a tie the tie goes to the cyclo alkane so if you had 6 carbons in both you would name it as an alkyl cyclohexane molecule