4.1 Naming AlkanesJan 25, 2022
alkanesif you want to go with the formal name that will be the subject of this lesson on my organic chemistry playlist now this starts a whole chapter on alkanes in the first half of the chapter , we will all be involved in naming alkanes we will name normal alkanes and then complex substituents and then bicyclic compounds also the second half of the chapter will deal with the different three dimensional confirmations of alkanes we will have a lesson on constitutional isomers one on what are called Newman projections and then we will finish it with the confirmations of the cycloalkanes, the most important of which is the confirmation of the president of cyclohexane now, if you are new to the channel, my name is chad, welcome to the preparation of chad, where my goal is Make science be understandable and even enjoyable now this is my new chemistry playlist to organic.
I will be releasing it weekly during the 2020-21 school year so if you don't want to miss a subs. create the channel click the notifications bell you will be notified every time i post a new video so naming alkanes now you can remember a couple of chapters back we learned a huge variety of different functional groups and alkane was just the first in fact we often thought of it as something like the absence of all other functional groups, so just a reminder, an alkane is a molecule that has all the carbons and hydrogens and nothing else and has no carbon-carbon double bonds or can be carbon-carbon triple bonds, those would be alkenes and alkynes respectively and we build a slightly different set of rules and we'll find out that all the different functional groups, whether they're alkenes, alkynes, alcohols, amines, ketones, aldehydes, etc. to put a little different spin on how we name them we'll name them and learn how to name them throughout the year but for now we'll just focus on the alkanes and all the rules we'll learn how to name these h It will still apply to all those others functional groups just with a little extra rule here and there for those too so it turns out we're going to talk about what's called the backbone and the backbone is going to be the longest continuous carbon chain in a structure so if we look at here, I have a bond line structure and I remember that each vertex represents a carbon, so here we have one, two, three, four, five, six and then seven or seven and then an eighth carbonyl so and what I mean by continuous chain, so I skipped the end a little bit, they have to be directly linked together in a kind of linear chain, so one two three four five six and from here being six I go up here to seven or down here to seven, but even though there are eight carbons total in the structure, the longest continuous chain will be seven carbons, and so if we circle that longest continuous chain here and I had the option of either r encircling this as part of the longest continuous chain or this and it's completely arbitrary wouldn't make a difference either way, but we're going to call that the main chain and then anything that's not part of the main chain will then be called a substituent, like so which turns out that the number of carbons in a in the parent chain or the number of carbons in a substituent will be indicated by a certain prefix now that it is the parent chain of an alkane use a suffix to n e but the prefix that comes before that goes to be a number prefix that describes the number so if we take a look you know that only single carbon alkane as backbone is just ch4 and that's methane so on the other hand if we have a two carbon chain that's just ethane and meth means a f means two and so on there's a whole list in your hand and i'll put it on the board here but probe three butte means four pence like a pentagon m means five hex like a hexagon means six hept means seven act like an octagon means eight none means nine dec like a decade is ten years dec means ten undec means uh eleven like a meaning one and cover meaning ten and ten plus one is eleven then dodec two plus ten means 12.
Now some of you may not be responsible for 11 or 12, but most people will be hooked by at least one to 10 here, so and again if you're the main string, we just add a and and and and so again a carbon chain is methane a two carbon chain is ethane propane with three carbons butane with four carbons pentane with five carbons hexane with six carbons hept anus with seven carbons octane with eight carbons nonino with nine carbons decane with 10 carbons undecane with 11 carbons dota cane with 12 carbons now obviously real molecules get bigger than this but you're only hooked up to a maximum of 12 for almost any class i've ever seen so cool so that's for the main chain now that the parent cha again always ends with that suffix a and e for an alkane but if you're a substituent if you're a branch off the main chain , we actually end it with a yl sound, so in this case it's just a carbon. right there instead of calling it a methane group we call it a methyl group let's draw it in red so if it were a substituent with two carbons it would be an ethyl group three carbons a propyl group four carbons butyl then pentyl hexyl heptal octal not aldecyl undical doticle and so on this is how it works now again students often struggle with these numbers from five upwards for the most part because five pents as a pentagon and hexadecimal for a hexagon with six and so on but the first few four and they are used a lot are probably pretty new to you so you have methyl ethyl propyl butyl and I like to say meat eats peanut butter so to get that mnemonic to get the first four so methyl ethyl propyl butyl yo i eat peanut butter so if it helps very well and if it doesn't work well sorry you will remember that forever.
I apologize ok so now we know how to name main chains with an ane ending and the substituent with an yl ending so let's kind of put this together in this rather simplistic example the way this works is we name the substituents first and the main chain last when we name an alkane so we're going to say methyl first and then in this case we have one two three four five six seven carbons on that longer chain and remember seven the prefix is hept and we'll call it like main chain hepdane and this will be called methylheptane but that's not good enough and the reason is not good enough if this methyl group could have technically been located anywhere on the chain so now we have to number that longest continuous chain , that parent string and you want to number it in this case you could go left to right or right to left and the way you ch oose is you want your s The substituent has what we call the lowest possible chain locant where the lowest possible number is in the parent chain so notice if I go from left to right it would be one two three four five would be attached at the sixth carbon of the chain but if I number this from right to left, we can see that it would actually be attached to the second carbon of the main chain, and that's how we're going to number this and before we name a substituent, always give its chain locator first and, Instead of just saying methylheptane, let's say 2-methyl and then heptane.
Now one thing to note: we always put a hyphen between a string locator and the substituent and I don't put a space or anything between the substituents and the parent string. Now, in this example, we only had one substituent, so in many examples, we'll see that we'll actually have multiple substituents, and the key is that there's no space. there is no hyphen between the letters of the substituents and the initial letters of the parent chain it's just one big word. I like to joke with students that the goal in organic chemistry nomenclature is to make the longest word in the world and we will see that there are certain rules that will ensure that the word we use for the name we give to a compound is as long as it is. possible but this is just 2-methylheptane but we can make this a bit more challenging so if we look at this one here again the first rule is find the longest continuous chain let's get this out of our way here so the longest continuous chain and I can definitely see that this is going to be part of the longest continuous chain so we'll go one two three four five and the question is do I go here or do I go up here well in this case it would be six seven or it would be like six seven or six seven but I have three different ways to get seven carbons out of this and so when there is a tie of how can you g and the longest string they break the tie by saying let's do the longest word again.
I like to think about it in the world but they say to break the tie by choosing a form that has more substituents coming off the main chain, so for example if it only goes one two three four five six seven well then the only substituent would be this branch that it comes off the main chain right there, just one, however if instead of having numbered it that way, I go like six seven, now all of a sudden I would have a branch. coming off carbon five which is not part of the main chain and a branch coming off carbon six which is not part of the main chain would have two substituents which will guarantee that we get a longer word and that is how they break the tie, like so which and in this case I could have made this guy also seven it's actually arbitrary in that case because if I have a one carbon substituent in six on the right hand side or on the left hand side I'll still get two substituents off of this thing and they're fully equivalent so it didn't matter in that case okay so now we find our longest parent string now that's not the actual way we're going to number this longest parent string as we'll see so the first thing I'm going to do is I'm going to circle that longest continuous chain that main chain one more time and then I'll circle the substituents here we also have one here and we have one here and so this will be the first time you have more than one substituent and there will be another rule associated with how you name it ok but the first thing I'm going to do is number the longest continuous chain again and in this case if number this is number one, this ends.
I would go one two three four five six seven and what we would see is that my first substituent that I find in the numbering system would be located with a string locator of five but if I number it backwards and I go one two three four five six seven the first substituent i will have will be located with string locator tw o and lower is better just like it was up here so you are looking for the first substituent you come across in numerical terms and you want to get the lowest number possible so so we'll number this starting with this end as number one so one two three four five six seven is ok this is where things get a little tricky but if we take a look at this it's still a string of seven carbons that will still be called heptane, the end of our name, the father. the chain will be called heptane but now we have two substituents we have a single carbon substituent which is still called a methyl group but then i have a two carbon substituent here and it's called an ethyl group don't forget i eat peanut butter one two three four is fine fine when you have several different types of substituents we actually named them in alphabetical order so we didn't actually name them based on where their chain locants are it turns out they decided the rule of thumb was going to be that your name t hem in alphabetical order and ethyl comes before methyl in the alphabet so we'll name ethyl before methyl and this will be a form of ethylmethylheptane but before you say ethyl or methyl you have to give me those string locators first please which ethyl will be first here and it's located on the main chain, the main chain from carbon 3, so we'll start by saying 3-ethyl, all good from there, n Our next substituent is the methyl group. and it's located on carbon two of the main chain and some will say two methyl note the use of hyphens here so our chain locants will be surrounded by hyphens so in all cases except where they're at the beginning, I guess I'll just have a hyphen afterwards, but if they're in the middle of the word, those string locators will be surrounded by hyphens.
I like to say that we are going to separate the numbers from the letters with hyphens. really that chain locants need a hyphen around them unless they're at the beginning of the name so two ethyl three methyl and then we stay on the main chain all substituents are named at this point and that's just heptane again and again no space here no hyphen just a big ugly word and again we would say this is 3-ethyl2-methyl heptane so we're making this more and more complicated as we go along so in this case we have to make it a again find the longest continuous chain, sostudents often have trouble with this early on so one thing I recommend is to look at branch points like this one right here look at those branch points and say ok in all the different directions well obviously go to the right. it's going to be part of this but the question is do i go up or do i go left here my longest chain to the left would be one two my longest chain up would be one two or one two and it's the same again but i can see that I'll have more substituents if I go up like we did in the last example so it'll be part of our longest continuous chain and I'll go around it until we get to the next branch point and at this branch point here I can go down and go one two or I can go to right here and go one two or one two or one two those are the three equivalents it doesn't matter which but it's two carbons down it's two carbons up to the right and the tiebreaker again is going to be if one of those gives me more substituents that won't be part of the main chain, yes, going to the right, so in this case, we'll go to the right and again I could have circled this carbon, this one, or this one, they would all be equivalent, there wouldn't be a difference I just chose go random one ok but there's our longest continuous main chain and it's two three four five six seven eight carbons in length so the main chain here will be called octane at the end of the name but let's take a look at all those lovely substituents we have one here one here one here one h Here and one here and all of a sudden this got a lot more fun so if you look this guy is a methyl group they are both methyl groups so this guy here is an ethyl group with two carbons and this is an ethyl group with two carbons too now when you have multiples of the same substituent you have to tell how many of them you have using a number prefix now they are not the same number prefixes that tell you how many carbons something has but now we are going to have some lovely number prefixes like mono di tri etc. we'll use them, so in this case we have three different methyl groups, so we're going to say trimethyl in the name, we've got two different ethyl groups, so we're going to say diethyl in the name, now what?
You should know though that when you alphabetize the different types of substituents here you don't include that as part of the alphabet you just alphabetize it under ethyl vs methyl and ethyl is still going to win so let's name the ethyls first but when let's do it we'll say diethyl but now we have to keep numbering the longest chain so if we number this from left to right it would be one two three four five six seven eight but I would see my first substituent would be the location of chain number two, but if I go from right to left it would be one, two and one in my first substituents would also be located at number two, it would be a tie, so what do you do to break the tie?
Well, then you go to the next substituent you find, and therefore if I number this way. go one two and my next substituent would be this ethyl which would be located at the number three chain location so if I go to the other side then one two but here I have two substituents that are attached at carbon two and so don't only the first substituent was attached to carbon two, so the second would also have a chain locator of two and that will break the tie since its second substitution is at two, its second substituent from this direction will be located at three, in Instead, this is the best way we want to get those lower numbers for the substituents where possible and so we'll number this from right to left, so we'll go one, two, three, four, five, six, seven, and eight, okay , now that we have the chain locations for everything, we're going to name the ethyls first and I can see they're located at 3 and 6, so on the main chain, so we're going to say 3 6 diethyl, okay, yeah watch when you have mu multiples of the same substituent between the locants of your string, you will use a comma. so your string locators are separated with a comma but all your string locators are separated with the actual substituents using hyphens so keep that in mind so I like to say we separate numbers from numbers with a comma and numbers from letters with a hyphen and that's usually enough for most or If the timing is right then 36 diethyl but then we have three methyls so let's say trimethyl and two of them are located at strand locator two and one of them is located at the chain locator seven now the truth is a lot of students lost early on is if two of them are located in the same chain location you have to number it twice so it will be 227 trimethyl and finally our main chain of octane now many students when they start forget one of the two and only say two seven trimethyl and when I see two seven trimethyl I'm like good one is in two ones at if ete and I don't know where the third one is so you have to explicitly state all the chain locations so if you're going to say trimethyl you better have three chain locators in front even if one of them repeats for show that there are two of them joined there so we have a couple more examples to go through here and again the key here is that there are all these special rules for you.
You know certain situations that always come up and I want to make sure that we do at least one example of every special situation you might run into that you really do have these rules to the letter so okay this is the first rule here again. is to find your longest continuous chain of carbon atoms and here if I start with any of these carbons it's the same length both ways from this branch point so I'll pick this one here and when I get to this branch point here Definitely i can say i can get more carbons going right than going up so we'll go right and then from here either going down or going right it's the same distance either way so it doesn't really matter which i choose , in this case it will leave us with three substituents and in this case the three identical substituents are all methyl groups, now we have to figure out how to number the longest chain if we go from left to right the f The first substituent we will come across would be in position two if we go from right to left, the first substituent we would encounter would also be in position two and it is a tie and, as in the last example, c When there is a tie, then you go to the next substituent you find and from left to right that next substituent would be at one two three four it would be in position five but if I go from right to left it would be in position one two three and that will be the top method here of numbering this so we'll number this from right to left 1 2 3 4 5 6 and 7 and in this case they're all methyl groups so we'll name them all at the same time we'll say trimethyl for three identical methyl groups and give again three chain locants to indicate where they are located so two three and six so two point three point six trimethyl and then the longest chain of seven carbons is heptane and again a big word all right another example of that' I'll repeat another special case that we'll run into in th is the case of the longest continuous chain from this branch point I definitely want to go left here not up and then we'll go to this point of fork and from here if I go up it's two carbons long and if I go to the right it's two carbons long it doesn't matter which one we choose so a lot of people like to choose right in the middle but they are in the habit of doing it right in the middle and they think they should always make it right in the middle so I'm just going to pick the other one this time just so you know it doesn't have to be right in the middle from left to right or right to left so which is the longest continuous chain in any way you can think of.
If it had been equivalent, either would have worked. I just picked the other one so you can see that it's right, so if I number this from left to right, the first substituent I'll find would be one, two, three, but if I number it from right to left. the first substitution I'll run into would be at one, two, three, so there's our two substituents that aren't part of the main chain, so it's a tie, so we move on to the second substituent and if number from left to right, it goes one, two, three, four and it would be under the second.
It would be five and if I number from right to left it would be one two three four five and if I go from right to left or left or right it's three and five it's where the substitutions are located no matter what and when you have a tie so well if everyone they are identical substituents great it doesn't matter but if they are not and these are not identical in this case one is methyl and the other is ethyl well you are going to use the alphabet to break the tie in that case not the size so let's choose the ethyl to have the lowest number in this case, not because ethyl is bigger than methyl, but because it comes earlier in the alphabet, so a lot of students see us working on this example and that's okay, so we're numbering this in such a way that ethyl is the lowest number to break the tie and they think, oh, it's because of the size, not again, it's because of the alphabet, ethyl comes before methyl in the alphabet, so let's name this, we'll still name it in alphabetical order ethical too. we're going to start with ethyl and then we'll start with 3-ethyl and then five methyl great and then once again our main chain which is seven carbons long is once again heptane and again one big long word with no space great three ethyl five methylheptane ok so now we are going to move some examples of what we call cycloalkanes and these are alkanes that form ring structures and when you have that ring structure you are usually going to name the ring as the parent chain in most examples which you'll see but we'll definitely work on an example where the ring isn't actually the parent chain we'll see what makes a difference in things like this but in these three examples we're going to name the ring the parent chain and when your ring has six carbons like this it does here instead of calling it hexane it puts a cycle prefix on it and calls it cyclohexane let's write that here so one of the most common mistakes that What students commit is when they rename cycloalkanes is that they forget to say cyclo at the beginning, so they just revert to hexane like they would a linear string, so keep in mind that you need to prefix cyclo there when you're done. naming the main chain as a cycle for an alkane cycle okay now the first example we're going to do here is going to have a single substituent and this is the first time I'm including a halogen as a substituent now only halogens are named as substituents so and simply, instead of saying chlorine, bromine, iodine, fluorine, you put an o at the end of the line, so it would be fluorochlorobromoiodo as a substituent, just as you would name a methyl ethyl propyl butyl pentyl, etc., it gives you the locate string o first and then it's just going to say fluoro or chloro or bromo or iodo then and again always named as a substituent not as part of the main chain ok now another thing that's going to be a bit ex The catch about cycloalkanes is that there's no end to the chain now, when we've numbered the longest chains in the past, we always start at one end or the other end or something like this and that's where carbon 1 is going to be, but in a ring there is no ending and so the key here is that you can choose or at least you have to figure out where the number one carbon is going to be in the case that there is only a single substituent in your cycle. the goal is to make sure the substituent gets to be at the lowest possible number well that happens if you make its location number one and that's where it will always be for a single substituent wherever your substituent is that's it where the number one has to be and if your g or one two three four five six clockwise or one two three four five six counter clockwise is the same either way now with a only substituent then because by definition and how we number things it has to be located with a number one chain locant we left it out of the name and instead of saying a chlorocyclohexane with only one substituent it just says chlorocyclohexane and that's what we'll do here and it's a big word and again if it says a chlorocyclohexane it's technically wrong so leave it out and again a lot of students get confused here so they say ok chad hey if I have something like this and number my longest chain one two three four five six and they say chad so this would just be chlorohexane right no no this is a chlorohexane because when it's in a linear chain chlorine could be located attached to carbon two instead or attached to carbon three instead and in a linear chain and you have to give me the chain locator because I don't know where it is, otherwise it's just in a ring with only one substituent that should be located in one by default and note that doesn't mean the one above could you know?
Put the chlorine here and then we would have defined it to be carbon one instead, soit's just for our cycloalkanes with a substituent which by default will have to place it at position one not true for our linear system so don't understand that common mistake students make at this stage ok now if you have a disubstituted cyclohexane in this case so well so your goal is to get as low numbers as possible but if you notice you can number this let's say I'll put where the chlorine is at the number one and that would put where the methyl group is here at the number two and our substitution would be located at positions one and two, but we could also have put the methyl group at number one and the chlorine at number two or and so either you number it this way clockwise or in this way counterclockwise, my two substituents are located at exactly the same numbers, the same string locants one and two both ways, so we have It's a tie and if you remember the tiebreaker is the alphabet and once again this is our chlorine substituent this is our methyl substituent and chlorine comes first in the alphabet so we'll do it clockwise here with chlorine at number one and methyl at number two so when you have more than one substituent it will now include all your chain locants it's just with a single substituent you'll just leave out this chain locator because you have to be in one but here you have to let me know where they re at and so in this case we'll still name this in alphabetical order so we'll do a chlorine and then the 2-methyl like this and then again the big mistake they make students is that they will just say hexane and forget they need dicyclohexane for the cool ring so one chloro 2 methyl cyclohexane in this example now students take what they got from a disubstituted cyclohexane and that often makes them co make a mistake when they get to a trisubstituted cyclohexane and, in this case, they look and say oh, let's see. we have a chlorine, we have a methyl and we have a bromine for our substituents, well bromine is first in the alphabet, we'll make it number one, well again, we're just again relying on the alphabet to break the tie. and we haven't checked that all the numbers are the same at all and so notice that we could actually make bromo number one and when you do this you always want to number that way clockwise or in counterclockwise to find the next lowest numbered substituent possible, so notice if I flip clockwise and he's one, it would be two three four for the next, where if i start with it as number one it would be two three and we do it this way and that would be one three and four for those chain locators well notice what if i start with chlorine as number one?
Well if I start with it as number one I definitely want to go clockwise because then my next chain locator to locate the substituent would be two and then three and then the bromine would be at four. and notice that we would be comparing one three and four to one two and four and in this case there is no tie with all the numbers and we don't have to go back to the alphabet at all there is a tie at one but two goes to beat three and that's fine , well, we had another option to put the number one, the number one had to be where one of these three substitutions is, so I guess the other one we could have tried is to put the one where the methyl group is located and if we make it number one we're definitely going to want to go counterclockwise so the next substituent we see is located at two and then three four and bromine would be located at five in this case instead and so we can see our sub the constituents would now be at one, two and five so for the first position we definitely have a tie at one for the second substituent we have a tie at two but for the third substituent four or it will be five so the numbering system is The one outlined in blue here will be correct so let's go back and remove some of these but keep in mind we didn't have to go back to the alphabet and that's a big mistake that kids make students here, as they often go back to the alphabet in a tri-substitute like this before they've verified that the numbers are all the same, it's only when all the substituents have exactly the same string locants that you then go back to the alphabet to break the tie , we never had a tie here one two and four was going to be the best possible number we could have so now we have the correct numbering system but when we go to name this we still name our substituents in alphabetical order and not numerical order , so bromine is going to be first in the alphabet t then chlorine and then methyl so that's another thing that students screw up is they know we don't use the alphabet to get what They're correct numbers here, but then they forget that they need to name them alphabetically when they list the substituents so but that's what we need to do here, so we'll start with 4-bromo, move to 1-chloro, then 2-methyl and then our main chain again is cyclohexane so once again four bromo one chloro2 methyl cyclohexane now alluded to the fact that we named one where the cyclo part was not the main chain so in the last few examples always it was but in this case it's not going to be the case because here we have a straight chain that is six carbons long and our ring is only four carbons in this case so the way it works is if one of them is longer or more complex with more substitutions that's what it's going to do well on the main chain this case if I name the straight chain as the p a different chain is six carbons long with a substituent the ring if I name the ani i see as the main chain is four carbons long with a substituent the straight chain and so i will choose a longer main chain in that case now many students make the mistake and ask me the question or they will say well chad i can get a longer chain i can go 1 2 3 4 5 6 7 8 9 10 and keep numbering through the ring well the rules say you can't do that so either your main chain is the ring or it's not the ring but you can't include the knot ring and the combined ring and some kind of longer backbone doesn't work so in this case it's just the knot ring and our backbone is this guy and our substituent is those four carbons right there now our backbone is six carbons long, so we're going to call it hexane and we're going to number it such that our substituents are at the lowest number possible so we're definitely going to number this from right to left and how to remember Also if this had just been a main chain we would call it cyclobutane butte it means four carbons but in this case because it is a substituent remember when you make the main chain you like methane ethane propane butane pentane but if it is the substituent it is methyl ethyl propyl butyl pentyl with a yl sound and so this case and again instead of being cyclobutane it's going to be a cyclobutyl group and in this case it's located in position one and when we name it we'll say a cyclobutyl and then the backbone is just hexane now i know some of you are thinking that you are like hey chad you have to give the good one i have to give the one it's only again when it's attached when a substituent is attached and it's the only substituent to a good ring in this case my dad the chain isn't even a ring and again it could have been six carbons and it could have had this cyclobutane attached to carbon two or the butyl group of the cycle. don't say a cyclobutyl hexane if I just say cyclobutyl hexane well then it was located at position one or position two or position three I don't know it definitely has to be stated in this case so when you have only one substituent on a ring main chain is when you don't include the one but again in this case we have a single substituent attached to carbon one of a linear chain you definitely have to include the one in that case now if this lesson was helpful to you please consider giving me a like and share is the best thing you can do to support the channel and if you're looking for the study guides that accompany these lessons or if you're looking for naming alkanes practice problems, please check out my premium course at chatsprep.com
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