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Common and systematic naming: iso-, sec-, and tert- prefixes

Common and systematic naming - Iso, Sec and Tert prefixes. Created by Sal Khan.

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  • blobby green style avatar for user keshavbagla
    When you say common and systemic naming do both come under the IUPAC naming system?
    (58 votes)
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  • blobby green style avatar for user Valentine Agranovsky
    I did not fully understand why it was dimethyl ethyl, I thought the longest chain would have 2 carbon chain rather than a 3 carbon chain; it would be great if I could get an explanations. Thank you!
    (4 votes)
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    • leaf green style avatar for user Raza Zia
      when trying to name the BRANCH you have to approach the compound from the point at which it's attached to the main structure (in this case the cyclopentane). Therefore, in the example, you can pick any toe on the chicken toe looking thingy and use that as the backbone of the BRANCH. The backbone will then be a 2 carbon chain (hence, ethyl). You have attached to this branch two methyls (dimethyl) attached to the 1 carbon (1,1).

      I hope this helps :)
      (29 votes)
  • starky ultimate style avatar for user ST
    It sounds like these names can get pretty huge. How big can they get?(In my chemistry book, there's a name 22 letters long, so it has to be pretty big, I guess)
    (9 votes)
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  • female robot ada style avatar for user Pragya
    why we say the branched butyl group as iso butyl group?
    (6 votes)
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    • blobby green style avatar for user John Gregory
      Hey I read this question and realized I didn't have a true in-depth understanding of how "iso" fits into everything. For example, tert-butyl has a carbon and 3 other carbons branching off... but so does iso-butyl. It seemed like they should be interchangeable terms but I know there's more reasoning behind it.

      So I reviewed it and took more comprehensive notes. I know the post I'm replying to is 2 years old but Ill toss this up for anyone else who might find it useful.

      Starting simple from the beginning:

      Butane is C4H10. A diagram would look like:
      / \ /

      This is a 4-carbon straight chain and is called n-butane ("n" stands for "normal"). However, there is one other unique way you can draw butane and still end up with C4H10. When you move a CH3 to the other CH2 (ie if you have positions 1-2-3-4, you would move 1 to 3 or 4 to 2), you end up creating

      ......... CH3

      This is still C4H10 but different arrangements can make a compound react differently. Same formula, different arrangement = isomer. Therefor, n-butane rearranged uniquely makes isobutane.

      It is important to note here that you can highlight a 3-carbon chain + methyl (CH3) group. (1,2,3) + (4). No matter which 3 carbons you pick as your main chain, you will always have the central CH numbered as the 2nd carbon. Attached to it will always be a branch chain of methyl (CH3) group no matter how you look at it. Therefor, isobutane can also be called 2-methylpropane. Remember this for later.

      Arbitrarily decided, highlighted is the 3 carbon chain (carbons numbered 1,2,3) and non-highlighted methyl group (carbon numbered 4)

      ......... CH3

      When you bond butane to a larger compound ("R"; short for functional group) with a single bond, you LOSE one hydrogen from butane. The specific carbon you bind R to can create different isomers.

      R + C4H10 ----> RC4H9
      R + butane ----> Rbutyl

      Lets number the 4 carbon chain n-butane /\/ (CH3-CH2-CH2-CH3) as 1-2-3-4 with the numbers representing the different Carbons respectively. R can bond in 4 different places but only TWO of them will make a unique compound. R bonded to 1 is identical to R bonded to 4 and R bonded to 2 is identical to R bonded on 3. Draw it out. Proving it it yourself on paper helps everything make sense.

      R1-2-3-4 or 1-R2-3-4.

      Remember the single bond to R replaces one of the hydrogens so:

      When R bonds to 1 we get "1-butyl". When R bonds to 2 we get "2-butyl".

      1 is the primary carbon. It also has only 1 other carbon attached to it. So the new compound with R on carbon #1 is just called n-butyl.
      2 is the secondary carbon. It also has 2 other carbons attached to it. So the new compound with R on carbon #2 is sec-butyl (s-butyl).

      This covers what happens when n-butane bonds to R. But remember our ISOMER that we made from butane? Isobutane. All you do is follow the same process that we just did for n-butane.

      With numbers replacing carbons, isobutane looked like this:


      Similarly to n-butane, R can bind to any 4 of the carbons but only 2 choices will make a unique compound... 1 and 2 (again, 1 is the same 4 and 2 is the same as 3; prove by drawing) Carbon 1 is CH3; carbon 2 is CH.
      Recall that isobutane can also be called 2-methylpropane because of its 3-carbon chain + methyl group on carbon #2.

      Carbon #2 (CH) in isobutane is known as tertiary because it has 3 other carbons (in this case: CH3, CH3, and CH3) attached to it. So we can simply say that when R bonds to carbon #2, a tertiary carbon, the new compound formed is tert-butyl (t-butyl).

      This brings us to our final possible compound. Bonding R to the carbon #1 of isobutane yields isobutyl.

      It's on the first carbon and this carbon has 1 other carbon attached to it so can we call it "n-isobutyl"? NOPE. Recall "n" stands for "normal" and isobutane is NOT "normal butane" it is an ISOMER. You cannot call something normal and iso ("n" and "iso") simultaneously. Though they may share the same formula, they may not share the exact same properties. One is a straight chain one isn't. So it is one or the other.


      straight chain butane = n-butane --------------can form-------------> n-butyl OR s-butyl
      reconfigured branch chain butane = isobutane ---can form---> i-butyl OR t-butyl

      In other words:
      1.) butane can be either normal butane or isobutane.
      2.) normal butane single bonding to a functional group and losing a hydrogen forms "normal butyl" or "secondary butyl"
      3.) isobutane single bonding to a functional group and losing a hydrogen forms isobutyl or tertiary butyl.


      With this fundamental understanding we can now clearly elaborate that "iso" is the prefix used to describe 2 separate methyl (CH3) groups attached to a common carbon. It looks like a Y shape.

      The smallest possible isoalk_ane_ is isobutane. From here you can go to isopentane, isohexane, etc. You just keep your two separate CH3's attached to the central carbon (that Y shape) and keep adding carbons to the base of the branch while subtracting hydrogens appropriately.

      The smallest possible isoalk_yne_ is isopropyl.

      Verify this by drawing on a piece of paper!

      Start by drawing propane (C3H8). The only way to draw it is linear (CH3-CH2-CH3). No matter how you arrange it you always end up with this. That is why there is no isoprop_ane_. Now bond this to R. You lose a hydrogen. Here you should visualize 2 different paths in your mind. You can bind on the 1st carbon (3rd carbon has an identical outcome) and continue your straight chain to make n-propyl. Or you can bond to the 2nd carbon to make isopropyl.

      Add CH3 to the base (bottom of the Y) of your isopropyl and you now have isobutane. Open up your isobutane with a bond on the base. This changes CH3 to CH2 and an open ended bond (to R). Now you have isobutyl. Add another CH3. That makes isopentane. Continuing in the same direction, bond the CH3 to R. That makes isopentyl. You get the idea. You can see the base of your "Y" shape getting longer and longer.

      If you choose to start on the 1st or 3rd carbon (doesn't matter) from propane it's the exact same idea but you will grow your chain like propane to n-propyl to n-butane to n-butyl to n-pentane, etc as long as you keep adding on the outside carbons.

      Side note it is important to note that R, depending on what it is, will change your name. Adding hydroxide (OH) on to butane, for example, makes butanol. Where we used just butyl before you would now have to say butanol. Or if you add chloride it would be something like butyl-chloride, chlorobutane, etc but this is another topic.
      (18 votes)
  • blobby green style avatar for user sameenansari
    At Sal talks about how the group would be called (1,1 dimethylethyl)... in the video Organic Chemistry Naming Examples 3 at , Sal says that for the systematic naming, you use the prefix "bis" not "di" to indicate more than one of the same group attached to the main backbone. My question is that if there if more than one carbon attached to one point on an alkyl group, is it always the case that the prefix "di" is used instead of "bis"??
    (9 votes)
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    • leaf green style avatar for user Dave M
      Sal is wrong. We do not say "bis" except in rare circumstances. That's because we're organic chemists. Go to biology or move on to biochemistry...they use the terms "bis" "tris" and "tetrakis" all the time. Chemists rarely use those terms. These are NOT IUPAC (systematic) prefixes. This happens a lot. Biologists/biochemists call alcohols "hydroxyl" groups. That's fine but no chemist calls them that, it's not IUPAC. They are hydroxy substituents to chemists or primary, sec, tertiary alcohols, nothing else.
      (2 votes)
  • blobby green style avatar for user shubham.kumar0109
    how are iso butyl & normal butyl different?
    (6 votes)
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  • blobby green style avatar for user Rudradutt Simpson Ranade
    which of the ways of naming are better?
    (3 votes)
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  • blobby green style avatar for user crx_curve_carver
    At , I thought a butyl group was a 4 carbon long chain, but Sal draws a a chain that looks to me like it's only 3 carbon's long with a methyl group attached. Where am I getting things wrong? Same thing at . The longest carbon chain I see there is 2. Help!
    (3 votes)
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    • leafers sapling style avatar for user Anita Hsieh
      The one at , he drew a giant Y. For each end points of the Y, it has C on it, then in the middle of the Y, there's another C there too. Making it a total of 4 carbons. at , the ends of each line is one C plus the C where the 3 lines connect, making it a total of 4 carbons too.

      Once you've established your longest carbon chain, which are the cyclos in this case, you don't need to count the groups attached to the cyclos according to their longest chain. You just need to have your main body (the cyclo), then add to the name the numbers of C, or functional groups attached to the main body.

      I hope this is somewhat clearer.
      (2 votes)
  • blobby green style avatar for user Arun Kanhere
    It seems to me that the systematic naming was much easier because it was branching off a ring structure. If it was a simple Butylpentane, we would've had to include which carbon the butyl was attached to. How does that fit into (2-methylpropyl)cyclopentane or (1, 1dimethylethyl)cyclopentane?
    (2 votes)
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    • blobby green style avatar for user ianfk3120
      I'm sure Sal will cover this in the later lessons but you have to be careful about your longest chain of carbons when your dealing with non-cyclo chains. In a non-cyclo configuration the base name reflects the longest chain of carbons. By adding a butyl group to a pentane chain you will have changed your longest chain and hence your base name. Draw it out and I think you'll see, there is actually no way to end up with a #-butyl-pentane configuration.

      For example: draw a butyl group off the second carbon of a pentane chain. You should come up with something like 4-methyl-octane. Add the butyl group to the 3rd carbon and you get 3-ethyl-heptane. Add it to the first or last carbon and you get Nonane.
      (4 votes)
  • leaf blue style avatar for user Sahil Kabra
    At , Sal says that the "iso" prefix can be only used below 5 or 6 carbons however compounds do have isopentyl, isohexyl and isoheptyl. How do they exist then?
    (2 votes)
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Video transcript

Let's see if we can get the molecular structure for butylcyclopentane. So you just break this up the way we've done it in the last several videos, the suffix is -ane, so it is an alkane, all single bonds. So single bonds. It's pentane, so we're dealing with five carbons on the base, or on the backbone. So this is five carbons and it's a cyclopentane, so it's five carbons in a ring. So its five-carbon ring is the backbone, and then we have a butyl group added to that five-carbon ring. Now, you might say, hey, Sal, how do I know which carbon to add it to? When you're dealing with a ring and you only have one group on the ring, it doesn't matter. Let me just show you what I mean. So let's draw the five-carbon ring. Let's draw the cyclopentane. So it'll just be a pentagon, so one, two, three, four, five, and it's a ring, so you can connect them. One, two, three, four, five. Now, it doesn't matter where I draw the butyl group. It's all symmetric around there. We just have a ring and it's connected to a butyl group at some point. It'll start to matter once we add more than one group. So we can just pick any of these carbons to add the butyl group to. Now, just as a review, the but- prefix, that refers to, remember, methyl, ethyl, propyl, or meth-, eth-, prop-, but-. This is four carbons. This is a four-carbon alkyl group. So let me just add it here. I could have added it to any of these carbons around this cyclopentane ring. So if I just add it right here, so I'm going to have four carbons. So one, two, three, four. That is the butyl part of this whole thing. And then let me just attach them up. So you might be tempted to just draw this right there. And actually, this would be right. This is butylcyclopentane. But a question might arise. I just happened to connect the cyclopentane to the butyl at this first carbon on the butyl right there. I could have just as easily done it like this. I could have just as easily had it like this, where-- let me draw my butyl again, so I have one, two, three, four. So, once again, this is a butyl, but instead of being bonded to the cyclopentane on my first carbon, maybe it's bonded right here. Let me do it with that yellow color. Maybe it's bonded right here. This seems like maybe this could also be butylcyclopentane. It looks like we have a butyl group. This is a butyl right here. I drew a butyl group right over here, and I also drew a butyl group right over here. But these are fundamentally two different molecular structures. I'm touching the first carbon here. I'm touching the second carbon over here. Now, there's two ways to differentiate this. One is the common naming and one is the systematic naming. So let me differentiate between the two. So in the common naming, and this can get a little bit involved, and this frankly is probably the most complicated part of naming organic compounds. Systematic is often more complicated, but it's easier to systematically come up with it. So there's a common and then there's a systematic. So the common way of doing it is, if you just say butylcyclopentane, that implies that you are bonding to the first or, depending on how you view it, the last carbon in the chain. So this right here is butylcyclopentane. This right here is not just butylcyclopentane. What you would do is you definitely have a cyclopentane ring, so this would definitely be a cyclopentane. Let me put some space here. This is definitely going to be a cyclopentane. And you do have a butyl group on it, so we do have a butyl group, but because we are bonded-- we aren't bonded to the first carbon. We're bonded to a carbon that is bonded to two other carbons. We call this sec-butylcyclopentane, so this is sec-. And everything I'm doing is obviously free-hand. If you were to see this in a book, the sec- would be italicized, or sometimes it would be written as s-butylcyclopentane. And this sec- means that we have attached to a carbon that is touching two other carbons. So you look at the butyl group, and say, well, which of these carbons is attached to two others? It's either that one or that one. And regardless of whether you're attached to this or this, if you think about it, it's fundamentally the same molecular structure. So that's what you do when you're attached to that guy right over there. But what about the situation where we're dealing with just the common names right here? What about the situation where it looks like this? So we have our cyclopentane right there, and we have a-- I guess we could call it a butyl group. It'll have four carbons in it, but let's say that the four carbons look something like this. Let's say there are four carbons, so we have one, two, three, four carbons and we're bonded to this one right over here. So whenever you're bonded to one end of the four-carbon group and it branches off at the other end, and it seems a little complicated, this only deals for alkyl groups below five or six carbons, this we call an isobutyl group. So let me write this down. So this right here, is sec-butyl, or s-butyl sometimes for short. This right here, this right there is iso-butyl. It's actually-- iso- that is an iso-butyl group. And then the last thing to worry about when you're dealing with butyl groups is something like this. Let me draw it. So you could also draw four carbons like this. You have one carbon, two, three, four. One, two, three, four carbons and you're attached over here. Now this naming, this group right here-- and you're going see the systematic naming is much easier for these compounds. This group right here, over here the carbon you're attached to is attached to two other carbons, so it is sec-butyl. When you're attached to three carbons, it is t-butyl, or tert-butyl, so this right here is a tert-butyl group, or sometimes called a t-butyl. And I really want you to understand the difference here. The common naming, it's easier to say and easier to spell, but it's sometimes a little confusing. This is just straight up butyl so you would call this butylcyclopentane. This is sec-butyl, because you have this guy connected to two carbons. That's where the sec- comes from. Sometimes it'll be s-butyl. So this could be called sec-butylcyclopentane or s-butylcyclopentane. This, because we're attached to the end away from this branching off, is still a butyl group, since we have four carbons. But since we're attached here, this is iso-butyl, so this is iso-butylcyclopentane, And then finally, since the carbon we're attaching to is attached to one, two, three other carbons, it is a tert-butyl or a t-butyl group. So this is t-butylcyclopentane. That's the common naming. So maybe I should clear out systematic here just so it's clear to you that everything we've done here is common naming. So let me write it down. It won't hurt to write them down again because the more familiar you are with these, the better. So this is just butylcyclopentane. This is s- or sec- butylcyclopentane. And this is iso-butylcyclopentane. I'm going off the screen here. And then finally, this is tert-butyl, or t-butylcyclopentane. Now, I said these are the common naming. What are the systematic naming? Well, in the systematic, this is still butylcyclopentane. So let me write this down. Systematic, this is still butylcyclopentane, which makes sense. This is very clearly a cyclopentane. This is very clearly a butyl group. But in the systematic naming, what we try to do is we try to name this group right here just as we would name a traditional chain, but we ended it with an -yl. So if you look at this right here, what we do is we just consider the chain where we attach. We attached over here, so the longest chain from that point is there and there. So if you look at it like that, it looks like you have one, two, three carbons, and you have one carbon attached on the beginning. So this little group right here in the systematic naming, this looks like a one, two, three. Three carbons, that's the prop- prefix, so we're dealing with a prop-, and it's all going to be one group, so it's a propyl group. This is a propyl group, but it has a methyl-- remember, meth- is one carbon. It has a methyl group attached on the first carbon. So this is 1-methylpropyl. Now, that describes just the group. 1-methylpropyl describes just this part right here. That describes just that right over there. And then to have the whole compound, to describe the whole compound, you put this in parentheses, so this is the systematic naming. So 1-methyl-- I put an L there. Let me do it in the same color. 1-methyl, because you're starting where you're attaching. So 1-methyl, you have a methyl group right there on that first carbon. It's a propyl chain. One, two, three, propyl, and then you would say cyclopentane. That's the systematic name for that. Now if you look at this one right here, and the common name is iso-butyl, what you do is you look at where we attach. Where we attached is one, two, three carbons. So once again, I'm doing that same one, two, three carbons. So once again, this is a propyl. Prop- is for three, but with a methyl group now is attached to the one, two, the second carbon. So this is 2-methyl. Let me make some space here. This is 2-methyl. So that describes this group right here. That describes this entire group cyclopentane. Remember, this is a systematic name. You might sometimes see this referred to as iso-butylcyclopentane or 2-methylpropylcyclopentane. And this is actually a -yl. I spelled it wrong. And then finally, we do the same exact idea here, but it becomes a little bit more interesting. Over here, we are attached to this carbon and the longest chain I can do starting with that carbon is just one chain right there. So we just have a two-carbon chain, right? One, two. The prefix for a two carbon is ethyl, or eth-. Eth-, and since it's a group, ethyl. And then we have two methyl groups attached right over there, and it's attached on the one carbon, right? This is one and this is two. We call the one carbon where we are attached to the broader chain. So what this is going to be, you would actually write one comma one to show that we have two groups attached to the first carbon and both of them are methyl. So we write 1,1-dimethyl, di- for two, dimethyl. So this entire group right here, which we also called t-butyl, in systematic naming is 1,1-. We have two groups attached to this first carbon. 1,1-dimethylethyl. That's this whole thing. This is the ethyl, and then we have two methyls attached there. that's why we write 1,1-. They're both attached to the one. We have two of them. That's why we wrote di- over there. So it's 1,1-dimethylethyl- and then finally, cyclopentane. So hopefully, that doesn't confuse you too much. I think if you watch the video over and over and try to practice it with your own problems, you'll see that the systematic name way is actually pretty, pretty logical. And actually, if you have more than five or six carbons in the group, they always or they tend to always use the systematic naming.