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Organic chemistry
Course: Organic chemistry > Unit 3
Lesson 1: Naming alkanes- Representing structures of organic molecules
- Naming simple alkanes
- Naming alkanes with alkyl groups
- Correction - 2-propylheptane should never be the name!
- Common and systematic naming: iso-, sec-, and tert- prefixes
- Naming alkanes with ethyl groups
- Alkane with isopropyl group
- Organic chemistry naming examples 2
- Organic chemistry naming examples 3
- Naming a cycloalkane
- Naming two isobutyl groups systematically
- Organic chemistry naming examples 4
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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.
Want to join the conversation?
- When you say common and systemic naming do both come under the IUPAC naming system?(58 votes)
- Actually IUPAC generally uses systematic naming, but for certain cases they have retained the older 'common' naming conventions. Butyl is one of those cases. So the IUPAC nomenclature is actually sec/iso/tert. This explains it: http://en.wikipedia.org/wiki/Butyl(11 votes)
- 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)
- That's not very big. The longest name is 189,819 letters long for titin (a protein). I fear, it's full name won't fit this window so you'd better google it.(40 votes)
- 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)
- Valentine,
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 :)(28 votes)
- why we say the branched butyl group as iso butyl group?(5 votes)
- 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:
/ \ /
CH3-CH2-CH2-CH3
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-CH-CH3
......... 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-CH
-CH3
.........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:
1-2-3
...4
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 its3-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.
Recap:
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.
Conclusion:
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.(17 votes)
- AtSal talks about how the group would be called (1,1 dimethylethyl)... in the video Organic Chemistry Naming Examples 3 at 13:17, 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"?? 8:12(9 votes)
- 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.(1 vote)
- how are iso butyl & normal butyl different?(6 votes)
- They are structural isomers of each other. n-Butyl (normal butyl) is a straight 4 carbon chain, and as a substituent it is attached from its first carbon (the one on the end). iso-butyl also has four carbons, but they are not arranged as a straight chain. iso-butyl is a straight 3 carbon chain that has a CH3 group attached at its second carbon, and as a substituent it is attached from its first carbon. This is probably easier to see by looking at structures: http://www.masterorganicchemistry.com/2011/11/10/dont-be-futyl-learn-the-butyls/(6 votes)
- which of the ways of naming are better?(3 votes)
- The systematic, because you don't have to remember too many prefixes(4 votes)
- 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 5:35. The longest carbon chain I see there is 2. Help! 6:36(3 votes)
- 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 5:35, the ends of each line is one C plus the C where the 3 lines connect, making it a total of 4 carbons too. 6:36
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)
- 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)
- 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)
- 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? 5:47(2 votes)
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.