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Organic chemistry
Course: Organic chemistry > Unit 3
Lesson 2: Naming alkanes, cycloalkanes, and bicyclic compoundsAlkane and cycloalkane nomenclature III
Practice naming hydrocarbons with branches and rings. Also discusses classification of carbons: primary, secondary, tertiary, and quaternary carbons. Created by Jay.
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- at, could it also be it be named 2-methylbutyl since you can number the chain so the methyl group can be located on the 2 carbon instead of the 3? 6:00(55 votes)
- Not as a substituent. Because we're assuming it's attached to a bigger molecule, you start numbering from the carbon that's bonded to the parent chain.(62 votes)
- Why is it named 1-methyl ethyl and not 1-ethly methyl . I thought it was named alphabetically?(17 votes)
- the ethyl is the largest chain so it will be at the end(29 votes)
- Athe used the prefic "neo," but never explains why. The substituent seems to follow the pattern which uses the "tert" prefix so I'm confused why it's not called tert-pentyl. 6:42(19 votes)
- Neo prefix is used for when the (CH3)3 branch is attached to another carbon/carbon chain.
Tert prefix is when the (CH3)3 branch is attached to any R group (i.e. any other substituent like Br/Cl etc).
If we look on the video for the neo-pentyl chain, if carbon number 1 were another substituent (i.e. not carbon/carbon chain) it would be tert-butyl rather than neo-pentyl.
Helpful image here:
http://www.personal.psu.edu/users/t/h/the1/shrpic1.gif(6 votes)
- i cant understand why do we use "iso"......
thanks in advance(3 votes)- When you see a "Y-structure" (Look at the example 2 and 4 in the beginning) as a substituent attached to your longest carbon-chain, the backbone of your molecule, you should immediately think "-Iso".
Why?: It´s because, as you can see in example 2 and 4, you have two protruding methyl-groups attached from a singe carbon (The second carbon in example 2 and the third carbon in example 4), you could call it "-Sec" but whenever you see a single "Y-structure" without any other branching it is more common to call it "-Iso".
Iso in organic chemistry means "the same", or "no change" and as you can see in the "Y-structure", it looks the same from each angle.(9 votes)
- Can the neopentyl also be text-pentyl?(2 votes)
- IUPAC Name of Tert-pentyl is 1,1-dimethylpropyl
IUPAC Name of neopentyl is 2,2-dimethylpropyl :D(8 votes)
- I still do not understand the "sec" and "tert" concept with naming complex substituents. Can someone please explain this to me.(4 votes)
- For classifying carbons; alpha, beta and gamma are also used instead of numbering them with a degree sign. Is this statement right?(1 vote)
- The alpha carbon is the carbon attached to a functional group; the beta carbon is the next one out; the gamma carbon is the next one after that, and so on. The furthest carbon atom from the functional group is called the omega carbon. For example, linolenic acid is an omega-3 fatty acid containing three C=C bonds. (Omega is the last letter in the Geek alphabet. The 3 means that the double bond furthest from the functional group is between the third and fourth carbon atoms from the end).(5 votes)
- At aboutIs there any logic behind the use of the -am(yl) suffix and the neo- prefix? 6:00(1 vote)
- Amyl is the common name for a 5-carbon group. Pentyl is the *IUPAC name.*
Just as iso means a methyl group on the next-to-last carbon, neo means two methyl groups on the next-to-last carbon.(6 votes)
- At aroundcan 2,2-dimethylpropyl is neo-pentyl , why not tert-pentyl ? 6:50(3 votes)
- At, isn't that carbon bonded to 4 carbons 12:48(3 votes)
- no it bonded to 3 carbons and the extra one available bond is attached to hydrogen , and this is why its called Primary(1 vote)
Video transcript
So how do we name this molecule? Well, we start with the
longest carbon chain. So there are seven carbons
in my longest carbon chain. So I would call this heptane. And I number it to
give the substituent the lowest number possible. So in this example,
it doesn't really matter if I start from the
left or from the right. In both examples,
you would end up with a 4 for your
substituent there. Now, this substituent
looks different from ones we've seen before. There are three carbons
in it, but those carbons are not in a
straight-chain alkyl group. So if I look at it, right
there are three carbons, but they're not going
in a straight chain. They're branching
of branching here. So this is kind of weird. How do we name this substituent? Well, down here, I have
the same substituent, and I'm going to draw
this little zigzag line to indicate that that
substituent is coming off of some straight-chain alkane. And when you're naming a
complex substituent like this, you actually use the
same rules that you would use for a
straight-chain alkane. So you first identify
the longest carbon chain, which in this case
is only two carbons. So that would be an ethyl group
coming off of my carbon chain. So I'm going to go ahead and
name that as an ethyl group. I'm going to go ahead number
it to give my branching group there the lowest
number possible. So I go 1 and 2. So what is my substituent
coming off of my ethyl group? Well, that's a methyl group
coming off of carbon 1. So I name it as 1-methylethyl. OK, so now, that
complex substituent is named as 1-methylethyl. So I could go ahead and
put that into my name. So coming off of carbon
4, I have 1-methylethyl. And I'm going to put
that in parentheses. And all of that is coming off
of carbon 4 for my molecule. So 4-(1-methylethyl) heptane
would be an acceptable IUPAC way of naming that molecule. So if your naming your complex
substituent as 1-methylethyl, that's the official IUPAC
way, but there are also common names for these
complex substituents. So the common name for
1-methylethyl is isopropyl. So isopropyl is the common name. And isopropyl is
used so frequently that it's perfectly
acceptable to use isopropyl for the name of this
molecule as well. So you could have said, oh,
this is 4-isopropylheptane, and you would have been
absolutely correct. So that's yet
another IUPAC name. So iso means same, and it
probably comes from the fact that you have these two
methyl groups giving you this Y shape that are the same. So that's one
complex substituent, one that has three
carbons on it. Let's look at a bunch
of complex substituents that have a total of
four carbons on them. So all these guys have
a total of four carbons. And let's do that same
trick with the zigzag line so we can ignore the
rest of the molecule and just think about them
as being alkyl groups. So how do I name these? The same steps. You find your
longest carbon chain. So for this one, the longest
carbon chain would be 3. It's an alkyl group
so it's a propyl. And when you number
it, this would get a 1, this would get a 2,
and this would get a 3. So you have a methyl group
coming off of carbon 1 here. So it would be
1-methylpropyl as the name of that complex substituent. The common name for
that is sec-butyl. So butyl because there are
a total of four carbons. Let's do the next
one, so 1, 2, 3 again. So when I number
it 1, 2, 3, I can see that it would be
propyl once again. So I go ahead and
write propyl here. And what is coming
off of that group? Well, I have a methyl group
coming off of carbon 2 this time, so it would
be 2-methylpropyl for this complex substituent. The common name for
this is isobutyl. So butyl again because
there are four total carbons in this complex substituent. Iso because, once again, you
have these two methyl groups, so they're like the same,
so you get that Y formation. So that's isobutyl. The next one,
longest carbon chain, there are two carbons in
my longest carbon chain so that would be ethyl. And when I number my
longest carbon chain, I can see that I have
two methyl groups, and each of those methyl groups
is coming off of carbon 1. So I would say this would
be 1,1-dimethylethyl. So 1,1-dimethylethyl would be
the IUPAC name for this complex substituent. You will also see tert-butyl. So tert-butyl is probably
used even more frequently. Again, butyl because there are
a total of four carbons here. So those are the
three possibilities for a complex substituent
with a total of four carbons. Let's look at just a
few of the possibilities for complex substituents
that have five carbons. There are actually
much more than this, but these are the ones that
are most commonly used. So let's just focus
in on these two. So once again, we'll
draw our zigzag line to represent the fact
that this is actually connected to some
straight-chain alkane. And once again, we find our
longest carbon chain-- 1, 2, 3, 4. So that would be butyl. And when I number that
carbon chain-- 1, 2, 3, 4-- I can see that I have a methyl
group coming off of carbon 3. So it would be 3-methylbutyl
for the IUPAC name. And this is also
called isopentyl. So you could say isopentyl since
there are five carbons now, and iso, because again,
you have this methyl group and this methyl group
looking like a Y. They're like the same thing. Or I've seen this
called isoamyl before. So isoamyl or isopentyl are
acceptable IUPAC names as well. What about this
one on the right? Longest carbon chain-- 1, 2, 3. So that would be propyl. And numbering it 1,
2, 3, immediately it is obvious that you have
two methyl groups coming off of carbon 2. So it would be
2,2-dimethylpropyl, otherwise known as neopentyl
since once again you have five carbons for these. So again, there are many more,
and we'll stop with those. And so that gives
you an idea about how to approach naming
complex substituents. And of course, when you
name complex substituents, you have to use them
when you're naming straight-chain or
cycloalkane molecules. So let's look at a
cycloalkane molecule, and let's see how
to name this guy. Well, I have four
carbons in my ring, and I have four
carbons in this group. So tie goes to the cycloalkane. So remember from the
last video, if you have an equal number
of carbons in your ring as with your chain,
you're going to name it as an alkyl cycloalkane. The cycloalkane wins the tie. So there are four carbons,
so this'll be cyclobutane. So let's go ahead and
write cyclobutane here. And once you've
determined that you're going to name it
as a cycloalkane, then you have to look at
this complex substituent and say, OK, well, that's 1,
2, 3, so that would be propyl. And then when you number
that complex substituent 1, 2, 3, obviously there is
a methyl group coming off of carbon 1. So you would write
1-methylpropyl. And if you wanted to,
you could identify that 1-methylpropyl
as coming off of carbon 1 of your cyclobutane. So you could put this
in parentheses and write 1-(1-methylpropyl)cyclobutane. Or you could just
leave the one off, and say
(1-methylpropyl)cyclobutane, because, again, it is implied. What is the common name for
this complex substituent? So 1-methylpropyl, and
we go back up here, and we find 1-methylpropyl
was also called sec-butyl. So we could also have named this
molecule sec-butylcyclobutane. So let's go ahead
and write that. So sec-butylcyclobutane is a
perfectly acceptable IUPAC name as well. So it just depends. Do you want to do it
the official IUPAC way, or do you want to memorize
some of these common names? Let's do one more
thing in this video. Let's classify carbons. OK, so we're going
to something called classification of carbons. This is a topic that comes
up over and over again throughout inorganic
chemistry course, so the earlier you learn this
concept the better off you are. So classification of carbons. So let's say I have carbon
connected to three hydrogens, and then I also
have it connected to one other carbon
in some R group. I want to know how to
classify this carbon. So this carbon is connected
to one other carbon. So therefore, we
say it is primary. So that is a primary
carbon right there. Let's take off one of
the hydrogens, and let's put on another R group. So I'll make it R
prime to distinguish it from the first R group. So this time, if I wanted
to know the classification of this carbon, it's connected
to two other carbons, so it is said to be secondary. So it is a secondary
carbon like that. And I'm going to once again
take off one of the hydrogens. So I'll make it an R
double prime group. And now if I wanted to
classify my central carbon, now this is connected to
1, 2, 3 other carbons, so it is said to be tertiary. So that is a tertiary
carbon like that. And finally, I have one
more example, of course. I take off the last hydrogen. So now I have R, R
prime, R double prime, and R triple prime. So what is the
classification of this carbon now connected to
four other carbons? So it is said to be quaternary. So that is a quaternary
carbon right here. All right, so if I'm
trying to think about where some of these common
names come from I can see, oh, well,
right here I have-- if carbon's bonded
to two other carbons, well, that would be secondary. So S-E-C for my prefix. So let's go back up
here, and let's see if we can find those examples. So here I have this carbon
bonded to two other carbons. So this carbon was
said to be secondary, so I think that's
where this comes from. I've never seen that explained
in a textbook or anywhere, but it just makes sense. So it's ignoring the fact
that this carbon is actually attached to a ring. It's saying this carbon
on my complex substituent is bonded to two
other carbons, so it is secondary on that
complex substituent. What about tertiary? So carbon bonded to three other
carbons is said to be tertiary. So if I go back up here
again I can say, well, that would make sense, because
if I look at this carbon, it's bonded to three
other carbons, right? So I could say that
is a tertiary carbon. And once again I'm
ignoring the fact that this carbon is actually
bonded to another carbon on the ring. So if you just look at
the complex substituent, that carbon is said
to be tertiary, which I think is where
the name comes from. Let's do one more example
of assigning classification of carbons to this molecule. So let's look at this
carbon right here. This carbon is bonded to
one other carbon and three hydrogens. So this carbon is
said to be primary. This carbon right here is
bonded to two other carbons, so it is said to be a secondary. This carbon right here is
bonded to three other carbons so it is tertiary. This carbon is bonded to one
other carbon so it is primary. This carbon is bonded
to three other carbons, so it is tertiary. And all of the carbons
on the ring right here are bonded to
two other carbons, so they are all said
to be secondary. So that's a very
important skill to develop classifying your carbons. Again, this will come
up in future videos with different
functional groups.