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Organic chemistry
Course: Organic chemistry > Unit 9
Lesson 1: Naming benzene derivativesNaming benzene derivatives
How to name benzene derivatives, with examples. Created by Jay.
Want to join the conversation?
- At,why did the aldehyde group got preference?Could it not be benzoic acid.If not why? 9:08(13 votes)
- because in order of preference of functional groups ''aldehyde'' comes before the group -OH
for more information refer to link below
http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry
also it gives the lowest no possible for the compound.(14 votes)
- Can I use Ortho, Meta, and Para if I use Benzene instead of xylene in the name? So Ortho Benzene?(5 votes)
- No, you cannot. Using "xylene" designates that there are two methyl substituents, so it is necessary in the name. Ortho-benzene would tell you that there are two substituents that are ortho to each other, but not what substituents they are.(10 votes)
- what is meant by lowest sum rule(1 vote)
- The lowest sum rule states that, when you can number a chain from two different ends and get two different sets of numbers for the substituents, you choose the name in which the sum of numbers is the lowest. For example, you might name a compound as 3,9-dimethyldecane or 2,8-dimethyldecane. The second name has the lowest sum of numbers, so that is the correct name.
Similarly, you might name a compound 3,4,9-trimethyldecane or 2,7,8-trimethyldecane. The sum of the numbers in the first name (16) is less than the sum of the numbers in the second name (17). But the first name is incorrect because another rule takes precedence: the rule of first point of difference. Here, the name that has the lower number at the first point of difference is the correct one. Thus, 2,7,8 is chosen over 3,4,9 because 2 is lower than 3. Note that this rule also gives the correct name for disubstituted chains.
The lowest sum rule is thus a special case of the rule of first point of difference. It works only for disubstituted chains. The rule of first point of difference works for all levels of substitution. Thus, it is preferable to avoid using the lowest sum rule and use instead the rule of first point of difference.(9 votes)
- AtJay says methoxy for ether. However at 9:39he mentioned something called an anisole that was the same. So why do we not use anisole? 3:29(4 votes)
- HI SweetShruti
Because when we name the organic compounds we have to see that which compounds possess greater priority ... The functional group with the highest priority will be the one which gives its suffix to the name of the molecule.
For your Question,, in that compound Aldehyde group has greater priority than the Ether and yes ofcourse the hydroxyl. therefore we have to write the Suffix of Aldehyde rather than ether (Anisole).
Hope it Answered your question!(3 votes)
- can I write a phenol as a hydroxybenzne?(3 votes)
- You can write it, and people will understand what you mean, but it is wrong. The only correct name is phenol.(5 votes)
- athe called it 2,4,6-Trinitro toulene, can we also call it sym-Trinitro toulene 11:00(2 votes)
- Yes, sym-trinitrotoluene is one of the names that has been used.
But you should avoid the name because it is not an official name.
2,4,6-Trinitrotoluene is an accepted name, but the IUPAC name is
2-methyl-1,3,5-trimethylbenzene.(3 votes)
- Atwhy is the -OH branching off carbon 4 named as a hydroxy instead of the "special" name phenol? 9:55(1 vote)
- The aldehyde group takes higher priority in the naming than the OH group, so the base name of the compound is benzaldehyde.
The OH and OCH₃ groups are then named as hydroxy and methoxy substituents.(4 votes)
- At,instead of being 4-hydroxy-3-methoxy benzaldehyde can it be 3-methoxy-4-hydroxy benzaldehyde? 10:03(1 vote)
- The first one is correct, as it is ordered alphabetically - the numbering does not come into play when determining what order they are in the name.(4 votes)
- What is the position of benzene in the priority table? How do we know whether it is a parent or a substituent in a given compound?(2 votes)
- At, did Jay just say 'dot' structure? Did he mean 'bond-line' structure or something like that? 0:45(1 vote)
- Bond line structures where we only focus on carbon atoms is a simplified version of Lewis dot structures. Really all the different ways to draw organic molecules are just variations of Lewis dot structures. So either name can be considered correct, but bond-line is specific.
Hope that helps.(3 votes)
Video transcript
Benzene was first discovered
by the great Michael Faraday. And at that time,
chemists could figure out the molecular
formulas of compounds, but they didn't know the
exact dot structures. And so chemists knew
that benzene contained six carbons and six hydrogens,
but they weren't sure exactly how those
atoms were connected. And so there were several
different proposals for structures for benzene. And the winning
structure was proposed by August Kekule who said
that benzene contains six carbons in a
ring, and then you have alternating single and
double bonds in that ring. And the story goes that Kekule
came up with this dot structure when he had a dream
and he saw some snakes bite each other's tails. And the snakes then
whirled around in a circle. And that gave Kekule
the idea for the ring. So this is one possible
dot structure for benzene but I didn't have to draw my
double bonds in this place. I could actually show
a resonance structure for benzene. I could take these electrons,
move them over here and push these
electrons over here. And then these electrons
would be over here. And so a resonance
structure for benzene. I could have my pi
electrons over here, over here, and over here. So either one of these
Kekule structures is an acceptable dot
structure for benzene. Now remember, in reality, since
these are resonance structures, the actual molecule is more of
a hybrid of these two molecules. And with that in mind,
sometimes chemists will prefer to represent benzene
with the six carbons in a ring and with a circle
here in the center to represent the delocalization
of those pi electrons. So maybe this is the lazy
way to represent benzene. And this is called
a Robinson circle after the great synthetic
organic chemist, Sir Robert Robinson. And so sometimes
the Robinson circle is a useful way to
represent benzene. And sometimes a Kekule structure
is over here on the left. And you would use
a Kekule structure if you're trying to show
the mechanism of reactions that benzene does. Let's look at how to name
derivatives of benzene. So here we have benzene with a
methyl group coming off of it. And so you could just call
this molecule methyl benzene. So that's one
possible name for it. But most people don't
call it methyl benzene. It's called toluene. So it's such a common
molecule in organic chemistry that toluene an
acceptable IUPAC name. And in this example, we
have an alkyl substituent that has only one carbon. This methyl group has only
one carbon versus the six in the benzene ring. Well, what happened if your
alkyl group had more carbons than your benzene ring, which
is the situation over here on the right. So if we count up how
many carbons we have, we have a total
of seven carbons. And so in this
case, we're actually going to mean this is an alkane
and name the benzene ring as a substituent coming
off of our alkane. So a seven carbon alkane
would be called heptane. So I can go ahead and
write heptane here. Then I have a benzene ring
coming off of carbon 4. And when you're naming a
benzene ring as a substituent, so it would be C6, H5
substituent coming off of my ring here, we
call it a phenyl group. So I have a phenyl group
coming off of carbon 4. So this would be
4-phenyl heptane as the IUPAC name
for this molecule. Let's look at some
other examples where we see benzene with
one group on the ring. And these are all very famous
monosubstituted benzenes. And because they are so
famous, their common name is acceptable in
IUPAC nomenclature. So a benzene ring with an OH
group on it is called phenol. And we can use that when
we are naming molecules. Benzaldehyde would be
an aldehyde coming off of a benzene ring. And benzaldehyde is, of
course, famous for having the smell of almonds. It's a really, really
wonderful smell which is also why
you'll hear some of these molecules referred
to as aromatic compounds. Originally, it was
because of the smell. And we're going to see in
future videos what aromatic means in a chemistry sense. Over here, we have
a benzoic acid, a carboxylic acid
functional group coming off of our benzene ring
here like that. And so, here I have seven of
the most famous and most common monosubstituted
benzene derivatives. And so these are molecules
that most professors will have you memorize because you
can use these names when you're trying to name more complicated
benzene derivatives. So commit these to memory. Let's look at some disubstituted
benzene derivatives. So this molecule
over here on the left has two methyl groups coming
off of the benzene ring. And so this first molecule has
a methyl group coming off carbon 1, and coming off of carbon 2. So we could call this
1, 2 dimethylbenzene. That would be an
acceptable IUPAC name. But whenever you
have a benzene ring with two methyl groups on
it, the common name for that is xylene. And so if we wanted to
call this molecule xylene, technically, all three of these
molecules will be xylenes. They're all benzene ring with
two methyl groups coming off of it. And so we have to distinguish
these xylenes from each other. And so when you
have two groups that are right next to each other
on a benzene ring, in this case my methyl groups,
my methyl groups are right next to each
other on my benzene ring, we say that that
relationship is ortho. So I could call this
molecule ortho xylene. And that would be
another acceptable name. And sometimes you'll
just see an O there. So you call it o-xylene. And that's fine too. We go over here
to this molecule. This is also xylene. But we can see the
methyl groups are in slightly different positions. Right now, we would have
1, 3 dimethylbenzene. So 1, 3 dimethylbenzene
would be an acceptable name for this molecule. And when you have
two groups that are a carbon away
from each other, so this relationship is said to
be meta in organic chemistry. So you could call this
a meta, let me take off that m here as I am
running out of room here. You can call this meta-xylene. So this is meta-xylene
or just m-xylene. And then finally
another xylene molecule. This time our two methyl groups
would be at carbons 1 and 4. So you call this 1,
4 dimethylbenzene. Let me go ahead
and write that in. 1, 4 dimethylbenzene would
be one IUPAC name for it. But again, most
people would name this as a xylene derivative. So it's a xylene derivative. This time my two groups
are opposite each other. So they're opposite
each other on the ring. And we call this relationship
para in organic chemistry. So you could say it's
para-xylene or also p-xylene. Let's look at some more examples
of disubstituted benzene rings. And so here we go. We're actually going to use
the monosubstituted derivatives that we talked about above. So if I look at this molecule
over here on the left, I can see that this is the
phenol portion of the molecule. So I can go ahead and
say that this is phenol. And then in terms of
identifying the bromine, I have two options here. I could use a number. I could say that
bromine is at carbon 4. So I could call
this 4-bromophenol. And that's an
acceptable IUPAC name. Or I could use the OMP system
that we talked about above. And when you have two groups
that are opposite on the ring, we call that para. So I could also call this
molecule para-bromophenol. And that's an
acceptable name as well. Or I could even shorten
it to p-bromophenol. Let's do this disubstituted
benzene ring over here. And if I look at it, I can
see that is benzoic acid. That's one of the ones
that we memorized above. So I can go and
write the parent name as being benzoic
acid right here. And I now have to
identify my substituent coming off of my benzene ring. So benzoic acid would
make this carbon 1. And then my substituent
is coming off of carbon 3, and my substituent
is a nitro group. So I could call this
3-nitrobenzoic acid. Or I could say that the
relationship between those two things coming off my
benzene ring would be meta. So I could call this
meta-nitrobenzoic acid. So let me see if
I have room here. So meta-nitrobenzoic acid. Or I could just say
m-nitrobenzoic acid. So all of those would
be correct IUPAC names. Let's look at polysubstituted
benzene derivatives now. And so these are actually two
of the more famous examples that we could do. Once again, we're
going to try to find a monosubstituted
parent name here. And if I look, I can see
that right here, this would be benzaldehyde. And so usually aldehydes have
precedence over alcohols. And so that's why
we're going to name this as a benzaldehyde
derivative. So I'm going to go
ahead and-- let's see, just to make sure I have
enough room to start naming it by saying the parent name
is benzaldehyde right here. And since the aldehyde
gets precedence, this would get a carbon 1. We want to get the lowest number
possible to our substituents coming off my ring. So I'm going to go this way. And I can see that I have a
group coming off of carbon 3. And it's an ether group. So we talked about how to
name ethers as substituents. This would be a
methoxy substituent. So this would be
coming off of carbon 3. So I'm going to write 3-methoxy
benzaldehyde like that. And then I go over
here to carbon 4. And I could say I
have an OH group. And if I'm naming an
alcohol as a substituent, I would call this a hydroxy
or a hydroxyl group. And I'm going to
say that this is a hydroxyl group at carbon 4. So 4-hydroxy-3-methoxy
benzaldehyde would be the IUPAC name for this molecule. This molecule is better
known as vanillin. So the smell of vanilla is
probably my favorite smell. So I really enjoyed
doing labs that involved the vanillin molecule. Let's do one more, another
extremely famous example. I can see that I'm trying to
find a monosubstituted benzene derivative, that would
be toluene right here. So I'm going to go ahead and
put toluene as my parent name. And that would
make this carbon 1. The methyl group
would become carbon 1. So I go ahead and number to
get the lowest number possible. And I can see that I have three
nitro substituents located at 2, 4, and 6. So to finish my IUPAC
name, it would be 2, 4, 6. I have three nitro, so that
would be tri- as my prefix, so trinitro toluene. And so this is also a
very famous molecule. It's not normally called
2, 4, 6 trinitro toluene. Most people in
the general public would, of course,
know this as TNT. So the famous explosive. So this would be TNT. And we just named it. Of course, the name comes
from the nomenclature for polysubstituted
benzene derivatives. So hopefully this video just
shows you an introduction as to how to name molecules
with benzene in them.