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Naming alkenes examples | Alkenes and Alkynes | Organic chemistry | Khan Academy - YouTube
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Everything we've named so
far has been an alkane.
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We've seen all single bonds.
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Let's see if we can expand our
repertoire a little bit and do
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some alkenes.
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So let's look at this first
carbon chain right here.
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And actually, here I drew out
all of the hydrogens just to
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remind you that everything we
were doing before with just
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the lines, it really
was representing
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something like this.
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When you start having the
double bonds, and we'll
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explain it in more detail later
on, it actually starts
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to matter a little bit more to
draw the constituents, because
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there's actually different ways
that you can arrange it.
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Because these double bonds, you
can imagine, they're more
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rigid, you can't rotate
around them as much.
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But don't think about that
too much right now.
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Let's just try to name
these things.
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So like we always do, let's try
to find the longest chain
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of carbons.
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And there's only one chain
of carbons here.
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There's one, two, three, four,
five, six, seven carbons in
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that chain.
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So we're going to be
dealing with hept,
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that is seven carbons.
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But it's not going
to be a heptane.
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Heptane would mean that we
have all single bonds.
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Here we have a double bond, so
this is going to be an alkene.
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So this tells us right here
that we're dealing with an
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alkene, not an alkane.
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If you have a double bond,
it's an alkene.
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Triple bond, alkyne.
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We'll talk about that
in future videos.
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This is hept, and we'll put an
ene here, but we haven't
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specified where the double
bond is and we haven't
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numbered our carbons.
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When you see an alkene like
this, you start numbering
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closest to the double bond, just
like as if it was a alkyl
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group, as if it was a side
chain of carbons.
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So this side is closest to the
double bond, so let's start
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numbering there.
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One, two, three, four,
five, six, seven.
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The double bond is between two
and three, and to specify its
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location, you start at the
lowest of these numbers.
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So this double bond is at two.
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This is actually hept-2-ene.
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So this tells us that we have a
seven carbon chain that has
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a double bond starting-- the
ene tells us a double bond.
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Let me write that down.
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So this double bond right
there, that's what
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the ene tells us.
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Double bond between two carbons,
it's an alkene.
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The double bond starts-- if you
start at this point-- the
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double bond starts at number two
carbon, and then it will
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go to the number three carbon.
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Now you might be asking, well,
what if I had more than one
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double bond here?
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So let me draw a quick
example of that.
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Let's say I have something like,
one, two, three, four,
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five, six, seven.
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So this is the same
molecule again.
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One, two, three, four,
five, six, seven.
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The way we drew it up
here, it would look
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something like this.
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What if I had another double
bond sitting right here?
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How would we specify this?
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Well, once again we have
seven carbons.
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One, two, three, four,
five, six, seven.
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So we're still going to
have a hept here.
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It's still going to
be an alkene, so
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we put our ene here.
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But we start numbering it, once
again, closest to the
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closest double bond.
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So one, two, three, four,
five, six, seven.
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But now we have a double bond
starting at two to three, so
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it would be hept-2.
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And we also have another double
bond starting from four
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and going to five,
so hept-2,4-ene.
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That's what this molecule
right there is.
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Sometimes, this is the-- I
guess-- proper naming, but
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just so you're familiar with
it if you ever see it.
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Sometimes someone would write
hept-2-ene, they'll write that
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as 2-heptene, probably because
it's easier to say 2-heptene.
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And from this, you would be able
to draw this thing over
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here, so it's giving you the
same amount of information.
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Similarly over here, they
might say 2,4-heptene.
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But this is the specific,
this is the correct
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way to write it.
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It let's you know the two and
the four apply to the ene,
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which you know applies
to double bonds.
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Let's do a couple more.
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So over here, I have a double
bond, and I also have some
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side chains.
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Let's see if we can figure
out how to deal with
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all of these things.
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So first of all, what is our
longest chain of carbons?
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So we have one, two, three,
four, five, six.
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Now we could go in either
direction, it doesn't matter.
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Seven carbons or
seven carbons.
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Let's start numbering closest
to the double bond.
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The double bond actually will
take precedence over any other
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groups that are attached
to it.
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So let's take precedence--
well, over any other
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groups in this case.
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There will be other groups that
will take precedence in
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the future.
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But the double bond takes
precedence over this side
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chain, this methyl group.
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But it doesn't matter in this
case, we'd want to start
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numbering at this end.
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It's one, two carbon, three
carbon, four carbon, five
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carbon, six carbon,
seven carbon.
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So we're dealing with
a hept again.
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We have a double bond starting
from the second carbon to the
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third carbon.
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So this thing right here, this
double bond from the second
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carbon to the third carbon.
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So it's hept-2,3-ene-- sorry,
not 2,3, 2-ene.
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You don't write both
endpoints.
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If there was a three, then there
would have been another
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double bond there.
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It's hept-2-ene.
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And then we have this methyl
group here, which is also
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sitting on the second carbon.
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So this methyl group right there
on the second carbon.
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So we would say
2-methyl-hept-2-ene.
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It's a hept-2-ene, that's all of
this part over here, double
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bonds starting on
the two if we're
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numbering from the right.
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And then the methyl group
is also attached
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to that second carbon.
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Let's do one more of these.
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So we have a cycle here, and
once again the root is going
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to be the largest chain or
the largest ring here.
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Our main ring is the largest
one, and we have one, two,
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three, four, five,
six, carbon.
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So we are dealing with hex as
our root for kind of the core
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of our structure.
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It's in a cycle, so it's
going to be cyclohex.
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So let me write that.
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So it's going to be cyclohex.
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But it has a double
bond in it.
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So it's cyclohex ene,
cyclohexene.
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Let me do this in a
different color.
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So we have this double bond
here, and that's why we know
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it is an ene.
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Now you're probably saying, Hey
Sal, how come we didn't
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have to number where
the ene is?
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So if you only have one double
bond in a ring, it's assumed
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that one end point of the double
bond is your 1-carbon.
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When you write just cyclohexene,
you know-- so
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cyclohexene would look
just like this.
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Just like that.
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You don't have to specify
where it is.
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It's just, one of these are
going to be the double bond.
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Now when you have other
constituents on it, by
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definition or I guess the proper
naming mechanism, is
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one of the endpoints of the
double bond will be the
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1-carbon, and if any of those
endpoints have something else
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on it, that will definitely
be the 1-carbon.
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So these both are kind of the
candidates for the 1-carbon,
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but this point right here also
has this methyl group.
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We will start numbering there,
one, and then you want to
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number in the direction of the
other side of the double bond.
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One, two, three, four,
five, six.
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So we have three methyl
groups, one on one.
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So these are the-- let me circle
the methyl groups.
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That's a methyl group
right there.
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That's a methyl group
right there.
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That's just one carbon.
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So we have three methyl groups,
so this is going to
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be-- it's at the one, the
four, and the six.
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So it is 1, 4, 6.
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We have three methyl groups, so
it's trimethyl cyclohexene.
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1, 4, 6-trimethylcyclohexene.
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That's what that is, hopefully
you found that useful.
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