Naming alkenes examples - Chemistry, Class 12 Video Lecture

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