Common and systematic naming: iso-, sec-, and tert- prefixes Video Lecture

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.