Alkane with isopropyl group | Organic chemistry | Khan Academy

Let's try to name this
molecule right over here.

And so the first
thing we want to do

is identify the longest
chain of carbons.

Let's see, it could be
1, 2, 3, 4, 5, 6, 7, 8.

Or let's see, maybe it's 1,
2, 3, 4, 5, 6, 7, 8, 9, 10.

Yes, indeed, that's
the longest chain.

And if you go 1, 2, 3, 4, 5,
6, 7, or 1, 2, 3, 4, 5, 6, 7,

so those aren't the longest.

So the longest one is this one--
1, 2, 3, 4, 5, 6, 7, 8, 9, 10,

10 carbons.

And this is going
to be an alkane,

because it's all single bonds.

And an alkane that's
a chain of 10 carbons,

we would use a prefix dec
for 10, so this is a decane.

Let me write that
right over here.

This is a decane.

Now let's think
about the groups that

are attached to this decane.

So we have this group
right over here.

This has two carbons in it,
one carbon, two carbons.

And so, because it
has two carbons,

we would use the prefix eth.

Remember, meth is one
carbon, eth is two carbons.

And since it's a group and we're
not talking about the backbone,

this is an ethyl group.

And we have another ethyl
group right over here.

Two carbons attached
right over here.

This is also an ethyl group.

And now, this group right
over here is interesting.

We can count the carbons in it.

So it has 1, 2, 3 carbons.

So you could think about,
well, this has three carbons.

Our prefix for three
carbons is prop.

So you could say, hey, maybe
this is a propyl group.

This right over here, you could
say maybe this is propyl group.

And you wouldn't be completely
off base by saying that.

But we have to be a little bit
more careful when we name it.

Because a propyl
group, you would

assume that you're attaching
to one end of the propyl group.

But we're not attaching to
one end of the propyl group.

We're attaching essentially
to the second carbon,

to the middle carbon.

And this is a secondary carbon.

The reason why it's
called a secondary carbon

is because it's attached
to two other carbons.

If it was attached to
three other carbons,

it would be a tertiary carbon.

If it was attached
only one carbon,

it would be a primary carbon.

Since we're attached
to the secondary carbon

right over here,
this is sometimes

called a sec-propyl group.

And it's also sometimes called
isopropyl, an isopropyl group.

And you'll actually see
isopropyl a little bit more

frequently.

And these would both be
referred to as common names

for this group.

Now, if you wanted to
name this systematically,

then you would do it very
similar to the way that you

would name the entire molecule.

You would look for the
longest chain here.

And the longest chain
in this molecule,

starting with where
you are attached,

is a chain of two carbons.

And so the backbone
right over here is ethyl.

Let me write this
right over here.

It's an ethyl backbone here.

And then you could
view this carbon

as a group attached to
that ethyl backbone.

And we would start
counting right

where we are attached
to the main chain.

So this is the one carbon,
this is the two carbon.

So this right over here, this
is just one carbon group.

This right over here
is a methyl group.

So you have a methyl
group attached

to the one carbon
of an ethyl group.

So the systematic name
for this, and this

is a little bit less
typical for a group as small

as a propyl group, but you
could call this 1-methylethyl.

1-methylethyl the
systematic name for this.

Now, the systematic
name, you might say, hey,

why go through the pain of doing
this for something so simple

that we could just
call isopropyl?

This is useful if this was
a much larger or a much more

complex group that was
attached to this main chain.

But more typically,
and this is why

it's called the
common name, you'll

see this thing right over
here just called isopropyl.

And sometimes you
would see it called

sec-propyl even s-propyl.

Now that we've named
all of the groups,

let's think about what
carbons they are attached to

and where we can
start counting from.

And the way that this
is done is that you

would start counting from
the end of your carbon

chain, this decane backbone,
and you'd count from the end

that bumps into the
most groups faster.

So, for example, if you
count from this end,

this would be the 1 carbon,
2 carbon, 3 carbon, 4 carbon,

5 carbon.

On the 5 carbon, we
bump into two groups.

If we started over
here, this would

be the 1 carbon, 2 carbon, 3
carbon, 4 carbon, 5 carbon.

On the 5 carbon, we do bump into
a group, but only one group.

And we'd have to wait
until the 6 carbon

to bump into two groups.

So we'd get to the
two groups faster,

we would start
counting on this end.

So this is the 1 carbon, 2
carbon, 3 carbon, 4 carbon,

5 carbon, 6 carbon, 7
carbon, 8, 9, and 10 carbon.

And so when we think
about which groups

are we going to refer to first,
do we refer to the ethyl groups

first or do we refer to
this isopropyl group first?

We just think about
what letter they

start with in
alphabetical order.

So, for example,
these ethyl groups,

they clearly start with an e.

E comes before in the alphabet
than the i in isopropyl,

before the p in sec-propyl--
you would normally

ignore the sec or
the tert when you're

thinking about
alphabetical order--

and e also comes before
the m in methylethyl.

So we will talk about
the ethyl groups first.

So remember, our backbone,
10 carbons, all single bonds.

It's an alkane.

Since there's 10 of
them, it is decane.

And so let's talk
about the ethyls first.

And since we have two ethyls,
we can say that this is diethyl.

So let me write that down.

So we can write diethyl.

And of course, we
need to specify

where those two ethyls are.

One is at the 5 carbon,
one is at the 6 carbon.

So we could call
this 5,6-diethyl.

The 5 refers to
this ethyl group,

6 refers to this ethyl group.

And now we could talk
about the isopropyl group.

The isopropyl group is
also on the 5 carbon.

So we could say
5,6-diethyl-5-isopropyldecane.

Or if we wanted to
use sec-propyl instead

of isopropyl, we could
write sec-propyl here.

Or if we wanted to do
the systematic naming,

we could call this 5,6-diethyl.

Instead of writing
isopropyl here,

we could write all of this.

1-methylethyl here.

So actually, let me just
copy and paste that.

Let me just copy this
1-methylethyl right over here.

Copy and paste, let
me stick that there.

1-methylethyl.

And of course, the main
backbone is decane.

So all of these are
reasonable ways to name it.

This would be the common
way, where we use isopropyl.

Here we're doing it
more systematically

by calling the
group 1-methylethyl.