Sn1 and Sn2: leaving group

- [Instructor] The S N
1 and S N 2 reactions

involve leaving groups.

Let's look at this pKa table

to study leaving groups in more detail.

On the left, we have the acid.

For example, hydroiodic acid, HI,

with an approximate pKa of negative 11.

Remember, the lower the pKa value,

the stronger the acid.

So on this table with the
pKa value of negative 11,

hydroiodic acid is the strongest acid

and the stronger the acid,

the more stable the conjugate base.

So the conjugate base to HI

is I minus the iodide anion.

And since this is the conjugate
based to the strongest acid,

this is the most stable base.

Let me write that down here.

This is the most stable base on the table,

which means that the iodide anion

is an excellent leaving group

because it is very stable.

Next, we have hydrobromic acid,

approximate pKa of negative nine.

So the conjugate base
would be the bromide anion,

so also a stable conjugate base.

So therefore, a good leaving group.

For HCl, it's the chloride anion,

also a good leaving group.

So you see these halide anions

as leaving groups all the
time in organic mechanisms.

Let me write this down here.

So these are all examples

of

good

leaving

groups.

Next, let's look at this
acid on the left here.

This is p-Toluenesulfonic acid

with a pKa value of negative three,

so it's still pretty acidic.

The conjugate base to
this is on the right here

and we call this anion a tosylate group.

Let me write this down.

This is called a tosylate group.

And since it's kind of a bulky group,

instead of drawing this out all the time,

you often see OTs written.

So, OTs,

like that,

and you could put a negative charge

on the oxygen here if you wanted to.

So you'll see the tosylate group

function as a leaving
group in many reactions.

Let's look at an example of another acid.

So if I move down here to
H3O+, the hydronium ion,

with a pKa value of negative two.

The conjugate base to H3O+ is H2O

and water is also a good leaving group.

So let's go back up here to the topic

and we can see that all the
acids that we talked about

have negative pKa values,

so negative 11, negative
nine, negative seven,

negative three, and negative two.

And notice all of the conjugate bases

are good leaving groups.

So you can say that if an acid

has a negative value for the pKa,

the conjugate base will
be a good leaving group.

Let's look at another example of an acid.

So, water.

Water's pKa value is positive 15.7,

so it's not a very strong acid.

The conjugate base to water

is the hydroxide anion, OH-,

and this is a bad leaving group.

So hydroxide ion is a bad leaving group

and that's because water
is not a strong acid.

Look at this value for the pKa,

positive 15.7.

So if we look at ethanol,
similar story here.

So ethanol has a pKa value of positive 16.

So the ethoxide anion is
not a good leaving group,

so this pKa values are in the positive

and these conjugate bases
must not be very stable

which means they are bad leaving groups.

Let me write that down here.

So these are examples
of bad leaving groups.

Both S N 1 and S N 2 reactions

need good leaving groups.

However, the S N 1 reaction
is even more sensitive.

Let's look at tert-Butyl chloride.

Let's say it's reacting
via an S N 1 mechanism.

The first step should be
loss of leaving group.

So these electrons come
off onto the chlorine.

We would form the chloride anion

which has a negative one formal charge.

We just saw on our pKa table

that the chloride anion is
a stable conjugate base.

So therefore, this is
a good leaving group.

We're taking a bond away
from the carbon in red,

so the carbon in red gets
a plus one formal charge

and we form a tertiary
carbocation as well.

Since this is the rate determining step

of our S N 1 mechanism,

the formation of our stable anion,

this formation of a good leaving group

helps the S N 1 mechanism occur.

Next, let's look at this alcohol here.

If we approach it the same way

as we did in the previous problem

and we said, "Okay, first step
is loss of the leaving group

"and these electrons come
off onto the oxygen."

Think about what leaving group that is.

That would be the hydroxide ion

which we know from our pKa table

is not a good leaving group.

So the hydroxide ion

is not as stable of an
anion as the chloride anion.

So the chloride anion
is a good leaving group.

The hydroxide anion is
a bad leaving group.

So that's not the first
step of this mechanism.

We need to make a better leaving group

and you can do that by
having a proton source.

Let's say we have a source of
protons, an acid in solution,

so let's say there's an H+ here.

The first step would be
to protonate our alcohol,

so our alcohol is gonna act as a base

and pick up a proton.

Let's draw the results of that.

We have our ring.

Let's put in that methyl group.

And now our oxygen is
bonded to two hydrogens.

There's still a lone pair
of electrons on this oxygen

which give the oxygen a plus,

which gives the oxygen a
plus one formal charge.

So the electrons here
in magenta, let's say,

pick up this proton to form this bond.

Now we're ready for
loss of a leaving group

because if these electrons
come off onto the oxygen now,

we form water as a leaving group.

Let me draw that in here.

So here is the water molecule.

Let me highlight those electrons in blue.

These electrons come off onto
the oxygen then we form water

and we know from our pKa table

that water is a good leaving group.

We're taking a bond away
from this carbon in red,

so we're also gonna form
a tertiary carbocation.

Let me draw that in here.

Here's our ring.

Here's our methyl group.

A plus one formal charge
on the carbon in red.

So by thinking about your pKa values,

you can determine the
stability of the conjugate base

and therefore, if a leaving group

is a good leaving group
or a bad leaving group

and that helps you out when
you're drawing mechanisms.