WEBVTT

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

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involve leaving groups.

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Let's look at this pKa table

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to study leaving groups in more detail.

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On the left, we have the acid.

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For example, hydroiodic acid, HI,

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with an approximate pKa of negative 11.

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Remember, the lower the pKa value,

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the stronger the acid.

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

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hydroiodic acid is the strongest acid

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and the stronger the acid,

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the more stable the conjugate base.

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So the conjugate base to HI

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is I minus the iodide anion.

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

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this is the most stable base.

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Let me write that down here.

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This is the most stable base on the table,

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which means that the iodide anion

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is an excellent leaving group

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because it is very stable.

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Next, we have hydrobromic acid,

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approximate pKa of negative nine.

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So the conjugate base
would be the bromide anion,

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so also a stable conjugate base.

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So therefore, a good leaving group.

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For HCl, it's the chloride anion,

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also a good leaving group.

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So you see these halide anions

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as leaving groups all the
time in organic mechanisms.

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Let me write this down here.

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So these are all examples

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of

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good

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leaving

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groups.

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

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This is p-Toluenesulfonic acid

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with a pKa value of negative three,

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so it's still pretty acidic.

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The conjugate base to
this is on the right here

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and we call this anion a tosylate group.

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Let me write this down.

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This is called a tosylate group.

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And since it's kind of a bulky group,

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instead of drawing this out all the time,

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you often see OTs written.

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So, OTs,

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like that,

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and you could put a negative charge

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on the oxygen here if you wanted to.

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So you'll see the tosylate group

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function as a leaving
group in many reactions.

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Let's look at an example of another acid.

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

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with a pKa value of negative two.

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The conjugate base to H3O+ is H2O

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and water is also a good leaving group.

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So let's go back up here to the topic

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

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have negative pKa values,

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so negative 11, negative
nine, negative seven,

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negative three, and negative two.

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And notice all of the conjugate bases

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are good leaving groups.

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So you can say that if an acid

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has a negative value for the pKa,

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the conjugate base will
be a good leaving group.

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Let's look at another example of an acid.

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So, water.

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Water's pKa value is positive 15.7,

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so it's not a very strong acid.

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The conjugate base to water

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is the hydroxide anion, OH-,

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and this is a bad leaving group.

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So hydroxide ion is a bad leaving group

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

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Look at this value for the pKa,

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positive 15.7.

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So if we look at ethanol,
similar story here.

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So ethanol has a pKa value of positive 16.

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So the ethoxide anion is
not a good leaving group,

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so this pKa values are in the positive

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and these conjugate bases
must not be very stable

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which means they are bad leaving groups.

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Let me write that down here.

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So these are examples
of bad leaving groups.

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Both S N 1 and S N 2 reactions

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need good leaving groups.

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

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Let's look at tert-Butyl chloride.

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

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The first step should be
loss of leaving group.

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So these electrons come
off onto the chlorine.

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We would form the chloride anion

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which has a negative one formal charge.

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We just saw on our pKa table

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that the chloride anion is
a stable conjugate base.

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So therefore, this is
a good leaving group.

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

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so the carbon in red gets
a plus one formal charge

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and we form a tertiary
carbocation as well.

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Since this is the rate determining step

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of our S N 1 mechanism,

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the formation of our stable anion,

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this formation of a good leaving group

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helps the S N 1 mechanism occur.

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Next, let's look at this alcohol here.

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If we approach it the same way

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as we did in the previous problem

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

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"and these electrons come
off onto the oxygen."

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Think about what leaving group that is.

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That would be the hydroxide ion

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which we know from our pKa table

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is not a good leaving group.

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So the hydroxide ion

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

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So the chloride anion
is a good leaving group.

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The hydroxide anion is
a bad leaving group.

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

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We need to make a better leaving group

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and you can do that by
having a proton source.

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

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

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The first step would be
to protonate our alcohol,

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so our alcohol is gonna act as a base

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and pick up a proton.

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Let's draw the results of that.

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We have our ring.

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Let's put in that methyl group.

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And now our oxygen is
bonded to two hydrogens.

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

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which give the oxygen a plus,

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which gives the oxygen a
plus one formal charge.

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

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pick up this proton to form this bond.

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Now we're ready for
loss of a leaving group

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because if these electrons
come off onto the oxygen now,

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we form water as a leaving group.

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Let me draw that in here.

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So here is the water molecule.

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Let me highlight those electrons in blue.

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These electrons come off onto
the oxygen then we form water

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and we know from our pKa table

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that water is a good leaving group.

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

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so we're also gonna form
a tertiary carbocation.

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Let me draw that in here.

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Here's our ring.

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Here's our methyl group.

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A plus one formal charge
on the carbon in red.

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So by thinking about your pKa values,

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you can determine the
stability of the conjugate base

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and therefore, if a leaving group

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is a good leaving group
or a bad leaving group

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