Leah here from Leah4Sci.com with video 2 of the nucleophile and base analysis for substitution
and elimination reactions.
Catch part 1 along with the entire substitution-elimination series at Leah4Sci.com/substitution-elimination.
In part 1of this video, we discussed how to differentiate between strong bases or nucleophiles
versus weak bases or nucleophiles by looking at charge.
The next thing you want to look at is the attacking atom.
What if you have two atoms that both have the same charge?
You have to look at which one is more reactive when holding that charge.
There are over a hundred elements on the Periodic Table but I highly recommend memorizing the
placement of the following ten atoms.
Knowing where these atoms occur on the table helps you remember the following two trends:
electronegativity goes up and towards the right and size goes down and towards the left.
If we take a look at atoms on the same period, for example, carbon, nitrogen, oxygen, fluorine
giving each of them a negative charge does not make them equally reactive.
Since fluorine is the most electronegative, it will be happiest with that charge and therefore
less likely to attack.
However carbon on the opposite extreme is the least electronegative will be the least
happy with this negative charge and therefore, even more likely to attack.
And since the trend for electronegativity is carbon less than nitrogen, less than oxygen,
less than fluorine, a negative carbon will be the strongest base or nucleophile one compared
to a nitrogen which is stronger than oxygen, which is stronger than fluorine.
However as you go down the Periodic Table, the atoms start to get larger and their electronegativity
decreases.
So not only do we have a less electronegative atom, but because it's bigger, it has the
ability to distribute its size making that negativity less effective.
And so, the trend for strength decreases as we go down the Periodic Table.
Since fluorine has the most concentrated charge, it's going to be the most reactive followed
by chlorine, followed by bromine, and followed by iodine given of course a negative charge
in each of these atoms.
There are exceptions to every rule and this occurs in polar protic solvents which we'll
talk about in the next video.
In part one of this video, I mentioned that some bases are terrible nucleophiles and some
nucleophiles are very weak bases.
So let's start with the bases that make terrible nucleophiles and these are typically referred
to as the triple B or Big Bulky Base.
The most common example that you're going to see is tertbutyloxide or tertbutoxide.
This molecule has an O- making it very strong and very reactive.
The O- is bound to a carbon which is bound to three other carbons making the entire molecule
very, very bulky.
In chemical terms, this is referred to as Steric Hindrance which tells you that the
molecule gets in its own way.
Have you ever tried to walk through a narrow door carrying a really big umbrella?
You as a person can get through the door but because you're carrying a very large umbrella,
it actually gets in your way and prevents you from getting through.
The same thing happens with a Big Bulky Base.
The oxygen is strongly negative and wants to attack but because the rest of the molecule
is so big, it tends to get in its own way.
Now, think back to substitution and elimination reactions.
In this substitution reaction, the nucleophile attacks the molecule and remains attached
to that molecule after kicking out the leaving group whereas in an elimination reaction,
the base simply grabs a hydrogen which collapses the bond and kicks out the leaving group.
If the Big Bulky Base tries to act as a nucleophile, it actually gets in its own way and prevents
itself from attacking a molecule as a nucleophile.
So instead, it has to resort to sneaking up on a side hydrogen of the molecule and pulling
off that hydrogen which causes the elimination reaction.
So anytime you see a Big Bulky Base like tertbutoxide, the first thing that should come to mind is
an E2 reaction.
We said that a molecule with a negative charge will be strong and reactive but there are
some cases where you have a very strong acid that gives you a very weak conjugate base.
For example, whenever you have an acid made of HX where X can be any halogen, it will
break up and give you H+ and X-.
These atoms have negative charges but they're very, very weak bases and therefore will not
participate in elimination reactions.
When you recognize a negative atom or molecule to be a very weak base, it can still act as
a nucleophile but will not act as a base.
Instead of memorizing all of these, try to understand what happens and why so that you
can apply it to many different molecules.
Let's take a look at iodide or I-.
I- is a very large atom with a distributed negative charge.
So even though there is a net charge of -1, if you look at any part of the molecule individually,
it will have a fraction of that charge.
However, if you bring this large molecule near a positive charge, the molecule or in
this case, the atom will polarize.
Meaning, there will be a slight separation of charge because all of the negativity will
be attracted towards the positive charge on the carbon leaving the other end of this molecule
slightly positive.
There will be enough negativity concentrating near that carbon allowing this molecule to
act as a nucleophile.
In other words, if you have a molecule that is highly polarizable, it can act as a nucleophile
even if it happens to be a very weak base.
This is not limited to halogens.
If you look down the Periodic Table under oxygen, you have sulfur which is less electronegative
but something like SH- which is a larger molecule or even as R- which is larger than its oxygen
counterpart, OH- and OR- will be a stronger nucleophile because sulfur is larger and more
polarizable.
This can also occur when you have a pi system given that the extra pi electrons are able
to polarize the molecule even more, for example, in CN- or N3-.
Be sure to join me in the next video where I show you how to identify polar protic, polar
aprotic, and non-polar solvents and show you how they apply to substitution and elimination
reactions.
Are you struggling with organic chemistry?
Are you looking for information to guide you through the course and help you succeed?
If so, download my ebook, 10 Secrets to Acing Organic Chemistry using the link below or
visit Leah4Sci.com/OrgoSecrets.
If you enjoyed this video, please give it a thumbs up and even share it with a friend
or two.
If you have any questions regarding this video, leave a comment below or contact me through
my Facebook page at Facebook.com/Leah4Sci.
There will be many related videos posted over the course of the semester so go ahead and
click the subscribe button to insure that you don't miss out