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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.
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