Leah here from leah4sci.com and in this video series we're going to look at the 4 basic
types of Organic Chemistry Mechanisms specifically focusing on the pattern of the reaction arrows.
When you first learn reactions, you simply look at the overview.
For example, molecule A reacts with molecule B to give you C plus D. A specific example
would be the SN1 reaction where 2 Butanol is reacted with hydrochloric acid to give
us 2 chloro butane plus H3O+.
When you're studying mechanisms you're looking at exactly what happens every step of the
way.
How does molecule A react with molecule B?
How is it that we get product C and product D and more importantly how would you classify
every step along the way in terms of how the electrons are moving and what they're doing
in the attack.
There are 4 main types of mechanism patterns you'll see in Organic Chemistry.
The Nucleophilic Attack, Loss of the Leaving Group, Proton Transfer, and Rearrangement.
While each one is a unique pattern, they all have one thing in common, they're looking
at how the electrons are moving.
If you're not comfortable with the concept of arrow pushing, review that video first
linked in the description.
But as a reminder, the arrow in a mechanism is showing us where the electrons are moving.
Never protons, never anything else, it's specifically the electrons.
We have two types of arrows.
Double headed arrow and the single headed arrow.
I like to think that each arrow head telling me how many electrons are moving.
With the double headed arrow we have a pair of electrons and this is the case for over
ninety percent of the mechanisms you'll see in Organic Chemistry.
The single headed arrow or the Fish hook is a lone electron and you'll only see this when
you're looking at radical reactions.
The tale of the arrow regardless of pair or single, tells us where the electrons were
and the head of the arrow tells us where the electrons are going.
For example, if I want to show that molecule A with the lone pair of electrons is attacking
molecule B, I would show the arrow as starting from the electrons of A and ending towards
the direction of B.
This tells me the electrons of A are reaching over to attack and form a bond with B then
I show my yield arrow for a final product which is A bound to B where that bond is formed
by the two electrons that did the initial attack.
You always want to show the electrons that are about to move, the arrow in the direction
of where they go where the arrow head lands on the atom you're attacking.
The yield arrow to show what happens as a result and then of course the final result
in this case where we have the bond sitting between the two atoms.
Now let's take a look at the individual patterns.
I don't want you memorizing these patterns when you're studying Organic Chemistry.
Don't memorize the mechanisms, instead I want you to ask yourself, what is going on here?
Why is it happening?
and what can I take away from this so that I can apply it every mechanism in the future.
In order to understand the nucleophilic attack we need to explain the Nucleophile and Electrophile.
If you see an unfamiliar word, ask yourself if you recognize something in that word.
In this case, I see the word Nucleo which tells me Nucleus and Phile meaning loving.
The nucleus is positive and something that loves the positive will seek it out so we
can define a nucleophile or nucleus loving as positive seeking.
What type of atom or molecule will seek positive?
Opposites attract so we're looking at something negative or partially negative.
The electrophile is the opposite.
Electrophile has electro which tells us electron and phile meaning loving.
The charge of an electron is negative and something that is an electrophile will be
negative seeking.
What is seeking out an electrophile?
Something positive or partially positive.
Throughout my videos you'll see the nucleophile represented as Nu with two electrons which
is to remind you that the nucleophile has that lone pair of negative or electronegative
electrons that want to do the attack.
An electrophile as E+ to show us that it's positive or partially positive.
Now don't mistake seeking as going after because it's never the positive that moves, it's always
the electrons so the nucleophile will seek out the electrophile and that gives us the
basics for our first pattern.
A nucleophilic attack is a nucleophile with its lone pair of electrons seeking out a positive
or partially positive electrophile to form a new bond.
For example, if you are working on a mechanism and you see a secondary carbocation with an
iodide in solution.
You'll see this intermediate in SN1 reactions or Alkene addition reactions.
The carbocation is positive, that makes it the electrophile.
The iodide is negative which makes it the nucleophile.
The pattern for this reaction is the nucleophile taking one of its lone pairs of electrons
and attacking the electrophile in this case the positive carbon atom.
The electrons start out on the nucleophile, the arrow head goes towards the electrophile
and that gives us a new bond as follows.
The iodine now sits next to that carbon, it still has three blue lone pairs of electrons
but it now has a new bond between itself and carbon where the electrons for the new bond
came from the lone pair that attack the carbocation.
Another common type of nucleophilic attack is when you have a carbonyl being attacked
at the carbonyl carbon.
This molecule is harder to identify as an electrophile because we don't see an obvious
positive charge the way we did with the first carbon.
However, if you recognize that carbonyls have resonance, you'll find that charge.
The resonance in this case happens where the pi bond between carbon and oxygen will move
up towards that oxygen atom so that the pi bond breaks and winds up as a lone pair.
Oxygen still has one bound to carbon, it has the initial two lone pairs but that third
pair comes from the pi bond that broke.
This give oxygen a formal charge of negative one and carbonyl formal charge of positive
one.
If you're confused with this make sure you review the video series linked below, the
first one on resonance to explain the concept and the second on formal charges to help you
understand where the plus and minus came from.
These are both resonance contributors but we can draw the resonance hybrid as carbon
double bound to oxygen with a dotted line representing the bond constantly moving up
and down between both forms and that gives me a partial positive charge at the carbon
and a partial negative charge at the oxygen.
If we look at this molecule again and remember that partial positive charge, we have now
identified our electrophile.
If we react this with a strong group for example a methyl Grignard the carbon atom electrons
that are attached to the grignard act like a carbanion.
These electrons act like they're sitting on a C minus making them very reactive, making
this molecule a nucleophile.
We show the mechanism with an arrow starting at the bond between carbon and magnesium.
These are electrons they are just sitting around carbon, don't get confused this is
actually a bond between carbon and magnesium.
We start the arrow here and then bring them all the way to the partially positive carbonyl
carbon that's our attack.
In this case because carbon would have five bonds we have to show another set of electrons
moving.
We show the electrons that were already resonating upward being permanently kicked up onto the
oxygen.
Draw the yield arrow to show the product and then redraw the skeleton as you see it.
In this case we have the green three carbon molecule with a single bond to oxygen, two
green lone pairs from the initial lone pairs and a third lone pair from the pi electrons
that collapse upward.
But what happened as a result of our nucleophilic attack, it's this methyl group that is now
attached to that central carbon atom and the electrons that form that new bond are the
very same electrons that did the nucleophilic attack.
The second common mechanism pattern is the Loss of the Leaving Group which as the name
implies is a leaving group getting lost.
For example, if we have a molecule where a chlorine is bound to a secondary carbon, in
this case two chloro propane and chlorine decides it's had enough.
There are two electrons forming that bond between carbon and chlorine, but chlorine
starts yanking on that bond, remember this is a polar bond, there's an unequal distribution
of charge and chlorine being more electronegative is hugging those electrons.
It pulls and pulls and pulls until it finally pops off taking the electrons with it and
breaking away.
You show the arrow starting at the bond where the electrons are about to break and ending
on the atom that will wind up holding those electrons as a lone pair.
As a result, the carbon is now missing a chlorine and has a formal charge of plus one and the
chlorine now has four lone pairs of electrons.
The three initial pairs that it had while bound to the molecule and a fourth pair that
came from that bond breaking and settling on to the chlorine atom, giving us chloride
with a negative charge.
If the leaving group breaks away and it's neutral, the resulting formal charge is minus
one.
If the leaving group breaks away when it's positive, the resulting charge is zero.
When you gain an extra pair of electrons, the charge goes down by one so that plus one
goes down to zero and zero goes down to negative one.
You'll see loss of a leaving group in a more advanced reaction when you study nucleophilic
substitution on aromatic rings.
If you haven't learned it yet, don't be overwhelmed by what's going on, instead, simply pay attention
to the leaving group in this case, the chlorine.
In this reaction, we're going to have a domino effect of electrons moving for resonance and
ultimately kick out the leaving group.
We show one of the two oxygens attacking the nitrogen to reform a pi bond.
This causes the electrons between nitrogen and carbon to resonate into the ring which
causes the pi bond nearest the leaving group to come directly towards the leaving group,
which puts too many bond on the carbon holding the leaving group and kicks out the leaving
group.
Let's redraw this exactly as we see it showing what has and hasn't changed.
The black pi bond on the right hasn't moved.
Nitrogen is now only single bound to the carbon on the ring.
The oxygen on the left hasn't changed so it still has three lone pairs and a negative
charge.
The oxygen on the right had three lone pairs and now has only two.
It had a single bond and now it has two from these electrons forming a pi bond.
On the left of the molecule, these electrons are now sitting as a pi bond in the ring and
the electrons in this pi bond which we'll now show in blue simply moved over one position.
The carbon that initially held the leaving group now has just an OH attached and chlorine
which had three lone pairs to begin with now has a fourth lone pair and a negative charge,
thanks to these electrons from the bond that broke and left with the chlorine atom so that
it's now sitting as four lone pairs on chlorine in solution.
Be sure to join me in part 2 in this series where we look at proton transfer and rearrangement
and then we'll look at this mechanism step by step, identifying the arrow patterns for
each individual step in the mechanism.
You can find this entire series on my website leah4sci.com/mechanism.