Leah here from Leah4Sci.com.
And in this video, I will give you a thorough breakdown of the solvent types and their effect
on substitution and elimination reactions.
Your solvents will fall into three categories.
You have polar protic, polar aprotic, and non-polar solvents.
You should be able to identify each type of solvent in order to determine what type of
reaction will take place.
We'll use water to explain polar protic.
The water molecule has an oxygen covalently bound to two hydrogen atoms.
While the bond is covalent, meaning the electrons are shared between two the atoms, because
oxygen is highly electronegative, it will hug the electrons between oxygen and hydrogen
and this extra electron density makes the oxygen partially negative.
Because the hydrogen atom has the bonding electrons pulled away from itself, the nucleus
is partially exposed giving each hydrogen a partial positive charge.
A polar molecule is a molecule that has opposite poles, meaning negative and positive.
If you analyze the water molecule, we have a distinct separation between the negative
and the positive where the oxygen portion of the molecule is considered the negative
pole and the hydrogen portion of the molecule is considered the positive pole.
Protic kind of sounds like proton and this has to do with hydrogen bonding.
So let me ask you a question.
Do you like to have fun?
Because hydrogen does, specifically, F, O, or N. If you have a hydrogen atom bound to
either a fluorine, oxygen, or nitrogen and you place it near another fluorine oxygen
or nitrogen, these atoms will attract the hydrogen so strongly that it's almost impossible
to tell which was the original atom bound to hydrogen.
Fluorine is less likely to show up in your polar protic solvents but definitely look
out for the oxygen and nitrogen such as what we have in water.
If I bring another water molecule nearby, the partial negative oxygen from the second
water will be attracted to the hydrogen on the first water molecule forming a very strong
interaction or a hydrogen bond.
Therefore, a polar protic solvent is a solvent that has either oxygen or nitrogen directly
attached to a hydrogen atom.
This will make the hydrogen partially positive and able to interact with any other molecule
that is dissolved in the solution.
Common polar protic solvents include water and ammonia or larger molecules that have
an OH or NH, for example, alcohols or amines.
Again, the key here is that the hydrogen has to be bound directly to the oxygen or to the
nitrogen and not to some other atom like carbon or sulfur.
Since polar protic solvents have partial negative and partial positive on each every solvent
molecule, they tend to favor charges dissolved in solution.
If you consider an SN1 or E1 re action, the carbocation intermediate that forms is not
very stable.
However, if you place that carbocation into a polar protic solution, the partial negative
atom of the solution will stabilize that positive charge by donating some of its electronegativity.
At the same time, when the leaving group leaves, it will form a negative charge in solution.
For example, if Br- is my leaving group, it forms a negative charge in solution which
is stabilized by the partially positive end of the polar protic solvent molecules.
So when you see a polar protic solvent, consider a one-type, meaning an SN1 or E1 reaction.
SN2 reactions will be unfavorable in this type of solvent because a strong nucleophile
will be blocked on its way trying to attack the molecule.
Polar protic solvents treat E2 and SN2 reactions completely differently.
If you have a molecule with a leaving group, that molecule will be partially positive and
therefore attract a nucleophile.
However, if I try to bring a very strong nucleophile into solution to attack that carbon, the solvent
tends to get in the way of the nucleophile thereby preventing the reaction.
The nucleophile, recall, is going to be partially negative.
This will attract the partially positive end of solvent molecules surrounding that nucleophile
and thereby preventing it from attacking.
In other words, an SN2 reaction will likely not take place in a polar protic solvent.
Not only will the nucleophile be hindered by the solvent, if it's negative enough, it
can actually react with the solvent attacking a solvent molecule instead of the reactant
thereby destroying the ability of that nucleophile.
But this is very different when considering an elimination reaction.
If you have a very strong base and you dissolve it in its own conjugate solvent, for example,
if we take OH- dissolved in H2O, water is simple the conjugate base of OH-.
So if the OH- steals a hydrogen from another water molecule, all that leaves me with is
another OH-.
So eventually one of these strong basic molecules will wind up attacking the beta hydrogen causing
that elimination reaction to take place.
So again, polar protic solvents will prefer the E1, SN1, or E2 reactions but will not
favor the SN2 reactions.
A polar aprotic solvent is still going to be polar, meaning will have that separation
of charge on the molecule.
However, the prefix "a" implies without or missing and that means that a polar aprotic
solvent even though it has that separation of charge will not have a hydrogen on oxygen
or nitrogen and therefore will not be able to donate a hydrogen for hydrogen bonding.
There are four common examples that you will likely have to memorize.
I will show you the structure so that you understand what you're looking at but you
don't have to know how to draw it as long as you recognize these molecules to be polar
aprotic.
The first is DMSO which stands for dimethyl sulfoxide.
We have a sulfur double bound to an oxygen and the sulfur is also bound to 2 methyl groups.
Since oxygen is a lot more electronegative than sulfur, oxygen will pull electron density
away from sulfur making it partially negative and leaving the sulfur partially positive.
However, notice that the hydrogens on this molecule are attached to carbon and not oxygen
or nitrogen, meaning that these hydrogens cannot participate in hydrogen bonding.
The DMF stands for dimethylformamide which is actually N,N-dimethyl given that the nitrogen
has 2 methyl groups attached.
Notice that we have with oxygen and nitrogen on this molecule which will be partially negative
leaving the carbon partially positive.
However, once again the hydrogens show up bound to carbon rather oxygen or nitrogen
making the solvent unable to donate protons for hydrogen bonding.
Acetone is simply the common name for propanone which has a carbon double bound to an oxygen
and once again, 2 methyl groups.
And if you're familiar with ketones, you'll know that oxygen is partially negative and
carbon partially positive.
Once again, a polar molecule with no hydrogens to donate for hydrogen bonding.
And lastly, we have acetonitrile which has a carbon triple bound to a nitrogen leaving
that nitrogen very partially negative and the carbon partially positive.
Now, these are not the only polar aprotic solvents but these are the most common ones
that come up when studying substitution and elimination reactions.
Here is an interesting trend that you want to consider.
When analyzing halogens as nucleophiles, the solvent will make a difference.
In the last video, we said that the smaller halogens tend to be stronger nucleophiles
and that's because their charge is more concentrated.
We also said that the larger halogen is more polarizable making it an ideal nucleophile.
So which is it, small or large?
In a polar aprotic solvent, there are no partially positive hydrogens to stabilize or slow down
a halogen from reacting making the smaller more concentrated charge the stronger halogen
that makes F- stronger than Cl- which is stronger than Br- and stronger than I-.
However, analyzing the same sequence in a polar protic solvent is going to give you
the reverse.
And that's because the polar protic solvent will surround or cage these halogens as follows:
fluoride which is tiny, when surrounded with a molecule like water will be completely caged
and unable to attack.
However, iodine which is rather large will still be partially surrounded by the water
molecules but because it's so big, it's still able to get out of that cage and attack.
That makes iodine the largest halogen, the strongest nucleophile in a polar protic solvent
but in a polar aprotic solvent where there is nothing to cage it, the fluoride or smallest
halogen will be your strongest nucleophile.
Given that the polar aprotic solvent does not have as strong of an ability to stabilize
attacking molecules, it will favor a stronger reaction like SN2.
Recall that a polar protic solvent can destroy a strong nucleophile by reacting with it but
the nucleophile has no hydrogen to attack in an aprotic solvent.
So whenever you see a polar aprotic solvent, consider the SN2 reaction over an E2 reaction.
And finally, we have the non-polar solvent which as the word implies is not polar at
all.
This occurs when a molecule typically has just carbon and hydrogen with no exciting
functional groups.
Common examples for solvents are going to include toluene which is simply a benzene
ring attached to a CH3 or you might see something like hexane which is 6 carbons in a chain.
These are ideal solvents when you have hydrophobic molecules meaning molecules that have no polarity,
no charge, and therefore fear water.
And a non-polar solvent does not stabilize any intermediate charges so you will have
to consider your two-type reactions meaning your SN2 and E2.
You want to think of the two-type reactions because the leaving group does not want to
leave on its own knowing that the charge that forms will not be stabilized.
But if you bring a strong base or nucleophile, it will act as a bully disregarding the happiness
of the leaving group and simply attack and kick it out.
Overall though, you're more likely to see polar protic or aprotic solvents rather than
non-polar solvents in your substitution and elimination reactions.
Be sure to join me in the next video where I show you how to analyze leaving groups in
substitution and elimination reactions.
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