In this video we'll do a quick SN1 SN2 E1 E2 reaction mechanism fly-through review.
For more details on the reaction, mechanism, and what to look out for, make sure to visit
my website for the complete substitution elimination tutorial video series, practice quiz, and
cheat sheet.
You can find them on the link below, or by visiting leah4sci.com/Substitution-elimination.
SN1 stands for Unimolecular Nucleophilic Substitution.
Keep in mind this reaction is in competition with E1.
The mechanism starts when the leaving group breaks away from the alkyl chain taking the
electrons with it even carbon with the positive charge.
The Cl minus dissolves in solution as an unreactive spectator ion.
And we have a carbocation intermediate.
This is the clue for an SN1 reaction: a carbocation intermediate that is slow to form and the
rate determining step of the reaction.
The solvent acting as a nucleophile in a solvolysis reaction will use its lone electrons to attack
that positive charge forming a bond between a nucleophile and carbocation.
The incoming oxygen now has three bonds and one lone pair for a positive charge which
we get rid off when another solvent molecule uses its electrons to deprotonate the oxygen
by taking the hydrogen and giving it back its electrons to give me a final product.
If your professor wants side products you can also show the protonated solvent molecule
as a CH3OH2+ with its counter ion the leaving group as Cl minus.
The clues to recognize an SN1 reaction takes place is as follows: Carbocation intermediate
as the rate determining slow step and a solvent nucleophile attached where the leaving group
used to be.
SN2 stands for Bimolecular Nucleophilic Substitution where bimolecular is two molecules reacting
at the same time and a one step, no intermediate reaction.
For this reaction,we have a primary leaving group reacting with NaSH in a polar aprotic
solvent DMSO.
Anytime you see Na recognize spectator positive ion to give us a negative S which is a good
nucleophile.
This is a fast reaction because we're not waiting for a carbocation to form.
Instead, the nucleophile will take its lone electrons directly attack the carbon holding
the leaving group to take that out so that in one single step we get the entire product.
SH is now attached where bromine leaving group used to be.
And the leaving group is floating around somewhere in solution.
This product is achiral but in the case of a chiral starting molecule you would get an
inversion of the initial configuration for SN2 reactions.
The clues for this reaction are as follows: We have absolutely no carbocation intermediate,
this is a fast reaction that happens in one step and the fact that the nucleophile sits
on the carbon where the leaving group used to be.
E1 stands for unimolecular beta elimination.
This is similar to the SN1 reaction and is often in competition.
In fact, we'll demonstrate this reaction using the same starting molecule and reagent, the
only difference is that we'll add heat because this will help us favor elimination over substitution.
The mechanism begins in the same way where the leaving group will grab the electrons
binding the carbon and break away to give us a carbocation intermediate.
This is the rate-determining slow step of the reaction with the leaving group just wandering
away somewhere in the solution.
For elimination, instead of a solvolysis where the nucleophile attacks the carbocation directly,
the nucleophile is looking for a beta hydrogen.
A hydrogen that sits on the carbon next to the carbocation and it’ll use its electrons
to pluck off that hydrogen.
The electrons the bind hydrogen to carbon will stay on the molecule and collapse towards
the positive charge to give us our final product, which has a pi-bond between the alpha and
beta carbon.
The clues that an E1 reaction took place is as follows: 1.
You have a carbocation intermediate and 2.
You have a pi-bond between the alpha and beta carbon.
E2 reaction is a bimolecular beta elimination.
‘Bi’ tells us two molecules reacting at the same time that means no intermediate for
a fast one step reaction.
For this E2 reaction, we'll look at secondary leaving group reacting with NaOCH3 in CH3OH
recognizing that Na is a positive spectator giving us a negative base, in this case CH3O
minus or methoxide.
We'll add in heat to help stabilize the pi-bond formation intermediates.
This will favor an E2 over SN2 reaction.
Like the E1 reaction we're not attacking directly, instead we're looking for the multi substituted
beta hydrogen so that the base can grab the hydrogen and pull it off the molecule.
This will cause the electrons between hydrogen and the carbon to collapse in the direction
of the leaving group kicking out the leaving group so that this is a one step, absolutely
no intermediate reaction, to give us the product.
The product is a pi-bond between the alpha and more substituted beta carbon for the Zaitsev
elimination.
The clues that this reaction took place includes absolutely no carbocation intermediate, because
we have a strong attacker and a fast one step reaction.
Our second clue is a pi-bond between the alpha and more substituted beta carbon.
Do you need to see this reaction in more detail, I've broken down everything regarding substitution
eliminations reactions in a twenty plus video series along with a practice quiz and cheat
sheet which you can find on my website.
Click the link in the description or visit leah4sci.com/substitution-elimination.
In order to master these reactions, you first have to learn how to evaluate the four criteria
that will help you determine if substitution or elimination will take place.
This includes, analyzing the alkyl chain, the incoming nucleophile or base, the solvent
the reaction takes place in and evaluating the leaving group.
The series starts with those criteria then follows up with a step by step breakdown of
the reaction, mechanism, intermediates, and everything you need to know along with practice
problems for each one followed by tutorial videos showing you how to choose, Is it SN1?
SN2?
E1, or E2?
Again you can find all these on my website: leah4sci.com/substitution-elimination.
I also have a substitution and elimination mini bootcamp which is a very thorough in-depth
crash course of all of this.
You can get details for that when you download the solutions to the substitution elimination
quiz.