Will these reactions undergo SN1, SN2, E1 or E2? Instead of a blindly memorized,
complicated flowchart, let’s use this simple 4 part checklist. This is all you have to memorize.
I go through each checklist item in detail on my website linked below at leah4sci.com/checklist.
And now we're going to put it all together. The alkyl halide is alkyl, carbon chain, and
halide halogen typically chlorine, bromine, and iodine that we’re attacking in the substitution
or elimination reaction. We're specifically looking for two carbons. The alpha, and the
beta. If I have an alkyl chain, the first thing I wanna do is recognize the location of the halogen,
in this case a bromine. The carbon that holds the halogen, that's the zero carbon or the alpha. And
the carbon one away from the alpha is my beta. If this is my alpha, one away, beta, another beta,
and another beta. The alpha carbon can be primary, secondary, or tertiary, a topic I explain with the
pencil trick linked below. If my leaving group is sitting on a tertiary carbon, the molecule is way
too sterically hindered for a nucleophile to come in and attack. This is why a tertiary alkyl halide
will not undergo an SN2 reaction. A primary alkyl halide will not give up its leaving group to form
a primary carbocation because that is way too unstable which automatically rules out our SN1
and E1. Since the secondary leaving group is not sterically hindered and will form a carbocation,
it tells us nothing except that your professor is evil, I mean your professor just wants you
to think harder. As we go through the checklist, if you're not comfortable with the specifics like
when a nucleophile attacks or a carbocation forms, make sure you go back and watch my
entire series which you can find on my website along with the substitution and elimination
practice quiz and cheat sheet linked below leah4sci.com/SNE, again that's leah4sci.com/SNE.
The beta carbon is relevant in the elimination reaction to help us determine 1. If the reaction
can even take place and 2 - if we have multiple beta carbons which one do we attack? For example,
methyl bound to a leaving group has an alpha carbon but no beta. If I have no beta carbon,
I have no beta hydrogen, no beta hydrogen, nothing to eliminate, no E1 or E2 reaction. But if I have
more than one beta carbon, specifically more than one beta hydrogen, Zaitsev’s rule tells
us the more substituted carbon will form the major product, and the less substituted carbon
will give us the minor product. The exception of course is with the big bulky base that'll
give the Hoffman rather than Zaitsev product. The most common question here is, how do I tell
the difference between a nucleophile and a base? And while I have not one but two videos on this
topic at leah4sci.com/checklist, I challenge you to change your question. Is it a strong or a weak
attacker? In other words, will I have a 1 type SN1 E1 or a 2 Type SN2 E2 reaction. The 2 in
SN2 and E2 tells us bimolecular reaction. Two molecules reacting at the same time so quickly
that I have no intermediates. This is because the attacker is so strong, it doesn't wait around for
a carbocation to form and doesn't allow for any intermediates. Remember, happy, stable,
unreactive. Unhappy, unstable, reactive. And what makes the molecule unhappy? CHARGE! Charge is a
burden. When you see a negative charge, that molecule wants to attack making it a strong
attacker, for example OH minus or any RO minus like methanol. Having that negative charge gives
us a 2 type reaction, ruling out SN1 and E1. SN1 and E1 are unimolecular reactions. That means one
molecule reacts at a time giving us multiple steps and one or more intermediates. This is
because we have weak molecules just taking their time waiting around and finally reacting. This
can only happen if I have no bully to rush it along with weak attackers. What makes a molecule
weak? something happier, more stable, typically neutral molecules. For example water, or alcohol
like methanol. Having no charge makes them weak, ruling out the 2 type reactions like SN2 or E2.
As for nucleophile versus base, I typically look at everything else to determine if it's
substitution or elimination with a couple of exceptions. The triple B or Big Bulky Base
is so sterically hindered, it can't approach the alpha carbon and plucks a hydrogen from the nearby
beta carbon and if possible, the less substituted beta hydrogen because that is so much easier to
reach. The most common example tert-butoxide has a negative oxygen and oh my goodness look at what’s
attached. It's a four part checklist but you'll probably have solved half your practice problems
with just these two. Are you with me so far? If yes. Make sure to give this video a thumbs up,
subscribe to my channel, and hit that bell notification icon so you don't miss
out on any new videos. And let's continue. Looking at this solvent is a little tricky,
so let's break it down. We're choosing between a polar protic versus aprotic solvent. I explain
the Solvents in detail on leah4sci.com/checklist. But for now recognize that a protic solvent has
very partially positive hydrogens. For example, the hydrogens on water and ammonia where the
aprotic has very neutral hydrogens sitting on carbon such as the hydrogens here on DMSO. The
partially positive hydrogens in protic solvents will attempt to stabilize any charge in solution
such as a carbocation or a negative charge. But the aprotic solvent without those partially
positive hydrogens won't do anything and in fact they make the molecules more angry. Because the
protic solvent will stabilize the carbocation, SN1 and E1 reactions prefer protic solvents. But that
does not rule out our 2 Type reactions because an E2 reaction will also prefer aprotic solvent. This
is where a lot of students get confused. If it's a 1 Type reaction, we can have SN1 and E1, if it's
a 2 Type reaction, it's just the E2 favored in a polar protic solvent. Let's take a look why. If I
have a strong attacker like OH minus, and it’s acting as a nucleophile. If I dissolve this in
a polar aprotic solvent, that Oxygen has nothing to react with except for the alpha carbon and so
it goes looking for that partially positive carbon to attack and create a substitution
reaction. But if I take this hydroxide, and now I dissolve it in a protic solvent like water,
the oxygen is still looking for something positive but now it gets distracted by the hydrogen on
water and when it grabs that hydrogen from water, and gives that oxygen back the electrons, I form a
new neutral water molecule and another hydroxide which now goes looking for another hydrogen
and another hydrogen. And if it's just looking for hydrogen it will eventually find that beta
hydrogen pluck it off and give me my E2 reaction. The leaving group is more a formality on the
checklist where it's not about choosing which reaction but rather will the reaction take
place. What is a Good Leaving Group? something that not only leaves but stays gone. If I kick
something out and it comes back, terrible leaving group. Picture that friend who leaves
but has just one more thing to say and just one more thing to say and they never leave,
terrible leaving group. and if that's the case, ask can I bribe this molecule? can I protonate
with acid, can I add a big bulky group like a tosylate. Even Though it’s a four part checklist,
you'll often see heat in the reaction. Do not memorize that heat means elimination. Even Though
heat does favor elimination. In the energy diagram series linked below at leah4sci.com/SNEdiagram, we
see that our reactants begin at a certain energy level and our products are more stable at a lower
energy level. But we don't simply go from reactant to product, instead, we see the energy go up and
then come down. This is called the activation energy and differs for every reaction. If you have
a slow reaction like SN1 and E1 they have a high activation energy. For a reaction like SN2 and E2.
The product of the SN2 reaction has more entropy and is therefore considered less stable where
the E2 product has less movement, less entropy and considered more stable. However, it takes a
lot more energy to form that more stable product, it's a higher upfront cost for a better pay out.
But if I put more energy in the form of heat into the system, I can afford that greater activation
energy investment to give me a more stable product which is why heat will favor E2 over SN2.
With all of this in mind, let's go back to our initial practice test. Let's start with problem
number one. I have a primary alpha carbon, this would give me unstable carbocations, I rule out
SN1 and E1. My beta carbon has hydrogen, nothing else to rule out. My attacker doesn't have a
charge or does it? Anytime you see an Na+ that's a positive spectator sitting with an OH minus.
Minus, negative charge, strong attacker, 2 type reaction, Well I've already ruled out SN1 and E1,
my solvent DMSO, Dimethyl Sulfoxide is a polar aprotic solvent which favors SN2 over E2,
bromine is a good leaving group easily kicked out for an SN2 product. For Number 2 I have a
tertiary alpha carbon which rules out SN2. I have beta carbons with Hydrogen telling us nothing so
let's move on to the attacker. Once again, NaOH is just Na+ and OH minus. OH minus is strong,
negative, a 2 type reaction, I rule out SN1 and E1. I'm not given a solvent but I already know
that it's not SN2 and Chlorine is a good leaving group so I'm good to go with an E2 reaction.
Number 3, same starting molecule, we already know tertiary alpha, no SN2, beta hydrogens are good,
leave them where they are. My attacker, wait a minute, I don't have an attacker,
or do I? My solvent CH3OH can also double as my attacker in what is called a solvolysis reaction
where the solvent participates in the reaction and CH3OH as an attacker is neutral, happy, stable,
unreactive, very slow which means we’ll have a 1 Type reaction. As a solvent, CH3OH is polar protic
which will stabilize carbocation intermediates. Chlorine is a good leaving group and I have no
heat. Am I missing something on the checklist? I'm stuck with two reactions, is it SN1 or E1? and
the answer is both. Substitution and Elimination especially unimolecular, are often in competition
with each other and so my answer will include SN1 with an OCH3 and E1 with a pi bond. Say I change
it up and give you the reactant and the product but ask you to figure out what reaction took place
and the step by step mechanism? This is exactly what I cover in my next video, which you can find
linked below or by visiting my website where you can find my entire Substitution and Elimination
video series, practice quiz, and cheat sheet, leah4sci.com/SNE, again that's leah4sci.com/SNE.