hey it's professor Dave let's talk terminology so let's take a look at some
so let's take a look at some terminology that is very important in organic chemistry and actually commonly
misused let's start out by looking at stereo specificity and stereo
selectivity a lot of students think that these that these terms are are
interchangeable and synonymous actually or not they mean something very very
particular so let's start with stereo specificity we've got we've got an sn2
reaction very basic reaction right we know what's gonna happen here
got our secondary substrates en minus is going to attack and we're going to
displace the bromo group and then we've got our CN group on there so regular sn2
and this is stereospecific because out of possible stereochemical
outcomes right we would think that the CN could be either on the dash or the
wedge it is going to be 100% on the dash okay we're only going to get this
product right we got 100% that we're never gonna get CN on the wedge now the
reasoning for this is because of the sn2 mechanism right we know that in order
for substitution to occur the incoming nucleophile is going to have to access
the lowest unoccupied molecular orbital of the substrate and as it happens that
is the anti-bonding orbital that is precisely 180 degrees from the carbon
leaving group leaving group bond so the nucleophile can only approach in one
very specific direction that's why we're only gonna get one very specific
stereochemical outcome so stereospecificity
or a stereo specific reaction means we specifically get only one outcome out of
any stereo chemical possibilities and not any other right so so specificity
you get a specific product so in contrast let's look at stereo
selectivity this is a little bit more nuanced but say for example we have we
remember hydrogenation over platinum metal this is a very basic addition
reaction we have this alkene here we know that we're going to hydrogenate and
we know that this is heterogeneous catalysis so we have that solid platinum
surface that is going to provide the hydrides the substrate is going to lower
down then receive the hydrides
and therefore getting hydrogenated but the thing is we've got this methyl so
the substrate can approach in one of two ways either the substrate can approach
with the methyl sticking up like that right the methyl sticking away from the
Platinum surface or it can it can approach this way with the metal
sticking down towards the Platinum surface now for very obvious reasons one
of these two situations is going to be more sterically favorable right the
methyl sticking down is going to be not quite as favorable energetically because
of the slightly the slight additional repulsion that is going to be occurring
so as we can see it if if it lowers on to the Platinum metal such that the
hydrogens add on the bottom and push that methyl up to give us the cis
dimethyl product that's going to be more favorable and and that is reflected in
the product mixture we've got 68 percent cis right we've got 68 percent there
whereas the other situation where it approaches with the methyl down so the
this original methyl is down and then the new methyl gets pushed up that would
give us the trans product that's only 32 and so the reason we call that
selectivity this is a stereo selective reaction because there are there's more
than one stereo chemical possible outcome and we do get both however the
system will select one of those products to be more more prominent in the product
mixture than the other so we do get both where we get more of one than the other
in a way that is statistically significant and reflects certain some
certain aspect about the substrate or about the reaction in general in this
case it was all about sterics right it was about the sterics of this methyl
group where that methyl group was pointing for the reaction to go just one
more example very similar but just to kind of get the hang of this we're gonna
do a reduction here we've got lithium aluminum hydride we've got a ketone so
we know that a hydride we know we know what's gonna happen here right this is
going to attack and this will pick up a proton and we're gonna get an alcohol
right but which way is this hydride going to attack from this is an sp2
center therefore trigonal planar so the hydride
can come in like this it's come this way or it can come from behind
it can come from behind there and so that's gonna that's going to create two
different products in terms of stereochemistry so one thing that could
happen the hydride could attack from the back and push the new hydroxyl forward
because we're going from sp2 to sp3 center so we're going from trigonal
planar to tetrahedral so it could push that new hydroxyl
forward and that would be this or it can attack from the front like this and push
that new hydroxyl to the back and so that would be this and so there is some
discrepancy here because of this methyl that is projecting out like this so the
sheer fact that this methyl is projecting in this direction with
respect to the plane of the rest of the molecule there is a little bit more
steric hindrance right associated with the hydride approaching from this side
it's going to feel more steric hindrance from this methyl than if it was to
approach on the other side and so that means that the activation energy
associated with reduction from one side versus the other is actually going to be
slightly different it will need a little bit more kinetic energy to be able to
approach from this side and successfully reduce and again that is reflected in
the product mixture so we're gonna get 76% cis on these groups because it's
more energetically favorable or a lower activation energy for the hydride to
attack from the back and that would give us this versus 24% getting the trans
where it will it will overcome this higher energy barrier and attack from
the front push that hydroxyl back and we get this so again it's stereo
selectivity because we do get more than one product it is just that one is
favored over the other or the rest if there are multiple within the product
mixture which is different from stereo specificity where we get only one product.
okay so now let's look at regio specificity and regio selectivity this
is going to be very similar to what we just looked at we're just talking about
regiochemistry now instead of stereochemistry
so first going over regiospecificity this is this should be very familiar
this is just a regular markovnikov hydrohalogenation again
very common addition reaction and we don't need to go over the mechanism we
know that this pi bond is going to protonate we're going to get the
secondary carbo cation instead of the primary and so we're gonna get 100% of
this structural isomer so we know that there are ways to go anti markovnikov
right if we did this in the presence of hydroxides we could get the other regio
isomer but the point is that here we're only going to get the bromine there and
we're not gonna get the bromine there at all we're gonna get 100% the markovnikov
product so we call that regiospecificity this is a regio specific reaction
because that's all we get now to talk about regio selectivity remember when we
looked at each elimination we looked at Zaitsev and Hofmann elimination
products and we looked at that in a couple different contexts but this is
actually a great way to show regio selectivity as well let's say that we
have this oh this should actually be actually a sorry that should be let's
call that bromine so that's gonna be a bromo group of something so we're gonna
do a regular elimination now we know that there are there's more than one
proton available for elimination and we can get different structurally different
products depending on which one we get so let's say we've got ethoxide here
let's go ahead and get this proton remember we're gonna heat that up a
little bit make sure we don't get any work we're gonna promote elimination
that way so let's go ahead and get this proton and we're gonna do that and then
we're gonna kick this off like that so if we do that we're gonna get the pi
bond right there and if we get the pi bond right there that's going to give us
the Zaitsev product that's the more substituted alkene product which as we
recall from another tutorial this is the thermodynamically favored product right
this is the more stable product sits low on an energy diagram and that's why
we're gonna get more of it right so the thermodynamics our driving is what's
driving this here but by contrast right we could get any of the protons there's
three protons here three protons here on those two methyls so six chemically
equivalent protons that we could also get to get to get the other product so
we could go over here and we could get that and we
could kick this off that way and that would give us the hoffman product and
we're only gonna get about 20% of the hoffman product remember that would be
kinetically favored this is the less thermodynamically favorable it's not as
low on an energy diagram so remember in order to get that we would have had to
use a much bulkier base like tert-butoxide
and so this is an example of regioselectivity because we're getting
we're getting two products here and and we're getting 80% one 20% of the other
so there is a selectivity the system will select one product over the other
in the product mixture and so with with stereoselectivity
we saw that sterics where maybe the primary factor there there can be
electronic factors that drive stereo selectivity as well but we saw sterics
in two examples here we're seeing electronic influence that's guiding the
regio selectivity and and so that's that's a that's a Zaitsev hoffman thing
where we're talking about a thermodynamically favored product versus
a kinetically favored product so again regiospecificity only getting one
product regioselectivity mixture of two or more products but one is selected for
by the system and it's more dominant in the product mixture so that's a little
bit of information about stereo specificity and stereo selectivity and
regiospecificity and regioselectivity.