Stereospecificity vs. Stereoselectivity and Regiospecificity vs. Regioselectivity
27
February

By Adem Lewis / in , , , , , , , , , /


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.


20 thoughts on “Stereospecificity vs. Stereoselectivity and Regiospecificity vs. Regioselectivity

  1. Dang, now that is going to be a lot to take in, but that is the upmost benefit if this informative channel. I was wondering, was there any news following the Space For Humanity application?

  2. I have no idea what you are talking about but you are the best at talking about stuff I have no clue about. Way to go Professor Dave you rock!

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  5. Professor, can you explain the term science? And pseudo-science? Is the possible application of the scientific method what differentiates the two?

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    I got very less marks in my January attempt 😭

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