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5.1 Overview of Isomers | Constitutional Isomers and Stereoisomers | Organic Chemistry | Chad's Prep | YouTubeToText
YouTube Transcript: 5.1 Overview of Isomers | Constitutional Isomers and Stereoisomers | Organic Chemistry
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Video Transcript
isomer is in stereochemistry that's
going to be the topic of this lesson and
we're going to start with an overview of
isomers in general and
we've got a lot of vocabulary to go
through in this first lesson we're going
to lay a really good foundation for the
entire chapter but
we'll talk about such terms like chiral
ache hyrule optically active enantiomers diastereomers
diastereomers
racemic mixture miso compounds just lots
and lots of vocab in this first one well
you got to kind of know the denotations
kind of like the textbook definitions
but you also need to know some
connotations kind of like how to
recognize these in context
and which molecules kind of have these
certain characteristics and things of
the sort so
a little bit of a pain in the butt so
but we're going to work really hard in
this first lesson
to kind of pave the ground work for the
rest of the chapter now in the rest of
the chapter
we'll talk about absolute configurations
we'll identify r and s
or assign r and s as we say to chiral centers
centers
we'll talk about molecules with multiple
chiral centers and how we sometimes
represent them with fischer projections
we will then move on to comparing
molecules and figuring out is there some
sort of isomeric relationship between
these molecules and then
you know how many different levels of
classification can we get how specific
can we get in
identifying that isomeric relationship
uh after that we'll kind of you do a
couple unique things we'll talk about
molecules that can be
chiral without chiral centers and that
probably means nothing to you right now
and that's okay
then we'll finish the lesson off talking
about what's known as optical activity now
now
for those of you that already seen maybe
some lectures on this topic in your
course in college or something like that
great some of these terms might sound
familiar and for the rest of you just know
know
lots of vocab coming and by the end of
this series of lectures for this chapter
hopefully all these things are much more
familiar than they are right now
now if this is your first time to the
channel my name is chad and welcome to
chad's prep
where my goal is simply to make science
both understandable and maybe even enjoyable
enjoyable
now this is my brand new organic
chemistry playlist i'll be releasing
these lessons weekly throughout the
2020-21 school year so if you don't want
to miss one subscribe to the channel
click the bell notifications
you'll be notified every time i release
all right so we'll start here with an
overview of isomers and so first of all
you got to realize that for two
compounds to be isomers they can't be
the same compound
so but they've got to have the same
chemical formula so the same number of
each type of
atom for them to be isomers and these
come in two varieties we come
constitutional isomers and stereoisomers
so and you got to kind of know the
difference now constitutional isomers
which are also called structural isomers
we say they have a different bond
connectivity so a different bond
connectivity and so
good example here would be like giving
you this guy here
and this guy here these both have the
formula c3h6
so but they have very different bond
connectivity here you've got three
carbons in a straight chain
whereas two of the carbons are joined by
a double bond so a sigma and a pi bond
whereas here we have no pi bonds and
instead of a straight chain we have
actually those three carbons in a ring
but both of these have three carbons six
hydro six hydrogens so they're isomers
they're not identical so and the atoms
are bonded in a totally different way so
a different
bond connectivity these are
constitutional or structural isomers
let me give you a little more subtle
example here as well so say i give you
these two as well they both have the
formula c3
h7cl and in this case we've got one
chloropropane and two chloropropane
and here chlorine is attached to carbon
one and here chlorine is attached to
carbon two
and so the atoms are bonded in different
ways they have a different
bond connectivity and once again these
would be constitutional or
structural isomers now like i said we're
actually spending the bulk of the
chapter talking about these
stereoisomers instead
the difference here is that your
stereoisomers are going to have the same
bond connectivity what's going to be
different though is the
three-dimensional arrangement of the
atoms so
all the atoms are going to actually be
bonded to the same atoms but how they're
uh kind of related to each other in
three-dimensional space is what's going
to be different
so and you've already seen an example of
a stereoisomer and that was our cis and
so if we look at cysts and trans isomers
so and you found out that if you had two
substituents and here i made them both
methyl groups but they could be two
different groups as well
but you found out if they're attached to
cyclohexane with both wedges or both dashes
dashes
or if you drew it on a chair
conformation both up or both down they
would be
and then you found out if one was a
wedge and one was a dash or
one was up and one was down on a chair
conformation that would be
trans to each other instead and if you
look at these these are not the same
molecule turns out they have exactly the
same formula
but you can also see that all the atoms
are bonded
to exactly the same other atoms they
have the same bond connectivity we say so
so
six atoms in a ring where carbons one
and four bonded to methyl groups
six atoms in a ring or carbons one and
four are brought into methyl groups but
the three-dimensional arrangement of
those atoms is indeed different
so cis again where they're on the same
side these two methyl groups are closer
in space than these two methyl groups
which are trans to each other so again
same bond connectivity but different
three dimensional arrangement of the atom
atom
cool so those are your cis trans isomers
now it's not just cyclohexane but these
can come with any
cycloalkane and again the two
substituents don't have to be the same
they could be different as well
and you're still going to have this cis
or trans relationship possible
alkenes
so in this case we've got cis2-butane
and trans-two-butane which
uh if you're on pace with this class you
have not learned to name yet
so but in this case you've got four
carbons in a chain four carbons in a
straight chain you've got a pi bond
between carbons two and three pi bond
between carbons two and three
so but it turns out these don't have the
same three dimensionality associated
with them so
if you've got a carbon-carbon double
bond it means you've got a sigma and a
pi bond
and that pi bond is the result of the
sideways overlap of p
orbitals and so if you try to rotate
that bond you would break the sideways
overlap with those p orbitals and so
it turns out that pi bonds you can't
rotate so you can't rotate a double bond
and as a result of not being able to
rotate this we actually have these two
different possibilities that can't
interconvert because the bond can't
rotate around
to allow them to interconvert and we
call these again cis and trans isomers
just like we did
up here they have the same bond
connectivity but a different three-dimensional
three-dimensional
arrangement of the atoms now if we draw
in the relevant hydrogens here on these
sp2 hybridized carbons
it'll get a little easier to see what's
going on here
so in this case we like to say when
you've got the two like groups
right next to each other only 60 degrees
apart that's cis
so here whether you look at these two
hydrogens or in this case these two
methyls we can see that they're only 60
degrees apart and that's this
so however in the next example you can
see that these two methyl groups are 180
degrees apart or you could look at the
hydrogens the same way
the hydrogens are 180 degrees apart so
and that is referred to as
trans so not all alkenes will be capable
of cis and trans
so it turns out each sp2 carbon besides
the double bond has to be bonded to
two different things that's a methyl
that's a hydrogen they're different
so far so good look at the other sp2
carbon that's a methyl that's a hydrogen
that's two different things
so there's going to be cis trans
isomerism so but the moment
you make you know either side of this
have two of the same thing so
methyl group and put another hydrogen
there well this guy wouldn't have cis
trans isomerism
because now i would say you know well am
i comparing these two well that's cis but
but
these are the same as well so that's
trans well then it's neither because
there's not two different ways this guy
could exist
to make sure that cis trans-isomerism is
possible so for an alkene cyst trans isomerism
isomerism
is not possible if either sp2 carbons
bonded to two of the same thing
so it is possible though if both sp2
carbons are bonded to two different things
things
and it can be the same two things on
this side and the same as as on this side
side
but as long as each sp2 carbon on its
side is bonded two different things it
will have cis and trans isomers
that are possible okay so that's cis
trans isomerism and this is
not super new so but the next part is
going to be a little bit new and these
chiral centers and this again we're kind
of whittling
our way to the most important part of
the chapter and this chapter really is
going to center a lot around these
chiral centers
cool so before we talk about chiral
centers we have to talk about what the word
word
chiral means and so chiral refers to
when you compare
something to its mirror image so if i
take my hand it's our classic example my
right hand and my left hand
you can see that they are the perfect
mirror image of each other now they're
not the same though
so like if i want to you know play
baseball if i want to get you know a
right-handed glove versus a left-handed
glove i have to get two different gloves
one for each hand they don't you know a
left-handed glove would not fit in my
you know my other hand and my right hand
global not on the other hand as well and so
so
because the hands are different now you
might be like well you can line them up
this way and you're right because
they're mirror images of each other
but they're not exactly the same at all
they're not superimposable we say
and so when you've got two objects that
are mirror image of each other
but are not the same we say
non-superimposables that use the word we use
use
i like to use the word non-identical but
non-superimposable is kind of the one we
really like to stick with with organic
chemistry but i'm going to use them both
quite a bit today
just to remind you that
non-superimposable means non-identical
and so when you have two objects that
are mirror images like your hands but
are not the same non-identical non-superimposable
non-superimposable
we say that your hand is an example of
an object that is
chiral and so however if you take two objects
objects
and compare them so and let's say
they're mirror images but they're identical
identical
so then that's going to be an example of
something that's a chiral and a good
example that would just be a
blank sheet of paper if i had a blank
sheet of paper in one hand or another
one on the other hand they would be identi
identi
you know both mirror images and
identical you couldn't tell them apart
so as long as there's nothing written on
them at that point so a blank sheet of
printer paper would be a good example of
something that is
a chiral and so now we have chiral and a
chiral and it
involves whether or not you're identical
to your marriage or not identical to
image if you're not identical to your
mirror image you are chiral
and if you're identical to your image
you're a chiral now let's take a look at
how this applies in kind of the
molecular world here
and so we're going to take a look at
some little models and for this to make
any sense we're going to have to zoom
in all right now you're going to put up
with my big head filling your screen
i apologize for that so but i've got two
molecules here
and these molecules are perfect mirror
images of each other and you can kind of
see that if i start lining them up
so you can see that they are perfect
mirror images of each other here
so line up right along a mirror plane we
like to say so in every respect
now the problem though is that if you
try to actually overlap them you'll see
that you can't
now i can overlap you know the bond on
the top and the blue bond
but if you notice the red and the black
are in opposite places here
and these are not the same molecule so
they are mirror images
but they're non-superimposable mirror
images they're non-identical mirror
images and so
we can say that this molecule is chiral
or we could say that this molecule is
chiral so the word chiral
can refer to an individual molecule and
it just means that this molecule
is not the same thing as its mirror image
image
okay so that's chiral now we have the
opposite word as well and that's called
a chiral let's see an example of that
all right so here i've got an example of
compounds that are achiral and we can
see that once again these are perfect
mirror images of each other
so they line up right along a mirror
plane here
so however though if you actually start
overlapping the atoms
they line up perfectly all the atoms match
match
in every respect we say they are superimposable
superimposable
and so when a compound and its mirror
image are the same
identical superimposable that compound is
is
a chiral now so again chiral means that
you and your mirror image
are not the same a chiral means you and your
your
mirror image are the same so okay let's
go back and take a
closer look at the chiral version
all right so here we've got these chiral
compounds and these are rather unique so
in that again they're mirror images but
they're not identical
and this is going to have some some
interesting consequences so it turns out
because they're perfect mirror images
they have the same polarity they have
the same dipole moment so to speak then
which also cause them then therefore to
have the same boiling point the same
melting point and pretty much
almost every physical property you can
think of is exactly the same
and that's a pain in the butt because we
usually manipulate some difference
between physical properties
between compounds in order to separate
them well enantiomers is what these are
it turns out when you've got two chiral compounds
compounds
those two mirror image versions are
called enantiomers of each other so
that's going to be our next keyword here
so these enantiomers
these non-superimposable mirror images are
are
really difficult to separate so they
have the same boiling point so you can't use
use
like distillation to separate them they
have the same dipole moment the same
polarity so you can't use some form of
chromatography to separate them so
we have to get pretty creative in
separating these enantiomers
so now i've got the chiral compound here
on my right your left
and i've got the a chiral compound on my
left your right
and the question is then what can we
look at from a physical perspective
that is the key to understanding why one
was chiral and one was achilles one was
different than its mirror image and one
was identical to its mirror image
so well it turns out it's the presence
of what we call a chiral
center so and if we look at the chiral
version of this
this has what's called a chiral center
right in the middle and a chiral center
which is also called it's got
lots of names chiral center chirality
center stereogenic center asymmetric center
center
so is an sp3 hybridized atom in an
organic chemistry most the time
we look at it's going to be carbon but
it technically doesn't have to be and it
shows up every once in a while something
other than carbon but
most of the ones we'll see in this class
will be carbon but you have an sp3
hybridized atom which makes it
tetrahedral in shape
that has four different groups attached
and we call them chiral centers because
if you have one of those that's the
single greatest indication that a
molecule is probably going to end up being
being
chiral being different than its mirror
image well if you notice the a chiral
one we looked at here
it did not have four different groups so
again with the chiral version we had red
blue black
and then the white four different things
so with this guy
notice i've got a a red and a black but
then i've got
two whites it's four things but not four
different things
and so this would not have a chiral
center whereas again this one with four
different things
does have a chiral center and having one
chiral center makes this guy
chiral having no chiral centers this guy
ends up being
a chiral so cool so these chiral centers
they're just called that because
simply they're the single greatest
indication we have that a molecule is
going to end up being chiral
so usually if i'm trying to identify if
it compounds chiral first thing i do is
look for these chiral centers somewhere
in the structure
you're not always going to have a model
in front of you you're going to
recognize it from you know a good bond
line structure something like that and so
so
you're going to look for some atom that
doesn't have to be carbon again but it
often will
so some atom that's sp3 hybridized it's
bonded to four
different groups now keep in mind this
is four different groups it doesn't mean
four different
atoms all four of those groups could be
carbon atoms but one could be the carbon
of a methyl one the carbon of an ethyl
one the carbon of a propyl and one the
carbon of a butyl
that would be four different groups and
that would be a chiral center
now we'll find out there are some
exceptions not every compound that has
chiral centers is going to be chiral but
most of them
will and we'll have a special name for
those exceptions a little bit later in
this lesson
all right i'm glad you made it through
having my head fill your screen we'll
get back to doing a little social
distancing now here
so we'll go back to these chiral
compounds one more time and so
uh once again it turns out these chiral
compounds are
you know mirror images but have nearly
identical physical properties in almost
every respect
it turns out except for one and we call
that property
optical activity and this is going to
get a little bit weird it turns out it's
a quantum mechanics thing so
but we got to talk about what's first
called plane polarized
light so light is you know has wave-like
characteristics at least i should say
and this wave can be a vertical wave it
can be a horizontal wave it can be a
diagonal wave
and when you have unpolarized light you
have light
that's directed in every possible
orientation for its wave anyways
so well plain polarized light you can
shine that unpolarized light through
what's called a polarizing filter and
polarizing sunglasses are an example of
a polarizing filter
and with that polarizing filter does it
blocks out all of the orientations
except for
one and so let's say i use a polarizing
filter and it blocks out all of the
different orientations except vertical light
light
and so after i shine it through that
filter the only light that makes it through
through
is the vertically aligned light we would
call that plane
polarized light and so it turns out when
you shine
plane polarized light through say a
solution of just pure water let's just
say it's not a solution but just pure water
water
it'll come out the other side and it'll
still be vertical light
now it turns out one interesting thing
is that if i dissolve an a chiral
compound in that water
it will still come out the other side as
vertical light but if i dissolve
a chiral compound in the water it won't
come out the other side
vertical anymore it'll either get
rotated one direction
or the other and come out at some other
angle besides the vertical
so kind of a weird thing and it turns
out it gets even weirder so
if you have the two different
enantiomers if i dissolve one of the
enantiomers in the water
in one case and let's say the right the
light gets rotated off to the left
well then if i go and dissolve the other
enantiomer in water at the same concentration
concentration
then it would now get dissolved to the
left i'm sorry to the right
my left your right so by exactly the
same amount though provided they were
the same concentration so
the two enantiomers for this chiral
compound the two different antimers
they rotate this plane polarized light
in opposite directions
so the first thing is that so chiral
compounds are optically active
a chiral compounds are optically
inactive and again that just simply
means that chiral compounds rotate plane
polarized light
a chiral compounds don't so that's the
first part
but that's the only way we really can
you know quickly distinguish between
two chiral compounds so
in kind of a macro way is we just stick
them what's called a polarimeter and we
just see which way the light gets rotated
rotated
so and again the key is those two
enantiomers will always rotate
light in opposite directions by the same amount
amount
again assuming they're in the same
concentration now let's say you took and
dissolved both of those compounds in
your water at the same time together in
equal concentrations
well it turns out that solution is no
longer going to rotate light that would
be an example of a
optically inactive solution now if you
put one chiral compound or the other only
only
so you have an optically active solution
but put them in there together in equal concentrations
concentrations
and the light doesn't get rotated
anymore because half the molecules want
to rotate light one direction
the other half of the molecules want to
rotate the light the other direction and
so on
average the light's going to encounter
an equal number of molecules as it makes
its way through the solution
and therefore on average not get rotated at
at
all and so they give a special name to a
solution that's got a 50 50 mixture of
enantiomers and that is a
racemic mixture cool so another
vocabulary racing mixture
50 50 mixture of enantiomers and a
receiver mixture is not an
optically active solution
all right so let's get back to our
discussion of isomers here so
in this case we've got our lovely chiral
centers here
uh for stereoisomers so we talked about
cistrans but if you've got chiral centers
centers
we're going to break you up into one of
two types then you're either going to be
enantiomers which is a word
we have already used and enantiomers
again are
non-superimposable mirror images only
chiral compounds have enantiomers a
chiral compounds don't have enantiomers
chiral compounds are different than
their mirror image and the two different
forms are called enantiomers
a chiral compounds are identical to the
mirror image so they wouldn't have enantiomers
enantiomers
but the other type here
are called diastereomers and so
enantiomers again so let's
let's actually go back up to
stereoisomers for a second so stereoisomers
stereoisomers
same bond connectivity different
three-dimensional arrangement of the atoms
atoms
and it turns out from stereoisomers i
broke this up kind of here
but technically a lot of textbooks will
actually break this up straight from stereoisomers
stereoisomers
into this kind of dichotomy instead
enantiomers are diastereomers
if two compounds are stereoisomers well
they are either going to be either
enantiomers or diastereomers
it turns out now enantiomers are stereoisomers
stereoisomers
that are mirror images of each other and
so guess what diastereomers are
they are stereoisomers that are not
mirror images of each other
and your cis trans isomers technically
would be a great example of that
these are not mirror images of each
other at all and so they are not enantiomers
enantiomers
but they are stereoisomers and so your
only other option then is then
stereoisomers that are not mirror images
and it turns out all of your cis trans
isomers are
technically diastereomers so and this is
something to file away in your head
because most of the time we
to look at the word diastereomers we're
going to recognize when compounds are
diastereomers in a different way for
most of the examples
but you should remember that cis and
trans isomers are technically also
examples of diastereomers as well
and again whether that's cycloalkanes or
alkenes same diff
so but most of the time when we look at
molecules that have chiral centers
we'll be able to identify them pretty
quickly of whether they're not the
relationship whether or not they're just
the same
identical compounds or whether or not
they are some form of steroids from
either enantiomers or diastereomers
and they'll be a quick way to kind of
figure out well are they you know
mirror image of each other or are they
not mirror images of each other so for example
example
at these two right here so i've lined
these up on
right across what we call a mirror plane
or sometimes
actually let's call it a mirror plane so
this mirror plane right here so you can
see that
the one on the left is the perfect
reflection of the one
on the right so however if you actually
rotated this one around
to try and line up the carbon chain well
by flipping it over notice right now
that chlorine is coming out of the board so
so
right now it's coming out of the board
by the time i flip the whole molecule
over to line up the carbon chains
that chlorine would be going into the
board and it wouldn't look like this
like this instead and those are not the
same they have the same bond connectivity
connectivity
all the atoms bonded the same stuff but
the three-dimensional arrangement of the atoms
atoms
is indeed different and so in this case
they are mirror images
but i can see that they're never going
to line up and so the key here though
in recognizing that these were
enantiomers is finding that chiral center
center
first so some atom usually carbon that's
bonded to four different things
and it turns out that's this carbon
right here so this carbon right here is
bonded to a chlorine
okay check it's also bonded to a
hydrogen that's not technically drawn
in and you should recognize that there's
a hydrogen
right there and then it's bonded to two carbons
carbons
but these two carbons are not the same
the one on the left is the carbon of a
methyl group the one on the right is the
carbon of an ethyl group they're
definitely not the same and so that's
four different groups that's
a chiral center and it turns out if you
have one chiral center you're chiral
done it's when we have many chiral
centers that will see some exceptions
creep in but if you've got one chiral
center you're chiral
no fans or butts life is good all right
so these are mirror images but not identical
identical
life is good one other thing you should
know is that a chiral center is
the most common example of what we call
a stereocenter
and a stereocenter is just an atom where
if you have two of the things attached
to it
trade places you get a different stereoisomer
stereoisomer
and it turns out chiral centers are
examples of these
stereocenters so it turns out if you
take any two of the groups attached
any two doesn't matter which two most
commonly you'll see it be the wedge in
the dash but it could be any two
on a chiral center if you have those two
groups trade places
you get the mirror image version of that
compound and so in this case if you have
the hydrogen and the chlorine trade places
places
well then it's going to look exactly
like this the chlorine would be the dash
the hydrogen which is not drawn in would
now i could have just if well have had
the methyl in the ethyl traded places or
something like that
so in that if i had the methyl and ethyl
traded places it looked exactly like this
this
but having the chlorine the hydrogen
took place it looks like this but don't
forget that this
was the same thing as this just rotated
to 180 degrees
around and so the way we recognize enantiomers
enantiomers
is oftentimes not by lining them up on a
mirror plane like this although that can
happen and you should recognize
enantiomers that way
but oftentimes we'll recognize it by
looking at chiral centers and seeing
that two groups of traded places oh
trade two groups on a chiral center you
get the mirror image
version of that chiral center
all right so now we've seen a couple
different ways we recognize this and so
again the definition enantiomers are
non-superimposable mirror images
but the way we'll often recognize them
is by looking to see if two groups have
been traded places
on a chiral center we'll find out that a
compound with many chiral centers
is you'll have to invert every single
one of those chiral centers to get the
mirror image the enantiomer
and so even if you can't quite see the
mirror image of that point
there's still mirror images so like
right now you can see that my hands are
mirror images
now if i do this you know and something
like this you might be like well are
they mirror images or are they not well obviously
obviously
they're my two hands so they are so
except for where i you know punched a
wall when i was like 18 and
broke my hand and there's a big bump
here but outside of that these are
perfect mirror images
so and again even if you are not lined
up in such a way that you can see that
they're mirror images they're still
mirror images
so same thing here if i give you these
two right here
you're going to want to get to the point
where you're like oh they had just the
wedge and the dash trade places
that's the reverse form the inverted
form the mirror image form
of that chiral center when you have any
two groups trade places on a chiral
center you're going to get that mirror
image form and so even though
if i compared these two right here and
said you know how are they related well
you can't see that they're mirror images
very easily
but now you've learned a new way to
recognize that they are indeed mirror
images even if i can't see it right now
i know that they're mere images and they
are enantiomers
so let's talk about diastereomers for a
second and so once again just reminder
cis and trans those are diastereomers
but the new type
is you're going to see that
diastereomers most commonly are going to
show up in the context of having many
chiral centers
actually let's go here let's say i give you
this compound
and then i'm going to give you its
so you have to first look and say okay
are these compounds mirror images yeah
that's the perfect reflection of this
do they have chiral centers well yeah
turns out there's
two there are two of these chiral
centers present
in this case we'll see that there's some
new relationships possible here but
these are
molecules that have chiral centers and
they're mirror images and they're not
the same if you
try and overlap this and if i flip this
over to try and get the bromine in the
right place by flipping it over
now the bromine would be here and the
chlorine here but they'd be dashed bonds
not wedge bonds
they're not superimposable and they're
mirror images these are enantiomers so
still under the enantiomers category
so and the idea though is that you have
to invert every single chiral center for
that to be possible
or just draw them as mirror images like
i've done here however there's going to
be another option
this one right here if you look at
comparing these two now you look at
these and say okay
it's a five carbon chain bromines bonded
one in from one end chlorine's bonded
one in from the other end
so the bond connectivity is exactly the same
same
but i look and i say are they mirror images
images
because they're lined up in such a way
that if they were mirror images you'd
see it right now
however the reflection of a wedged
chlorine should be another wedged
chlorine not a dashed one now the
bromines look mirrored
but the chlorines don't so these are not
mirror images so they are
stereoisomers that are not mirror images
which means they can't be
enantiomers so then the only other
option is diastereomers
and the definition is really non-superimposable
non-superimposable
non-mirror images so they're just
stereoisomers that aren't mere images
cool and that's what we see here and so
usually what you'll find is
you know i could draw this a little bit
different way if i
rotate this one around and flip it over
so and i'll do that in red so same
compound on the right here i'm just
going to rotate it out of the plane here
180 degrees
see these lined up just a little bit
different you'd see that the chlorine
lines up
just like the one above it but the
instead so again this is the same exact
compound as the one on the upper right here
here
and so we already decided that these
were diastereomers now i just want you
to be able to recognize it in a
different way
if you look at the chiral center on the
left the one that has the chlorine attached
attached
it's in exactly the same configuration
the same three-dimensionality there's no
difference okay
but if you look at the other one so
one's a wedge one's a dash so they've
had the wedge in the dashboard places
here the bromines on the wedge the hydra
on the dash
here the bromine is on the dash the
hydra on the wedge we've inverted it
again it's a stereocenter
you trade two groups places you get the
opposite mirror image version of this
so we have one chiral center in the same configuration
configuration
we have one chiral center in the
opposite configuration and that's going
to be the most common way you
you actually recognize diastereomers
you're going to have
many chiral centers and at least one of
them needs to be in the same configuration
configuration
and at least one of them needs to be in
the opposite configuration that'll be
the key
for two compounds to be enantiomers it
turns out all of the chiral centers have
to be inverted
they all have to be in the opposite
configuration but for diastereomers at
least one the same
at least one different then they're
going to be diastereomers now i say this
and students get used to recognizing
diastereomers this way but
don't forget that cis and trans isomers
again are
also considered diastereomers so there's
one last thing we need to do in this
lesson and
one of the common questions you'll get
on this section of your course
is they'll give you a big molecule and
they'll just ask you to identify
the number of chiral centers on the
molecule so there's a couple tricks
for how you do this now one of the keys
is going to be actually eliminating
atoms that are definitely not chiral
centers so first thing is you realize
that all chiral centers are sp3
hybridized again they don't technically
have to be carbon
but most the ones or even all of the
ones you'll see will be carbon so but technically
technically
don't just you know assume it can't be
an oxygen or something like that
so we'll have to rule it out but it has
to be an sp3 hybridized atom so again a
chiral center is an
uh sp3 hybridized atom a tetrahedral atom
atom
so with four different groups so as a
result then i can cross off all the sp2
and sp hybridized atoms
so this one's out this one's out this
one's out this one's out this one's out
this one's out
this one's out this one's out those are
all sp2 hybridized atoms having
pi bonds right so they can't be chiral
centers so we'll rule them out
the next thing i'll go through and do is
i'll take a look at some of the
heteroatoms like so in this case
this oxygen right here doesn't bond into
four different things now it turns out
in some rare cases a lone pair can count
as one of four different things on a
tetrahedral atom that was a three
four different things so however this
has got two identical lone pairs there's
not four things coming off that auction
he is sp3 hybridized but he doesn't have
four things so
we'll eliminate him as well and so from
here on out it's just going to be
comparing carbons
and so again these carbons have to have
four different groups so next time we'll
look and say well any
carbon that's bonded to more than one
hydrogen would not have four different
things then
so because if you have more than one
hydrogen then they're going to be
identical so
for example like this is right here is a
methyl group a ch3 and with three
identical h's there's not four different
things there
same thing's true there there and there
and there so but they're not the only
ones it's not just ch3s i can look for
ch2s as well so
notice this guy's got two identical
hydrogen so he's not a chiral center
same with this one here
and that's it so then i'm going to want
to go through and look at what's left
so i got rid of all the sp2s i got rid
of anybody bonded to more than one hydrogen
hydrogen
so who's left well we've got this guy left
left
right here this carbon right here and if
i look what's he bonded to well he's
bonded to the carbon of a methyl
he's bonded to this carbon over here
responded to this carbon over here and
then there's a hydrogen not drawn in
that's a dash
well the hydrogen's definitely different
the question though is what about all
three of these carbons
well i can definitely tell that this
carbon of the methyl is definitely
different than either one of these carbons
carbons
but these carbons are definitely
different as well and you might be like
well how do i know chad well
look this one's part of the five and six
membered ring whereas this one's just
part of a five-membered ring and then
some chain coming off
they're definitely just bonded different
stuff now one of the other things you
can do sometimes
students often struggle for some that
are like bonded as part of a ring
well it's part of a ring so for them to
be equivalent you could line up a mirror plane
plane
so and the entire right side of the
molecule would have to be exactly the
same as the left-hand side of the molecule
molecule
so for two atoms in a ring anyways for
them to be equivalent the right side and
the left side of this molecule
centered around uh that chiral carbon
are not the same so these are definitely different
different
carbon groups and so as a result this
carbon's bonded to three different types
of carbon groups and a hydrogen
that's four different groups and he indeed
indeed
is a chiral center all right we'll go
over to this guy as well and he's also
going to be a chiral center he's bonded
to the oxygen right here
he's bonded to a hydrogen as with a
wedge bond that's not drawn in
and then he's bonded to these two
carbons and these two carbons are
definitely not equivalent this one's
part of ring this one's not
this one's got no h's this one's got two
ages they're just different and so as a
result that also
is a chiral center and then we'll move
on to this carbon right here
and this one is bonded to this carbon
this carbon this carbon and again a
hydrogen as a dashed bond
so and in this case i can see that this carbon
carbon
is not the same as this methyl carbon so
that's cool this carbon's bonded to two
methyls and this just is a methyl
and it's definitely different than this
one as well because this one's a ch2
that's bonded to you know the rest of
the molecule and stuff so all three of
these carbons are definitely unique
that's three different types of carbons
and a hydrogen that's four different groups
groups
and that is also a chiral center and
then last but not least the only carbon
we have looked at is that guy right there
there
so but this guy and this is the tricky
one in this example this is the one
where i'm expecting to fool a few students
students
so this carbon right here is actually
bonded to a methyl group right there and
another methyl group right there
those are both methyl groups and so this is
is
not a chiral center
now a lot of students will learn this
and they'll say well just look for
wedges and dashes because if it's a
chiral center then they have to show the
three dimensionality of it
so which is typically true and sometimes
they won't show the three-dimensionality
and still expect to identify as a chiral
center but oftentimes
that's exactly right they're going to
show you three-dimensionality so that
because it you know technically could
exist in two different mirror image
forms right and so
if they're showing you
three-dimensionality that's a key of
also where to focus in
but don't forget to rule some other
stuff out and also don't fall for this
trick right here
it's not just that they show you
three-dimensionality you still have to
verify that there's
four different groups that was not the
case here and so he wasn't a chiral
center and so
overall we can see that this molecule
just has a total of
three different chiral centers now if
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