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5.3 Molecules with Multiple Chiral Centers | Enantiomers, Diastereomers, and Meso Compounds | OChem

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molecules having multiple chiral centers now

now

we're in a chapter on isomers and

stereochemistry and this is the third

lesson in now we started off

by looking at isomers in general and

constitutional versus stereoisomers

and we introduced the term chiral center uh

and then in the second lesson we went

through and showed how we could assign

absolute configurations to these chiral

centers and we saw that there

is going to be designated r or s and now

we want to take a little bit closer look

at molecules that have

multiple chiral centers and some of the

unique things to them and some of the

vocab that comes up specifically

in this context now if this is your

first time visiting the channel

my name is chad welcome to chad's prep

where our goal is simply to make science

both understandable and maybe even

enjoyable now this is my brand new

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so for molecules that have multiple

chiral centers first thing i want to

deal with is just

how many possible stereoisomers are

there going to be here so

if you look so if i put let's say a

chlorine right here

so all of a sudden this is a carbon

that's bonded to four different groups

he's got a chlorine

a hydrogen not drawn in and then the

carbon of a methyl and this carbon of

the bigger chain here and that's four

different groups

and so this could exist in two different

forms and i could represent those two

forms if i make the chlorine

attached with a wedged bond versus

having the chlorine attached with a

dashed bond that would be one way so the

idea is that when you've got a chiral center

center

it exists in two forms r and s as we

learned in the last lesson

and so in this case there'd be two

different stereoisomers but what if

i add another

chiral center so this carbon's bonded to

four different groups so bromine a

hydrogen and then these two carbons are definitely

definitely

not equivalent the right hand side of

the molecule on the left-hand side that

has a chlorine are totally different so

four different groups another chiral

center and then this chiral center can

now exist in two different forms r

and s and so all of a sudden the total

number of stereoisomers

now goes up to four because you have two

different versions

the way the molecule could look at this

chiral center two different that you

could you know have it look here

and they're four different overall so

notice it could be r r s

s r s s r four different versions and

this is kind of

the same principle behind like flipping

a coin when you flip a coin there's two

possible outcomes you can

you know flip a heads or you can flip a

tails if you flip a coin twice in a row

you can get four different possible

outcomes heads heads tails tails heads

tails tails heads

so if you flip it three times it'd be

eight possible outcomes four times

sixteen possible outcomes

and it turns out the num the more you

flip it the more possible outcomes and

it works on a principle of 2 to the n

power where n is the number of flips

well in this case we're not flipping a

coin but we're just adding chiral

centers to the molecule

and n here's going to represent the

number of chiral centers and the more

chiral centers you have the more

different stereoisomers that could

possibly exist

for a given structure and so in this

case with two

chiral centers we figured out it's going

to be four possible stereoisomers

with three chiral centers let's say we

put like a fluorine right here

now all of a sudden we've got another

chiral center now it'd be eight possible

you know stereoisomers and it's always 2

times 2 times 2

over and over and over again with a

couple exceptions though as we'll see

and so the way we phrase this is that 2

to the n where n is the number of chiral centers

centers

equals the maximum possible number

now it's interesting that we say the

maximum possible number

and we'll find out that you're not

always going to hit that maximum it

turns out in certain cases

when you're dealing with molecules that

are fairly symmetrical

you might actually end up with less than

that maximum so it wouldn't apply to

this molecule here but we will see

some examples here before we get to the

end of this lesson

now i don't actually want to deal with

this lovely molecule

the one i want to deal with is this guy

right here

that has two chiral centers so and with

two chiral centers and no chance for

symmetry we would expect to have the maximum

maximum

four possible stereoisomers and i want

to draw all four here

the chlorine could be on a wedge and the

bromine on a wedge

okay and there's our four different stereoisomers

stereoisomers

that are possible now the next thing

we've got to deal with are some

relationships here now we've already

already learned the terms enantiomers

and diastereomers so but just to review

enantiomers are stereoisomers that are

mirror images

diastereomers are stereoisomers that are

not mere images

and to rewind back a little further stereoisomers

stereoisomers

are isomers that have the same bond connectivity

connectivity

but a different three-dimensional

arrangement of the atom so

all of these have exactly the same bond

connectivity but definitely the

three-dimensional arrangement of the

atoms is different

now in this case none of these are lined

up in such a way that you can see if

they're mirror images

so but a couple of them definitely are

so if we took this molecule right here

and rotated it around 180 degrees it

would look like

so 180 degrees would put the bromine

over here

and now it would be a wedge so and again

180 degrees out of the plane of the board

board

would put the chlorine over here and now

it'll be sticking out of the board

instead of in

and now i can see that oh yeah they are

mirror images

that is totally the mirror reflection

right across the mirror plane here

so it's hard to see that their mirror

image is right here but they are

and again stereoisomers that are mirror images

images

are enantiomers and i'm going to kind of

put a

e here to represent that they're

enantiomers and one thing you know with

with chiral centers if you know you got

one chiral center your chiral

so and if you have the r version then

the enantiomer is going to be the s version

version

well when you've got many chiral centers

you have to invert

all of them and so if we assign r and s

here real quick so here chlorine is

number one

this carbon is number two this carbon's

number three one two three

number four hydrogen's in the back so a

right-handed turn is gonna correspond to

r notice how quickly i did that you're

one maybe not quite that quick but you

want to

get very quick at assigning rns it will

be key

to finishing this next exam involving

this involving isomers and stereochemistry

stereochemistry

same thing here bromine's number one

this carbon's number two this carbon's

number three one two three

that's a left-handed turn and it's s

because again the hydrogen that's

priority number four

is in the back just like it's supposed

to be now if you notice here

the chlorines on the wedge the hydrogen

on the dash but in the next molecule

over the chlorine's on the dash the

hydrogen's on the wedge

and when we learn that when we have two

groups trade places

it inverts it so if this one was r then

this one here is going to be

s and if this one's s again bromine is

on a wedge hydraulic dash now bromine's

on the dash hydrogen on the wedge

if the original here is s then this one

is inverted

and is going to be r and we've just

demonstrated a valuable point

so we again know this is the mirror

image of this compound we just couldn't

see it the way it was

however new way to recognize it if you

invert every chiral center

in a chiral molecule you'll get the

enantiomer and so here i had two groups

trade places at both chiral centers

that's going to be the enantiomer cool

so let's look at this

once we've assigned one of these

compounds the rest are pretty easy

because i can see if any chiral centers

in the same

or opposite configuration of the ones

i've already signed so

in this case the chlorine's on a wedge

still on a wedge down here

exact same configuration that's going to

be r but here the bromine's on the wedge

hydra on the dash here

bromine on the dash hydrogen on the

wedge so instead of s this one's going

to be

r and then finally down here again i can

compare this one to either this one or

this one or this one

but here i can see that both of these

are inverted the chlorines the wedge the

chlorines the dash the bromine's the

dash the bromine's the wedge

so these must both be s instead of r

and once again we can see that when you

have multiple chiral centers if you invert

invert

all of them you get a relationship that

is enantiomers now when you use that

word enantiomers it's always comparison

between two molecules and

in fact that's true if you use the word

stereoisomers enantiomers or

diastereomers any of those three words

you're comparing two molecules so if i

asked you if my

left hand is the enantiomer of my right hand

hand

you'd say yes great if i said is my left hand

hand

an enantiomer and i stopped and you say

okay chad finish the question

is my left hand the enantiomer of my

right butt cheek no it is not it is the

enantiomer of my right hand

so you've got to always compare

something you can't just say something

and enantiomer now i could say is my left

left

hand chiral and that's a valid question

chiral the description for one compound

it just means that this compound

if i did compare it to its mirror image

would not be the same thing

so cool so now we've learned very

quickly with multiple chiral centers

we can actually recognize whether or not

they're enantiomers

if we actually assign r and s which will

be key in a little bit now these are

drawn in such a way that it's not too

bad to see that they're inverted

even without assigning rns but this will

become handy later on

now on the other hand what if i were to

do a different comparison here so let's

say i was going to compare

these two well

in this case they've got one chiral

center in the same configuration

one chiral center in an opposite

configuration so this means they're not

the same molecule they're not identical

so they're still stereoisomers but

they're also then not going to be mirror images

images

and if you're a stereoisomer that is not

a mirror image then you are and you know

what i should probably do this in a

different color

let's just keep this scheme going so

that means these guys

are going to be d for diastereomers and

so this would be a common way we

recognize diastereomers as well

if you have many chiral centers if at

least one of them is in the same

configuration but at least

one of them is in the opposite

configuration then they're not mirror

images so they can't be enantiomers

which means they have to be

diastereomers by default

so cool so that's the case here if i

compare these two

it's also the case here if i compare

these two so one of the chiral centers

in the same configuration one is in the

opposite configuration

those are diastereomers same thing if i

compare either

these two so one different

one the same or

these two as well again one different

one the same

cool and so that's a common way we get

to recognizing diastereomers if you have

many chiral centers and at least one is

in the same configuration and at least

one is in the opposite configuration

those are diastereomers

but don't forget diastereomers the

technical definition

was non-superimposable or non-identical non-mirror

non-mirror

images and we also saw in the first

lesson in this chapter that cis and

trans isomers

were also considered diastereomers so

don't forget this is going to be a

common way we often recognize

diastereomers from

here on out but don't forget that cis

and trans isomers are also diastereomers

cool so so far so good so

but this is going to get just a little

more complicated here with the

introduction of what we call

meso compounds all right so if we're

going to alter it just a little bit here

this original

structure we had and i'm going to take

that chlorine and make it a bromine

instead and what this is going to do is

it's going to make it that symmetry

could possibly you know could be

possible here

depending on the the

three-dimensionality of these two

bromines whether they're both wedges

both dashes or if one's a wedge and

one's a dash

so and that's going to be important

because we're going to learn a new way

to recognize whether or not you have an

achiral molecule so let's say we take a

look at

cyclohexane for a minute and my question

for you might be is it chiral or a chiral

chiral

well one of the things we learned is to

recognize when i've got chiral

molecules is look for chiral centers

well this guy's got no chiral centers

and that's true well one other thing we

could do though is just look at the

definition say well would he be the same

or different than his mirror image

well if we draw his mirror image

so reflected right across a plane of

symmetry here

so i can see that he is totally

identical to his mirror image there is

no difference okay

definitely could identify him as a

chiral well it turns out somebody

figured out

one additional way and they said

if you can find what's called an

internal mirror plane so instead of

taking and reflecting

you know the entire structure across a

mirror plane they said if you can find an

internal mirror plane and they called

these sigma planes

in a molecule and that's not the only

one where again the right hand side is

the perfect reflection of the left hand

side within the molecule and that's not

the only one i've got one going through

this bond one going through this bond

one going through these atoms these atoms

atoms

cyclohexane's got a ton of them but if

you have at least one

you're automatically going to be a

chiral it turns out if you have an

internal mirror plane

then you're going to be able to not help

but be identical to your overall mirror image

image

so now we've got a new test for finding

a chiral compounds

any compound with this internal mirror

plane this

sigma plane is achiral it's not the

definition of a chiral

but it's a new way to recognize achiral

compounds and this is going to be

important a second that's why

in dealing with symmetry here in a

minute we might end up with some a

chiral compounds now

we said early on that the easiest way to

recognize a chiral compound

and i should say a chiral compound

is look for chiral centers but there are

exceptions and those exceptions are

called miso compounds

miso compounds are achiral compounds

even though they have chiral centers so

here cyclohexane is a chiral

but it's not a miso compound he's a

chiral and we can see he's the same as his

his

mirror image or we can find this

internal mirror plane either one of

those would indicate that he's a

chiral but he doesn't have any chiral

centers so we could not say he's a miso compound

compound

so two parts of the definition of meso

compound he's a chiral

and has chiral centers or a chiral even

though he has chiral centers the way i

like to say it

all right so let's look at the different

stereoisomers of this guy and see the relevance

relevance

all right so here's our four possible

stereoisomers or so we

think in a second we'll see that that

actually is not the case

when we've got a plane of symmetry

possible like we do in this molecule

and we've got a perfect plane of

symmetry if we drew it right

here or this one right here but we don't

have that same plane of symmetry here

because one's a wedge and one's a dash

the three dimensionality means there's

no plane of symmetry down the middle

between them but that's going to have

important ramifications here so

if we assign rns kind of like we've done

before here

one two three this guy's r so one two

three this guy's

s one two three it looks like a r but

it's really s because the hydrogen is a

wedge the number four priority is a

wedge coming out

so and same thing here this one's gonna

be r and i can just match them up this

one's r

as well but this one instead of being s

is also r this one's

s and this one's the opposite so it's

also s and now we've assigned all our

r's and s's

and if we go and define some

relationships here like we've done

so these two are enantiomers

so i can see that i've got two chiral

centers and they're both

inverted and that's going to get me an

enantiomer okay life is good

the problem is if you try to relate

these two you're going to come to a

wrong determination

because it looks you want to say well

chad you inverted both chiral centers

they should be enantiomers

well the key is if you have a compound

with multiple chiral centers

and i should say if you have a compound

that is chiral with multiple chiral centers

centers

and you invert all of them you get the

enantiomer but the problem is this

compound's not

chiral with that internal mirror plane

right there

it is an example of an a chiral compound

just like we saw with cyclohexane

so it's a quick way to recognize if i

can find that internal plane this

automatically is a chiral

and that's going to have an important

ramification here so being a chiral

it doesn't have an enantiomer so i can't

call these nanometers because he doesn't

have one

so for an a chiral compound i mean this

is his mirror image don't get me wrong

these are mirror images totally mirror

images but

an achiral compound is not the

enantiomer of its mirror image

it's identical to its mirror image these

are actually the same compound

if you flipped this one over it would look

look

and now the bromines would both be wedges

wedges

and yeah you could say these are mirror

image of each other but they're not just

mirror images they're the same

thing and so as a result here we didn't

actually draw anything new

by drawing this one right here we drew

this one over again

and that's why we're not going to hit

the maximum possible number of

stereoisomers with two chiral centers

and we definitely have

two chiral centers here so both got four

different groups

it turns out they have the same four

different groups but each has four

different groups

and so with two chiral centers we expect

to get a maximum of four stereoisomers

we only get three in this case and so

when symmetry is possible and when one

of your stereoisomers is a miso compound

you're not going to hit that maximum and

so it turns out this guy

is the miso compound here

and again there's two parts to being

miso he's a chiral and i can see that

with that

sigma plane again so and also has

chiral centers if he didn't have chiral

centers he'd just be a chiral but not

miso but because he's

a chiral and has chiral centers or even

though he has chiral centers that's what

makes him miso

now we introduced this idea of finding

that mirror plane because it's often

convenient for recognizing these miso compounds

compounds

but a lot of students because it's the

first time we ever brought up that

internal mirror plane was for recognized

meso compounds

they think that any molecule that has

the internal mirror plane here is miso

not true any molecule that has the

internal airplane again is simply

a chiral you then have to also look for

chiral centers

for it to be further classified as miso

so note that all meso compounds are

achiral by definition

but not all a chiral compounds are miso

okay so let's take advantage of one

other thing here so

and i'll draw it like so and all of a

sudden here i've rotated some of the

carbon-carbon bonds in here and i've

represented this thing in a way

that makes it difficult to see if

there's an internal mirror plane or not

you might have to rotate some carbon

carbon bonds around to figure that out

and so here's the deal so at the beginning

beginning

of this chapter we you know briefly

alluded to optical activity we said that

chiral compounds rotate plane polarized

light they're optically active

a chiral compounds don't they're

optically inactive so keeping that in

mind what i'm going to tell you now is that

that

chiral centers rotate light that's

what's actually rotating the light so

chiral centers rotate

light period so does this chiral center

rotate light yes

does this chiral center rotate light yes

does this molecule rotate light yes

because overall it's a

chiral molecule now in this case with

the miso compound it's a little bit

crazy because

does this chiral center rotate light yes

it does does this chiral center rotate light

light

yes it does does this molecule rotate

light no

it doesn't so

this chiral center rotates light this

one rotates light the molecule doesn't

rotate light

well the key again is realizing that

you've got the same four different

groups attached to this chiral center as

this one

this one's got a bromine a hydrogen this

methyl and this half of the molecule

this guy's got a bromine a hydrogen this

methyl and then this half of the molecule

molecule

it's the same four different groups but

one of them's r and one of them's s

one of them is going to rotate the light

one direction one's gonna rotate light

exactly the opposite direction

so that on average every photon of light

passing through the solution

will have encountered an equal number of

these lovely chiral centers

and overall won't be rotated so that's

kind of the deal miso compounds even

though they have chiral centers

don't rotate light so

i just want to keep that in mind because

this is going to be a key to also

finding another way to recognize a miso

compound because here i look at this

molecule i'm like

well it has chiral centers and i know

it's one of the stereoisomers of this so

there's a

chance it could have an internal mirror plane

plane

if the three-dimensionality is right i

just can't tell in this cont

in this confirmation if it is one of

those well in this case the first thing

i had to realize

is did it even have a chance of having

symmetry notice if i add

one more carbon onto the left-hand side

here like adding one more carbon right here

here

i wouldn't be worried about this having

symmetry the left-hand side is different

than the right-hand side

there's no chance it could be miso

anymore but again let's take that off

and now i see oh yes it has a chance of

being symmetrical and same thing here

yes it has a chance of being symmetrical

and when it has that chance of being

symmetrical we can see another way to

recognize that it's the meso version

look at your chiral centers here and for

for meso compounds that have two chiral centers

centers

because it's going to be optically

inactive one of them has to be r

and one of them have to be s if they're

both r

or both s those are chiral compounds

they will be optically active

and so in this case even though i can't

see if there's a mirror plane i could

figure it out by just simply assigning r

and s

i can recognize it as a chance for

symmetry so now i'll assign r and s

in this case one two three this guy's r

hydran's in the back so one two three

this guy looks like r but the hydrogen's

a wedge coming out so it's really

s and this is indeed the meso compound

it is a chiral it will not rotate light life

life

is good cool so

for compounds that are not uh that are

on a straight chain like this for

recognizing them as miso

so as long as we're just talking about

two chiral centers one's gonna have to

be r

one's gonna have to be s again for

molecules with a chance of symmetry

now if you've got a molecule that is a ring

so let's say i gave you something like

this one here so it turns out

recognizing meso compounds is is going

to be

a common question you'll encounter on

the exam for this chapter and if you

look at this lovely molecule here there

are chiral centers

so two chiral centers however you can

also see that there's a lovely internal

mirror plane a

sigma plane right down the middle of

that molecule and that makes it a meso compound

compound

and meso compounds are much easier to

recognize on cyclo

alkanes than they are on straight chains

because for a cycle alkane you can't

rotate the carbon-carbon bonds in any

kind of crazy way so that you can't see

the mirror plane

for a cyclic compound like this if

there's a mirror plane you're going to

see it

but for one that's on a straight chain

you can rotate all those carbon carbon

bonds and put it in any kind of crazy

conformation you want

and so maybe it's me so and here i can

totally see it's miso

two chiral centers mirror plane done but

i could put this in a confirmation where

we couldn't see that mirror plane and

only has to have that mirror plane in

one of the many confirmations possible

for it to be a chiral and so once again

so the rings are much easier the

straight change though

so you might assign r and s as another

way to figure out notice you could do

the same thing here but i can see it

so but if you assign rns you'll find out

that one of these has to be r and one of

these has to be s as well and so in this case

case

this bromine is number one this carbon's

number two this carbon's number three

and this guy's in the r configuration

bromine is number one this carbon's

number two this carbon's number three

and this one is in the

s configuration and lo and behold that's

going to be true for our meso compounds again

again

as long as they have a chance for

symmetry and if you've got exactly two

chiral centers you go to assign rns

life is good now if you've got more than

two chiral centers and this is not

something you will come across

you can't just say one's r and one's s

because you've got more than two so

but again that's not likely something

you encounter you're going to see meso compounds

compounds

most often and maybe even all the time

for what you see in an undergraduate

organic chemistry class

in this kind of setting where you have

exactly two chiral centers

now if you've benefited from this lesson

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so if you're looking for the study

guides that go with this lesson or if

you're looking for practice problems

in dealing with isomers and

stereochemistry consider taking a look

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YouTube Transcript:5.3 Molecules with Multiple Chiral Centers | Enantiomers, Diastereomers, and Meso Compounds | OChem

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5.3 Molecules with Multiple Chiral Centers | Enantiomers, Diastereomers, and Meso Compounds | OChem