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5.1 Overview of Isomers | Constitutional Isomers and Stereoisomers | Organic Chemistry

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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

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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

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

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

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

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

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

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

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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YouTube Transcript:5.1 Overview of Isomers | Constitutional Isomers and Stereoisomers | Organic Chemistry

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5.1 Overview of Isomers | Constitutional Isomers and Stereoisomers | Organic Chemistry