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R and S Configuration Using Cahn Ingold Prelog Priority Rules Leah Fisch

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Leia hair from leh Versailles calm and

in this video we'll start the discussion

on RNs configurations for chiral

molecules you can find this entire

series alone with my practice quiz and

cheat sheet by visiting my website layer

for sicom slash chirality in the last

video we introduced the concept of

chirality and said that we classify a

pair of enantiomers as having the R or s

configuration and how do we do that we

need to find the absolute configuration

around each chiral atom where each

chiral carbon where the absolute

configuration looks at the chiral carbon

the atoms around it and then the order

of the atoms to give you R or s and the

way we rank these atoms is by following

the cahn-ingold-prelog system after the

inventors how do we rank the atoms by

atomic number not by the size of the

chain not by the weight by the atomic

number of the individual atom that is

attached to your chiral Center this is

where a lot of students get confused so

once again the atomic number of the atom

directly attached to that central carbon

that chiral carbon so you get a chiral

molecule you open your periodic table

you waste a good few seconds looking for

the atoms don't do that in my intro to

or go video series I show you the 10

atoms that you have to memorize in your

organic chemistry course so that you

don't waste your time constantly looking

at the periodic table we have in period

1 hydrogen area 2 carbon nitrogen oxygen

fluorine period 3 phosphorus sulfur

chlorine and then going down bromine and

iodine why is this important we have

electronegativity going up towards the

right size going down to the left which

is great for reactions but here's

another reason to know them this is the

order that we're going to use to rank

these atoms the atomic numbers will

increase from left to right on period

and then increase as you go down the

groups this tells you that iodine is

your absolute top priority

these are the atoms in your molecule

again these are the 10 most common atoms

that are going to show up in your

molecule so you start with iodine then

bromine then chlorine sulfur phosphorus

and fluorine oxygen nitrogen carbon and

hydrogen is always your lowest priority

the one other atom that will sometimes

come up to trick you is deuterium

deuterium where is that on the periodic

table it's a little bit of a trick the

professors like to use to see if you

know how to differentiate and also to

pinpoint the specific hydrogen in

question remember from general chemistry

that isotopes are atoms that have the

same atomic number meaning the same

number of protons but a different number

of neutrons which gives them a different

mass you don't have to know the specific

names for different isotopes but you

should recognize hydrogen when you think

of hydrogen you're thinking of the

hydrogen isotope that has a mass of 1

and an atomic number of 1 because it has

one proton no neutrons and that gives

you a mass of 1 this is protium or what

we refer to as hydrogen but there is

another hydrogen atom that has an atomic

number of 1 but a mass of 2 and that's

because it has one proton and one

Neutron for a total mass of 2 this is

the heavy hydrogen called deuterium

where the prefix du stands for the

number 2 this is the only time where

you're going to look at atomic mass

instead of atomic number to rank

substituents on a chiral Center so when

present hydrogen is always your lowest

priority than deuterium here are the 4

steps I like to go through to make sure

that I'm ranking my chiral centers

correctly step 1 rank the atoms using

the cahn-ingold-prelog system and we'll

try a few examples shortly step to

ensure that number 4 is in the back when

we're ranking atoms we're looking at

three-dimensional structures we have a

chiral Center with 4 groups coming out

of it but it's very hard to rank in 3 dimensions

dimensions

so if we have four substituents we say

number four is the lowest priority let's

not think about it let's put number four

in the back ignore it take the top three

remaining contenders and figure out if

they will give me R or s so what you

have to do is ensure that number four is

in the back I'll show you what to do if

it's not in the back but you always have

to think of it as back step three we're

going to cross out number four because

again we only care about the top three

priorities we put number four in the

back so that we can get rid of it and

finally step four you want to trace an

arc from one to two to three because

that is well we care about chirality is

a tricky topic and it's easy to get

overwhelmed so as you're studying this

study them in phases and make sure that

you master the first phase before you

move on to the next your four phases are

as follows when you have number four in

the back that's pretty simple when you

have number four forward it's harder but

still relatively simple when you have

long chains so that you have to rank

deep into the molecule and finally when

number four is not forward or back and

we'll go through this one at a time so

make sure you master it try the

questions and the practice quiz then

move on to the next stage try the

questions move on to the next stage and

that is how you're going to master

chirality we'll start with a simple

example looking at two chiral carbons

where the substituents are the same but

the order is slightly different I'm

including the little cheat guide so you

can see the ranking of atoms but I want

you to memorize it so they have this

ingrained in your head let's start with

the structure on the left anytime you

see hydrogen that is always number four

nothing can rank lower than hydrogen

because it's the lowest Adam even

deuterium outranks hydrogen so when you

have a hydrogen present you know it's

number four and when you don't have

deuterium which is often the next

absolute lowest atom is carbon and

what's the absolute lowest substituent

you can put on a carbon hydrogen so

here's the trick hydrogen is always

number four if both hydrogen and methyl

are present methyl is always number

three if hydrogen is not present but you

have a methyl it'll always be number

four zero in on that market and then

rank the other atoms you're not as

familiar with in this case we have

iodine and bromine iodine is lower down

it has a higher atomic number so it

outranks bromine that'll be number one

number two iodine is number one bromine

is number two step one rank your

substituents good step to ensure that

number four is in the back I set this up

as a simple example with hydrogen as

number four in the back step three you

want to cross out number four because we

put it in the back so we don't have to

think about it and step four we're going

to trace an arc from one to two to three

an arc from one to two the three

everyone has their way of doing this I

like for my arrow head on the arc and on

top of the molecule and that way I can

see if the arc curves towards the right

it is R if the arc does not curve

towards the right it is not R and the

opposite is s other ways you'll hear it

is clockwise this one is R and

counterclockwise is s now I personally

will confuse clockwise and

counterclockwise or have to think of a

clock which is why I don't like it but

you can think of R as the right way of

the clock and s is counterclockwise it's

not the right way of the clock and

that's the absolute simplest type of

structure you'll see but what if it's

not this simple what if you have

multiple atoms to look at where it's not

that easy to rank in one shop

for example what if the molecule has a

ch3 a ch2 ch3 and here we'll have a

ch2oh and

let's put a bromine so we look at the

atoms and we see we have carbon carbon

carbon and bromine bromine outranks all

the carbons great that's priority number

one but what do we do with the rest of

them first thing you'll recognize is

when you don't have hydrogen but you

have a methyl it is your lowest priority

so that's number four

but how do you know to rank the other

two carbons and this is where you have

to what I call go deep into the molecule

I'm going to show you how to break this

up for when you're first learning it but

as you get more confident you'll be able

to look at the structure and without

drawing anything immediately tell what

is the higher priority and what is lower

the three substituents that we have to

rank here are the ch2 ch3 I'll put a

black carbon to represent the chiral

carbon we're comparing that to the ch2oh

and finally if you didn't recognize

methyls number four let's compare that

as well methyl is ch3 the first thing

you want to do is look at the atom

directly attached to your chiral center

that's a carbon carbon and carbon

because the carbons are exactly the same

no carbon wins no carbon loses we have

to cancel them all out and go deeper

into the molecule specifically we're

looking for the highest priority atom

attached to the carbon that we rolled

out for the first one we have hydrogen

hydrogen or carbon where carbon is the

highest priority for the second one we

have hydrogen hydrogen oxygen where

oxygen is higher priority and for the

last one we have nothing but hydrogen's

so we'll pick any random one and this is

what we're comparing oxygen is the

highest priority since we already have a

number one the group with the oxygen

becomes number two carbon is the second

priority and the next number down would

be group number three and finally

hydrogen is the lowest priority giving

me the number four that we already knew

from our trick but just in case now you

understand why it's number four step one

ranked step two make sure number four is

on the back good to go step three cancel

out number four and now trace an arc

from one to two to three and in this

case it's counter why's the top does not

go to the right so it is not our instead

it is s but what if your molecule

doesn't look this pretty and isn't set

up this nicely what if you're given a

molecule in line structure for example

2-butanol where you have an alcohol on

carbon number 2 and you're asked to find

if it's RN s we're going to go through

the same four steps step number one rank

we have our little cheat guide we

identify the chiral carbon the sp3

hybridized carbon with four unique

substituents weight four substituents

one two three where's number four in

wine or skeletal structure don't forget

you have invisible hydrogens and instead

of writing invisible hydrogens i'll put

it as an invisible number four because

we know it's there it's the lowest

priority we're going to cross it out

anyway just keep it in the background as

a number four so let's rank we have an

oxygen which we're comparing to carbon

and carbon oxygen outranks carbon making

the alcohol priority one the two carbons

are the same they cancel out but

remember you have to look at what else

is on the chain the highest priority on

the Left carbon are three hydrogen atoms

the highest priority on the right carbon

is another carbon atom if the

substituents are linear chains with

nothing else on it no branches no

functional verbs the longer chain wins

not because it's heavier or longer but

simply because at the point of

difference when we have hydrogen's

versus another carbon the carbons win in

this case the ethyl group is going to be

priority number two because carbon has a

carbon on it the methyl group remember

it's always second to lowest if you have

a hydrogen present that is priority

number three and we already have number

four we prioritized major number four is

in the back cancel out number four and

trace an arc from one to two to three

the top of the arc goes to the right

giving me R now what happens if priority

number four is in the front of the

molecule rather than the back how do you

find RNs that is exactly what we'll

cover in the next video and you can find

that alone with the stereochemistry

practice quiz and cheat sheet by

visiting my website layovers sicom slash chirality

chirality

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