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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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