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R and S configuration for Lengthy Complex Substituents - Stereochemistry Vid 5
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laya hair from layer four sicom and in
this video we'll look at how to find our
nest configurations for chiral centers
with complex or lengthy substituents you
can find this entire video series along
with the stereochemistry practice quiz
and cheat sheet by visiting my website
layer for sicom slash chirality at this
point I'll assume you're comfortable
with finding RNs configurations for
symbol molecules and if you're not make
sure you go back and watch those videos
before you continue with this one but
now we want to look at what happens if
you have longer substituents or the
different substituents start with the
same atom to remind you we're ranking
substituents based on their atomic
number and this is the guide that I have
on the cheat sheet to help you remember
which atoms will outrank which so we
look at our molecule and determine that
we're comparing carbon to carbon to
carbon to carbon we look back at our
little guide I realize we're just
comparing carbon how do we figure out
which substituent will rank higher or
lower if every single one starts with
carbon what you have to do in this case
is go deeper into the molecule and to
show you how to do that I'm going to
rewrite each substituent and show you
how to analyze it here we have the four
substituents keeping the same color so
you can tell what's what but before we
continue I want to point out we're not
looking at the number of atoms bound
we're looking at bonds to the atom wait
isn't that the same thing in most cases
it is but when you have a PI bond for
example a double or a triple bond you
have to imagine every bond as its own
priority so looking at this aldehyde
where we have a carbon double bound to
oxygen that's like having a carbon bound
to two oxygen atoms then oxygen is going
to rank twice so I'll rewrite the
carbonyl as two separate oxygen atoms to
help you remember that and to help you
rank the first thing we do is look at
the atoms directly bound to the chiral
carbon and compare them but in this case
every single one is carbon so we
cancel it out if you cancel it out you
have to look at what is the one absolute
highest priority on the atom you just
cancelled out why do I say on the atom
because for example we have a sulfur
here but it's jumping we can't skip the
carbon we have to instead look at what
is attached to the carbon that we just
crossed out in red for the aldehyde we
have an oxygen for this group we can
choose any of the two carbon atoms it
doesn't matter carbon is carbon for the
green group we have an oxygen and for
the orange group we have a hydrogen now
we have something to compare to we're
comparing oxygen to carbon to oxygen to
hydrogen hydrogen is always your lowest
priority and that means carbon to
hydrogen will be number four or you can
remember the trick I told you in a
previous video where if there's no
hydrogen or deuterium methyl is always
your lowest priority and we just proved
it then we have carbon which is out
ranked by oxygen and since the purple
and green each have oxygen the entire
blue substituent is the second lowest
because it's carbon bound to carbon
making this monstrosity number three and
one and two will be between the purple
and green since they each have one
oxygen don't forget the purple oxygen is
really just one bond to oxygen equaling
the green bond to oxygen so we cancel it
out and then we look at the next highest
priority not on the atom we just
cancelled out but the initial carbon
atom that we cancel out for all of them
in the purple group we have another
oxygen atom as the highest priority
because the double bound counts twice on
the green we only have hydrogen as our
next highest priority oxygen outranks
hydrogen therefore the aldehyde is
priority number one outranking the ch2oh
which is priority number two the rest is
pretty straightforward cancel out number
four trace the path from one to two to
three and since the top goes to the
right or clockwise this molecule is our
but what if the atoms are not written
out so clearly what if instead you're
given a molecule that looks like this
here we have a molecule with one chiral
carbon and coming out of it we have
three carbon containing substituents a
phenyl group a cyclohexyl group and an
isopropyl where's the fourth substituent
don't forget we have an invisible
hydrogen going to the back whenever
you're given skeletal structure and
there is an invisible hydrogen going to
the back you should automatically know
that hydrogen is number four so instead
of writing hydrogen just put a number
four towards the back you're going to
cross it out it's going to be out of
your way if you're comfortable enough
with this you can even ignore it
completely and just look at the three
you're given rank those three fine Arnaz
never bothering to draw the hydrogen
mentally acknowledge it but don't draw
it so let's take a look at our priority
we're comparing carbon to carbon to
carbon they're all the same so we cancel
them out we have to find the next
highest priority on the red carbon that
we cancelled out for the one on the Left
we have a carbon for the phenyl group we
have any carbon for the cyclohexyl we
have a carbon once again they cancel out
so we don't look at the green carbon
instead we go back to the red carbon and
see what is the next highest priority on
the carbon the isopropyl has another
carbon the cyclohexyl has another carbon
the phenyl group has another carbon but
don't forget the PI bond implies yet
another bond to carbon but once again
they all cancel out so we look for the
next highest priority remember the
phenyl has a PI bond which means this
counts as a second carbon the isopropyl
has an invisible hydrogen the cyclohexyl
has an invisible hydrogen over there
because it should have four bonds and
that means the phenyl group having that
second carbon due to the PI bond
outranks the isopropyl and cyclohexyl
making this group priority number one
going back to the ice to purple and
cyclohexyl they both have the hydrogen
so they cancel out what we do now if we
cancelled out every sin
Adam on the initial red carbon is we go
to the next carbon atom and see what is
highest priority on it and that means we
would go to any one of the green carbons
that we rolled out so further the
isopropyl will show one of the hydrogen
atoms that's the highest priority
because all I have left are hydrogen
atoms it doesn't matter which one I
choose so we'll choose any one hydrogen
atom but for the cyclohexyl the highest
priority is a carbon in addition to the
hydrogen atoms with carbon is higher
priority so that's what we're looking at
carbon outranks hydrogen making it
higher in priority we have group number
one making this the second as group
number two isopropyl as group number
three and then we simply cancel out
number four trace from one to two to
three and our molecule is our for even
more practice finding RNs on simple and
tricky molecules make sure you try the
stereochemistry quiz posted on my
website LAIV for sicom slash chirality
for even more chirality be sure to watch
the upcoming videos where I show you how
to find RNs for Fischer projections
Newman projection cyclohexane and chair
confirmations and then show you how to
compare molecules to determine if
they're the same or if there are named
humors using a combination of logic this
swap method and probably never redrawing
the molecule you can find all this with
my practice quiz and cheat sheet on my
website at layer four sicom slash chirality
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