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Finding R and S When Group #4 is Forward - Stereochemistry Vid 3
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Leah here from Lea for.com and in this
video we'll look at how to find RNs
configurations when priority group
number four is coming forward on your
molecule you can find this entire video
series along with my stereochemistry
practice quiz and cheat cheet by
visiting my website Lea for.com
cality in the last video I showed you
the four-step method for finding RNs
configurations step one prioritize your
groups step two ensure number four is in
the back step three cross out number
four and step four trace an arc from one
to 2 to 3 if the top of your Arc is
going clockwise or towards the right you
have an R configuration and if the top
of your Arc is going towards the left or
counterclockwise you have an S
configuration that is the simple way to
approach it but now what do you do if
you have a molecule that doesn't have
group number four in the back what do
you do if group number four is coming
forward notice that when I prioritize
this molecule I have 1 2 3 and four so
that group number four is coming out of
the page straight at me the goal with
prioritizing is to rank the top three
substituents when they're facing forward
with number four out of the way because
we're not considering that now your
professors may tell you to redraw the
molecule or to build a model kit it and
then visualize it that way but honestly
that's a waste of time instead what I
want you to do is imagine that you're
presenting this molecule to someone and
you're looking at them face to face and
how you understand this I'll share a
quick story I was the acfl in my
military unit which means I helped lead
the warm-ups and the workouts so here I
am standing in front of an entire group
of people you're looking at the back of
me since I'm facing them so you can see
my hair instead of my face to start the
workout I said take your right hand and
stretch it above your head and to
demonstrate I lifted my right hand
stretching it above my head expecting
them to follow now what do you think
happened about half of the people were
watching me do the demonstration and
since they were facing me my right hand
matched up to their left hand and they
all raised that left hand to stretch but
the other half of the group heard me say
right hand and so they raised their
right hand
which matched up to my left hand and
what we had was a whole lot of confusion
with half the people stretching their
right hand and half the people
stretching their left hands the first
thing I want you to understand is how
this happened when you're standing face
to face with another person your right
side matches their left side and their
left side matches your right side if
you're holding something up to them
what's facing you is facing away from
them and what is facing them is facing
away from you now how does this relate
to a chyal molecule let's take a look
here I have a model kit which is great
for learning stereochemistry you can
pick one up on Amazon through lfor
side.com kit and I'll get a small
commission notice the carbon here is
tetrahedral and has four unique
substituents let's set it up so that
we're prioritizing the color over white
white will be number four and we'll
position that to the back and I have the
colors facing forward so you can trace
the path from 1 to 2 to three that's how
it's supposed to be right but wait what about
about
me when I turn I see number four facing
forward while you see it facing back and
that's how we have to visualize when you
have priority number four in the front
so when I show it to you I have to
imagine that even though to me it's
forward to you it's back to you it's
correct now let's try this this we'll
prioritize from blue to green to Red
trace of half blue to green to Red what
do you see well you probably see this
counterclockwise but I see it
clockwise blue to green to Red that's
clockwise but you're seeing it
counterclockwise if I have number four
facing forward which I have and 1 to 2
to 3 is clockwise I have to think I'm
showing it to you your you're seeing it
backwards and so I have to turn clockwise
counterclockwise I switched it up and
now blue to green to Red is in the
opposite direction I am seeing this
counterclockwise you are seeing it
clockwise so I have to think number four
is facing forward to you it's facing
back blue to green to Red to me is
counterclockwise therefore to you the
correct way it's
clockwise let's let's take this back to
our example if I have a molecule where
number four is coming forward I will
imagine that I am teaching you and I am
showing you this molecule and in holding
this up in front of you I see number
four coming forward to me but you see
number four towards the back the way
you're supposed to see it if I'm trying
to demonstrate if it's RNs I will still
cancel out number four and trace the
path from 1 to 2 to 3 which appears to
be S but because you are seeing it
backwards I have to remember that if I
see it counterclockwise and S you are
seeing the same exact molecule with
number four towards the back going
clockwise as R if you understand what I
did it becomes very simple so anytime
you see a molecule where number four is
coming forward simply cross it out trace
a path from 1 to 2 to 3 and if if it
appears to go towards the right
clockwise R reverse it so that you get
counterclockwise s and if you cross out
number four and then tracing an arc from
1 to 2 to 3 appears to go
counterclockwise s you simply switch it
so that it's now clockwise and R so in
review if number four is forward rank
the same way as you did for a standard
molecule trace an mark from 1 to 2 to 3
and see if it's clockwise or
counterclockwise and then if it's R turn
it into s and if it's S turn it into R
because number four forward means that
you're showing it to someone else they
see it backwards and they see it in
reverse and this doesn't work just on a
clean molecule like what we've shown
here let's take something more complex to
to
analyze what if you're asked to find the
RNs configurations for every chyro
Carbon on this molecule I see a lot of
ch2s in the ring I see this carbon here
that has two methyl groups that means it
does not have four unique substituents
it is a chyal and then I see this one
over here which is chyal it has a
hydrogen it has an o and then going
around the ring I have an asymmetric
pathway CU going down I have a ch2 and
then carbon hitting the methyl groups on
the other side I have ch2 then just
another ch2 just another ch2 there there
is no symmetry on this molecule making
it chyal in ranking my substituents I
know that hydrogen is always going to be
number four oxygen will outrank carbon
giving me number one and then for the
carbons we have to look deeper and
determine that the lower carbon is
number two and the upper carbon is
number three if you're not comfortable
with this make sure you see video five
that tells you how to deal with
substituents that have the same atom but
a difference down the line but for now
we have our priorities we notice number
four is forward so we cross it out trace
an arc from 1 to 2 to 3 that appears to
be R but because number four is forward
we simply switch it and it becomes s and
this is an S
configuration but this only works when
group number four is coming forward
rather than back now what do you do if
group number four is in the plane of the
page do you treat it as a number four
back or do you treat it as a number four forward
forward
and the answer is neither and this is
exactly what we'll cover in the next
video remember you can find this entire
series along with my stereochemistry
practice quiz and cheat cheet by
visiting my website Lea for.com
carity are you struggling with Organic
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