0:03 Leah here from Lea for.com and in this
0:06 video we'll look at how to find RNs
0:08 configurations when priority group
0:10 number four is coming forward on your
0:12 molecule you can find this entire video
0:15 series along with my stereochemistry
0:17 practice quiz and cheat cheet by
0:21 visiting my website Lea for.com
0:23 cality in the last video I showed you
0:26 the four-step method for finding RNs
0:29 configurations step one prioritize your
0:32 groups step two ensure number four is in
0:34 the back step three cross out number
0:38 four and step four trace an arc from one
0:40 to 2 to 3 if the top of your Arc is
0:43 going clockwise or towards the right you
0:45 have an R configuration and if the top
0:48 of your Arc is going towards the left or
0:50 counterclockwise you have an S
0:52 configuration that is the simple way to
0:55 approach it but now what do you do if
0:57 you have a molecule that doesn't have
0:59 group number four in the back what do
1:01 you do if group number four is coming
1:04 forward notice that when I prioritize
1:10 this molecule I have 1 2 3 and four so
1:12 that group number four is coming out of
1:15 the page straight at me the goal with
1:18 prioritizing is to rank the top three
1:21 substituents when they're facing forward
1:23 with number four out of the way because
1:26 we're not considering that now your
1:27 professors may tell you to redraw the
1:30 molecule or to build a model kit it and
1:32 then visualize it that way but honestly
1:35 that's a waste of time instead what I
1:38 want you to do is imagine that you're
1:40 presenting this molecule to someone and
1:43 you're looking at them face to face and
1:44 how you understand this I'll share a
1:47 quick story I was the acfl in my
1:49 military unit which means I helped lead
1:52 the warm-ups and the workouts so here I
1:54 am standing in front of an entire group
1:56 of people you're looking at the back of
1:58 me since I'm facing them so you can see
2:00 my hair instead of my face to start the
2:03 workout I said take your right hand and
2:05 stretch it above your head and to
2:07 demonstrate I lifted my right hand
2:10 stretching it above my head expecting
2:12 them to follow now what do you think
2:14 happened about half of the people were
2:17 watching me do the demonstration and
2:19 since they were facing me my right hand
2:21 matched up to their left hand and they
2:24 all raised that left hand to stretch but
2:27 the other half of the group heard me say
2:29 right hand and so they raised their
2:30 right hand
2:32 which matched up to my left hand and
2:35 what we had was a whole lot of confusion
2:36 with half the people stretching their
2:38 right hand and half the people
2:40 stretching their left hands the first
2:42 thing I want you to understand is how
2:44 this happened when you're standing face
2:47 to face with another person your right
2:50 side matches their left side and their
2:53 left side matches your right side if
2:56 you're holding something up to them
2:58 what's facing you is facing away from
3:02 them and what is facing them is facing
3:04 away from you now how does this relate
3:08 to a chyal molecule let's take a look
3:10 here I have a model kit which is great
3:12 for learning stereochemistry you can
3:14 pick one up on Amazon through lfor
3:17 side.com kit and I'll get a small
3:19 commission notice the carbon here is
3:22 tetrahedral and has four unique
3:24 substituents let's set it up so that
3:27 we're prioritizing the color over white
3:28 white will be number four and we'll
3:30 position that to the back and I have the
3:32 colors facing forward so you can trace
3:35 the path from 1 to 2 to three that's how
3:37 it's supposed to be right but wait what about
3:39 about
3:43 me when I turn I see number four facing
3:46 forward while you see it facing back and
3:48 that's how we have to visualize when you
3:50 have priority number four in the front
3:52 so when I show it to you I have to
3:55 imagine that even though to me it's
3:57 forward to you it's back to you it's
4:00 correct now let's try this this we'll
4:03 prioritize from blue to green to Red
4:06 trace of half blue to green to Red what
4:09 do you see well you probably see this
4:13 counterclockwise but I see it
4:17 clockwise blue to green to Red that's
4:19 clockwise but you're seeing it
4:21 counterclockwise if I have number four
4:25 facing forward which I have and 1 to 2
4:28 to 3 is clockwise I have to think I'm
4:30 showing it to you your you're seeing it
4:32 backwards and so I have to turn clockwise
4:39 counterclockwise I switched it up and
4:41 now blue to green to Red is in the
4:45 opposite direction I am seeing this
4:47 counterclockwise you are seeing it
4:49 clockwise so I have to think number four
4:51 is facing forward to you it's facing
4:54 back blue to green to Red to me is
4:56 counterclockwise therefore to you the
4:58 correct way it's
5:00 clockwise let's let's take this back to
5:04 our example if I have a molecule where
5:06 number four is coming forward I will
5:09 imagine that I am teaching you and I am
5:12 showing you this molecule and in holding
5:14 this up in front of you I see number
5:18 four coming forward to me but you see
5:19 number four towards the back the way
5:22 you're supposed to see it if I'm trying
5:25 to demonstrate if it's RNs I will still
5:27 cancel out number four and trace the
5:30 path from 1 to 2 to 3 which appears to
5:33 be S but because you are seeing it
5:36 backwards I have to remember that if I
5:39 see it counterclockwise and S you are
5:42 seeing the same exact molecule with
5:44 number four towards the back going
5:48 clockwise as R if you understand what I
5:51 did it becomes very simple so anytime
5:53 you see a molecule where number four is
5:57 coming forward simply cross it out trace
5:59 a path from 1 to 2 to 3 and if if it
6:01 appears to go towards the right
6:05 clockwise R reverse it so that you get
6:08 counterclockwise s and if you cross out
6:11 number four and then tracing an arc from
6:13 1 to 2 to 3 appears to go
6:17 counterclockwise s you simply switch it
6:21 so that it's now clockwise and R so in
6:25 review if number four is forward rank
6:27 the same way as you did for a standard
6:31 molecule trace an mark from 1 to 2 to 3
6:33 and see if it's clockwise or
6:36 counterclockwise and then if it's R turn
6:39 it into s and if it's S turn it into R
6:41 because number four forward means that
6:43 you're showing it to someone else they
6:45 see it backwards and they see it in
6:47 reverse and this doesn't work just on a
6:49 clean molecule like what we've shown
6:52 here let's take something more complex to
6:52 to
6:55 analyze what if you're asked to find the
6:57 RNs configurations for every chyro
7:00 Carbon on this molecule I see a lot of
7:03 ch2s in the ring I see this carbon here
7:05 that has two methyl groups that means it
7:07 does not have four unique substituents
7:09 it is a chyal and then I see this one
7:12 over here which is chyal it has a
7:16 hydrogen it has an o and then going
7:18 around the ring I have an asymmetric
7:21 pathway CU going down I have a ch2 and
7:24 then carbon hitting the methyl groups on
7:26 the other side I have ch2 then just
7:29 another ch2 just another ch2 there there
7:32 is no symmetry on this molecule making
7:35 it chyal in ranking my substituents I
7:37 know that hydrogen is always going to be
7:40 number four oxygen will outrank carbon
7:42 giving me number one and then for the
7:44 carbons we have to look deeper and
7:47 determine that the lower carbon is
7:49 number two and the upper carbon is
7:52 number three if you're not comfortable
7:53 with this make sure you see video five
7:55 that tells you how to deal with
7:57 substituents that have the same atom but
8:00 a difference down the line but for now
8:02 we have our priorities we notice number
8:05 four is forward so we cross it out trace
8:07 an arc from 1 to 2 to 3 that appears to
8:10 be R but because number four is forward
8:13 we simply switch it and it becomes s and
8:15 this is an S
8:17 configuration but this only works when
8:20 group number four is coming forward
8:22 rather than back now what do you do if
8:25 group number four is in the plane of the
8:27 page do you treat it as a number four
8:29 back or do you treat it as a number four forward
8:30 forward
8:32 and the answer is neither and this is
8:34 exactly what we'll cover in the next
8:36 video remember you can find this entire
8:39 series along with my stereochemistry
8:41 practice quiz and cheat cheet by
8:45 visiting my website Lea for.com
8:47 carity are you struggling with Organic
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