0:02 Leia hair from leh Versailles calm and
0:05 in this video we'll start the discussion
0:07 on RNs configurations for chiral
0:09 molecules you can find this entire
0:12 series alone with my practice quiz and
0:14 cheat sheet by visiting my website layer
0:18 for sicom slash chirality in the last
0:20 video we introduced the concept of
0:23 chirality and said that we classify a
0:26 pair of enantiomers as having the R or s
0:30 configuration and how do we do that we
0:31 need to find the absolute configuration
0:35 around each chiral atom where each
0:37 chiral carbon where the absolute
0:40 configuration looks at the chiral carbon
0:43 the atoms around it and then the order
0:47 of the atoms to give you R or s and the
0:49 way we rank these atoms is by following
0:52 the cahn-ingold-prelog system after the
0:56 inventors how do we rank the atoms by
0:59 atomic number not by the size of the
1:02 chain not by the weight by the atomic
1:05 number of the individual atom that is
1:07 attached to your chiral Center this is
1:09 where a lot of students get confused so
1:12 once again the atomic number of the atom
1:14 directly attached to that central carbon
1:17 that chiral carbon so you get a chiral
1:19 molecule you open your periodic table
1:21 you waste a good few seconds looking for
1:24 the atoms don't do that in my intro to
1:27 or go video series I show you the 10
1:29 atoms that you have to memorize in your
1:31 organic chemistry course so that you
1:33 don't waste your time constantly looking
1:36 at the periodic table we have in period
1:38 1 hydrogen area 2 carbon nitrogen oxygen
1:42 fluorine period 3 phosphorus sulfur
1:44 chlorine and then going down bromine and
1:48 iodine why is this important we have
1:50 electronegativity going up towards the
1:52 right size going down to the left which
1:55 is great for reactions but here's
1:58 another reason to know them this is the
2:00 order that we're going to use to rank
2:03 these atoms the atomic numbers will
2:05 increase from left to right on period
2:08 and then increase as you go down the
2:11 groups this tells you that iodine is
2:13 your absolute top priority
2:16 these are the atoms in your molecule
2:18 again these are the 10 most common atoms
2:19 that are going to show up in your
2:22 molecule so you start with iodine then
2:25 bromine then chlorine sulfur phosphorus
2:27 and fluorine oxygen nitrogen carbon and
2:30 hydrogen is always your lowest priority
2:33 the one other atom that will sometimes
2:35 come up to trick you is deuterium
2:38 deuterium where is that on the periodic
2:41 table it's a little bit of a trick the
2:43 professors like to use to see if you
2:45 know how to differentiate and also to
2:47 pinpoint the specific hydrogen in
2:49 question remember from general chemistry
2:52 that isotopes are atoms that have the
2:54 same atomic number meaning the same
2:57 number of protons but a different number
2:59 of neutrons which gives them a different
3:02 mass you don't have to know the specific
3:04 names for different isotopes but you
3:06 should recognize hydrogen when you think
3:08 of hydrogen you're thinking of the
3:12 hydrogen isotope that has a mass of 1
3:15 and an atomic number of 1 because it has
3:20 one proton no neutrons and that gives
3:24 you a mass of 1 this is protium or what
3:27 we refer to as hydrogen but there is
3:29 another hydrogen atom that has an atomic
3:32 number of 1 but a mass of 2 and that's
3:34 because it has one proton and one
3:38 Neutron for a total mass of 2 this is
3:40 the heavy hydrogen called deuterium
3:43 where the prefix du stands for the
3:46 number 2 this is the only time where
3:48 you're going to look at atomic mass
3:50 instead of atomic number to rank
3:53 substituents on a chiral Center so when
3:55 present hydrogen is always your lowest
3:59 priority than deuterium here are the 4
4:01 steps I like to go through to make sure
4:03 that I'm ranking my chiral centers
4:07 correctly step 1 rank the atoms using
4:10 the cahn-ingold-prelog system and we'll
4:12 try a few examples shortly step to
4:17 ensure that number 4 is in the back when
4:19 we're ranking atoms we're looking at
4:21 three-dimensional structures we have a
4:24 chiral Center with 4 groups coming out
4:26 of it but it's very hard to rank in 3 dimensions
4:27 dimensions
4:31 so if we have four substituents we say
4:34 number four is the lowest priority let's
4:35 not think about it let's put number four
4:39 in the back ignore it take the top three
4:41 remaining contenders and figure out if
4:44 they will give me R or s so what you
4:47 have to do is ensure that number four is
4:49 in the back I'll show you what to do if
4:50 it's not in the back but you always have
4:53 to think of it as back step three we're
4:56 going to cross out number four because
4:58 again we only care about the top three
5:00 priorities we put number four in the
5:02 back so that we can get rid of it and
5:05 finally step four you want to trace an
5:09 arc from one to two to three because
5:11 that is well we care about chirality is
5:13 a tricky topic and it's easy to get
5:16 overwhelmed so as you're studying this
5:19 study them in phases and make sure that
5:21 you master the first phase before you
5:23 move on to the next your four phases are
5:26 as follows when you have number four in
5:29 the back that's pretty simple when you
5:32 have number four forward it's harder but
5:34 still relatively simple when you have
5:37 long chains so that you have to rank
5:40 deep into the molecule and finally when
5:43 number four is not forward or back and
5:46 we'll go through this one at a time so
5:47 make sure you master it try the
5:49 questions and the practice quiz then
5:51 move on to the next stage try the
5:53 questions move on to the next stage and
5:55 that is how you're going to master
5:57 chirality we'll start with a simple
5:59 example looking at two chiral carbons
6:02 where the substituents are the same but
6:04 the order is slightly different I'm
6:06 including the little cheat guide so you
6:07 can see the ranking of atoms but I want
6:09 you to memorize it so they have this
6:11 ingrained in your head let's start with
6:13 the structure on the left anytime you
6:16 see hydrogen that is always number four
6:19 nothing can rank lower than hydrogen
6:21 because it's the lowest Adam even
6:24 deuterium outranks hydrogen so when you
6:25 have a hydrogen present you know it's
6:27 number four and when you don't have
6:29 deuterium which is often the next
6:32 absolute lowest atom is carbon and
6:35 what's the absolute lowest substituent
6:38 you can put on a carbon hydrogen so
6:40 here's the trick hydrogen is always
6:42 number four if both hydrogen and methyl
6:44 are present methyl is always number
6:47 three if hydrogen is not present but you
6:49 have a methyl it'll always be number
6:52 four zero in on that market and then
6:53 rank the other atoms you're not as
6:56 familiar with in this case we have
6:58 iodine and bromine iodine is lower down
7:01 it has a higher atomic number so it
7:03 outranks bromine that'll be number one
7:07 number two iodine is number one bromine
7:10 is number two step one rank your
7:13 substituents good step to ensure that
7:15 number four is in the back I set this up
7:17 as a simple example with hydrogen as
7:20 number four in the back step three you
7:22 want to cross out number four because we
7:24 put it in the back so we don't have to
7:27 think about it and step four we're going
7:30 to trace an arc from one to two to three
7:34 an arc from one to two the three
7:37 everyone has their way of doing this I
7:39 like for my arrow head on the arc and on
7:42 top of the molecule and that way I can
7:45 see if the arc curves towards the right
7:48 it is R if the arc does not curve
7:51 towards the right it is not R and the
7:54 opposite is s other ways you'll hear it
7:57 is clockwise this one is R and
8:01 counterclockwise is s now I personally
8:02 will confuse clockwise and
8:04 counterclockwise or have to think of a
8:06 clock which is why I don't like it but
8:09 you can think of R as the right way of
8:12 the clock and s is counterclockwise it's
8:13 not the right way of the clock and
8:16 that's the absolute simplest type of
8:19 structure you'll see but what if it's
8:21 not this simple what if you have
8:23 multiple atoms to look at where it's not
8:25 that easy to rank in one shop
8:29 for example what if the molecule has a
8:34 ch3 a ch2 ch3 and here we'll have a
8:37 ch2oh and
8:40 let's put a bromine so we look at the
8:44 atoms and we see we have carbon carbon
8:47 carbon and bromine bromine outranks all
8:50 the carbons great that's priority number
8:52 one but what do we do with the rest of
8:54 them first thing you'll recognize is
8:56 when you don't have hydrogen but you
8:58 have a methyl it is your lowest priority
9:00 so that's number four
9:03 but how do you know to rank the other
9:06 two carbons and this is where you have
9:08 to what I call go deep into the molecule
9:11 I'm going to show you how to break this
9:13 up for when you're first learning it but
9:15 as you get more confident you'll be able
9:17 to look at the structure and without
9:20 drawing anything immediately tell what
9:22 is the higher priority and what is lower
9:24 the three substituents that we have to
9:28 rank here are the ch2 ch3 I'll put a
9:30 black carbon to represent the chiral
9:33 carbon we're comparing that to the ch2oh
9:37 and finally if you didn't recognize
9:39 methyls number four let's compare that
9:43 as well methyl is ch3 the first thing
9:45 you want to do is look at the atom
9:48 directly attached to your chiral center
9:51 that's a carbon carbon and carbon
9:54 because the carbons are exactly the same
9:58 no carbon wins no carbon loses we have
10:00 to cancel them all out and go deeper
10:03 into the molecule specifically we're
10:06 looking for the highest priority atom
10:08 attached to the carbon that we rolled
10:11 out for the first one we have hydrogen
10:14 hydrogen or carbon where carbon is the
10:17 highest priority for the second one we
10:20 have hydrogen hydrogen oxygen where
10:22 oxygen is higher priority and for the
10:24 last one we have nothing but hydrogen's
10:27 so we'll pick any random one and this is
10:29 what we're comparing oxygen is the
10:32 highest priority since we already have a
10:34 number one the group with the oxygen
10:38 becomes number two carbon is the second
10:41 priority and the next number down would
10:43 be group number three and finally
10:46 hydrogen is the lowest priority giving
10:48 me the number four that we already knew
10:50 from our trick but just in case now you
10:53 understand why it's number four step one
10:56 ranked step two make sure number four is
11:00 on the back good to go step three cancel
11:03 out number four and now trace an arc
11:07 from one to two to three and in this
11:10 case it's counter why's the top does not
11:13 go to the right so it is not our instead
11:16 it is s but what if your molecule
11:18 doesn't look this pretty and isn't set
11:22 up this nicely what if you're given a
11:25 molecule in line structure for example
11:28 2-butanol where you have an alcohol on
11:30 carbon number 2 and you're asked to find
11:32 if it's RN s we're going to go through
11:35 the same four steps step number one rank
11:38 we have our little cheat guide we
11:40 identify the chiral carbon the sp3
11:42 hybridized carbon with four unique
11:45 substituents weight four substituents
11:48 one two three where's number four in
11:51 wine or skeletal structure don't forget
11:54 you have invisible hydrogens and instead
11:56 of writing invisible hydrogens i'll put
11:58 it as an invisible number four because
12:01 we know it's there it's the lowest
12:03 priority we're going to cross it out
12:06 anyway just keep it in the background as
12:09 a number four so let's rank we have an
12:12 oxygen which we're comparing to carbon
12:16 and carbon oxygen outranks carbon making
12:19 the alcohol priority one the two carbons
12:21 are the same they cancel out but
12:23 remember you have to look at what else
12:25 is on the chain the highest priority on
12:27 the Left carbon are three hydrogen atoms
12:30 the highest priority on the right carbon
12:33 is another carbon atom if the
12:34 substituents are linear chains with
12:37 nothing else on it no branches no
12:39 functional verbs the longer chain wins
12:42 not because it's heavier or longer but
12:44 simply because at the point of
12:46 difference when we have hydrogen's
12:49 versus another carbon the carbons win in
12:52 this case the ethyl group is going to be
12:55 priority number two because carbon has a
12:57 carbon on it the methyl group remember
12:59 it's always second to lowest if you have
13:01 a hydrogen present that is priority
13:03 number three and we already have number
13:07 four we prioritized major number four is
13:09 in the back cancel out number four and
13:12 trace an arc from one to two to three
13:15 the top of the arc goes to the right
13:19 giving me R now what happens if priority
13:21 number four is in the front of the
13:24 molecule rather than the back how do you
13:26 find RNs that is exactly what we'll
13:29 cover in the next video and you can find
13:31 that alone with the stereochemistry
13:33 practice quiz and cheat sheet by
13:36 visiting my website layovers sicom slash chirality
13:38 chirality
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