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Organic chemistry
Chiral examples 1
Chiral examples 1. Created by Sal Khan.
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- Can I know the name of the last example molecule?(8 votes)
- 1-Bromo-1-chloro-2-fluroethane.(30 votes)
- At, why is it not clear whether the two CH2s on either side of the CH are the same or not? Doesn't the C in the CH have to be bonded to Cl, H, and 2 Cs? I don't understand why Sal talked about looking at all 4 CH2s counterclockwise and clockwise from the CH before determining whether that C was a chiral carbon. 1:33(4 votes)
- Okay, I think I got it (check out the next video, Chiral Examples 2, at)... If carbon is bonded to a group, that is what you look at, rather than the individual atoms to which the carbon is bonded. So by looking at the pseudo-butyl groups (those 4 CH2s), we can tell that the carbon we're checking for chirality is bonded to H, Cl, a butyl group, and another butyl group, not just H, Cl, C, and C. 1:30
A central C bonded to a hydrogen, propyl group, ethyl group, and pentyl group would be chiral, even though that central C appears to just be bonded to H, C, C, and C.(4 votes)
- The first example (at), do we consider that the molecule is planar. Do we consider that if the molecule (any molecule) is shown like that to be planar and find if it's chiral or not..??? 2:22
Also can we say that if a molecule has a chiral center then it is a chiral molecule... or it doesn't have to be?
Thanks!(4 votes)- In general, if you have a small molecule with no sp3 centers in the molecule, the molecule is planar. But there are many exceptions. Here are a few common ones:
- Water is planar. The oxygen is sp3 but there are only 3 atoms so there's nothing to stick out of the HOH plane.
- Allene is not planar. See the wikipedia page and Sailor Astra's explanation below.
- Cis-hexatriene is not planar because of steric hindrance. This shows the "no sp3 centers" rule doesn't work for larger molecules.
The only way not to get caught by these "freak cases", is to draw out the orbital structure and the 3D structure (or at least imagine it).(4 votes)
- In the first example at around, if you rotate from the CH-Cl 180 degrees to the left, wouldn't the two bottom CH2's then be on the top and the one CH2 on the bottom? So how is it supper impossible? 4:44(3 votes)
- That's what it looks like, doesn't it?This is one of the problems caused by drawing things free-hand. You have to remember that the ring is a regular pentagon, and he is really rotating the ring by only ²/₅ of a circle (144°), not half a circle (180°). Try it with a regular pentagon, and you will see that they really are superimposable.(5 votes)
- Can someone tell me if a C-atom can still be Chiral when he has 3 different groups and one is double-bonded?
For example: 1-bromo-1,2-dichloroethylene(3 votes)- No; a C atom needs to have four different groups bonded to it to be chiral. In that example, the molecule will instead have E/Z isomerism across the double bond.(3 votes)
- At, how come we cannot flip the molecule at 180 degrees and superimpose it on the other one if all the atoms are in the same plane? 10:42(2 votes)
- Keep in mind these molecules exist in 3D. The chiral carbons are sp3 hybridized and therefore have tetrahedral geometry. Sal drew them in 2D to make it easier to draw their mirror images which yes looks like we could just do a flip and superimpose them. However, if you draw them in 3D with the wedges or build a molecular model of the enantiomers you'll find they can't be superimposed. Hope that helps.(5 votes)
- Why doesn't the nitrogen in a tertiary amine with three unique substituents form a chiral center? Wouldn't the lone pair on the nitrogen be equivalent to the fourth unique group on a carbon atom?(3 votes)
- Good question – and the answer is yes tertiary amines are chiral, however due to "inversions" only certain types can be isolated as the pure enantiomer. For the ones that aren't fixed you almost instantaneously get a racemic mixture, which masks the chirality.
(Secondary amines are also chiral, but inversion occurs so rapidly that you can't isolate one enantiomer from the other.)
https://en.wikipedia.org/wiki/Amine#Alkyl_amines(1 vote)
- how can we find the no of isomer a compound has?
Plz help..
thank u.(2 votes)- I think you just have to practice till you get the vision, but here are some simple Unbranched Alkanes inc. the # of isomers
methane --> 1 isomers
ethane -----> 1 isomers
propane -----> 1 isomers
butane ------> 2 isomers
pentane -----> 3 isomers
hexane -----> 5 isomers
heptane ---> 9 isomers
octane ----> 18 isomers
heptane(3 votes)
- If a molecule has two chiral centers can it still be a chiral molecule?(3 votes)
- (S,S) may be the non superimposible mirror image of (R,R) and therefore its enantiomer.
Likewise (S,R) may be enantiomer of (R,S). Then (S,S) is a diastereomer of (S,R) and also of (R,S). And (R,R) is a diastereomer of (S,R) and also (R,S). All 4 molecules could be chiral or optically active. See http://en.wikipedia.org/wiki/Diastereomer(3 votes)
- what is chiral axis and how could we identify the chiral axis(3 votes)
Video transcript
In the last video we learned a
little bit about what a chiral molecule or what a chiral carbon
or a chiral atom is. What I want to do in this video
is go through a bunch of examples and see if we can
identify if there are any chiral atoms and to also
see if we're dealing with a chiral molecule. So let's look at our
examples here. So here I have, what is this? This is chlorocyclopentane. So the first question is do
we have any chiral atoms? And when we look at our
definition that we thought of chiral atoms, it all comes from
this notion of handedness and not being able to be
superimposable on your mirror image, but we said that they're
usually carbons bonded to four different groups. Let's see, do we have any
carbons here bonded to four different groups? Well, all the CH2's, they're
bonded to another CH2 and then two H'2. I could draw it like
this: H and H. So they're bonded to two of
the same group, so none of these CH2's are good candidates
for being a chiral center or chiral carbon. They're both bonded to-- or all
of them are bonded to two hydrogens and two other very
similar-looking CH2 groups, although you have to look
at the entire group that it's bonded to. But they're all definitely
bonded to two hydrogens, so it's not four different
groups. If we look at this CH
right here, we could separate it out like this. We could separate the H out like
this, and so since it's bonded to a hydrogen. This carbon is bonded to a
chlorine, and then it's bonded to-- well, it's not clear when
you look at it right from the get-go whether this group is
different than this group. But if you go around, if you
were to split it half-way like this, or maybe another better
way to think about is if you were to go around this molecule
in that direction, the counterclockwise direction,
you would encounter a CH2 group, and then you
encounter a CH2 group, and then you would encounter a
third, and then you would encounter a fourth CH2 group,
then you would come back to where you were before. So you would encounter four
CH2's and then you'd come back to where you were before. If you go in this direction,
what happens? You encounter one, two, three,
four CH2's and you come back to where you were before. So all of this, this bottom
group, depending on how far you want to extend it, and this
top group, are really the same group. So this is not a chiral center
or not a chiral carbon. It's not bonded to four
different groups. And this is also not a chiral
molecule, because it does not have a chiral center. And to see that it's not a
chiral molecule-- let me see if I can backtrack this
back to the way I wrote it right before. So you see that it's not
a chiral molecule. There's a couple of ways you
could think about it. The easiest way, or the way my
brain likes to think about it, is just to think about
its mirror image. Its mirror image will
look like this. So if that's the mirror, you
would have a chlorine. Then you have a CH, CH2, CH2,
and you have a CH2, CH2, and then you complete your
cyclopentane. Now, in this situation, is there
any way to rotate this to get this over there? Well, if you just took this
molecule right here and you just rotated it 180 degrees,
what would it look like? Well, maybe a little over--
yeah, well, not quite 180 degrees, but if you were to
rotate it so that the chlorine goes about that far, you would
get this exact molecule. You would get something. It would look a little
bit different. It would look like this. Let me see if I can
do it justice. It would look like this. You would have a CH2. So let me let me do it up
here where I have a little bit more space. If I were to rotate this about
that far, I would get a CH. You get the chlorine and then
you have your CH2, and then you have another CH2, CH2,
and then you would have your CH2 up there. If you were to rotate this all
the way around, or actually this is almost exactly
180 degrees, it would look like this. And the only difference between
this and this is just how we drew this bond here. I could have easily, instead
of drawing that bond like that, I could draw it facing
up like that, and these are the exact same molecule. So this molecule is
also not chiral. So let's go to this
one over here. So what is this? This is a
bromochlorofluoromethane, just to practice our naming
a little bit. But it's very clear that
we are bonded to four different groups. All of the different groups, or
the atoms in this case that are bonded to this carbon, are
different, so this carbon is a chiral center. And it should also be pretty
clear that it is also a chiral molecule. If you were to take its mirror
image, and this is very similar to the example we did
in the first video on chirality, but its mirror image
will look like this. You have the bromine
on the right now. The hydrogen is still in
back, and you have the fluorine above it. No matter how you try to rotate
this thing, if you try to get the bromine all the way
over there, all the way to that position, then the hydrogen
would be in this position and the chlorine would
be in that position. And no matter how you try to
flip this around or rotate it or shift it, you will never be
able superimpose this molecule on that molecule right there. So that is a chiral center and
this is a chiral molecule. And there's a word for
these two versions. We're going to go into the
naming of them later on. It's a little bit
more involved. We'll have a whole separate
video on it. But these two versions of
bromochlorofluoromethane, they sometimes have different
chemical properties. And these are called
enantiomers. And enantiomers are just
the mirror images. Each enantiomer is a mirror
image of each other, but they are stereoisomers. This is all just terminology. Stereoisomers. You're familiar with the word
isomer, and isomer just means that you have the same atoms
in your molecule. But then you have different
types of isomers. You have constitutional
isomers that say, OK, different things are connected
to different things. Stereoisomers, the same
things are all connected to the same things. You have a carbon connected to
only a fluorine, a chlorine connected to the carbon, a
hydrogen connected to the carbon, a bromine connected
to the carbon. So all of the same things are
connected to the same things, but they're a three-dimensional configuration. That's where we're dealing
with the stereo part. Stereochemistry is the study
of three-dimensional chemistry, as essentially
understanding the actual three-dimensional structure
of things. So stereoisomers mean that we
have the same constituents, the same atoms. They
have the same connections to each other. Bromine is still connected
to carbon, which is still connected to hydrogen. That's all true over here. But their three-dimensional orientation is still different. And in this case where they
are mirror images of each other, we call them
enantiomers. And I should probably make
one clarification. In the last few videos, I've
been a little bit, you know, sometimes I'll say configuration
and sometimes I use the word conformation. So sometimes I'll use the word
configuration and sometimes I use the word conformation, and
I actually should be a little bit more, or I should have been
a little bit more exact about these. When you're talking about a
configuration, you're actually talking about a different
structure. To go from one configuration to
another configuration, you would actually have to
break bonds and kind of reassemble them. So these are different
configurations. Because in order for them to be
able to be the same thing, you would have to swap maybe the
bromine and the hydrogen in there where they are relative
to the carbon, so these are different
configurations. Confirmations are really just
different shapes or different orientations of the
same molecule. So when we talked about
cyclohexane being in a boat, so this is cyclohexane being
in a boat conformation, or this is cyclohexane being in a
chair conformation, it's the exact same molecule with the
exact same connections. We didn't detach any bonds
or reattach any bonds. They just flipped around
a little bit. So these are two different
conformations. These are two different
configurations. To go from one configuration
to another, you have to rearrange bonds. Now let's look at this
molecule over here. Can we identify any
stereocenters or chiral carbons or chiral atoms? And you have this carbon
right here. Let's see, this carbon right
here is bonded to a chlorine, a hydrogen, a bromine, and
then another carbon. So this is bonded to four
different things, so this is a chiral carbon. Sometimes they put a little
asterisk there. If we look at this carbon right
here you can-- well, it's bonded to a fluorine and
another carbon, but it's bonded to two hydrogens,
so it's not chiral. It has two of the same things
that it's bonded to. You can even see a little axis
of symmetry through it. If you look at that, you can
kind of flip it over, and it's going to be the same thing. But this one right here, that
is a chiral center. That is a chiral center, or
chiral carbon, or chiral atom, or a symmetric carbon. You'll see it used in
different ways. And because this molecule has
got that chiral center, you'll see that if you were to take its
mirror image, it would be an enantiomer. This is not superimposable
on its mirror image. We could even try to draw it. And just so you know, you don't
always have to do the mirror image on the
right side. We can draw the mirror image
on the left side. So if we want to draw
its mirror image, it would look like this. You would have a fluorine,
carbon, carbon, chlorine. You have your two hydrogens,
and then you would have a hydrogen here, and then you
would have your bromine here. No matter what you try to do, if
you try to flip this around or whatever, you will never be
able superimpose this on top of this, so these
are enantiomers. These are both stereoisomers
relative to each other. And either of these, regardless
of which one you pick, are chiral molecules. I'm over the time that I
normally want to go in the video, so in the next video
I'll do even more.