Introduction to chirality | Stereochemistry | Organic chemistry | Khan Academy
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Introduction to chirality | Stereochemistry | Organic chemistry | Khan Academy


If I were to draw a hand, and
let me just draw a hand really fast, so I’ll draw
a left hand. It looks something like that. That is a left hand. Now, if I were to take its
mirror image, let’s say that this is a mirror right there,
and I want to take its mirror image, and I’ll draw the
mirror image in green. So its mirror image would look
something like this. Not exact, but you
get the idea. The mirror image of a left
hand looks a lot like a right hand. Now, no matter how I try to
shift or rotate this hand like this, I might try to maybe
rotate it 180 degrees, so that the thumb is on the other side
like this image right here. But no matter what I do, I will
never be able to make this thing look like
that thing. I can shift it and rotate it,
it’ll just never happen. I will never be able to
superimpose the blue hand on top of this green hand. When I say superimpose,
literally put it exactly on top of the green hand. So whenever something is not
superimposable on its mirror image– let me write this down–
we call it chiral. So this hand drawing
right here is an example of a chiral object. Or I guess the hand is an
example of a chiral object. This is not superimposable
on its mirror image. And it makes sense that it’s
called chiral because the word chiral comes from the
Greek word for hand. And this definition of
not being able to be superimposable on its mirror
image, this applies whether you’re dealing with chemistry,
or mathematics, or I guess, just hands in general. So if we extend this definition
to chemistry, because that’s what we’re
talking about, there’s two concepts here. There are chiral molecules,
and then there are chiral centers or chiral– well, I call
them chiral atoms. They tend to be carbon atoms, so
sometimes they call them chiral carbons. So you have these
chiral atoms. Now, chiral molecules are
literally molecules that are not superimposable on
their mirror image. I’m not going to write
the whole thing. You know, not superimposable–
I’ll just write the whole thing. Not superimposable
on mirror image. Now, for chiral atoms, this is
essentially true, but when you look for chiral atoms within a
molecule, the best way to spot them is to recognize that these
generally, or maybe I should say usually, are carbons,
especially when we’re dealing in organic chemistry,
but they could be phosphoruses or sulfurs, but usually are
carbons bonded to four different groups. And I want to emphasize
groups, not just four different atoms. And to kind of
highlight a molecule that contains a chiral atom or chiral
carbon, we can just think of one. So let’s say that I have a
carbon right here, and I’m going to set this up so this
is actually a chiral atom, that the carbon specific is a
chiral atom, but it’s partly a chiral molecule. And then we’ll see examples
that one or both of these are true. Let’s say it’s bonded
to a methyl group. From that bond, it kind of
pops out of the page. Let’s say there’s a
bromine over here. Let’s say behind it, there is a
hydrogen, and then above it, we have a fluorine. Now if I were to take the mirror
image of this thing right here, we have your carbon
in the center– I want to do it in that same blue. You have the carbon in the
center and then you have the fluorine above the carbon. You have your bromine now
going in this direction. You have this methyl group. It’s still popping out of the
page, but it’s now going to the right instead of to
the left, So CH3. And then you have the hydrogen
still in the back. These are mirror images, if you
view this as kind of the mirror and you can see on both
sides of the mirror. Now, why is this chiral? Well, it’s a little bit of a
visualization challenge, but no matter how you try to rotate
this thing right here, you will never make it exactly
like this thing. You might try to rotate it
around like that and try to get the methyl group over here,
to get it over there. So let’s try to do that. If we try to get the methyl
group over there, what’s going to happen to the other groups? Well, then the hydrogen group is
going– or the hydrogen, I should say. The hydrogen atom is going to
move there and the bromine is going to move there. So this would be superimposable
if this was a hydrogen and this was a
bromine, but it’s not. You can imagine, the hydrogen
and bromine are switched. And you could flip it and do
whatever else you want or try to rotate it in any direction,
but you’re not going to be able to superimpose them. So this molecule right here is
a chiral molecule, and this carbon is a chiral center, so
this carbon is a chiral carbon, sometimes called
an asymmetric carbon or a chiral center. Sometimes you’ll hear
something called a stereocenter. A stereocenter is a more general
term for any point in a molecule that is asymmetric
relative to the different groups that it is joined to. But all of these, especially
when you’re in kind of in introductory organic chemistry
class, tends to be a carbon bonded to four different
groups. And I want to to stress that
it’s not four different atoms. You could have had a methyl
group here and a propyl group here, and the carbon would still
be bonded directly to a carbon in either case, but that
would still be a chiral carbon, and this would still
actually be a chiral molecule. In the next video, we’ll
do a bunch of examples. We’ll look at molecules, try
to identify the chiral carbons, and then try to
figure out whether the molecule itself is–

About James Carlton

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100 thoughts on “Introduction to chirality | Stereochemistry | Organic chemistry | Khan Academy

  1. Thank you sir for helping to simplify the only chemistry I've taken where I can feel the air stir above me as the entire lesson flew over my head.

  2. I sit in class for 1 hour and 20 minutes hearing the droning of a refrigerator and learn nothing. I sit here for 6 minutes and it starts to make sense.

  3. Dear Salman Khan,

    We shall throw out all the other crapy teachers let them flip burgers and give their salaries to you.
    God Belss You

  4. You are litterally saving me with these videos. I don't get anything in Chemistry but this is wow. So thank you!!

  5. how do i rotate the molecules ….i mean if o have rotated 180 degree it woulf fit and become superposable…plz explain it drives me crazy this rotation part

  6. When I give a speech on my med graduation day, I will say 'I would like to thanks those who has always been there for me, and those are, Mr Google, KhanAcademy and Mr YouTube'.

  7. now i feel like I can teach others organic chemistry! A nice refresher course, wish I continued with organic chemistry in my degree…

  8. I'm a little confused, at 4:02 you mentioned carbon is bonded with CH 3 a methyl group ? isn't it an ethyl since it's two carbon and not one ? eth= 2 carbon, and meth=1 carbon ? or do i got it wrong ? 

  9. Why the carbon atom has to be surrounded by 4 diff. grps ? ie can't it be like
    ch3
    |
    c
    / |
    f. br. ch3
    Is this isn't chiral ?

  10. wow, I really can't thank you enough sooooo helpful! Thanks for taking time out of your day to make these videos, you help tons of students everywhere!

  11. if you take the mirror image of the chiral carbon, wouldn't the methyl group go toward the back and the hydrogen go toward us (the front)?

  12. Makes me think of a short story by Arthur C Clarke about a man who accidentally fell into a 4th dimension of space, resulting in all his molecules changing chirality. The result was that he could not attain nourishment from anything he ate and so started starving.

  13. Apparently this free drawing blackboard software is quite popular, as I have seen it used by Stanford online courses as well. Newsflash: it sucks. It is painful to watch instructor slowly handwriting words letter by letter. Look, there is a keyboard where you can type it. Or better yet, reveal the whole word or phrase at once. Like powerpoint slides do.

  14. if we rotate it in x axis 180° and then rotate it in z axis 180° we can superimpose anything…even.hands….why not molecules…they have rotational freedom in all three axes HELP

  15. I was really worried: I've heard EVERYONE say how difficult oChem is. So when I heard of chirality, I was scared, but this concept comes extremely easy to me.

  16. 4:40 i think sal messed up the mirror image drawing of the molecule. the methyl group should have a dashed line and the hydrogen should have a wedged line

  17. You need to stop repeating things over and over as you write… "4 different… 4… 4 different… different atoms". I love the videos, but this is quite irritating.

  18. Just one question, the hydrogen in the example( in between the other two groups) is in the centre it will still be in the centre which ever way it is rotated ???

  19. Chiral centres are applicable in amino acids, though some are likely to be Oggston's 3-point attachment theorem. Hence, they are essential in enzymes (in nature L-form). So, primary, secondary, tertiary structures of enzymes would be altered by D-form, I believe.

  20. L-form and D-form, are how the molecule "bends light". R- and S- designation, are shorthand to distinguish between the two enantiomers, left or right handedness of molecule. Eg, when the smallest side group attached to a chiral carbon, is placed at rear of molecule, then the three other groups may rotate clockwise (R-), or anticlockwise (S-), going from heaviest side group, to second heaviest, then third.
    Oggston discovered that nature can tell the difference between 2 different, and two similar side groups, by the nature of attachment. The example Biochemists quote is cis-aconitate in TCA cycle. Might well apply to Glutamic acid, eg MSG. Hope it wasn't synthetically sourced, Chemists just don't get 100% Yields in such reactions (Racemisation)!!!

  21. "I'm not going to write the whole thing"
    (Dead silence. No one arguing with you)
    "I'll just write the whole thing" hahaha
    Anyways this video/playlist is super helpful. Thanks.

  22. tnx again for this another helpful video…I jst have this one doubt regarding chiral carbon…so..if there is one ethyl group and one butyl group at two sides of one carbon nd two different groups on other two sides..will it be chiral carbon..or not??I mean.. are ethyl and butyl are two different groups here??advanced tnx..

  23. Am I the only one who finds that all examples given are super imposable if you rotate them 180 degrees?

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