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MCAT
Course: MCAT > Unit 9
Lesson 5: Stereochemistry- Stereochemistry questions
- Chiral drugs
- Structural (constitutional) isomers
- Chiral vs achiral
- Stereoisomers, enantiomers, and chirality centers
- Identifying chirality centers
- R,S system
- R,S system practice
- Optical activity
- Enantiomers and diastereomers
- Cis–trans isomerism
- E–Z system
- Conformations of ethane
- Conformational analysis of butane
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Chiral vs achiral
Chirality is an important geometric property relating to a molecule's symmetry. A chiral molecule is non-superimposable with its mirror image, and has a "handedness" (think of shoes, which specifically go with a right or left foot). An achiral molecule is superimposable with its mirror image and do not have "handedness" (think of a baseball bat, which can be used with either hand).. Created by Jay.
Want to join the conversation?
So, are achiral molecules always identical molecules?(21 votes)
Yes, because if you rotate them you can see that they are superimposable, meaning they are exactly the same.(17 votes)
At2:31Sal takes his right hand and puts it over his left hand. They seem to be superimposable. But he says they are not superimposable as "his palms are not in the same position." What does this mean?(3 votes)- They have to facing the same direction to be superimposable on each other, because when his palms are together they are facing different directions. Hope that helps.(16 votes)
- Where cna i get a good molecular model set?(4 votes)
Amazon has a ton of cheap and efficient molecular model sets! Just look around!(12 votes)
- The annotation at the end of this video said these represent "two different molecules." Is that the correct way of saying it? Or should it be "two stereoisomers, or two enantiomers of the same molecule?"(4 votes)
You are more specific, but Jay is right. Two stereoisomers are two different molecules. This is due to different configurations (like how two constitutional isomers are two different molecules due to connectivities).
If at4:03, Jay pulled out the bonds of one the molecules and switched them around to make it transposable with the other, he would have changed the bonds of the molecule and therefore would have made a different molecule.(1 vote)
- What about chirality in inorganic coordination compounds? How do we identify it in octahedral or square planar molecules? What if there are bidentate or polydentate molecules that are ligands?(3 votes)
So what is the difference between a meso and an achiral?(2 votes)
A molecule that is meso has two or more chiral centres but, overall, the molecule is achiral because it can be superimposed on its mirror image. Therefore, meso compounds have no optical activity.
Furthermore, meso compounds always have an internal plane of symmetry.
These points are illustrated here - http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch07/meso03.gif. Despite this molecule having two chiral centres, molecule A is superimposable on B - hence A and B are identical molecules. Also there is an internal plane of symmetry so that the left hand side of the molecule reflects the right hand side.(3 votes)
- At2:35, Jay said that no matter what he does he can't superimpose his right hand on his left hand but we just saw that if he'd rotate his right hand it did perfectly match his left hand?(1 vote)
Why don't you try it yourself? Put your left and right hands together like you're praying, then flip your right hand 180 degrees. It doesn't match perfectly, in fact nothing you can do in terms of rotation can make them exactly the same.
The only thing that works is if you looked at your right hand in a mirror.
That is sort of the point of this, chiral molecules are different molecules and can have very different effects in the body.(4 votes)
- How do you do any of this if you are not given any stereochemistry? Ie if there are no dashed or wedged bonds to begin with?(2 votes)
They should always be given.
If they are not, you can assume that it is a racemic mixture (50% R and 50% S)(2 votes)
- Can we have more difference between the Chiral molecules and Chiral Atom?(2 votes)
Your question isn't very clear. You need to elaborate. Unless you're asking what is the difference between a chiral molecule and a chiral atom. In which case a molecule can be chiral and have enantiomers if one of its atoms is chiral.(1 vote)
- Are diastereomers chiral molecules? I know they have chiral centers and differ at some but not all chiral centers but as molecules would you call them chiral molecules?(1 vote)
Generally, yes.
When a molecule has two or more chiral centers, it is usually chiral. The exception is with meso molecules, which are not chiral.
These are molecules that due to symmetry have chiral centers that 'cancel' each other out.(2 votes)
2:31
Video transcript
Voiceover: Achiral objects are objects that are superimposable
on their mirror images. And in a minute, I'm gonna show you that a coffee cup is an
example of an achiral object. Chiral objects are objects that are not superimposable on their mirror images. And the word "chiral" comes
from the Greek word for "hand". And so I'm gonna show you how your hands are not superimposable on each other but your left and right hand are
mirror images of each other. Let's take a look at a coffee
cup reflected in a mirror. So, you can see on the left,
here is the actual coffee cup. In the mirror is the mirror
image of the coffee cup. And I'm gonna pull out the
coffee cup to make some space. And I'm gonna put what
I saw in the mirror, the mirror image, right next to it. Here I have another coffee cup,
so that's it's mirror image. And I'm going to take the coffee cup on the right and I'm going to rotate it. And so, as I rotate it,
you can see that it is superimposable with
the object on the left. So the mirror image is superimposable. And that's the definition
of an achiral object. So, we say that a coffee cup is achiral. Now let's try the same
thing with a molecule. So, this is difluoromethane. So, the green are the fluorine atoms. And in the mirror, you
can see the mirror image. So, once again, I'm going
to pull the molecule away and put what I saw in the mirror, the mirror image right next to it. And I'm going to rotate the mirror image. So the one on the right,
I'm going to rotate it to see if it's superimposable
with the one on the left. And so, I take it and as
you can see as I rotate, right there you can see that it is superimposable with the
molecule on the left. And since the mirror
image is superimposable, we say this is an achiral molecule. So these are actually two of the exact same molecules represented here. Now let's look at my hands. So, my left hand and in
the mirror you can see my right hand or what
looks like my right hand. So I'm gonna take my left
and my right hand together. We just showed that they are
mirror images of each other. Then I'm going to try
to rotate my right hand to see if it's superimposable
with my left hand. So you can see here, I
have my palms both up but my thumbs are not pointing
in the same direction. So they're not superimposable here. So I'm going to try again,
I'm going to try to rotate it. So now my thumbs are in the same position but my palms are not in the same position. So no matter what I do,
I can never superimpose my right hand on my left hand. So, the mirror image is not superimposable upon the original object. So no matter what you do,
you cannot accomplish this. And since the mirror image
is not superimposable, we say your hands are chiral. Now, finally let's take
a look at a molecule. So the white is hydrogen,
green is fluorine, red is bromine and yellow is chlorine. So the molecule is on the left and in the mirror is the mirror image. So I'm gonna pull out that molecule and once again leave some space. And put what I saw in the
mirror right next to it. So there's the mirror image. So I have these two mirror
images of each other. I take the one on the right
and I try to rotate it to see if I can superimpose
it with the one on the left. And so here I just rotated
it a little bit there. And you can see that the red, the bromine atoms are in the same position. However, the chlorine and the fluorine, the yellow and the green are
not in the same position. So I cannot superimpose it here. I try again and I rotate it some more. And you can see the yellow
is in the same position, the chlorine and also the hydrogen is. But, the red and the green are not. And so, no matter how I
rotate the mirror image, the one on the right, I can never get it to look like the one on the left. And since the mirror image
is not superimposable, we say that this is a chiral molecule. And this carbon here, this is a very important carbon, with four different substituents attached to
it, four different groups. We call this a chiral center
or a chirality center.