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Chiral help?

by eax
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eax
#1
Nov19-05, 07:59 PM
P: 57
From what I understand a chiral compound is one of which that when mirrored it is not identical.

Not identical in what way's? Is it not superposable? Not have same properties?

I have a multiple choise question asking which of the following is a chiral compound and the ones I choose are not on the list.

Textbook: SOLOMONS sixth edition [Thats the textbook I have to study from]
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gravenewworld
#2
Nov19-05, 08:55 PM
P: 1,404
Thats right a chiral molecule is one that has a non superimposable mirror image. For example, your hands are "chiral" objects. They are obviously mirror images of each other when you face both palms of your hands together. However if you try to lay one hand on top of the other hand you will see that they aren't the same. one of your thums points to the left and the other to the right.
eax
#3
Nov19-05, 09:44 PM
P: 57
This questioln is tricky. I still can't get it. After I create a mirror image I can rotate in 3D right? or 2D only? I am probably visuallizing the atoms in the wrong spatial arangement and thats why maby I can't answer the question. They are simple carbon compounds except one is a cyclo compound.

I want to see more examples of these my textbook has only a few examples. Any website have examples of these (With answers)?

Thank you!

bomba923
#4
Nov19-05, 10:03 PM
P: 736
Chiral help?

Quote Quote by eax
I want to see more examples of these my textbook has only a few examples. Any website have examples of these (With answers)?
In other words: NO MORE bromochlorofluoromethane FOR YOU!!!!

Here...are some *fun* examples of chiral molecules (that I made up ):
(They're really fun! )

1) 2-fluorobutane
2) 3-methylhexane
3) 2-bromomethyl-2-chloromethyl-1-fluoropropane
4) All amino acids except 'alanine' (i.e., alanine = aminoacetic acid = nonchiral)
5) N-ethyl-N-methyl-N-propylbutan-1-aminium
6) m-dichlorocyclohexane & o-dichlorocyclohexane
7) amino(hydroxy)acetic acid
8) 1-aminoethanol
9) 2-[pyridin-3-yl(pyridin-4-yl)methyl]pyridine
10) 2-amino-2-hydroxy-3-oxoacetic acid

Just adding some examples to what gravenewworld said
ShawnD
#5
Nov20-05, 02:42 AM
Sci Advisor
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P: 987
Quote Quote by eax
I have a multiple choise question asking which of the following is a chiral compound and the ones I choose are not on the list.
Chiral compounds are the ones where a particular carbon has 4 different groups on it. As mentioned, bromochlorofluoromethane is chiral because the 4 groups are bromine, chlorine, fluorine, and hydrogen (they're all different). If you took something like dichlorofluoromethane, this would not be chiral because 2 of the groups are the same (chlorine).
The trickier chiral molecules to see are things like 2-fluorobutane. On the second carbon, one of the groups is fluorine, one is hydrogen, one is methyl, and one is ethyl. Although that second carbon is bound to 2 carbons, those carbons are not the same because they are connected to different things.
Chiral carbons are always bound to 4 things. If it has only 3 things connected to it, it can't be chiral.
It's also important to remember that you generally cannot make chiral products from nonchiral starting materials.

In answer to your question "Not have same properties?", chirality can sometimes change the properties of things. Great example is amphetamine. Note the minor differences between nonchiral and S amphetamine.

Amphetamine +- (this means a mixture of chiralities)
boiling point: 203.0
density: 0.9306
refractive index: 1.518
slightly soluble in: water, ether
soluble in: chloroform, ethanol

S-Amphetamine (also called dexamphetamine)
boiling point: 203.5
density: 0.949
refractive index: 1.4704
slightly soluble in: water
soluble in: ethanol, ether

The most interesting thing to note is how the solubility changes slightly, or at least I think that's the most interesting thing.
ShawnD
#6
Nov20-05, 04:41 AM
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P: 987
I think someone's post got deleted, but not to worry, I have a copy of it.

Quote Quote by deleted post
Also enantiomers will rotate plane-polarized light in opposing directions.
If a chiral substance is dextrorotary, its enantiomer will be leuvorotatory.
This is actually one of the ways to tell if you have a chiral compound or not. If it doesn't polarize light; it's probably not chiral. Also, Dextrorotary and Levorotary are the "d" and "l" you see infront of things like d-glucose.
gravenewworld
#7
Nov20-05, 10:56 AM
P: 1,404
Chiral compounds are the ones where a particular carbon has 4 different groups on it.

This is not always true. There are chiral compounds that don't have a carbon bound to 4 different things. The defintion of a chiral compound is that the molecule only have a non superimposable mirror image. A compound can be chiral and have a "chirality axis."
movies
#8
Nov20-05, 11:09 AM
Sci Advisor
P: 284
That amphetamine example is a little misleading because you are comparing the racemate to an optically pure compound. If you compare just the two enantiomers in pure form, then the properties will all be identical except for the rotation of plane polarized light. Once you start mixing chiral molecules with other chiral molecules then all bets are off because you have the possibility of making diastereomeric complexes, which is probably what happens with amphetamine.

Also, with the d and l corresponding to dextrotatory and levorotratory, this is true, but note that if the preceding letters are capitalized (D and L) this is not always the case. There is a different convention for the D and L assignments. An example is tartaric acid. The natural form is (R,R)-(+), and therefore d, but with the other convention, natural tartaric acid is actually assigned L. So the full name is very confusing: d-(L)-(+)-(R,R)-tartaric acid.
ShawnD
#9
Nov20-05, 01:40 PM
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P: 987
One of those d and l conventions is how something is drawn using a fisher projection. That should probably just be ignored by everybody because it makes everything much harder than it should be.
movies
#10
Nov21-05, 10:44 AM
Sci Advisor
P: 284
Quote Quote by ShawnD
One of those d and l conventions is how something is drawn using a fisher projection. That should probably just be ignored by everybody because it makes everything much harder than it should be.
The D and L are actually how they relate to glyceraldehyde via chemical degradation. Of course, this can depend on how you degrade the molecule, so it's not a good convention. It's rarely seen now, except for amino acids and some random others.


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