[darmel]

is there a difference between a chiral center and a chiral carbon?


Edit: edited title for better indexing. Mitch

[Mitch]

A chiral carbon is a chiral center. A chiral center could found anywhere and does not have to be on a carbon

[AWK]

Chiral compound does not have to possess any chiral atom. Crystal may be chiral because of chiral arragement of non-chiral molecules or stabilised chiral conformation by solid state.

[AhmedEzatAlzawalaty]

a chiral ceter could be any ,as P ,N  and chiral carbon has to be attached to four different groups and there is not plane of symmetry and no axis of symmetry to be chiral molecule

[lavoisier]

is there a difference between a chiral center and a chiral carbon?


Edit: edited title for better indexing. Mitch

My stereochemistry teacher used to say that very few people have really clear ideas of what they're talking about when it comes to this subject. He was right, apparently.

I will just write here the rules he gave us:

0. Stereochemistry deals with two separate (even though interrelated) subjects: one is molecular symmetry, the other is stereoselective/stereospecific syntheses.

1. Molecular symmetry is based on rigid models of the molecules, where all lengths and angles are fixed. Stereoselective syntheses are based on breaking and/or making bonds, therefore terms such as 'asymmetric synthesis' are meaningless.

2. Chirality is a property of a molecule as a whole, not of any of its parts. There is no such thing as a chiral 'center', 'carbon', 'atom' or 'bond'. A molecule is chiral when it has no symmetry elements of the second order.

3. Two isomeric molecules are stereoisomers (enantiomers or diastereomers) when their atoms have the same connectivity but are differently arranged in space. Enantiomers are one the mirror image of the other; for diastereomers on the contrary, there's no symmetry operation converting one into the other.

So the answer darmel's question is:

a tetrahedral carbon atom with four different substituents is said to be 'stereogenic' because swapping any pair of substituents leads to a stereoisomer of the parent molecule. Any (configurationally stable) atom having the same property is a stereogenic center.

[Custos]

2. Chirality is a property of a molecule as a whole, not of any of its parts. There is no such thing as a chiral 'center', 'carbon', 'atom' or 'bond'. A molecule is chiral when it has no symmetry elements of the second order.

This is strictly true but it has become common practice to use the term "chiral centre" as a shorthand for stereogenic centre. Try searching for "chiral centre" on Google if you don't believe me. 

[lavoisier]

Sorry Custos, but I must disagree.

First, stereocentre or stereogenic centre are not much longer expressions than chiral center, so using the second you make a mistake while not really shortening anything.

Second, the fact that many people use/used the term 'chiral center' doesn't make it correct. There are very important scientists who titled their papers using meaningless terms such as 'asymmetric synthesis', 'asymmetric induction' and the like, but this doesn't make them right.
You may notice that there's been an 'evolution' in the terminology used for stereochemistry, because IUPAC put a lot of effort in standardising the way scientists describe their findings. The substitution of the old threo/erythro naming with the anti/syn relative stereochemistry descriptors is a perfect example. But if you type 'threo' in Google you will still get a load of old papers containing it.

You may think that I'm making a silly terminologic argument here, but science is not about 'common practice', it is about using precise definitions, reporting observations in a universally understandable way, and especially, not being afraid of changing your mind for the better.
Authority doesn't mean much in science, which is what makes it great. Look for the mathematician Galois in Google and you will see what I mean.

[movies]

I don't understand your argument for why "asymmetric induction" is incorrect.  Surely you could compare two enantiomeric transition states and see how the would "induce" the formation of a asymmetric molecule by means of installing a new stereocenter.

Indeed, I have an entire book called "Stereochemistry of Organic Compounds" which has definitions for both "asymmetric synthesis" and "asymmetric induction."  What is the problem here?  There can be new terms that define new things, right?

[english]

I think by asymmetric or chiral center we mean center that generates chirality or asymmetry, not in the sense that that particular point is chiral in itself.

I think that's what your professor was trying to get across lavoisier.  Maybe.

[Custos]

You may think that I'm making a silly terminologic argument here, but science is not about 'common practice', it is about using precise definitions, reporting observations in a universally understandable way, and especially, not being afraid of changing your mind for the better.

I'm not arguing that "chiral centre" is right just that it is commonly used and, hence, students should understand what is being referred to. As an even more trivial example, IUPAC nomenclature was rigorously taught at my high school and when I reached university I had to learn all the common names that people still use. Acetone for propanone, t-butanol for 1,1-dimethylethanol, xylene, glycerol and so on. They are not right either but you still need to know them or you will not understand what people are talking about.

[lavoisier]

I will try to answer everybody.

Let's start from the assumption that in science we want to call things with their proper name, in order to avoid confusion and to know exactly what we mean. Otherwise we can all invent our own autistic language and stop understanding one another.

movies, the argument was not mine, but my teacher's; but I agree with him, so I adopted his point of view. There are plenty of books, papers and people using incorrect terms, so their existence doesn't prove anything.

k.V., my prof.'s point was that you can't mix symmetry considerations with stereogenic centers or stereoselective syntheses, because in the first case you refer to rigid, unchangeable objects and their geometric properties (actually defined by mathematical operators), whereas in the last two cases you break and form bonds, so you have completely different things to deal with.

I remember he explained that 'asymmetric induction' is wrong because you don't really induce asymmetry, you need to have it from the beginning. And in fact when you do enantioselective syntheses, you don't really generate a single stereocenter from nothing: you go through diastereomeric transition states, AND this is only possible thanks to some adduct of preexisting chiral species. When you do diastereoselective syntheses you don't really do anything special, because diastereomers are inherently different (they are not correlated by any symmetry operation), therefore it's perfectly normal that they behave differently in the transition state.

Custos, IUPAC allows the use of many common terms for chemicals, especially heterocycles and simple chemicals. It strongly discourages the use of those terms which can generate confusion and/or don't follow a rational system. For instance, 'benzene' is always preferred to 'benzol' because the second may make people think of some aromatic alcohol, whereas the first is more consistent for an unsaturated hydrocarbon.

So at the end of the day, nobody will be shot for using inexact terminology, but I do believe my prof. was right when he pointed out that having clear ideas on what you're talking about is the basis of rational, scientific thinking.
I'm sorry I can't explain it as effectively as he did, but I'm sure that if you could talk to him he would convince you.
And yes, without using hypnosis.

[movies]

You're absolutely right that people using a term wrong doesn't make it right, but this definition seems to be precluding the possibility of having new terms that use the same words, which is a totally backward approach that doesn't allow for new development of thought!  Also, this book I have referred to is not just some book.  It is a very authoratative source on all things stereochemical by some very prominent chemists.  I can't take that lightly!

So, I still don't buy this.  Isn't a reaction "asymmetric" because I put an asymmetric molecule in there as a catalyst or reagent?  Asymmetric induction makes sense to me because you are taking an achiral molecule and making it into a chiral molecule while selecting for one enantiomer of product by means of an asymmetric reagent.  You induce asymmetry into what was a symmetric molecule.  Yes, you need an asymmetric molecule in there to begin with, but so what?  The substrate molecule was still symmetric.

I can't think of why the term "asymmetric" should only apply to single molecules.  Why can't it apply to a group of molecules in a transition state?  Can't you think of a transition state with a chiral molecule as a rigid object with defined geometric properties?  The fact that bonds are forming or breaking doesn't make the symmetry operations go away, you can still apply them.  The symmetry operations will change throughout the course of the reaction, but at any given point you can find them.  You could say the same thing for any molecule that has conformational lability.  The gauche form of butane is chiral, but we know the reality of butane.  Limiting yourself to just one snapshot of individual molecules is missing a lot of the picture.

You sound dismissive of the fact that you access diastereomeric transition states, but that is the key feature here.  The point of asymmetric synthesis is that you enforce a situation where you have to go through diastereomeric transition states because the enantiomeric transition states aren't accessible (because you have an enantiopure reagent).  That sounds like an asymmetric process to me because you are precluding that symmetry.

I'm sorry if it sounds like I am coming down on you pretty hard here, I just think that your professor's definitions are a little close-minded.  I have the same problems with certain other terminologies, especially the term "regioisomer" which I think is totally incorrrect, so I do understand where you are coming from.  I just don't think that the definitions fit here.

[lavoisier]

Now I am sure that I haven't been able to explain things effectively. The arguments against my point are based on the very same theoretical background as mine, so there must be a methodological mistake on one side. I don't pretend that I am on the right one, by the way. I may be totally wrong.

It's not a big problem, though, because I know why I adopted my prof.'s point of view, and that's enough for me. If this makes me a worse chemist, no worries, I can live with it.

My prof. was not close-minded at all; he was actually one of the best teachers I had. That's why I took everything he said very seriously. And in fact, the clarification of these terminologic issues took at most the first 1-2 hours, the rest was dedicated to the subject.

The only thing that puzzles me at this point is why someone bothered going through the nomenclature of stereochemistry and coming up with a new, rational system, if the one we had before was SO nice and perfect.

movies, just a quick note about what you wrote. Don't worry, I am not taking anything personally here, and I assume the same is true for you. And however, you wrote nothing particularly harsh or offensive; as you might know I had much worse posts against me in the past.

So please let's consider this matter sorted out.

[Custos]

We could be getting bogged down with semantics here. While I agree we should use precision whenever possible in scientific writing and speaking, it's also true that the language of chemistry is complex and cumbersome and so we take shortcuts the "everybody understands" - like my example of IUPAC versus traditional naming systems.

I'm quite comfortable with the term asymmetric induction because, as movies pointed out, if you start with an chiral substrate and end up with a homochiral product you have introduced asymmetry into the molecule. It's true that somewhere in the reaction there must have been asymmetric reagents or catalysts (ignoring the wild and since disproved claims of asymmetric chemistry performed in strong magnetic fields ), but to disqualify the term on that basis seems trivial and semantic. Sometimes the asymmetric induction comes from within the molecule itself. For example I did a series of cycloaddition reactions (1,3-dipolar and Diels Alder) with the homochiral methylene oxazolidinone shown and the cycloadducts were single diastereomers - the new chiral centre being "induced" by the one across the ring. Now it's correct to say that stereochemistry wasn't "created" because it was present to begin with, but from the perspective of the carbon that was sp2 and is now sp3 and homochiral, it is a fair description of what occurred.

I have the same problems with certain other terminologies, especially the term "regioisomer" which I think is totally incorrrect, so I do understand where you are coming from.  I just don't think that the definitions fit here.

Just curious, what's your problem with "regioisomer"? It's a convenient term for describing molecules that are molecular isomers with alike structures but for the position of substitutes. Do you prefer "positional isomers"?

[movies]

I have the same problems with certain other terminologies, especially the term "regioisomer" which I think is totally incorrrect, so I do understand where you are coming from.  I just don't think that the definitions fit here.

Just curious, what's your problem with "regioisomer"? It's a convenient term for describing molecules that are molecular isomers with alike structures but for the position of substitutes. Do you prefer "positional isomers"?

Yes, I do prefer "positional isomers."  My dislike of "regioisomer" is because regioselectivity is a mechanistic preference and therefore two things that are "regioisomers" are dependent on what reaction you are talking about and it's not a property of the compound itself.

I'm fine with saying that a reaction is regioselective, because that refers to the mechanistic preference.

I guess I think of it as the fact that two molecules are isomeric is a state function, it only matters what they are not how they got to be that way.  Enantiomers are enantiomers even if you made them in two different ways.  "Regioisomer" is dependent on how you go there.

Think about 2-methyl-cyclopentenone.  If you had a way to make that by dehydrogenation of 2-methylcyclopentanone then you might have "regioisomers" where the alkene part is at the 2,3 position, the 3,4 position, or the 4,5-position.  Now if you made the same compound by oxidizing 1-methylcyclopentene the "regioisomers" would most likely be 2-methyl-cyclopentenone and 3-methyl-cyclopentenone.  The error really comes when people say things like "We made [this compound] but it was contaminated with its regioisomer."  But I could make the same compound a different way, make the same statement, but I would be referring to different regioisomeric compounds.