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Author Topic: Stereospecific reaction  (Read 10291 times)

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Dolphinsiu

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Stereospecific reaction
« on: March 31, 2007, 12:45:50 AM »

Explain whether addition of bromine to 2-(E)-pentene is a stereospecific reaction.

I think it is stereospecific as one particular stereiosmer react to one specific stereoisomer.

so it gives only R,R and S,S conformation, but not R,S and S,R

Am I right?
« Last Edit: March 31, 2007, 01:38:24 AM by Dolphinsiu »
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Sam (UoN)

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Re: Stereospecific reaction
« Reply #1 on: March 31, 2007, 11:18:14 AM »

When the first bromine atom adds, causing the formation of the bromonium ion, this controls the addition of the second bromine atom (left from the bromine molecule) which occurs by SN2 (stereospecific) and the centre under attack undergoes inversion (see this diagram from wiki):


The atoms always add in this fashion (resulting in the inversion of one centre, which will be easier to see if the substituents on the double bond are marked).

The reaction you are doing (product is meso-): (attachment).

Remember that the incoming nucleophile will attack the carbon with the best ability to stabilise positive charge (regioselectivity) (as long as that carbon isn't too sterically hindered) because the reaction isn't pure SN2 (some SN1 character).  This doesn't matter as much in this case, but matters if you are carrying out the addition in Methanol, or some other solvent that will compete with the bromide as the nucleophile.

N.B. use of the Z-isomer yields a product with different stereochemistry, meaning that the reaction is stereospecific.
« Last Edit: March 31, 2007, 01:36:49 PM by Sam (UoN) »
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lavoisier

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Re: Stereospecific reaction
« Reply #2 on: March 31, 2007, 10:21:41 PM »

Explain whether addition of bromine to 2-(E)-pentene is a stereospecific reaction.

I think it is stereospecific as one particular stereiosmer react to one specific stereoisomer.

so it gives only R,R and S,S conformation, but not R,S and S,R

Am I right?

I think Sam's reply was very accurate, but it didn't directly answer your question.

A reaction is 'stereospecific' when its mechanism leads specifically (and not 'selectively') to one type of stereoisomers.

The classic example is OsO4 dihydroxylation of alkenes. The inorganic reagent binds to the alkene in a specific way (syn, in this case), leading to a specific type of diols depending on the stereochemistry of the alkene (E or Z). Of course, it can add to either side of the double bond, but the relative stereochemistry of the introduced hydroxyls remains the same.
Catalytic hydrogenation of alkenes is another stereospecific reaction.
The reaction you cited is stereospecific as long as it goes as described by Sam, i.e. via a bromonium ion intermediate. If it doesn't, e.g. if the bromonium ion opens to a carbocation, then the incoming bromide will be able to attack from either side, and you will have at best a stereoselective reaction.

So yes, you were right. But please note that (R,R) and the like are descriptors of configuration, not conformation! The conformation has to do with rotational isomers, where no bond is broken.
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Dolphinsiu

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Re: Stereospecific reaction
« Reply #3 on: March 31, 2007, 11:50:01 PM »

Do you mean that cis/trans and (R,R) (S,R) (R,S) and (S,S) are the description of configuration
whereas atropisomer and rotamer (rotation isomer) are description of comformation!

But drawing benzene into chair-shaped involving in flipping is description of comformation.

How about Newman, Fischer projection and flying wedge drawing, are all of them descriptions of conformation?
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lavoisier

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Re: Stereospecific reaction
« Reply #4 on: April 02, 2007, 09:59:28 AM »

Sorry, you're mixing up everything here.

R, S, E and Z are 'descriptors of configuration'.
As far as I'm aware there are no specific descriptors of conformation, apart from eclipsed, gauche...
The projections and drawing you mentioned are not 'descriptors', they are just pictures we use to visualize molecules. A descriptor is just a word.

The basic difference between configuration and conformation is that the configuration changes when you break and form bonds, whereas the conformation changes when you merely rotate bonds.
The stereochemistry of a double bond in an alkene requires configurational descriptors because you can't change E to Z or viceversa without breaking an re-forming bonds. Same story for R to S interconversion. Just check it with molecular models if you don't believe me.

Of course there are borderline cases (atropoisomers, tertiary amines, some phosphines...), but in this case it's the compound itself that either shows or doesn't show configurational stability.

I will not comment about benzene, which is a planar molecule and has no conformers! Check it again. But if you meant cyclohexane, it's easy to see that you can interconvert its various conformers without breaking or forming any bond, but just rotating them.
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Yggdrasil

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Re: Stereospecific reaction
« Reply #5 on: April 02, 2007, 09:12:32 PM »

A reaction is 'stereospecific' when its mechanism leads specifically (and not 'selectively') to one type of stereoisomers.

This is what I thought the definition of stereospecific was, but apparently it is not.  Here's a good explanation of stereospecificity v. stereoselectivity (plus an answer to the original question):

http://coronene.blogspot.com/2007/03/i-hate-o-chem-terminology-sometimes.html
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Mitch

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Re: Stereospecific reaction
« Reply #6 on: April 02, 2007, 09:49:12 PM »

I was going to reference the same post. :P
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Yggdrasil

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Re: Stereospecific reaction
« Reply #7 on: April 03, 2007, 05:15:19 AM »

Sweet, I scooped Mitch.
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lavoisier

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Re: Stereospecific reaction
« Reply #8 on: April 03, 2007, 07:12:34 AM »

A reaction is 'stereospecific' when its mechanism leads specifically (and not 'selectively') to one type of stereoisomers.

This is what I thought the definition of stereospecific was, but apparently it is not.  Here's a good explanation of stereospecificity v. stereoselectivity (plus an answer to the original question):

http://coronene.blogspot.com/2007/03/i-hate-o-chem-terminology-sometimes.html

I looked at this link. Their definition is:

Stereospecificity: A reaction is stereospecific when one stereoisomer of a reactant gives a stereoisomer of a product, while another stereoisomer of that reactant gives a stereoisomer of that product.

I'm sorry, but I think this statement is at best incomplete, if not positively wrong.
It looks a little too generic: what does 'another stereoisomer of that reactant' mean?

Let's consider for instance the osmilation of an unsymmetrically substituted alkene (like 2-pentene). From the Z isomer you get two enantiomers, from the E isomer you get another two enantiomers (because the two sides of the alkene are enantiotopic). But all 4 products are 'stereoisomers' to one another anyway, so what does that definition add to our understanding of the matter?

With all due respect to the authors of the site you mentioned, it doesn't look (to me) like the most authoritative source in the world.

I was taught that stereospecificity has to do with the mechanism of a reaction, and actually it's because of the mechanism that you get a definite set of stereoisomers of the product from a definite stereoisomer of the reactant, and a complementary set of stereoisomers of the product from a a complementary stereoisomer of the reactant.

I don't see the point of giving a sort of vague and dogmatic definition that diverts your attention from the underlying principles.
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movies

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Re: Stereospecific reaction
« Reply #9 on: April 03, 2007, 09:36:57 AM »

The difference in your example with 2-pentene is that you get a specific set of enantiomers depending on the stereochemistry of the starting material, not a mixture of diastereomers.

Mind you the definition cited in that post is virtually identical to the definition in the Eliel stereochemistry book (cited in the post), which is as authoritative a source as there is when it comes to stereochemistry.

I do agree, however, that I favor the mechanistic dependence in defining stereoselective vs. stereospecific.  I think that it really ends up being a different way of defining the same thing.  If the mechanism dictates a syn-addition in dihydroxylation, then that will manifest itself in the test of stereoisomeric alkenes as well.  What does bother me a little is the definition of an alkyne reduction to an alkene as stereoselective.  It seems that the mechanism would dictate a specific outcome of that reaction, not a selective one.
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lavoisier

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Re: Stereospecific reaction
« Reply #10 on: April 04, 2007, 07:19:02 AM »

Who defines the reduction of an alkyne to an alkene as being 'stereoselective'?

If you use catalytic hydrogenation, it is stereospecific; probably it's only stereoselective if you use methods which generate radicals (like Birch or similar).

For the general definition of stereospecificity, fair enough, if they like it... but still I believe that it's incoherently formulated (i.e. the two-way correlation between educt and product is not clear enough).
Anyway, I'm glad that my prof. took the time to explain us the actual meaning of the thing, in terms of mechanism, instead of treating us as parrots (and I know a lot of people who went through university getting full marks all the time by just mechanically repeating sentences read in books - very good memory instead of understanding of the subjects).
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movies

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Re: Stereospecific reaction
« Reply #11 on: April 04, 2007, 09:56:57 AM »

If you use the definition that stereoisomeric starting materials give stereoisomeric products, then alkynes hydrogenation can't be included because there are no stereoisomers of an alkyne.

I understand your frustration with your classmates who regurgitated rather than reasoned through their classes.  I have had similar experiences.  I also share your skepticism about this definition.  I find it troubling that there is no inclusion of an actual difference in the way that the reaction occurs mechanistically.  However, as I said, I'm not sure if the definition found in these books, though explained differently than they way that you (lavoisier) and I learned the concept, is actually truly a different definition.  It may just be that the statement is a consequence of the mechanistic difference, and so really means the same thing.  If that is the case, then I certainly agree with you in stating that the definition should be more clear.
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