[iScience]

i thought i understood what the difference was but i guess not..

i thought stereoselectivity was when the reactants yield several products but that one was just preferred over the others so that there would then be a higher proportion of that preferred product.

and i thought stereo specificity was when the reactants undergo a specific reaction such that only one type of reactant is yielded. i had some people even confirm these for me but then i look at this (image below) and i'm lost again.

reaction A: it seems that only one reactant is yielded and the others are NOT yielded from the reaction. why is it stereo'selective' as opposed to stereospecific?? and would anybody know what is meant when it says "only two of this"? the image below is the key to one of the old practice exams




thanks

[Dan]

The mark scheme looks pretty unclear to me.

S_N2 reactions are stereospecific (the mechanism allows only one product), so if these reactants are undergoing S_N2 displacement (which is what it looks like), then the reactions are stereospecific. That is my understanding anyway, I agree with you.

Can you ask the person who drew up the mark scheme to clarify?

[iScience]

Can you ask the person who drew up the mark scheme to clarify?

it's my professor, i emailed him about it;

also at the bottom, both stereo selective and stereospecific are checked. Well, i could only understand if there were two reaction mechanisms going on, but.. either way it looks like for reaction B, still, only one product is made. so how can it be stereoselective?

[iScience]

so i asked him why rxn A and B are both stereoselective and this is what he replied:

"Reaction A is stereoselective because it gives just one out of several possible stereoisomeric products. To prove a group to be stereospecific, stereoisomeric starting materials must give stereoisomeric products; since reaction A involves only one starting material, it doesn't do this."

i am still confused... the products ARE stereoisomeric aren't they?? and how does this distinguish stereoselective vs stereospecific?? stereoselective reactions can also yield stereoisomeric products can't they?

[Dan]

Your professor is referring to the fact that in a stereospecific reaction the sterochemistry of the product is dependent on the stereochemistry of the starting material. So for the reaction to be stereospecific: if you start with S,S-diiodocyclobutane you get R,R-dibromobutane, but if you start with R,R-diiodobutane you get S,S-dibromobutane as the product. This is true for an S_N2.

Your professor's argument seems to be that because you only have data for the transformation with S,S-diiodobutane, you can't say it is stereospecific because you don't have the data for the corresponding reaction of R,R-diiodobutane.

Personally I think he's missed the point. The reactions are stereospecific (and note that all stereospecific reactions are also stereoselective - stereospecificity is a special type of stereoselectivity).

IUPAC Link: http://www.chem.qmul.ac.uk/iupac/gtpoc/StSy.html#08

Here is a excerpt taken from Sterechmistry of Organic Compounds - E.L. Eliel, S.H. Wilen (Wiley)

[iScience]

okay that helped alot! thanks!!