Orgo Question

[CurryPower]

hey guys,

Whats the difference between stereospecific reactions and stereoselective reactions....+ can you give me an example.


thanks.

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[HawkeyePostOp]

Sterospecific: A reaction that for some reason, can only produce one specific stereoisomer. Always 100% yield.
eg: any reaction with 100% yeild

Stereoselective: A reaction that yeilds two stereoisomers, but one of these is favored.
eg: any reaction with two yields, one larger than the other.

 

[161927]

Stereoselective and stereospecific are the same thing, but stereospecific is the more common term. Regioselectivity is something different, because it refers to the site where a reaction takes place relative to other structural features of a molecule.

 

[HawkeyePostOp]

Stereoselective and stereospecific are the same thing, but stereospecific is the more common term.

wrong

 

[161927]

wrong

no u

 

[HawkeyePostOp]

no u

look it up

 

[sleepy425]

hawkeyepostop is absolutely right. they are different and the difference is very important.

as hawkeye said, the mechanism of a stereospecific reaction allows formation of only one stereoisomer. it's not like the other isomer could form but just doesn't because the other is favored (that would be stereoselectivity). it doesn't form because, given the mechanism, it's not physically possible to get the other product. the yield in a stereospecific reaction is always 100%.

a stereoselective reaction can form both stereoisomers (based on its mechanism), but formation of one isomer is favored over the other isomer for some reason. the yield of a stereoselective reaction can be 100%, but is generally less than 100%. So a stereoselective reaction can form the other isomer but prefers not to, a stereospecific reaction cannot form the other isomer.

incidentally, by yield, i don't mean the total yield, i mean the percent of the yield that is the given stereoisomer.

 

[161927]

look it up

A stereoselective reaction gives a majority of one stereoisomer. When you say a reaction is stereospecific, it's in reference to the fact that you started with a chiral product and end up with different stereochemistry in the product. Neither terms refer to, as you would have me believe, 100% control in forming a specific stereoisomer.

The semantic difference between stereospecificity and stereoselectivity is such a small one that it only has meaning to organic chemists. This will not be asked on the MCAT.

 

[161927]

hawkeyepostop is absolutely right. they are different and the difference is very important.

as hawkeye said, the mechanism of a stereospecific reaction allows formation of only one stereoisomer. it's not like the other isomer could form but just doesn't because the other is favored (that would be stereoselectivity). it doesn't form because, given the mechanism, it's not physically possible to get the other product. the yield in a stereospecific reaction is always 100%.

a stereoselective reaction can form both stereoisomers (based on its mechanism), but formation of one isomer is favored over the other isomer for some reason. the yield of a stereoselective reaction can be 100%, but is generally less than 100%. So a stereoselective reaction can form the other isomer but prefers not to, a stereospecific reaction cannot form the other isomer.

incidentally, by yield, i don't mean the total yield, i mean the percent of the yield that is the given stereoisomer.

Here we go, the official IUPAC definition:

stereospecificity (stereospecific)
1. A reaction is termed stereospecific if starting materials differing only in
their configuration are converted into stereoisomeric products. According
to this definition, a stereospecific process is necessarily stereoselective but
not all stereoselective processes are stereospecific. Stereospecificity may
be total (100%) or partial. The term is also applied to situations where
reaction can be performed with only one stereoisomer. For example, the
exclusive formation of trans-1,2-dibromocyclohexane upon bromination
of cyclohexene is a stereospecific process, although the analogous reaction
with (E)-cyclohexene has not been performed.
2. The term has also been applied to describe a reaction of very high
stereoselectivity, but this usage is unnecessary and is discouraged.


http://www.iupac.org/goldbook/S05994.pdf

 

[HawkeyePostOp]

The term has also been applied to describe a reaction of very high
stereoselectivity, but this usage is unnecessary and is discouraged.

IUPAC can discourage it all they want, but it's mainstream for college textbooks. When an MCAT question involving stereospecificity shows up, it will involve only one product.