regio- vs stereo- specific

[CrimsonMirage]

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hi,
can anyone offer a good explanation of regiospecific vs. stereospecific reactions? and give an example of each?
i'm having a hard time thinking of an example reaction that is stereospecific but not regiospecific...

thanks!

 

[kish180]

hi,
can anyone offer a good explanation of regiospecific vs. stereospecific reactions? and give an example of each?
i'm having a hard time thinking of an example reaction that is stereospecific but not regiospecific...

thanks!

stereospecific is concerned with orientation in space, whether your talking about anti vs. syn or R vs. S. Regiospecific is concerned with the basic carbon structure, it specifies which substituents are bound to which carbons in the products.
Examples:
Stereospecific: simple Sn2 rxns are all stereospecific. If the carbon where the substitution takes place is chiral, the configuration is inversed in the products. this is also an example of a reaction which is not regiospecific b/c there is only one carbon in consideration. Another example of something stereospecific but not regiospecific is the formation of a 1,2-diol from an alkene with KMnO4. Since this reaction creates a 1,2-diol, there is no regiospecificity since no matter what both of the double-bonded carbons receive a -OH substituent. However it is stereospecific since it results in syn addition accross the double bond.
Regiospecific: A simple elimination reaction is regiospecific because it can result in multiple different substituted alkenes.