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
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.