Return to Contents Chapter 7 : Stereochemistry Ch 7 contents
Answers to Absolute and Relative Configurations
 
Qu1
(a) Propene will react to give 1,2-dibromopropane which contains 1 stereocentre at C2, therefore the product will be a racemic mixture of (R)- and (S)-1,2-dibromopropane.
(b) (E)-but-2-ene will react to give 2,3-dibromobutane which has 2 stereocenters at C2 and C3. The reaction is stereospecific in nature via an anti process but we should review the possibilities. The starting point is to think of the alkene as being oriented as a horizontal plane and hence has a top and bottom face. Then the bromonium ion can form on either face. Steric effects force the bromide ion nucleophile to attack from the opposite face, but in the symmetrical case, this can occur from either end (blue pathway versus green pathway). Overall this gives four possible pathways....
 
addition of Br2 to e-2-butene

Since the 2 chiral centers in the product have the same 4 groups attached (-H, -Br, -CH3, and -CHBrCH3), then we have a meso compound, and all 4 of theses structures are actually the same. If you aren't convinced, rotate the JSMOL image to look like each of them !

(c) (Z)-but-2-ene, has the same permutantions of four pathways are described above, but the outcome is slightly different. will react to give 2,3-dibromobutane which has 2 stereocenters at C2 and C3. Now the possibilities are:
 
addition of Br2 to z-2-butene
(S,S)
(R,R)

Since the 2 chiral centers in the product we will get the two compounds (R,R) and (S,S) compounds that are enantiomers so we have a racemic product mixture. If you aren't convinced that there are only two different compounds here then rotate the JSMOL images to look like each of them !

Qu2 If the alkene from method A gives a pair of enantiomers, then it must be the E-alkene.
Therefore the addition of H2 must have occured with syn stereochemistry.
If the alkene from method B gives a a single meso compound, then it must be the Z-alkene.
Therefore the addition of H2 must have occured with anti stereochemistry.

organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary