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Stereospecific Reaction Questions

Article ID: 140
Last updated: 25 Jan, 2013
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 Check out this interactive stereochemistry tutorial on the ChemEdDL

 

Q1:  Bromine reacts with alkenes to give dibromides via a stereospecific reaction.

Draw the dibromide products produced by the addition of Br2 to the following alkenes, 
  (a) propene

(b) Z-2-butene

(c) E-2-butene


Q2: As you will learn later, alkynes can be reduced to alkenes in two different ways.

Using the data provided below and what you learned from Q1, deduce the stereochemistry of the two methods of reduction.
A : 2-butyne + H2 + catalyst gives an alkene that reacts with Br2 to give a racemic mixture
B : 2-butyne + Na / NH3 gives an alkene that reacts with Br2 to give a single achiral compound.

Answers to Questions:
 

Q1:

(a) Propene will react to give 1,2-dibromopropane which contains 1 stereocenter at C2, therefore the product will be a racemic mixture of (R)- and (S)-1,2-dibromopropane.
 

(b) E-2-butene will react to give 2,3-dibromobutane which has 2 stereocenters at C2 and C3. The reaction is stereospecific in nature in an anti fashion but we should review the possibilities:
 

Since the 2 chiral centers in the product have the same 4 groups attached (-H, -Br, -CH3, and -CHBrCH3) and a mirror plane between C2 and C3, then we have a meso compound, and all 4 of these structures are actually the same and all are achiral molecules.




(c) Z-2-butene, as above, will react to give 2,3-dibromobutane which has 2 stereocenters at C2 and C3. Now the possibilities are:
 


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.



Q2: If the alkene from method A gives a pair of enantiomers, then it must be the Z-alkene.

Therefore the addition of H2 must have occured with syn stereochemistry by using hydrogen gas and a metal catalyst.



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, as seen in the sodium ammonia reduction mechanism.

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Absolute and Relative Configurations     Labeling Enantiomers

Department of Chemistry & Chemical Biology