Organic Chemistry - "Change": Part 14 - Addition of HBr to an Alkene Leading to Optical Isomers

 Greetings,

This post explains the formation of optical isomers from the addition of Hydrogen Bromide (HBr) to an alkene.  The alkyl groups bonded to the alkene carbon atoms need to be "just right" for the production of a pair of mirror image optical isomers, also known as enantiomers.  When we have a molecule with a "central" carbon consisting of four different groups, or atoms, bonded to it, then we say that the carbon atom is chiral.

If we have a compound with one chiral carbon then only mirror image (enantiomer) isomers are possible.  A compound consisting of two or more chiral carbon atoms will have enantiomers and sets of optical isomers known as diastereomers (non-mirror-image optical isomers).  Examples of compounds with both types of optical isomers (enantiomers and diastereomers) are sugars (aka carbohydrates). 

The particular example explored in this post is the hydrogen bromide addition to 3-methyl,1-hexene.  There are two possibilities for the addition of HBr having to do with bromine adding to the "top" or "bottom" of the alkene structure.  This is demonstrated in the diagram, presented below.

The diagram also shows how the enantiomers are designated (R) and (S).  The circular diagrams on the right are projections with the lowest M.W. group ("Meth") behind, the chiral carbon, represented by the black ring, with the other three entities ("Br", "Prop" and "Eth") projecting out toward the viewer. The entities are color coded to show how they relate to different portions of the molecule.

That's all for now.  As always, thank you for reading!