A characteristic of reactions in organic chemistry where one constitutional isomer of the product is favored over all others. This is extremely important in synthetic organic chemistry where it is desirable to control your reaction product as much as possible.

One example is the following reaction. There are two possible ways for the HBr to add:

    Propene                       H  H  H
    H     H                       |  |  |
     \   /                   A: H-C--C--C-H (2-Bromopropane)
      C=C      + HBr  =>          |  |  |
     /   \                        H  Br H
 H -C     H                     
   / \                            H  H  H
  H   H                      B:   |  |  |
                                H-C--C--C-H (1-Bromopropane)
                                  |  |  |  
                                  H  H  Br

While it may seem like these products would be equally likely, product A is formed in significantly greater quantities than product B (The reasons for which are discussed in more detail under Markovnikov Addition). Some common variations on this type of reaction include two methods of alcohol synthesis namely, Oxymercuration-Reduction and Hydroboration-Oxidation1

In general, the more reactive your starting materials are the less regioselective your reaction will be. In the reaction shown above, should one substitute a more reactive HCl or HF for the HBr, the second product have an increasing probability of the second product being formed (though still not as much as Product A).

Another common contributing factor for regioselectivity is how stable the intermediates are. Given that two possible reaction pathways can occur, the one with more stable intermediates is more likely. In the case of Hydroboration, the pathway that attempts to put the Hydrogen on the terminal carbon (as happens in the HBr addition) would involve an intermediate state with a partial positive charge on a primary carbon. On the other hand, the favored reaction pathway draws electrons from a secondary carbon, passing through a more stable intermediate as the secondary carbon has two neighbors that can support the deficiency as opposed to just one. Since one transition state is favored over the other, this results in the corresponding product to be favored over the other.

1: Stictly speaking Hydroboration is sometimes considered an Anti-Markovnikov addition as the functional group selectively added to the terminal carbon, but it still remains as an important regioselective reaction.

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