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7.4: Reaction specificity and product selectivity

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  • When chemists describe the relationship between similar structures (for example in section 4.4), we need to use some elaborate terms such as enantiomer, diastereomer, configurational isomer, meso form, etc.  When we consider the possible products of a reaction, we often need a set of terms to describe the relationship between how they are formed.  It is most useful to define reactions in terms of their reaction specificity – do they differ in how the products are oriented in space (i.e., are they stereoisomers?), or in where the bonds are formed (i.e., are they positional isomers, a type of constitutional isomer?).

    Consider this reaction scheme from a final exam, where the question asked: “Which product will be formed?”

    As you will learn, this reaction in fact only gives product C as one single pair of enantiomers (it is racemic).  This is because (a) the OH goes on the less substituted carbon (it is an “anti-Markovnikov addition”), and (b) the H and OH are added from the same side (called a “syn addition”).  This is an excellent example of reaction specificity.

    Note that A and C are positional isomers, and they differ in where the OH goes.  In contrast, A and B are stereoisomers, so they differ only in how the new bonds are added in 3D-space.  As we work through chapters 8-10, we will hear terms such as “back side attack,” “Zaitsev’s Rule” and “Markovnikov’s Rule” – these are all rules that tell us information about reaction specificity.  Some of the most common terms are:

    Regioselectivity means that one direction of bond making or breaking occurs preferentially.  In practice, this tells us where the new group will end up.  In the above example, it tells us whether we get A/B or C/D.

    Stereoselectivity is defined by IUPAC as “the preferential formation in a chemical reaction of one stereoisomer over another.”  There are a whole series of related terms, for which the meaning should be obvious: Diastereoselectivity, enantioselectivity, etc.  In the example above, this tells us whether we get syn addition (A/C) or anti addition (B/D).

    Chemoselectivity tells us whether or a reaction occurs at the correct functional group.  In a chemoselective reaction, we would see one specific functional group reacting while others do not.  In the example above there is only one functional group, the C=C, so chemoselectivity is likely to be very high.  However, if we were to take the alkene in the above example and simply set it on fire – a reaction which occurs with both the alkane part and the alkene part of the molecule – that would not be chemoselective, as everything would turn to CO2 and water!

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