Ch 6 : Regio- and Stereoselectivity

Chapter 6: Reactions of Alkenes : Addition Reactions

Regioselectivity

Addition of non-symmetrical reagent to a non-symmetrical alkene, then two isomeric products that are constitutional isomers can be obtained. For example, the reaction of HCl with propene gives 1-chloropropane and 2-chloropropane.

reaction of propene and HCl

Normally, 2-chloropropane is the major product.
Since one product is favoured over the other, the reaction is said to be regioselective.
If 2-chloropropane were the only product then the reaction is said to be regiospecific.

We will talk about this particular reaction in more detail later.

Stereoselectivity

When an alkene undergoes addition, two new s bonds form. If we think of an alkene as having two faces, then the two new s bonds can either both form on the same face, which we call syn addition, or they can be formed on different faces which we call anti addition. Do you remember these terms from Chapter 3 ? A simplified illustration is shown below.

(Note : In the case of an alkene like ethene, the free rotation of the C-C bond means that the two product structures are conformational isomers and rapidly inconvert. However with cyclic alkenes, the two products are stereoisomers).