Orientation in Addition to Alkenes

Exercise 10-12 The hydration of fumaric acid catalyzed by fumarase in D20 leads to malic acid with only one C-D bond, which is selectively removed when malic acid is enzymatically reconverted to fumaric acid. The configuration of deuteriomalic acid prepared in this way has been shown to correspond to the following projection formula  

Deduce the stereochemistry of both the forward and backward reactions (hydration and dehydration) from this information. 

Addition of an unsymmetrical substance such as HX to an unsymmetrical alkene theoretically can give two products, 

10 Alkenes and Alkynes I. Ionic and Radical Addition Reactions 

To understand the reason for the pronounced selectivity in the orientation of addition of electrophiles, it will help to consider one example, hydrogen bromide addition to 2-methylpropene. Two different carbocation intermediates could be formed by attachment of a proton to one or the other of the doublebond carbons  

---