Alkene 5:

Problem

Solution

This hydroboration of (E)-3-methyl-2-pentene gives the racemate 3-methyl-2-pentanol. Two important features of this reaction are: 1) only a single diastereomer is formed, 2) only one regioisomer is formed. The B-H bond of BH₃ adds to one face of the double bond. In the oxidation step, the C-B bond is replaced by a C-O bond with retention of stereochemistry. Thus, item 1) is satisfied. In the hydroboration step, the boron adds to the less substituted end of the double bond and the hydrogen, consequently, to the more substituted end. When (Z)-3-methyl-2-pentene 2 undergoes the same set of reactions, racemic 3-methyl-2-pentanol 3 is formed; however, it is a diastereomer different from 1. Structure 4 differs from 2 by rotation about the C₂-C₃ bond. Convince yourself. Knowing the structure of the products in the hydroboration of these two alkenes, tells us the structure of the alkenes from which they came. This is not the case with the ozonolysis of these two olefins. See Problem 4 in Alkenes. For more on the the mechanism of hydroboration of 1 and 3, click here.