c) circle the carbon in the cyclic mercurinium ion in part (a) above that will be attacked by the nucleophile. Why might that carbon be more attractive to the nucleophile? 10. The second step in Model 3 is called demercuration. What bond is being broken and what bond is being formed on the molecule shown? (we will not cover the mechanism for this step). 11. Once your group has reached agreement on the above quesitons, discuss what product might result if the reaction shown in Model 3 was done using CH OH as a solvent instead of H20. by Suzanne Ruder PhD Model 4. Hydroboration H Hea BHI + HC -H Hd HCM Questions 12. (a). In the Lewis structure of borane (BH) above, how many valence electrons does boron have? (b). The electronegativity value for H = 2.3 and for B = 2.1. Which is more electronegative, (circle one) B / H? (c). Draw arrows on the structure for BH, above to show the dipole of the B-H bond. (d). Which atom of BH, is more likely to act as the electrophile? 13. In the mechanism for hydroboration, the B and H add to the alkene at the same time (concerted shown below). Although a full carbocation is not formed, the transition state shows that a partial positive charge develops on the carbon that is not bonded to the electrophile, boron BH Home 8+ HC H.CH HC (a). Why does the addition occur to give the regiochemistry shown above? (consider the stability of partial positive charge) (b). According to this mechanism, does addition occur SYN OF ANTI? (c). Hydroboration is stereospecific because during the first step the boron and hydrogen are added at the same time to the double bond. Would you expect addition to occur from the top face, the bottom face, or both faces equally