Biological substitution reactions

Predict whether each of the following substitution reactions is likely to be S l or S m2  

Following this initial SnI reaction, loss of the pro-R hydrogen gives geranyl diphosphate, itself an allylic diphosphate that dissociates a second time. Reaction of the geranyl carbocation with water in a second SnI reaction, followed by loss of a proton, then yields geraniol. 

7 Elimination Reactions of Alkyl Halides Zaitsev s Rule 383 

Test your knowledge of Key Ideas by using resources in ThomsonNOW or by answering end-of-chapter problems marked with 4. 

Strategy Look at the substrate, leaving group, nucleophile, and solvent. Then decide from the 

Solution (a) This is likely to be an SnjI reaction because the substrate is secondary and ben-zyiic, the nucieophiie is weakly basic, and the solvent is protic. 

Two Sisjl reactions occur during the biosynthesis of geraniol, a fragrant alcohol found in roses and used in perfumery. Geraniol biosynthesis begins with dissociation of dimethylallyl diphosphate to give an allylic carbocation, which reacts with isopentenyl diphosphate (Hgure 11.15). From the viewpoint of isopentenyl diphosphate, the reaction is an electrophilic alkene addition, but from the viewpoint of dimethylallyl diphosphate, the process in an S l reaction in which the carbocation intermediate reacts with a double bond as the nucleophile. 

Sim2 reactions are involved in almost all biological methylations, which transfer a -CH3 group from an electrophilic donor to a nucleophile. The donor is usually S-adenosylmethionine (abbreviated SAM), which contains a positively charged sulfur (a sulfonium ion Section 5.10), and the leaving group is the neutral S-adenosylhomocysteine molecule. In the 

FIGURE 12.16 Biosynthesis of geraniol from dimethylallyl diphosphate. Two Sni reactions occur, both with diphosphate ion as the leaving group. 

---