Organic Chemistry Chapter Eight Introduction to Nucleophilic Substitution Reactions Critical Thinking Questions (The harder ones are marked with a .) 1. What determines whether 2-bromobutane undergoes SN1 and/or SN2 reactions? * 2. 1-Bromoadamantane is a tertiary halide, yet it is 10,000 times slower than tert- butyl bromide when reacting with silver nitrate in ethanol. Consider the structure of 1-bromoadamantane and explain this observation. (Hint: Consider the geometry of a carbocation. It might be helpful to build a model of 1-bromoadamantane.) 3. On the same graph, draw reaction coordinate diagrams for the nucleophilic substi- tution reactions of sodium iodide with: Br adamantane 1-bromobutane. a. b. 2-bromobutane. 4. On the same graph, draw reaction coordinate diagrams for the nucleophilic substi- tution reactions of ethanol with: 2-bromobutane. a. b. 2-bromo-2-methylpropane. 5. Benzyl bromide reacts rapidly with sodium iodide in acetone to yield a white precipi- tate. Benzyl bromide also reacts rapidly with ethanol and silver nitrate. Bromoben- zene does not react with sodium iodide or with ethanol. Explain these observations. Br CH,Br bromobenzene benzyl bromide * 6. Allyl bromide is a primary alkyl halide, yet it undergoes rapid reaction with silver nitrate in ethanol. Explain. The structure of allyl bromide is shown here. Н.С -СНСH,Br 7. Are alkyl fluorides good substrates in SN1 or SN2 reactions? Explain. 8. What are the characteristics of a good leaving group? 9. Why doesn't the nitrate ion react to give SN1 substitution products? * 10. A student measures the dry weight of AgBr produced in the SN1 reaction of 2- bromo-2-methylpropane and calculates the moles of AgBr formed. The student proposes that the amount (mol) of tert-butyl ethyl ether formed via Sy1 reaction will be the same as the amount (mol) of AgBr formed. Critique this conclusion. 11. Molecular modeling assignment: SCN is an ambient nucleophile, capable of bonding at either the sulfur or the nitrogen end of the molecule. Construct a model of this molecule and examine the electrostatic potential map of this anion. Which atom is the most electron-rich? What product would be obtained from the reaction of ethyl iodide with this nucleophile? 12. Molecular modeling assignment: 1-bromo-3-chloropropane can potentially react with a nucleophile at two different sites. Construct a model of this molecule and examine the electrostatic potential map of this electrophile. Which site has the greater positive charge? Now examine the lowest occupied molecular orbital (LUMO) of the electrophile. Which site is most likely for nucleophilic attack? What product would most likely be obtained from the reaction of 1-bromo-3- chloropropane with sodium azide (NaN3)?