PLEASE DO NOT OPEN THIS EXAM UNTIL YOU ARE INSTRUCTED TO DO SO.

Transcription

1 CEMISTRY 0 :5 AM Section EXAM 2 Nov 2009 Name: Note: Your exam should consist of 5 pages including the cover page and grade tabulation sheet. Skim the entire exam, and solve the easiest problems first. Exams not returned when time is called will not be graded. PLEASE D NT PEN TIS EXAM UNTIL YU ARE INSTRUCTED T D S.

2 . (6 pts) Provide the structure of the expected major product(s) in each of the following reactions. You must clearly show stereochemistry where appropriate. a. o strong K, Me Δ or b. 2, Pd c. o NaCN, acetone NC d. weak K, Me 2 o 2. (2 pts) Design an alkyl halide reactant that will give the 2,4-diphenyl-2-pentene (shown below) as the only product upon treatment with potassium t-butoxide. Show stereochemistry where appropriate.? KtBu tbu or The β-hydrogen and leaving group are shown anti, giving the correct stereochemistry of the alkene product. This consitutional isomer won't give the correct product because the bulky base will remove the most accessible β-hydrogen from the methyl group. 2

3 . (2 pts) The following reaction is called the pinacol rearrangement. Propose a reasonable curved arrow mechanism to account for the product. C +, 2 Δ C catalyst regenerated C C C C 4. (8 pts) Draw the structures of the following alkenes: (E)-2-pentene, (Z)-2-pentene, 2-methyl-2-butene, -pentene. Match the alkenes with the following heats of combustion: kcal/mol, kcal/mol, kcal/mol, kcal/mol (4 pts) When (S)-2-bromo-2-fluorobutane reacts with sodium azide, the major product is (S)-2-azido-2-fluorobutane. Na (a) iefly explain why no 2-azido-2-bromobutane is formed. (b) Draw a mechanism for the reaction (S)-2- bromo-2-fluorobutane plus sodium azide to give (S)-2-azido-2-fluorobutane. as this reaction occurred with retention or inversion of configuration? iefly explain. (a) - is a weaker base and better leaving group than - (b) - S 2 The mechanism is SN2. Inversion of configuration does occur. It turns out that the priorities of the groups around the chirality center result in S configuration for both the reactant and product. 2 S -

4 6. (2 pts) Shown below are three proposed methods for synthesizing isopropyl methyl ether using substitution chemistry. Circle the letter of the preferred method. iefly explain what is bad about the nonpreferred syntheses. (a) Na preferred / not preferred because: Secondary RX. Strong base. E 2 elimination would be the major reaction. (b) Methyl RX. Weak base. A slower SN 2 reaction would occur. Elimination not possible. Na (c) Methyl RX. Strong base. A faster SN 2 reaction would occur. Elimination not possible. Note: is the leaving group in all three scenarios. 7. (0 pts) Propose a synthesis of pentane starting with,-dibromopropane. You may use any other needed reagents. More than one reaction may be necessary. Mechanisms are not required. 2 NaN 2 liq NaN 2 liq 2 Ni 8. (6 pts) Answer the questions below for the reaction shown to the right. Na Et a. Write a kinetic rate expression for the reaction: rate of product formation = tertiary RX, must be SN, first order rate = k b. Draw a labeled reaction coordinate for the reaction. You may assume that the products are lower in energy than the reactants. G TS TS2 This SN mechanism has two steps. - - c. Will the rate of the reaction change if some water is added to the solvent? Explain. Be specific. reaction coordinate SN reactions are faster in polar protic solvents because the developing charges in the RDS are more stabilized. The solvent shown above is ethanol, a polar protic solvent. Adding water should increase the rate of the reaction; it is also polar protic, but it is more polar than ethanol and has more sites for hydrogen bonding. 4

5 CEMISTRY 0 :5 AM Section EXAM 2 Nov 2009 Name: Page Points Score Total 00 5