1. Which statement is not true of nucleophillic substitution reactions?!

Transcription

1 Mock Exam 3 CH 235-2F Ch. 7, 8, 9, and 11 Multiple Choice: 1. Which statement is not true of nucleophillic substitution reactions? a. SN1 proceeds through a cation intermediate. b. SN2 occurs with inversion of stereochemistry. c. Rates of both SN1 and SN2 increase with higher concentration of the nucleophile. d. Rate of both SN1 and SN2 increase with higher concentrations of the nucleophile. e. Rates of both SN1 and SN2 increase with higher concentrations of RX. 2. Which alkyl halide would react the fastest under substitution conditions with HO-? a. 2-bromo-2-methylpentane b. 2-bromo-3-methylpentane c. 1-bromo-2,2-dimethylpentane d. 2-bromopentane e. 1-bromo-4-methylpentane 3. Why are polar protic solvents so effective at promoting SN1 reactions? a. They are effective at stabilizing cations and anions equally well. b. Only the negative ends f their dipole moments participate in salvation of ions. c. They readily for hydrogen bonds. d. They solvate cations effectively but not anionic nucleophiles. e. Both a and c a. 10 b. 9 c. 7 d. 5 e Predict the major organic product of the following reaction. CH3CH2O - 4. How many different carbons are present in the molecule shown below? CH3CH2OH

2 a. b. c. d. e. 6. Which reaction is faster (I) or (II) and what is its mechanism? + CH3O - Na + DMF + Br - + CH3S - Na + + Br - a. reaction 1, SN1 b. reaction 2, SN2 c. reaction 2, SN1 d. reaction 1, SN2 e. No correct answer 7. Choose the order that has the following compounds arranged in order of increasing (least to most) reactivity under SN1 conditions. a. I < III < II b. I < II < III c. II < I < III d. III < II < I e. III < I < II 8. Based on the reaction shown below, which solvent would yield the fastest reaction? + CH3OH a. 100% CH3OH b. 50% CH3OH: 50% CH3CH2OH c. 50% CH3OH: 50% H2O d. 75% CH3OH: 25% H2O e. 25% CH3OH: 75% H2O DMF

3 9. What is the reaction product of the SN1 reaction of water with (S)-3-bromo-3- methylhexane? a. (R)-3-methyl-3-hexanol b. (R) and (S)-3-methyl-3-hexanol, but R configuration dominates c. (R) and (S)-3-methyl-3-hexanol are formed in equal amounts. d. (R) and (S)-3-methyl-3-hexanol, but S configuration dominates e. (S)-3-methyl-3-hexanol 10. Which of the following is a true statement? a. Strong bases do not share their electrons as well as weak bases. b. The weaker the base, the better it is as a leaving group. c. The more electronegative the group, the better it is as a leaving group. d. The stronger the base, the better it is as a leaving group. e. The smaller the group, the better it is as a leaving group. 11. Why is the IR absorption for the stretching motion of internal triple bonds rarely observed? a. They do not form cations. b. The force constant for the stretching of a triple bond is too large. c. Triple bonds are too strong. d. The stretching of an internal triple bond does not change the dipole moment. e. No correct answer. 12. According to Hammond s Postulate, which of the following statements is correct? a. All transition states are more reactant like than product like. b. The transition state of an endothermic reaction step will be more reactant-like than product-like c. All transition states are more product-like than reactant-like d. The intermediate of an endothermic reaction step will be more reactant-like than product-like e. The transition state of an endothermic reaction step will be more reactant-like than product-like

4 13. Which of the following represents a pair of tautomers? I II a. I only b. II only c. III only d. Both II and III e. All of the above 14. What is (are) the products of the following reaction? III NBS CH2Cl2, hv I II III IV a. I b. II c. III d. I and IV e. I and III

5 15. Which of the following alkanes does not undergo radical bromination to form a single monobromoalkane as the major product? a. I b. II c. III d. IV e. More than one 16. Which of the following is not a true statement? a. A triple bond is shorter than a double bond. b. A triple bond consists of two sigma bonds and one pi bond. c. Disiamylborane, (Sia)2BH, is used as a reagent for hydroboration of a terminal alkyne. d. An electron in a sp orbital is (on average) closer to the nucleus than an electron in a sp2 orbital. e. 1-butene is more acidic than butane. 18. A mixture of 1-heptyne, 2-heptyne, and 3-heptyne was hydrogenated in the presence of a Pt catalyst. If one assumes that the hydrogenation went to completion, how many distinct seven-carbon alkanes are produced? a. 1 b. 2 c. 3 d. 6 e. 8 Short Answer: 1. Provide the structures for A-E in the scheme below. Show correct stereochemistry (geometry) where appropriate. I II III IV A + B HBr H2O2 C A Lindlar s Catalyst H2 D NaNH2 NH3 (l) 1) OsO4 B 2) NaSO3, H2O E

6 2. For each pair of reactions below a) indicate which reaction would be faster and b) briefly explain your choice. CH3CH2OH a. (CH3)2CHCH2Cl + N3 - (CH3)2CHCH2N3 + Cl - CH3CH2OH (CH3)2CHCH2I + N3 - (CH3)2CHCH2N3 + I - b. CH3CH2Br + OH - CH3CH2OH + HBr CH3CH2Br + COOCH3 - CH3CH2COOCH3 + HBr c. CH3CH2CHCH3CH2Br + CN - CH3CH2CHCH3CH2CN + Br - CH3CH2CH2CH2Br + CN - CH3CH2CH2CH2CN + Br - d. CH3I + OH - H2O CH3OH + I - H2O CH3I + SH - CH3SH + I - H2O e. CH3CH2Br + SH - CH3CH2SH + Br - DMF CH3CH2Br + SH - CH3CH2SH + Br -

7 3. The following questions pertain to the two molecules shown below: (25 pts) a. Draw the chair conformations for both molecules above. b. For each molecule, indicate which chair conformation (if either) is more stable. c. Indicate which chair conformation for each compound will readily undergo E2 elimination. d. Draw the alkene(s) which will be formed when the chair conformations you chose in part c are reacted with a strong base under E2 conditions. e. Which molecule shown above (if either) will react faster under E2 conditions? Briefly explain your answer.

8 4. Match the compounds A-D below to the correct IR spectrum. (20 pts)

9 5. Complete the following synthesis problems.

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