organic chem I E2

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Topics include: stereochemistry, reactions (thermodynamics and kinetics), alkyl halides, substitution reactions (SN1 and SN2), elimination reaction (what is a general product only), and nomenclature of alkenes.

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CHE 236 Org Chem I Summer 2019 Exam 2 Name ____KEY__________ 100 pts True/False: indicate which of the following statements are true or false. Correct all False statements so that they are TRUE. (2 pts each, total 18 pts) _F__ 1. An increase in temperature will result in an increase in the activation energy (Ea) of a reaction. More molecules will have enough energy to overcome the activation energy. __ F ____ 2. Iodide Fluorine is a better leaving group than fluoride iodine. __ T ___ 3. Methoxide is a better nucleophile than t-butoxide. __ F ___ 4. The cation formed by methane is less more stable than the cation formed on carbon 2 of 2methylpropane. __ F ___ 5. A meso compound does not have has a non-superimposable mirror image. _ F __ 6. Thermodynamics Kinetics is the study of the energy need for a reaction to occur. _ T ___ 7. All Lewis acids are electron deficient and can be called electrophiles. __ F ___ 8. Numerous halide compounds have been used to make life better, they are not all safe. __ T __ 9. Polar protic solvents increase nucleophilicity going down the table due to size being more important than basicity. Multiple Choice: Pick the best answer (2 pts each) 1. Label each pair of compounds as constitutional isomers (C), stereoisomers (S), or not isomers of each other (N). A) a: N, b: S, c: N, d: C B) a: C, b: N, c: S, d: C C) a: S, b: S, c: N, d: C D) a: N, b: S, c: C, d: N 1 2. The [a] of pure D-glyceraldehyde is +14o. Calculate the ee of a solution with [a] = +6.0o. +6 =x(14) A) 4.3% B) 8% C) 43% D) 57% D -glyceraldehyde 3. Splenda(tm) is an artificial sweetener made form Sucrose by addition of chlorine. Why is this chorinated compound water soluble? A) The Cl-C dipoles cancel B) Polar Cl readily forms H bonds C) Polar OH groups H bond to water D) Chlorine lowers the melting point 4. The C-O bond is very polar (more polar than the C-Cl bond) so carbon should be very polarized in alcohols which possess a C-O bond. Yet substitution of the -OH group in alcohols is NOT favored. Why? A) Oxygen forms a very strong C-O bond that is not easily broken. B) Oxygen is hydrogen bonded. C) The hydroxy leaving group is too basic. D) Nucleophiles react with the OH group instead preventing substitution 5. Which compound below CANNOT undergo β-elimination? Remember that all carbons β to the alkyl halide carbon must have an H on it A) CH3CH2C(CH3)2CH2Br C) (CH3)3CBr B) D) CH3CH2CH2Br 6. Which of the following is a polar protic solvent? A) CHCl3, chloroform B) (CH3)2NCHO, dimethylformamide C) CH3CHOHCH3, isopropyl alcohol D) (CH3)2SO, dimethyl sulfoxide 2 (CH3) 3CCH2Br E) Both a and b 7. A transition state A) is a well defined molecular species that is long lived (microseconds). B) is an unstable high energy species which is very short lived. C) is formed in a rapid equilibrium between a nucleophile and an alkyl halide. D) is formed irreversibly and must progress on to form a product. 8. Which of the nucleophiles below is the weakest Brønsted-Lowry base? A) OH- B) H3N C) CN- D) I- Weakest base will have the strongest acid: H2O pKa ~16, NH4+ pKa ~9, HCN pKa ~9, HI pKa ~-10 9. The E1 reaction shares which characteristics with the SN1 reaction? A) a common carbocation intermediate B) the first step is endothermic C) the first step is exothermic D) both a and b E) both a and c 10. Which dihalide would not produce 2-methyl-3-hexyne? IV 11. What is (are) the starting material(s) in the reaction below? 3 I 12. Consider the following E2 reaction. What rate equation would be observed for this reaction? A. rate = k[CH3CH2CH2Br] B. rate = k[CH3CH2CH2Br][KOC(CH3)3] C. rate = k[CH3CH2CH2Br][KOC(CH3)3]2 D. rate = k[CH3CH2CH2Br]2[KOC(CH3)3] Short answer (62 pts) 1. (16 pts) Nomeclature A. Name the following molecules (include any stereochemistry) (2S,3R)-2,3-dichloro-3-methylpentane 3-bromo-4,4-dimethyl-1,5-hexadiene B. Draw the following molecules (1R)-1-bromo-1-cyclopentylethane (3Z)- 2,4,6-trimethyl-3-octene 4 2. (4 pts) Draw the products of homolysis or heterolysis of each labeled bond. Classify each carbon reactive intermediate as a radical, carbocation, or carbanion. a. Homolysis of b. Heterolysis of 3. (10 pts) Locate the sterogenic centers in the following molecules. What is the maximum number of stereoisomers possible? Draw all possible isomers. Label each pair of enantiomers or diastereomers. Are any isomers meso, if so why? Max isomers: 22 = 4 Enantiomers: A&B, C&D Disatereomers: A&C, A&D, B&C, B&D CH3CHClCHClCH3 Max isomers: 22 = 4 Enantiomers: A&B, Meso due to plane of symmetry C Disatereomers: A&C, B&C 5 4. (5 pts) Draw the mechanism for the SN1 reaction of 2-bromobutane with CH3CH2OH. 5. (7pts) Use the molecule below to answer the following questions, feel free to redraw the molecule if your answer becomes too messy. a. Label the most electrophilic carbon in the molecule with an asterisk. b. Predict the substitution product of the molecule using ethanol (CH3CH2OH) as a reagent and solvent. What mechanism is occurring and are there any stereoisomers? Be sure to draw all possible products. SN1 reaction, 3o carbon and weak nucleophile, there will be stereoisomers, they are diastereomers, no change of stereochemistry at the non-substitution sites c. Identify the β hydrogens that would be deprotonated in an elimination reaction. d. Assume the elimination reaction proceeds via an E2 mechanism draw the products and give an explanation as to why the products are the ones that will result from the reaction. For an E2 reaction to occur the Cl and H must be anti, therefore the expected major product using Zaitsev’s rule will not occur, the Cl and H are not anti 6 6. (10 pts) Analyze the reactant(s) and reaction conditions, predict the predominant mechanism (SN2, SN1, E2, or E1) and the product(s) (remember stereochemistry) 3o Carbon Weak nucleophile, E1 or SN1 Strong base or nucleophile E2 7 7. (6 pts) Using the energy diagram answer the following questions. A. How many steps are involved in this reaction? Two steps B. Label ΔHo and Ea for each step, ΔHo for the overall reaction, transition states and intermediates. C. Which step is the rate determining step? Step 2 D. is the overall reaction endothermic or exothermic? endothermic TS Ea2 TS Interm ΔHo2 ΔHo overall Ea1 ΔHo1 8
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