322as08_e3_key

322as08_e3_key - Chemistry 3228L Name l E @r n Basic...

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Unformatted text preview: Chemistry 3228L Name l E @r n Basic Organic Chemistry March 31, 2008 EXAMINATION 3 Answer the following questions in the space provided. Please organize your thoughts and attempt to make your answer as legible as possible. If we cannot read your answer, it cannot be given credit. Please remember that C-C-C-C without hydrogens and “stick” structural representations will be given N0 credit (except for rings only). 1 (38) H 08) 111 (18) IV (28) EC. (08) TOTAL (110) Chemistry 3222L Examination 3 March 31, 2008 page 2 I. (38 pts) (a) Circle the molecule that reacts fastest with Kl in DMF. CH3 CH3 (b) Circle the most stable alkene with respect to its corresponding alkane hydrogenation product. That is, which alkene has the smallest enthalpy of hydrogenation? 9”“ (c) What does it mean to say a reaction is “under thermodynamic control”? Th 9 NM JUIMAJU—Q Jr/vav c3} fin Bro W (d) What does it mean to say a reaction is “under kinetic control”? div! 9% OTs OTs CH3 CH3 %ui lilovallzm (e) What is the order of rates of dehydration of alcohol with respect to 1", 2°, and 3° structure? In one sentence explain why. 3°>2°71°4m VBOIJ I‘m W 3° 72° > 1" RG) M ‘ ® AWmfluflah Chemistry 322aL Examination 3 page 3 March 3], 2008 (1‘) What is the order of rates of dehydrohalogenation of alkyl halides in base with respect to 1°, 2°, and 3° structure? In one sentence explain why. Elammmm m EZ)M mi» mu g°>1°>l W Cl ‘3 flu M I W 399% (Namath) WW WWW mm fli-l’s) (g) Increasing the temperature of a chemical reaction usually increases greatly the rate of the reaction. The most important reason for this is that increasing the temperature: A) Decreases the collision frequency. Bi Decreases the probability factor. ) Increases the fraction of collisions with energy greater than Em. Decreases the energy of activation. E) Makes the reaction more exothermic (h) Elimination reactions are favored over nucleophilic substitution reactions: A) at high temperatures. ‘/ / B) when tert-butoxide ion is used. C) when 3° alkyl halides are used as substrates. '/ D) when nucleop‘yles are used which are strong bases and the substrate is a 2° alkyl halide. @ in all of these cases. (i) The hybridization state of the charged carbon in a carbocation is A) sp4 B sp3 sp2 ) sp E)s Chemistry 3228L Examination 3 page 4 March 31, 2008 (j) Which is a true statement concerning the transition state of the rate-determining step of an SNl reaction? Structurally, it closely resembles the carbocation intermediate. ) Both covalent bond-breaking and bond-making are occurring. C) Formation of the transition state is an exothermic reaction. D) Necessarily, the transition state has zero charge overall. B) More than one of the above (k) Select the potential energy diagram that represents a two—step endothermic (endergonic) reaction. PE /{ C) 111 D) IV E) v (1) The Hammond postulate when applied to nucleophilic substitutions and elimination reactions states that: A) a negatively-charged nucleophile is stronger than its conjugate acid. B) polar aprotic solvents strongly accelerate the rate of 8N2 processes. bimolecular nucleophilic substitutions are 2nd order kinetically. the transition state for an endergonic reaction step (one accompanied by an increase in free energy) structurally resembles the product of that step. E) elimination reactions will always compete with nucleophilic substitution reactions. Chemistry 322aL Examination 3 page 5 March 3|, 2008 (m) The stabilizing effect of alkyl substituents on carbocations can be explained through what two effects? (Just name them.) (l) percowsu aim/i (V®sonav1€@\ CD :‘éAchnuegd‘onailon (polariiakili4753i Carbon (n) Select the potential energy diagram that represents an exothermic (exergonic) reaction. A) I B) 11 C) 111 IV V (0) Which will be true for M nucleophilic substitution reaction? A) AH° is positive. B AH° is negative. AG: is positive. ) AG° is positive. E) AG° is negative. (p) When an elimination reaction gives the less substituted alkene as the major product, we say that the major product is the B (i) ima X I product. Chemistry 322aL Examination 3 page 6 March 3|, 2008 (q) Select the potential energy diagram that represents a single-step endothermic (endergonic) reaction. PE ) 11 C) 111 D) IV E) v (r) The difference in the bond energies of reactants and the transition state of a reaction is designated by the notation: A AH° t3) AHI C) AG° D) AGI E) AsI Chemistry 3223L Examination 3 March 31, 2008 page 7 (s) Zaitsev’s rule states that: A) In electrophilic addition of an unsymmetrical reagent to an unsymmetrical alkene, the more positive portion of the reagent will become attached to the carbon of the double bond bearing the greater number of hydrogen atoms. B) An equatorial substituent in cyclohexane results in a more stable conformation than if that substituent were axial. C) E2 reactions occur only if the B-hydrogen and leaving group can assume an anti- periplanar arrangement. @ When a reaction forms an alkene, and several possibilities exist, the more (or most) stable isomer is the one which predominates. E) The order of reactivity of alcohols in dehydration reactions is 3° > 2° > 1°. 11. (18 pts) Provide structures for the organic product(s) formed in significant amount in the following reactions. Be sure to show regiochemistry and/or stereochemistry when appropriate, including R, S, E, and/or Z designations if warranted. Please remember that C—C-C-C without hydrogens and “stick” structural representations will be given no credit (except for rings or structures that are given in stick form). (a) H WOH conc. HBr :4 H 3 cool 74A + \ \ w) CH3 CH3§C_Br + CH3CEC(‘)N3(+) I. . _, i‘ CH3 '33: (543 l J. CH? (0) CH3 / HEC‘B’ —+KCN N’in—C'mH Jr “3N _C’”(H‘i’ CH3CH2 (R) DMSO | /Q‘ “A (Show the two most likely products.) )1 \CH3 < a cubm = on CHB) ) Chemistry 322aL Examination 3 page 8 March 3!, 2008 3 a( L | NaOEt /\ aCH—Cl—lj O/Clcha EtOH,Tl 1— QCM: Cfilt Show only the main (elimination) product. _ M if «’40 71% 07):: III. Chemistry 322aL March 3], 2008 Examination 3 page 9 (18 pts) Provide reasonable syntheses (i. e., minimum number of steps, good yields, good regioselectivity, stereoselectivity, stereospecificity, etc.) for the molecules specified below from the indicated starting materials and any other needed solvents or reagents (organic or inorganic). Be as specific as you can regarding reagents and conditions. Show the product formed from each important step of your syntheses. You can use any intermediate from one synthesis in other syntheses if a r0 riate. pp p CHz—Br 0%chon + Er --------------- --> OCH20H20H2<> m / CECQMQQ /\ t.1 ._ \ L/ CIC H H Br ./>u5cud—0Ts 0 ll 2 3 WP 2) C] \ Roma Meat) or 2 1450* {mm /\‘ 0M” (b) O—CHZCHZOH + (c) O—CHZCHZOH + /\- V / CH:CH2 Q / C=C 2> 0‘” Hsol @ CI Hz mg m ’5 ch @ *[Pol Haws ' gu""°l“"i Chemistry 322aL Examination 3 page 10 March 3], 2008 IV. (28 pts) Complete the following table of characteristics and variables related to substitution and A elimination chemistry. ' le sNz E. E W? 2 me expression (rate = k[ ‘27? ) 1;; if {ex—j” l§;;;:k[ Mimi] jme = k [a x] = kmq [ij Number of steps in mechanism :2 l ill 2 J1 \ Stereochemistry: SN: Change in configuration at reaction center E: Any conformational requirements mam [mi )bn {Li \\\1 / Tillman»; 93/“ /V C0 Chemistry 322aL Examination 3 page ll March 31, 2008 Extra Credit (8 pts) (a) Write a detailed mechanism for the following reaction. The mechanism should show all intermediates and indicate the direction of charge flow using the curved arrow formalism. CH3 CH3 CH1 ——-——» [i1 t l t' CH caaylc \\\\ H2304 /'\ L/\| r L/oLQ’ " / IJ/g H2. 45 T / m) w (b) Neopentyl triflate and neopentyl-d9 triflate, where the three methyl groups have the heavy isotope deuterium in place of hydrogen, are heated in formic acid (a highly polar protic solvent) to cause an elimination reaction. The rate constant for the non-deuterated (do) CH CIJH3 substrate is measurably larger than 3>C\ (I? HCOZH Otis /C H3 that of the dg material (i.e., do reacts CH3 CHz—O—fi-CFs ——> /C=C\ faster than d9). 0 CH3 H This rate difference is called a kinetic CD $03 isotope effect. What can you conclude 3>0\ (3 HCOZH CR3 /C D3 from the isotope effect about the CD3 CHz—O—S—CF3 ——> ::C\ likelihood of a primary carbocation (“3 C63 H being an intermediate in this reaction and why? k(CH3)3CCH20Tf > k(CD3)3CCH20Tf ‘ - ‘ (flic—> Q RD S m {341 alum} -——7 {all CHZG) + Jam gamma Mn Wire» / No 1° ‘36») 043,— a—c H2 —_oTJ; BMW ...
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322as08_e3_key - Chemistry 3228L Name l E @r n Basic...

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