322as08_e2_key - Chemistry 322aL Name\—==Vvl‘ S lSC(qg...

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Unformatted text preview: Chemistry 322aL Name \—==Vvl‘\ S \\ lSC/(qg ‘(\ Basic Organic Chemistry EXAMINATION 2 K E y March 3, 2008 Answer the following questions in the space provided. Please organize your mougflts 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 N 0 credit (except for rings only). I (25) II (06) III (18) IV (45) v (06) TOTAL (100) Chemistry 322aL Examination 2 page 2 March 3, 2008 I. (25 pts) (a) Give the IUPAC names of the following structures. Be sure to specify stereochemistry, including (R) or (S) where appropriate. (a) ._ CH3— CH2 trans- 5 “ [GU/i “‘ Ll— bVOlMO - E)?- S\ CHrC<ZC_C:Br diva — VlO em CH3 /CH2—CI-{2 CH2— CH2— CH;; (4: \ ' l ClS~ 2R R ~Ctlloro— S /C\ /C\\ /H CH3 [Ca 33 Ll— e ~‘i in '5 ol HO H CHch3 LC) CH2CH3 _, _ L ‘ VOWO ‘ _‘ __ $ @MIIBT Clo (d) neopentyl alcohol (e) sec-butyl bromide 9 CH; 6 W __ c __ 0 043 & CHloH CHBCLJ CH CH3 ”3 (f) isobutyl amine (g) vinyl fluoride CH4; CH3P in CH NH; 04504 P (h) allyl alcohol Cl42:Cl-l — CH]: OH Chemistry 322aL Examination 2 page 3 March 3, 2008 II. (6 pts) Circle the letter of the single best answer for each of the following questions. a) The most stable conformation of 3-bromo-2-methylpentane, viewed through the C-2—C-3 bond (i.e., C-2 in the front, G3 in the back): H 521% H CH3 K “UAW Br CH3 c H CH3 CH3 2C 3 ICH<CH3)2 II CH3 CH3 Br CHZCHa Br H H CH 3 H CH3 H CHZCH3 IV V A) I B) II C) III D) IV @ b). The most stable conformation of 3-bromo-2-methylpentane, viewed through the C-3—C—4 bond (i.e., C-3 in the front, G4 in the back):H CH3 3.1% HAW TEE: CH CH3 CH<CH3)2 ‘ 3” CH<CH3>2 III H CH3 Br CH(CH3)2 Br H H CH 3 H CH3 H cnzcn3 IV v B) 11 C) III D)I E) v V "IVA“, / l Chemistry 322aL Examination 2 March 3, 2008 page 4 III. (18 pts) (a) Designate the relationship between each pair of structures shown below by placing the letter of the correct answer in the blank provided. Letters may be used more than once or not at all. A. identical molecules (superimposable in the conformation/configuration shown by only rotation of the intact molecule in space, if necessary) B. enantiomers C. diastereomers D. conformational enantiomers E. conformational diastereomers F. constitutional isomers H OH E 0H __ OH and H . H H OH H H E. c] and it 31 H H and H CH3 H and CH3 H Br and m H and Chemistry 322d. Examination 2 page 5 March 3, 2008 1” (b) For each pair of compounds below, circle the one which is more stable as it is written. Note that ' the axial/equatorial AG° (“A value”) for CH3 is 7.5 and for Cl is 1.7 kJ/mole. /" H H 2‘ CH3 " CH3 W H mg“ H H3 H C H K CH3 C 3 CI IV. (45 pts) Circle the letter of the single best answer for each of the following questions. a) Which cycloalkane has the least ring strain? A) Cyclopropane B) Cyclobutane Q Cyclopentane QaI/L] Cyclohexane ) Cycloheptane b) Which cycloalkane has the greatest ring strain? @ Cyclopropane ) Cyclobutane C) Cyclopentane D) Cyclohexane E) Cycloheptane Chemistry 322aL Examination 2 page 6 March 3, 2008 c) Which isomer would have the largest heat of combustion? Propylcyclopropane : Ethylcyclobutane C) Methylcyclopentane D) Cyclohexane E) Since they are all isomers, all would have the same heat of combustion. d) Which is the least stable conformation of cyclohexane? A) Chair B) Twist Boat One-half chair E) Staggered e) Which of these Cmng isomers is predicted to be the most stable? OHCOO> I II III H H H fl Iv v A) I B) II C) III DIV V 0 Which of the following is NOT true of enantiomers? They have the same: A) boiling point. melting point. specific rotation. ) density. E) chemical reactivity toward achiral reagents. Chemistry 322aL Examination 2 page 7 March 3, 2008 g) Which compounds do NOT possess a plane of symmetry? CH3 Hm :0 a: 9 CH 3 H30 I V A) 1,11 andV B) I, III and IV II, III and IV III and IV V h) Which statement is not true for a meso compound? A) The specific rotation is 0°. B) There are one or more planes of symmetry. C) A single molecule is identical to its mirror image. @ More than one stereogenic center must be present. The stereochemical labels, (R) and (S), must be identical for each stereogenic center. i) What can be said with certainty if a compound has [a] £01 = -9.25° ? A) The compound has the (S) configuration. The compound has the (R) configuration. @ The compound is not a meso form. ) The compound possesses only one stereogenic center. E) The compound has an optical purity of less than 100%. j) Which of the following is true of any (S)-enantiomer? A) It rotates plane-polarized light to the right. B) It rotates plane-polarized light to the left. It IS a racemlc form. It is the mirror image of the corresponding (R)-enantiomer. ) It has the highest priority group on the lefi. Chemistry 322aL Examination 2 page 8 March 3. 2008 k) Enantiomers are: A) molecules that have a mirror image. B) molecules that have at least one stereogenic center. C) non-superposable molecules. non-superposable constitutional isomers. non-superposable molecules that are mirror images of each other. 1) If a solution of a compound (30.0 g/ 100 mL of solution) has a measured rotation of +15° in a 2 dm tube, the specific rotation is: ; +50° 9&5° ‘ ) +15° D) +7.5° E) +4.0° m) Which of the following reactions is most likely to occur with retention of configuration? A) (R) KMnO4 A2; Alf B) POCI3 (R) /Y “‘“" /Y NH? 0 E) P, C12 cu Cl (S) /ll\:,0H —-—> )firOH o n) One of the stereoisomers of the following molecule is cholesterol. How many chirality centers does cholesterol have? g HO Chemistry 322aL Examination 2 page 9 March 3, 2008 o) The most stable conformation for 1,2-ethanediol (ethylene glycol) is shown below. It is the most stable conformation because: H H OH H OH H A) this corresponds to an anti conformation. in general, gauche conformations possess the minimum energy. @ it is stabilized by intramolecular hydrogen bonding. it is a staggered conformation. E) it has the highest energy of all the possibilities. p) The twist boat conformation is the preferred conformation for this compound. @‘is—l,4-Di-tgr_t—butylcyclohexane : m— l ,4-Di-t_e;_t-butylcyclohexane C) gis-l,3-Di-t_e_r_t—butylcyclohexane D) m4 ,2—Di-t_er_t-butylcyclohexane E) None of these q) What is the percent composition of a mixture of (SH+)-2-butanol,[a] 2—; = +13.52°, and (R)—(-)-2-butanol,[a]2-’5 = -l3.52°, with a specific rotation [0.1% = +6.76°? : 75%(R) 25%(S) fl) 25%(R) 75%(3) ) 50%(R) 50%(S) D) 67%(R) 33%(S) E) 33%(R) 67%(S) Chemistry 322:1. Examination 2 March 3. 2008 r) Which structure(s) represent(s) diastereomer(s) of 1? CH3 CH3 H OH HO OH HO H Ho H OH HO H OH H OH CH3 CH3 1 11 111 IV . 11 \[email protected] 11 and 111 11 and IV D) 111 and IV B) All of them s) The device that is used for measuring the effect of optically active compounds on plane- polarized light is called a Poiar {Mé \‘6 d‘ t) True or false: A sample consisting of the pure R enantiomer of a compound will always rotate plane-polarized light in a clockwise direction. ‘ True False u) An equimolar mixture of two enantiomers is called a mm \ C WI \ XJY U F6 / L, :25:@ v) Circle the number of any of the following which are ways that have been commonly used page 10 to obtain optically pure enantiomeric forms of organic molecules. @lsolation from natural sources such as plants, animals or microorganisms. B) Manual separation of enantiomeric crystals of a crystalline racemic mixture (Pasteur’ s method). @Separation of diastereomers made from a racemic material and an optically pure “resolving agent,” then chemical separation the pure enantiomers from the resolving agent. D) Crystallization from chiral but racemic solvents. Conversion of achiral molecules into enantiomeric ones by catalytic transformations using optically pure catalysts (catalytic asymmetric induction). The catalysts may be man-made, or natural enzymes. Chemistry 322aL Examination 2 page 11 March 3, .2008 V. (6 pts) We have just learned in lecture about the S N2 reaction and the fact that it proceeds with inversion of configuration. The two reactions below are typical of the kind of evidence used to support this generalization. H SCH3 CH CH l 3 Br CH-asH l 3 H ______. 0 CH5? CH5? CH3 A CH3 3 Br H CH3 , H CH3- - a: H_c_§__, lmLHH mm C CH§7 CH3 CH3 D In one short sentence, why is the reaction of A more important than the reaction of C in claiming inversion? Hmmm{w will fifiWMM%Wg+MPVM In one short sentence, why is the use of the t~butyl group important in this experiment? T} to w 112/32 Di W W I (0% “from mo 50 H1 (7 I n3 Cam JL “ SUP” COW $0M“ mi \OVIS ...
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