Chem3A_Sp02_Mt1_Frechet

Chem3A_Sp02_Mt1_Frechet - Chemistry 3A — Spring 2001...

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Unformatted text preview: Chemistry 3A — Spring 2001 Midterm 1 I a— Professor Jean Fréchet Your full signature Pl February 28, 2002 Print your full namg“! L (Last name, First name. Middle) Your 8ng I I Please check the section number and name of your GSi/T A. 161 Margot Pauliok 311 David Barry 171 Danielle Dube 321 Christina Brown _ 181 Matt Pratt _ 461 Aaron Stutz 191 Laurie Schenkel ___ 471 Alex Kollias 111 Cathleen Yung 411 Andrew Chi 121 Priya Sonik 421 Shahed Ghoghawala 131 Paul Furuta 431 Mike Slater 141 Zach Fresco 561 Jean Han 261 Aaron Stutz 571 Ognjen Miljanic A 271 Vanessa Sun 511 Joe Kwon 211 Jamey Kain 521 Catherine Chan 221 Laura Anderson 531 Olga Fedin 361 Reema Thalji 541 Jason Serin 371 Warren Wood If you are making up an l-grade, indicate the semester you took 3A _ D" “0t “The i" this mm and the Professor L 1. 15) This exam has 10 pages; make sure that you have them all. Please be sure to use the very useful data given on page 10. We will only grade answers that are in the designated spaces. Please do your scratch work on the backs of the exam pages. Write only one answer to each problem; multiple answers will receive no credit, even if one of them is correct. Note: This examination runs for a total of 90 minutes. No questions will be answered by proctors after the exam begins. Please write legibly; ambiguous or messy answers will receive no credit. © 2002 J.M.J. Fréchet. Berkeley, CA @2002 J. Frechet, Berkeley Chem. 3A 2002 Midterm Exam #1 Page 2 of 10 1- ( 15 Paints) (a) The elemental analysis of an unknown alcohol reveals that it ‘ contains 50% oxygen, 37.5% carbon. and 12.5% hydrogen. What is its empirical formula L)(show your calculation). Use the following atomic weight values: 0 = 12. H =1. 0 = 16. _ | , "-l ‘I' ‘. C. I qt. ., {7.13. I -. I- ‘ r.' -:'- _{j ., , . k— , u I “l ii a | l | ['7 Y ’ - L l I I i . r' , J {I l l l “l - 1 r h - - . , a i L ' r= / , T 1 C “a U '1‘ C‘qkl f I! ‘ l i H j I" . Answer: C H ((0 (b) Write Kekule structures for three resonance forms of [CH2NCH2]+ Show the location of @the charge. circle the resonance form that contributes most and explain your choice. 1 \ , J— _ \x _ l \ 4‘ / i :._ 7&3: L I. g 7 F- _ r- / a / h N C . . 3/ ll . / tr / ( \ Li M H“ P ' ONTi—i and; II . I 5".”- l; I' " (J‘ l5” +{NJ ‘iv 5L 'r. 1.; _.r if; .. , L it .4 u»; I .9 H Li “Hag .112“! \r 0:» f F1 A. ..j(j :\ Q‘I'C l 14.“ _,' Qt: av vi [5»qu t 1L .. ,1 K?» W (K S m r U). Lax “raw VA r I. r (0) Draw the Lewis-dot structure of the methylene dianion CH22' as well as a 3-D structure showing all the orbitals and state the hybridization of the 0 atom in this dianion. &/ Answer. Hybridization of C is: qu I H Lewis-dot 3-D with orbitals @2002 J. Frechet. Berkeley Chem. 3A 2002 Midterm Exam #1 Page 3 of 10 2. ( 10 points) Name (lUPAC) or draw, as appropriate. the following molecules. 1 . \a'r Elke"! (a) d - TARA?" r‘ 3.1 16- \‘ 5.41; uwlll) -— 1- {£51}! LIQ ' ' Il‘LKlfill'l/‘HEQCIQ CG. VJ C- x _ ‘ r.l| I l‘ fl 0" S — ‘l IY— Aim-9‘1." Erie-Voli‘leK-U . -3 .0] .1 (Jill MM "Zr (c) ole-1-ohloro-2—ethylcyclohexane (Most stable chair conformation) ."II I. -' \‘ \ \_ gte ‘7 | (d) 1,7.7-trlmethylbicyolo[2.2.1]heptane (e) 2,2-dibromo-3-cyclopropylnonane @2002 J. Frechel. Berkeley Chem. 3A 2002 Midterm Exam #1 Page 4 of 10 3. ( 14 points) (a) What is the SHAPE of the methyl cation CHS". explain using VSEPR (1 sentence!) _2’ Shape: (b) what is the approximate H—C-H bond angle in the methyl anion CH3' f) Bond angle: \ [Ti .17 K (c) How many stereoisomers are possible for menthol? Answer: 25 (1/ (see structure below) ‘ ((1) Draw the two chair conformations of menthol (peppermintoil) and circle the most stable. Note: show all axial and equatorial bonds corresponding to those on the structure below. Menthol C it (e) How many optically active stereoisomers are obtained by monobromination of pentane C5H12? Draw a clear stereochemical representation of each stereoisomer using the template below as a model Do not write the same structure twice! _I _' U‘r @2002 J. Frechet. Berkeley Chem. 3A 2002 Midterm Exam #1 4f] gage 5 of 10 4. ( 12 points) (a) Removal of two protons from propyne HCEC—CHa lea he propyne dianion [CCCH2 ]2'. Write Lewis-dot structures for two resonance forms of the propyne dianion in which all three carbon atoms have an octet. Do not forget to show the location of any charges! Circle the most important resonance form and explain why it is the most important. Tm; 1's wow llmxi-N‘kfldfi - e- ' ~ ~- - __ - - .1 __—“' ‘ _-—__‘_L-""--.. ghucfl like :-‘ f [,1 r' m (V?) , 55 A211”! (to) Calculate the change in free energy (expressed in Kcal mole") needed to change the equilibrium constant of a reaction that takes place at 27°C from 1 to 100? Show the F equation(s) you use and the detail of your work. H H I._ Krat__. 1’ T: 11% —. Look “(eff it. . Perk -' [no ' I"-3‘:¥"9"-":-- if,“ I'lk '1 L \ If. Hi K\ o L- i I “Ul- ‘1 4r I'J I) ' ¢r .\ r n r _ P Q 1 ‘L {37L j}... — 1 .. 40,“: r‘ AL 3 C- : H] _ EM rl/ 5 “I Knit, Answer. - ,L. it: we". @2002 J. Frechet, Berkeley Chem. 3A 2002 Midterm Exam #1 Page 6 of 10 5. ( 13 points). (a) Name the scientist who discovered that the plane of polarization of plane polarized light is rotated when the light is passed through a solution of camphor Answer: got 4: \ (b) Name the scientist who discovered and quantified the dependence of reaction rates on temperature. Answer: (D (c) Write the Lewis-dot structure for the sulfate anion 8042' in which each oxygen is bound to sulfur in an equivalent manner and indicate the formal charge (if any) assigned to each atom. .JII‘ ' I .. '90.; '97 *5 ,5" (d) Consider the equilibrium: HA + H20 A' + H3+O Determine the concentration of [H3+O], [A'], and undissociated acid [HA] for a 1M solution of acid having pKa = -2. Show the details of your work. H _ _ _ fl . " i ' , til-M * ii. .. i. "4.. r“ l 3. tr- r it " . -: ' 1 I" - \I,-; L I ; _q _-.=\‘.( gag t " —-'-— 1X i.~‘:~< loo Answers; [H3+O]= a . f- @2002J. Frechet. Berkeley Chem. 3A 2002 Midterm Exam #1 Page 7 of 10 f 5- i 13 POintS) (a) circle the functional groups in the molecule below and name them . K L {/— ~ 1; |._.Ilr_ ll. 0 rd" " -\ m (b) Draw the more stable chair conformation of cis—1-ethynyl-2—methy|cyctohexane (show all bonded atoms on Ci and (32) and explain why this chair is more stable than the other chair conformation. (c) A sample containing both enantiomers of lactic acid has a specific rotation [ab = +0.95“ Given that the specific rotation of PURE (-)-|actic acid is [ab = -3.8°. what is the percentage of (-)-lactic acid contained in this sample? Explain and show your detailed work. ) . .lka‘HC‘l'tifi'fi‘LE'rli' 4' O ' . .r‘: ("—\ '{fitlnc adi Emil: :- :- (il— " l __ .. = _(iI1gF-iuo:15fl I : (.39; ' lucll’i-c 5'1; 9-1:} : 4- It'- ‘~' Jul i: [0Q D . 0i 7'1 infra 'JF‘t'bihf I o— ‘ l. .‘J o ViM‘IWl' 'J l ‘l l in Salt, H G) Answer: 1. fll . 6 © 2002 J. Frechet, Berkeley Chem. 3A 7. ( 13 Points) _.—n. of strain in each. and circle the most stable chair. Answer: 2002 Midterm Exam #1 Total strain = Page 8 of 10 (a) Consider the two chair conformations of the 1,2,4-trimethylcyclohexane shown below. Name and tally the strains present in each conformation. calculate the total amount Answer: Total strain = 1 E ism!) { (b) Match the tetters of the staggered and eclipsed Newman projections below to the indicated positions on the diagram describing the relative energies of different conformations of 2-methylbutane. CH3 {A H3CCH3 l ,r’ A H H.» B H30 H H H CH3 H30 H H .._I_ c D H H CH3 H CH r" CH3 H30 3 Staggered Eclipsed conformations conformations 240° Angle ofrotation 0° 60° 120° 180° 300° ,350’0 @2002 J. Frechet. Berkeley Chem. 3A 2002 Midterm Exam #1 Page 9 of 10 3- (10 POintS) (a) The free radical chlorination of alkanes with sulfuryl chloride Cl802Cl does not produce the same mixture of monochlorinated products as would be obtained using 0'2 and light. Using the product compositions below, calculate the selectivity of the 'SogCl radical for primary. and secondary hydrogens. Show the details of your work. CH30H20H2CH20H20| (28%) A 1.5 0:80ch CH30H2CHZCH20H3 —h~—r~ CHacHClCHQCHZCHg (48%) B \ v E H CHSCHchCICHZCHg (24%) c U1 GIL: flat- "“'r {"57" 1 —e: =4 l?" <3 4' my " e Answer. Relative reactivity secondaryl'primary = (b) Consider the chlorination of alkanes with chlorine in the presence of light. (i) Aikane A (C5H12) reacts to produce only one monochlorinated product. Write the IUPAC name of the monochlorinated product. Answer: (ii). Alkane B (07H15) reacts to produce seven monochlorinated products (ignore stereoisomers). Write the structure of alkane B. Answer: {Fl-I i' I @2002 J. Frechet. Berkeley Chem. 3A 2002 Midterm Exam #1 Page 10 of 10 fi Note: There are no questions to be answered on this page. it only contains data that may be of use in solving the questions contained in this exam. Not all of the data given is needed. Value of gas constant: R = 2.0 cal deg'1 mol‘1 Value of absolute zero (kelvin) = -273°C Value of e (base for natural logarithms) e = 2.718 Bond dissociation energies (in kcal mole”): RCHz—H 93; RZCH—H 95; R3c—H 91; CI—‘Cl 58; Br—Br 46; l—I 36; H‘Cl 103; H—Bl' 88; H—‘l 71; RCHz—Cl 81', RCHz—Br 68; RCHz—I 53; RzCH-Cl 80; RZCH—Br 68; R3C—Cl 79; R3C—Br 65'. Values of strain energies: Each CH3 - H eclipsing interaction: 1.5 kcal mol’1 Each H - H eclipsing interaction: 1.0 kcal mol‘1 Each CH3 - CH3 eclipsing interaction: 2.5 kcal mol'1 Each CH3 - CH3 gauche interaction: 0.9 kcal mol'1 Each t-Butyl - CH3 gauche interaction: 2.0 kcal mol‘1 Each CH3 - H 1,3-diaxial interaction: 0.8 kcal mol'1 Each Cl - H 1.3-diaxial interaction: 0.25 kcal mol‘1 I Each CH3 - CH3 1,3-diaxial interaction: 1.6 kcal mol'1 Each H - CN 1,3-diaxial interaction : 0.1 kcal mol'1 Each H - C(CH3)3 1.3-diaxial interaction: 2.5 kcal mol'1 Formulae used in solving quadratic equations: axz + bx + c = 0 x = [-b :1: (square root of (b2 - 4 ac)] / 2a Partial periodic table of the elements. ...
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Chem3A_Sp02_Mt1_Frechet - Chemistry 3A — Spring 2001...

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