chem3A-spring06-mt1-Frechet-exam

chem3A-spring06-mt1-Frechet-exam - Chem 3A Feb 22, 2006...

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Unformatted text preview: Chem 3A Feb 22, 2006 Page 2 of 10 1. ( 11 Points) (a) Choose one answer to the question: which of the following pairs are not resonance structures of each other? Circle the letter corresponding to your answer in the appropriate box. (D and I_ B All are pairs of resonance structures E 09 O 9 9 c— C H2C—CEC—H and H2C=c= H 9 2 (b) Consider the following molecules gor each, write your answer in the box provided). How many carbons are in the sp How many carbons are in the sp3 hybridization state? hybridization state? 0 ' C51 o ocn3 (c) Which of the following molecule is not chemically identical to the others. Circle the letter corresponding to your answer in the appropriate box. H3C—CH2 CH2—CH3 B H3C—(I'JH2 | HZC—CH CH2 l | H C—CH —CH—CH A CH3 ‘ 3 2 3 H3C—CH2 (lez-CH3 'H-$F—$H C CH3 H 9+3 H3C—CH2 CH2 “ac—CH2 CH3 ,1 ll HSC_C__CH2 H3C—C—CH2~CH3 D ['4 E H H3C—CH2—CH—CH2CH2CH3 CH3 F (d) What is the value of the H-O-H bond angle in water? Answer: (e) Using VSEPR deduce the shape of the molecule of BSHz. Answer: Chem 3A Feb 22, 2006 Page 3 of 10 2. ( 13 Points) (a) What is the hybridization state of each atom highlighted with an arrow pointing to it. Place each individual answer in the box provided at the end of the arrow. CH3 H3C— P: C-—-C:C/ d "3% 2. (b) An unimportant resonance form of acetonitrile oxide is shown below. Write two additional resonance forms that are more important in which C, N, and O atoms have an octet and some have +1 and —1 formal charges. Which is the most important resonance form? Circle it and explain your choice in the box provided below. H3C—C—N=O 1/ Explanation: The circled structure is the most important resonance form because 2. (c) How many 0 (sigma) and how many Tl' (pi) bonds are present in each of the molecules below. Write your answers in the appropriate boxes. H—CEC—CEC~H [hCZCH—CEN Chem 3A Feb 22, 2006 ' Page 4 of 10 3. ( 12 Points). (a) Draw three resonance forms (showing all unshared doublets and charges) for methyl isothiocyanate H3C-NCS (in which the atoms are linked in this order). All C, N, and S atoms must have an octet and some may have formal charges of 0, +1, or -1. (Do not show more than 3) Circle the most important resonance form and explain in one sentence why it is the most important. Explanation: (b) Using the curved arrows suggesting the mechanism of the reactions, show the product(s) obtained in each of the reactions below (show all unshared doublets and charges if any). '59; H II :03“ H—Q—lgC—CHg, ___» H Chem 3A Feb 22, 2006 Page 5 of 10 4. ( 12 Points). (a) DRAW a clear structure for each of the following molecules: (i) Cubane (ii) 2-chlorobicyclo[4.2.2]decane (b) Name the following compounds (IUPAC nomenclature): Br Chem 3A Feb 22, 2006 ' Page 6 of 10 5. ( 13 Points). (a) Draw a potential energy diagram for the rotation about the CZ-C3 bond of butane. Show Newman projections for all conformations located at the maximum and minimum points on your graph. Start the potential energy curve with the lowest energy conformation at the location marked with an x on the graph below and assign relative energy values to all the conformations at energy maxima and minima on your diagram. 0 60 1.20 180 240 300 360° * Torsional angle (b) Which C—C bond would break first in a pyrolysis reaction: a C-C bond of ethane or a C-C bond of 2,2-dimethyl propane? Provide a Quantitative explanation using the data on page 10. Chem 3A Feb 22, 2006 Page 7 of 10 6. (12 Points). (a) Show a mechanism (equations with the appropriate curved or “fishhook” arrows) for the formation of HzC=CH-CH3 during the thermal cracking (pyrolysis) of butane H3C-CH2-CH2-CH3 (b) The reaction of cyclopropane with chlorine (Clz) in the presence of light is known to produce 1,3-dichloropropane. Write an overall equation for this process and show a step by step mechanism for the formation of this unexpected product. Calculate the change in enthalpy for this reaction (Answer) Change in Enthalpy = Chem 3A Feb 22, 2006 Page 8 of 10 7. ( 12 Points) (a) Calculate the optical purity (% ee) and the percentage of each enantiomer in a mixture of (+) and (-) isomers of alanine with an optical rotation of -1 .70° given that pure (+) alanine has a specific rotation of +8.5°. Show the details of your calculation Optical purity = % of (+) isomer = % of (-) isomer = (b) A solution of coniine (0.79 per 10 mL) placed in a 5cm long polarimeter cell exhibits a clockwise optical rotation of 06°. Calculate its specific rotation [(1]. Answer: [a] = (0) Calculate AG° for the dissociation of fluoroacetic acid at 25°C (pKa = 2.66) given that Keq = Ka. Show the details of your calculation. Answer: (0) Draw a structure of the compound below in its more stable chair conformation Chem 3A Feb 22, 2006 Page 9 of 10 8. ( 15 Points) (a) Show a clear structure for the major product(s) obtained in each of the following monochlorination reactions initiated with light at room temperature. _—’ hv CH3 CH3 (b) Name the scientist who established the scale of electronegativity of elements Answer: (c) Circle the functional groups in the molecule below and name them. SH ch—CEC—<‘ 0 (d) Name the scientist who proposed that bonds are made by the in-phase overlap of atomic orbitals. Answer: (e) Write a balanced equation showing the catalytic reforming of heptane CH3(CH2)5CH3 into a C7 aromatic hydrocarbon and a gaseous by-product. Chem 3A Feb 22, 2006 Page 10 of 10 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 K'1 mol'1 Value of absolute zero (kelvin) = -273°C Value of e (base for natural logarithms) e = 2.718 ‘ THBZ nae; torithtiitiorttg ' Compound" ' 3 ~ ,(kcal moi”) V Gompnund CH3-Hi ~CH3+CH3 C2H5+H _ 101 : gas-tam; _ C3H7-l—H I 101 I - _tCH3)zCHCH2+H : ; 9101 , , _('t"é..if>zcfi+CHsi . 9845 > . " y , Hsiiirtfitcmg ;. « ' " Values of strain energies: Each CH3 - H eclipsing interaction: 1.5 kcal mol'1 Bond dissociation Energies CI—g—CI 58 Kcal mole'1 RCHz-g-CI 81 Kcal mole'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 f3? H2C—§—’CH2 Each t—Butyl - CH3 gauche interaction: 2.0 kcal mol'1 65 Kcal mole'1 Each CH3 - H 1,3-diaxial interaction: 0.8 kcal mor1 Each Cl - H 1,3-diaxial interaction: 0.25 kcal mol'1 Each CH3 - CH3 1,3—diaxial interaction: 1.6 kcal mor1 Each H - CN 1.3-diaxial interaction : 0.1 kcal moi'1 ...
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chem3A-spring06-mt1-Frechet-exam - Chem 3A Feb 22, 2006...

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