Bertozzi_Chem3A_Midterm1_1997_Key

Bertozzi_Chem3A_Midterm1_1997_Key - Chemistry 3A - Fall...

Info iconThis preview shows pages 1–13. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 8
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 10
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 12
Background image of page 13
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Chemistry 3A - Fall 1997 First Midterm - September 30, 1997 Professor Carolyn Bertozzi Your full signature Professor James Leahy Print your full name (Last name, First name, Middle) Please circle the section number and name of your GSl/T A. 111 Clarke,Krisfin 231 Grunwell,Jocelyn 341 Pindzola,Bradford 121 Schmidt,Joseph 361 Janzen,Daron 41 1 Purdy,Matthew 131 Desaire,Heather 371 Crawford,Nathan 421 Skyler,David 141 Tripp,Jennifer 381 Holland,Andrew 51 1 Marcaurelle,Lisa 21 1 Yin,Jun 31 1 Sadow,Aaron 521 Koritnik,Anjanette 221 Grayson,Scott 321 Patin,Joshua 531 Blazey,Charles 331 Duncan,Andrew Making up an l-grade Indicate semester you took 3A Professor This exam has 14 pages; make sure that you have them all. 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 m answer to each problem; multiple answers will receive no credit, Do Not Write in this Box. even if one of them is correct. ' _ 1' ( 16 ) . . . . 2. (20) Note: This examination runs for a total of 80 minutes. No questions will be answered by proctors after the exam 3- (20 ) begins. Please write legibly; ambiguous or messy 4 25 answers will receive no credit. —( ) 5. (20 ) Partial Periodic Table 6 (26 ) VII VIII 7 ( 18 ) He 8. (20 ) 9. ( 35 ) Total (200) l. [16 points] Complete the best Lewis octet structure for each of the following molecules by showing all of the remaining bonds, lone gairs of electrons and formal charges where necessary. Rug/H 61- [Cl‘lstl+ \ l ’ H\;/H L \N4L\ka / ll V " H 0° C 90 H l H \N/ \N/ l' ' l l l + H H \,, /C§ M/ N \ l H ~4- H b. 03 60 ‘ J'— .a ¢ \ -» _ -° .9 9—" no/O§o'. Ola c. CH2N2 W + .7 H /C:N:N', d. [NCSJ— “, a. ___ ,N:C:S: 1N3 L—Vé: [20 points] Using curved arrows (i.e., "electron pushing"), show the movement of electrons that converts the resonance structure on the left to the resonance structure on the right (arrows should be shown only on the structure on the left). Circle the letter below the structure that is the ma'or contributor to the overall structure of the molecule. H30 H3 H30 H3 \ f \ +/C H -B=o- tun om: H3C H3C ~ A b. H H c H3C/C\tIl/C 3 H3C/ \fiL/CHS no dwng CH3 CH3 5a om‘Rm 1 ('9 B F _' (H /H (1' H _ .C>:N: Okofga on (ID 8 more. QUCWR/ am d. i' 'i l'\—.®C§ H KC¢C\6_/H MC\Q’\\\rg T UT/ l , l ' C,\r\a ran F H F H 0 ® B closai +13 {\stdmamfi . OA‘omg 3. [20 points] a. Draw a molecular orbital picture of carbon monoxide (CO) showing all of the h bridized and unh bridized orbitals on both carbon and ox en. Label each orbital as s, p, sp, Sp2 or sp3 and clearly show the geometry of the orbitals in three dimensions. Indicate which orbitals are involved in o—bonding and n— bonding and which hold lone pairs of electrons. (Hint: first draw the best Lewis S Y Wtructure for CO and then translate that into a molecular orbital picture). (=9 9:7 lll b. Carbon monoxide reacts with electron-deficient molecules such as BF3 to form a new c—bond. Which orbitals on the carbon and boron atoms of the starting materials are involved in bond formation circle one choice er atom)? + _. co + BF3 —-—> OC—BF3 Carbon: 3 p sp2 sp3 . - a: Boron: s ® sp sp2 sp3 (‘2 c. Using the energy scale below, construct an orbital interaction diagram to Show how the atomic orbitals on carbon and boron that ou identified in art b can overlap to form molecular orbitals. Place the electrons involved in bonding in the atomic and molecular orbitals. 4. [25 points] The change in energy upon rotation around the central C—C bond in 2,3- dichlorobutane can be depicted on a potential energy diagram using Newman projections. In the diagram below, show the relative energies of the given staggered and eclipsed conformers and the curve representing the change in energy upon bond rotation (the energies of eclipsing and gauche interactions among different atoms and groups are provided in the table below). cli sin Koal/mol uche Kcal/mol G rou s interactin E CH3, CH3 2 CH3, Cl 2. Cl, Cl 1 CH3, H 1 Cl, H 1 H, H 1 5. [20 points] Alkanes can be converted to alkylamines by a free radical amination reaction with hydrazine (NHgNHg). The free radical amination of cyclohexane is shown below. h NH2 V O + HzN—NHZ ——> 0/ + NH3 a. Show the initiation step for the reaction. ho HLNKNHlfl HLNo *- 'NHZ b. Show the two propagation steps for the reaction. Q ‘r “2”” "7 CV i” H3N Step 1: 0 NH”; ° m +— Hz’txl—N‘rlz A U + NH; V c. Show one example of a termination step for the reaction. 0 o O + Q 7 E) d. The change in enthalpy for the reaction (AH°) is —28 Kcal/mol. Using the h\ table of bond dissociation energies provided below, calculate the stren th of the N—N bond in hydrazine. Place our answer in the box rovided below. Bond DH° Kcal/mol é B WVAS bmbcfi -— é B mob Nil W 1°C—H 98 R o ,__>/2°C-—H 95 "‘ AH ng—H gg [own + as] _ [so + (01}- ~28 N—H 102 . 6/ H DH° (N—N)= Kcal/mol 6. [26 points] Amines such as ammonia (NH3) undergo a rapid process termed "pyramidal inversion" in which the three atoms attached to the central nitrogen atom "flip" from one side of the nitrogen atom to the other, similar to the inversion of an umbrella. The transition state (i) for this process is a planar species (structure B) as indicated in the following scheme. a. What is the hybridization on nitrogen in the ground state (structure A)? s? 3 b. What is the hybridization on nitrogen in the transition state (structure B)? Sp 2 c. Amine C shown below also undergoes a pyramidal inversion but at a slower rate than ammonia. Draw the structure of the transition state for the pyramidal inversion of compound C. Clearly indicate the geometry of the nitrogen atom in your structure. Label the atomic orbital that contains the lone air of electrons as s Q, SQ, SQ2 OF $23. g H —— 0 (WW) \‘fli d. The activation energy (Ea) for the pyramidal inversion of NH3 (A) is 6 kcal/mol, while the Ea for the pyramidal inversion of C is 12 kcal/mol. Depict the Chan es in otential ener durin the ramidal inversions of A and C on the two potential energy diagrams below (i.e., show the energies of the starting material, product and transition state for each inversion, and show the curves that connect them). Clear/y indicate the relative activation energies of the two inversions. Pyramidal inversion of A Pyramidal inversion of C \Z \Ccafl/mol e. Provide a brief explanation for the difference in the activation energies of the two pyramidal inversions. Consider the geometry of the transition state in your argument. W Mmc‘em 03min ’H/Ul (S 5353" Mbvicv w sigma bmé “are: M Tu MASCAD‘M SR—oA—Q) w CMQf‘X‘ mlOCio. ’\"D S 2’ l~n\f0\\/(_S Q/XPQAS{M O‘QW’FW‘C‘LLMQS \OO‘AA 6W COVV‘FOW‘A CJW - mmkmcl \OU‘AA anal/L 0‘? 60 .0 Emma} star ,5 +\/ULTS,’ h‘\¥mC‘Qfl ‘ls v Q. Q\ l (1061 le‘Af“ Ck C *NNQ I 5 50‘ PC ado-X \— m flaw/Q Stats “HMS l5 0m?“ Slih\‘%‘co~h+kA/\ «a s [A Mi M g 5 7. [18 points] Show the structures obtained b "rin fli " of the c clohexane derivatives on the left b corn letin the tem lates rovided on the ri ht (make sure that both structures represent conformational isomers of the same molecule). Using the table of A—values below, calculate the change in Gibbs free energy (AG°) in kcal/mol for each conformational isomerization. Place your answers in the boxes provided. +0'5’ .1 moi :* mdl AWE Cl l , ., o - 3 CH3 / F 5 H30 CH3 (/Q‘ <_.__—** was l CH; T + o S ' ' +/ 7 foc/UJ‘J W 1 H30 3 6— + 3 q ’\ l CH3 ’l’ 21 2— Groug A—value (Kcal/mol) —CH3 1.7 —C(CH3)3 5 ——CH(CH3)2 2.2 —CI 0.5 8. 10 [20 points] a. Ephedrine is a potent dilator of the air passages of the lung and is used in the treatment of asthma. Identi all of the stereocenters in e hedrine with an asterisk (*l and clear/K assign each stereocenter the R or 8 configuration. Ephednne b. Circle all the chiral molecules amon the set of structures below. Do n_ot circle any achiral molecules. CH3 9. [35 points] In the following multiple choice problems, mark the single answer which you deem most correct. a. The value of AS° for the following transformation is expected to be: H3C‘\ (st F0 H2 + HBr —+ H3C‘—'Cl:—BT H30 CH3 Large and positive 23 Large and negative Close to zero Approximately equal to AG° b. Increasing the temperature of a reaction will: Increase the value of AH° for the reaction Decrease the rate of the reaction 2g Increase the value of the specific rate constant Have no effect on the value of AG° for the reaction c. The role of a catalyst is to: Reduce the value of the specific rate constant Make an unfavorable reaction favorable 2; Reduce the activation energy of a reaction Reduce the value of 138° for a reaction 11 d. The following potential energy diagram depicts the propagation steps of which one of the light-initiated reactions below: CH4 +X- CH4 + F2 —> CH3F + HF CH4 + Cl2 ——> CHsCl + H0! CH4 + Brg —> CHgBr + HBr CH4 +12 —> CH31+ HI e. Rotation around the indicated C—C bond in the molecule below does not occur spontaneously at room temperature. Therefore, the barrier to rotation must: X Be greater than 23 kcal/mol Involve eclipsing interactions Be similar in magnitude to the rotational barrier in butane Be greater than the 0—0 bond dissociation energy 12 f. The strain energy associated with the boat conformer of cyclohexane originates from: Transannular interactions Eclipsing interactions Gauche interactions X All of the above 9. Which of the fluoroalkanes below is the major product of the following free radical halogenation reaction: A Xe A, B and C are formed in roughly equal amounts 13 ...
View Full Document

Page1 / 13

Bertozzi_Chem3A_Midterm1_1997_Key - Chemistry 3A - Fall...

This preview shows document pages 1 - 13. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online