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midterm2spring09key

midterm2spring09key - Name Last First(PLEASE PRINT CLEARLY...

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Unformatted text preview: Name: Last, First (PLEASE PRINT CLEARLY) CHEMISTRY 8A Spring 2009 Quiz 2 May 6, 2009 Professor Sheila S. David 7 ( 10 11 8 (8' (6' 6. (8' (6' 4. 12 Total ( 100 .oints) Extra Credit -_ For students to read at exam time: Closed book and notes. You may use a calculator. Please read exam carefully. Partial credit may be awarded if you show your work. For students to read after exam is returned: If you have any problems with the grading of a question, return your exam to Dr. David by May 18 a note attached indicating which problems should be regarded and a brief explanation. Do not approach the teaching assistants directly about regrades. The TAs and the professor will sit down together to do all re- grades at one time. Tests will not be regarded after May 18. Student ID Number: ID Check |:' Name: Last, First (PLEASE PRINT CLEARLY) 1. (2 points each) Appetizers (put letter answer in box at the right). i) Which of the following statements is not true about benzene: A. Each of the six carbons in benzene is sp2 hybridized B. There are six 11 electrons that are shared by all six carbons C. The most stable conformation of benzene is the chair conformation D. A p orbital on each carbon is involved in making the delocalized IT bond ii) Hipposuduric acid, which is found in hippo sweat, is a red compound. The origin of the red color of the molecules is due to its: COZH O o “o O HO Hipposuduric acid H020 A. conjugated pi electron system B. ability to stack within DNA bases of duplex DNA C. presence of OH groups D. large number of carbon atoms iii) Which of the following correctly describes intermediates and/or transition states? A. Transition states occur at minima on reaction coordinate diagrams B. Both transition states and intermediates occur at maxima on reaction coordinate diagrams C. An intermediate is always produced after the rate-detennining step of a reaction mechanism D. Transition states have partially formed bonds whereas intermediates have fully formed bonds E. None of the above iv) Which of the following is incorrect? A. Resonance stabilization is the extra stability a compound gains from having delocalized electrons B. Delocalized electrons destabilize a compound C. The greater the number of relatively stable resonance contributors, the greater the reasonance stability D. A and C E. B and Name: Last, First (PLEASE PRINT CLEARLY) 2. ( 10 pts) Listed in the table below are the acid dissociation constant values (Ka) for a number of weak acids. Arrange these acids in the order of increasing acidity with 1 = most acidic, 5 = least acidic [this may help: pKa = -logKa] Place numbers in column marked.“rank.” Weak Acid Q eK‘L Rank Ascorbic Acid (Vitamin C) 7.9 X 10‘5 ‘I. I & Acetic Acid (diluted to make vinegar) 1.8 X10‘5 4'? .3 Propionic Acid (smell of dirty socks) 1.3 X 10'5 4 fl :5 Formic Acid (the sting of an ant bite) 1.0 X 104 1+, 0 l . . . . .5 Caproic Acrd (characteristic smell of goats) 1.4 X 10 q‘ 35 Q 3. (10 pts) FE ‘ AC'lllthL'b 'th f dbl: :" 5 ) 1rc e a e ew1s ases in e group 0 compoun s e ow | 2 ”m6 1 —-) 4 3 -9 5 HCI BCI3 . M92+ 6 B) Put a BOX around all the Lewis acids in the group of compounds below 94: 1.92 :_ 2-93 ”mg \ fl bEfiQE Name: Last, First (PLEASE PRINT CLEARLY) 4. (20 pts) Classify each compound below as a nucleophile or electrophile and briefly explain your choice. Hint: Draw out the Lewis Dot structure, showing all lone pairs and charges, this will make the choice more readily apparent. a) Cyanide, CN’ Circle One: Electrophile Nucleophile Explain your choice: 66—”. Wm, haywénLL/gtur‘ 2r!“ 9" “C“M'VW me? 5 W 1 WJ (Ma 0!. W 4.12. ; 6/” eagle, 4677me #1:” M m. (Lp‘gmb) Hydronium ion, H3O“ CirclSOne: Nucleophile Explain your choice: la" M’/ .e W“ “W“? “"3“ a. k W \4 fl WA 26¢“,— fwuutfiw l/\ c) Cyclohexanol Circle One: Electrophile (Nucleophile 3 OH Explain your choice: d) BF3 Circle One: Electrophile Nucleophile Explain your choice: e) Alkene Circle One: Electrophile (Nucleophile :, Explain your choice: ____ (L\ CZC/K WM”; fill/Z44 P~/ P‘ /fiv MW°AW w Name: Last, First (PLEASE PRINT CLEARLY) 5. (8 pts) Give the major product that would form in the hydrobromination shown below. HBr Br / > 13> 6. (8 pts) Draw w n ru tu for the species below: c.0‘ / o z; 7. (6 pts) In the following acid-base reaction would the species on the right or the left be the major species present at equilibrium? CH3SH + CH3COO‘ = CH3S‘ +CH3COOH l , pKa = 9 pKa = 5 left or right 8. (6 pts) Identify the electrophile and nucleophile in the following reaction and then draw curved arrows to show bond-making and breaking process. (Hint: draw in all electrons and lone pairs). Q. Q "05‘ 0 CN :C _N‘- Name: Last, First (PLEASE PRINT CLEARLY) 9. (6 pts) Consider the reaction coordinate diagram below to answer the following questions: How many intermediates are present in the following reaction diagram? How many transition states are present in the following reaction . diagram? Which is more stable, reactants or products? 10. (8 pts) Lactic acid (also know as milk acid) has two acidic protons (A and B). Explain which proton is more acidic and draw the conjugate base that will result upon deprotonation. Name: Last, First (PLEASE PRINT CLEARLY) ll. (6 pts) In the following pairs, CIRCLE the structure that will make the greater contribution to the resonance hybrid. 69 12. (4 pts) Circle the compound that is the weakest acid (i. e. most difficult to remove a proton from)? .Qom: Name: Last, First (PLEASE PRINT CLEARLY) EXTRA CREDIT (5 points): Phenolpthalein (compound shown below) is an indicator that exhibits dramatic color changes upon changes in pH. In solutions of pH < 8.5, it is colorless, in solutions of pH > 8.5, it is deep red-purple. Account for the color change. (Hintishow how structure changes with pH, and explain the consequence of that change). ...
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