CHE311Exam1Key2011 - CHE 311 EXAM 1 2011 EXAM KEY Answers...

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Unformatted text preview: CHE 311 EXAM 1, 2011 EXAM KEY Answers in BOLD RED Multiple Choice (60%; 2% each) Clearly mark the letter of the best answer to each question on your answer sheet. Thank you! 1. In organic compounds, carbon usually has _______ bonds and _______ lone pair electrons. a. 3 ; 2 b. 2 ; 4 c. 4 ; 0 d. 6 ; 2 2. What type isomerism is present when two organic compounds vary in the order of connection of the atoms? a. stereoisomerism b. conformational c. connective d. constitutional 3. Which is NOT allowed to vary in the drawing of contributing structures of a chemical species with resonance? a. locations of electrons b. locations of atoms c. electron pairing d. formal charges 4. VSEPR geometry is often useful in the study of organic substances. Which number atom of the structure shown at the right is most likely to have a trigonal planar geometry, based on VSEPR rules? a. 1 b. 2 c. 3 d. 4 5. Which of the following does NOT use the correct drawing of a curved arrow to represent electron movement? wrong because it would give the O more than 8 e– ! 6. The pKa of CH3CH2OH is 16 while that of CF3CH2OH is 11.3. This is an example of ____________ effects in acidity. a. inductive b. conjugative c. London d. hybridization 7. The hydrocarbons in which there is at least one double bond between carbon atoms are called: a. alkenes b. alkanes c. alkynes d. arenes 8. Natural gas consists of mostly a. CO2 b. CH4 9. The hydrocarbon C5H12 has a total of _______ possible isomers, each with a unique IUPAC name. a. three b. two c. four d. five c. C8H18 d. C6H6 10. The complete combustion of C6H14 would require _______ moles of O2 per mole of the hydrocarbon. a. six b. nine and a half c. thirteen d. twelve 11. When a double bond is formed between two carbon atoms, the normal situation is that there is a sigma bond formed by overlapping two _____________ orbitals plus a pi bond formed by overlapping two ___________ orbitals. a. s ; p b. sp3 ; p c. p ; sp3 d. sp2 ; p 12. Which pair of hydrocarbons shown below are actually isomers of one another? these two are both C5H10 species and thus are true isomers ! 13. Which name below is the proper IUPAC name of this hydrocarbon: (CH3CH2)3CCH(CH2CH3)2 a. 1,1,2,2-­‐tetraethylethane b. 3,3,4-­triethylhexane c. 2-­‐ethyl-­‐2,4-­‐diethyloctane d. 1-­‐ethyl-­‐diisobutylpentane 14. Of the following C6H14 compounds, which one should have the most exothermic heat of formation? a. hexane b. 2,3-­dimethylbutane c. 2,2-­‐dimethylbutane d. 2-­‐methylpentane 15. A “normal” alkane has only one chain of carbon atoms. What conformation dominates that chain of carbons? a. eclipsed b. gauche-­‐staggered c. anti-­staggered d. anti-­‐eclipsed 16. The text’s Summary lists 4 types of strain in alkanes & cycloalkanes. The main strain in cyclohexyl-­‐chairs is… a. angle strain b. Van der Waals strain c. torsional strain d. bond length distortion CHE 311 Exam 1 Page 2 The next THREE questions refer to the following set of cyclohexane conformations: Note each conformation structure has an identifying letter (A, B, C, D) and several labeled positions (1-­‐11). 17. Which of the above is NOT a reasonably accurate depiction of any of cyclohexane’s common conformers? a. A b. B c. C d. D 18. Which position is an axial position on a cyclohexane chair conformation? a. ring A’s #2 b. ring B’s #5 c. ring C’s #8 19. Which position, on which ring, is a position which is eclipsed with one of an adjacent carbon’s positions? a. ring C’s position #8 b. ring A’s postion #1 c. ring B’s position #5 d. ring B’s position #4 20. Which of the following is the best illustration of the most stable conformer of trans-­‐1,3-­‐dimethylcyclohexane? 21. Which is neither a bicyclo nor a spiro compound? d. ring D’s #10 Answer d is not either type. 22. Which of the following should give off the MOST heat per methylene group when it is burned to CO2 & H2O? a. cyclopentane b. cyclopropane c. cyclohexane d. cyclohexadecane (16 C) 23. The name ethyl bromide is an example of ___________ nomenclature, which works best for __________ structures. a. functional ; complex b. substitutive ; complex c. functional ; simple d. substitutive ; simple 24. Which of the following is an example of the newest IUPAC name format of the alcohol CH3CH2CH(OH)CH2CH3? a. isopentyl alcohol b. isopentanol c. 3-­‐pentanol d. Pentan-­3-­ol 25. Which of these is a secondary alcohol and a tertiary alkyl halide, in one compound? a. CH3CH(OH)CBr(CH3)2 b. CH3C(OH)2CBr3 c. (CH3)2C(OH)C(CH3)2CH2Br d. CH3CH2(OH)CBr3 26. The most reactive hydrohalic acid in reactions with alcohols is _________, while __________ alcohols react fastest. a. HI ; tertiary b. HF ; primary c. HCl ; tertiary d. HBr ; secondary 27. Although it is possible to react alcohols with HCl to make alkyl halides, one may also use ___________ instead. a. SOCl2 b. HClO4 c. NaCl + HF d. Cl2 28. Which of these is most likely to react with HBr to form a bromide via a carbocation intermediate? a. CH3CH(OH)CH2CH3 b. CH3CH2CH2OH c. (CH3CH2)3COH d. CH3OH 29. In the SN1 reaction of an alcohol with HBr to form an alkyl bromide, what is the nucleophile involved? a. H+ ion b. H2O c. the carbocation d. Br – 30. The direct reaction of halogens and hydrocarbons has a variety of know characteristics; which is NOT one? a. the reaction has a chain reaction mechanism b. the reaction requires that some halogen reacts first c. the reaction operates via an ionic mechanism d. most hydrocarbons react multiple times if possible CHE 311 Exam 1 Page 3 Problems (40%; 20% each) Answer each as requested. You may wish to use your models to help you. I. DRAW OUT two Newman projections of pentane. In your drawing, focus on the geometry of the C2-­‐C3 bond and the groups attached to that. Draw an eclipsed and a staggered conformation. State which is most stable, then explain why using the drawings you have made. A partial initial Newman projection is given to start each of your drawings. Be sure to label which projection is eclipsed and which is staggered. Staggered Eclipsed CH3 H H H H H H H3C CH2CH3 II. This is the more stable of the two conformers because the largest groups on the C2 and C3 are then the maximum distance from one another. This minimizes van der Waals repulsions. This is actually the anti-staggered structure, which is more ideal than the gauche. CH2CH3 H H This is the less stable conformer because the larger groups are closer when in the eclipsed conformation. That would make for stronger van der Waals repulsions and result in lower stability for this conformer. Even with the largest groups far apart as drawn here, they still have more repulsion. Using proper DRAWINGS which you make, explain why cyclopropane is much less stable than cyclopentane. In your answer, you should have at least two specific, different factors which you cite and you should be able to point out features related to those factors on your drawings. Only brief explanations are necessary. H 60° angle NOT eclipsed H H H H H H H Folding of the ring reduces the H eclipsing & makes H better bond angles H H In cyclopentane, some twisting of the ring reduces the eclipsing of H atoms; in addition, the ring can fold up like an envelope which eliminates any significant eclipsing. H H eclipsed H H In cyclopropane, each H is fully eclipsed by its neighbors. This makes for very high torsional strain as the bonds twist to try to reduce this repulsion. Secondly, the angles are all 60°C of necessity in this equilateral triangle shape; this is a huge angle strain in comparison to the 109.5° ideal. The folding of the cyclopentane ring also allows the bond angles in the ring to be very close to 109.5° (even if flat, they would be 105°). This reduces any angle strain significantly. ...
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  • Fall '16
  • Dr. Kroll
  • Organic chemistry, d., Ring, Van der Waals force, largest groups, huge angle strain

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