CHEMISTRY MIDTERM # 1 answer key October 05, 2010

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1 CEMISTRY MIDTERM # 1 answer key ctober 05, 2010 Statistics: Average: 73 pts (73%); ighest: 99 pts (99%); Lowest: 31 pts (31%) Number of students performing at or above average: 61 (52%) Number of students performing below 55%: 8 (7%) Number of students performing at or above 90%: 17 (15%) 1. (12 pts) Mark as true (T) or false (F) the following ments. Do not explain! (T) Single bonds are always σ-bonds; (F) Resonance structures are always in a of rapid equilibrium; (F) The gauche conformation of butane is a ; (T) The chair conformation of cyclohexane is a global minimum; (F) Cyclopropane does not have any torsional strain; (F) Molecules with polar bonds always polar; (T) All Arrhenius acids are also ønsted acids; (F) Lewis acids are proton donors; (T) Increasing oxidation number indicates an oxidation process; (T) Intermediates are not formed in concerted reactions; (T) The ammond postulate relates the energies structures of two adjacent species on the potential energy profile; (T) Carbocations are electron-deficient species; 2. (5 pts) Provide the structural formula for each of the following molecules. 3. (2 pts) Provide a structure for each of the following compounds: a. Bicyclo[1,1,0]butane. b. Spiro[2,2]pentane. 4. (2 pts) Draw curved arrows to rationalize the following conversion. 5. (2 pts) Draw the structure of the carbocation produced from 2-methyl-2-butanol. 6. (6 pts) For each of the following molecules, complete the Lewis structure provide two additional valid resonance forms. Use the curved arrow formalism to show the flow of electrons. Rank the resultant resonance structures. a. Acetonitrile oxide;

2 3 C C N 3 C C N 3 C C N b. Methyl acetate; 3 C C 3 C C 3 C C (4 pts) Each of the following species has a resonance structure that is higher ranking than the one shown. Show that structure very briefly explain why it is higher ranking. N N more bonds more bonds, less charge separation 8. (6 pts) For some of the substances listed below the acid-base equilibrium for the reaction with K is shifted to the left, while for others it is shifted to the right. Using Table 1.8, decide the equilibrium shift for each structure. 9. (4 pts) Predict the shift of equilibrium (to the left or right) for the following acid base reactions. C C + N 2 C C + N 3 to the right N + CN + CN to the left N 10. (6 pts) Label the reactants in the following acid base reactions as Lewis acids or Lewis bases. + Al 3 Al 3 Lewis base Lewis acid

3 + Lewis acid Lewis base N 2 Lewis base + N Lewis acid (6 pts) For each of the following reactions, determine if it is a reduction, oxidation or not redox with respect to the organic compound. 3 C + 2 hν + oxidation 3 + not redox 3 C C MCPBA 3 C C oxidation 12. (4 pts) Draw an example of each of the following classes of compounds: a. An amine with 4 C-atoms; b. An acid chloride with a 5-membered ring; c. A carboxylic acid with 5 C-atoms; 13. (4 pts) For each of the following compounds, draw an isomer that has the same functional group. More than one possibility exists in each case! S S

4 14. (7 pts) Circle name all functional groups in the following structures. sulfide S carboxyl C N 2 amine methionine, an amino acid benzene ring amine amide N 2 sulfide N S amide N C alkene carboxyl cephalexin, an antibiotic 15. (6 pts) Give the relationship between the following pairs of structures. There are four (4) possible relationships: same compound, constitutional isomers, cis-trans isomers, not isomers (i.e. different molecular formula). a. same compound d. not isomers b. same compound e. f. same compound constitutional isomers c. cis-trans isomers 16. (4 pts) Draw the structure of the principle organic product of each of the following reactions. P 3 S 2 heat 17. (6 pts) Using Newman projections, draw the conformations arising upon a full 360 o turn (in 60 o steps) around the C1 C2 bond in isobutyl bromide. Represent the energy changes on a qualitative potential energy/dihedral angle diagram. Assign the proper term (i.e. minimum,, etc.) to each conformation. See next page!!

5 θ = 0 o θ = 60 o θ = 120 o θ = 180 o θ = 240 o θ = 300 o θ = 360 o = 0 o one --- staggered, two --- gauche s one -- staggered, one -- gauche zero -- s staggered, one --- gauche one --- E local minimum global minimum global minimum 0 o 60 o 120 o 180 o 240 o 300 o 360 o θ

6 18. Consider menthol: a. (3 pts) Draw the most stable chair conformation of menthol. a,a,a 3 C e,e,e most stable conformation b. (4 pts) ow many stereoisomers of menthol are possible? Draw their structures using a bold--dashed wedge representation. 19. Consider 1-methylcyclohexanol cyclohexylmethanol. a. (2 pts) Does the energy below better describe the reaction of 1-methylcyclohexanol or cyclohexylmethanol with? iefly explain! The reaction with would result in substitution formation of the corresponding alkyl bromide. The process could occur following either an S N 1 or an S N 2 mechanism. Based on the structure of each of the alcohols, one can make a prediction on the mechanism of its reaction with, as shown above to the right. The profile, on the other h, definitely reflects a stepwise reaction, i.e. one that follows an S N 1 mechanism. The conclusion is that the profile must describe the reaction of 1-methylcyclohexanol with.

7 b. (5 pts) Assign the correct chemical structure(s), corresponding to each minimum in the diagram. A= + E= C= 2 + G= (2 pts) BNUS PRBLEM (In order to receive credit for this problem, it has to be solved entirely!!). Ethylene glycol (see below!) is unusual in that the gauche conformation is more stable than the anti conformation. ffer an explanation. C C ethylene glycol anti conformation intramolecular hydrogen bond gauche conformation The gauche conformation of ethylene glycol is strongly stabilized by hydrogen bonding. Ethylene glycol is a good example of a species whose conformational preference is affected by intramolecular hydrogen bonding.