2311A and B Practice Problems to help Prepare for Final from Previous Marder Exams. Disclaimer.: Use only to help learn what you need to know and don t expect the final to be in the same form. 1
1. Short answer questions. a) Underline the compound with 2 (two) (and only two!) stereocenters ( 5 points) Br H 3 C HO HO HO O OH OH b) Which of the following equilibria are favored to the left?: (5 points) CH 2 OH CH CH 2 O 3 CH 2 CH 2 O HCl CH 2 OH Cl CH 2 S CH 2 OH CH 2 SH CH 2 O NH CH 2 O 3 NH 2 CH 2 OH (i) I, II (ii) II, III (iii) III, IV (iv) I, III c) What are the pka s (±3 units) of the following molecules (1 point each) (i) (ii) (iii) (iv) (v) (vi) HI Acetic acid Ammonia Water Ethylene Ammonia 2
d) Underline the least stable carbocation (5 points) CH 2 F e) Underline 5,5-Dimethyl-2-hexyne. (5 points) HC CH 2 CH 2 CH( )CH( ) 2 C CCHCH( ) 3 f) Indicate the formal charge of nitrogen and oxygen in the following structure. If an atom is formally neutral, indicate a charge of zero. (4 points) N O N: O: 3
g) Circle the energy- reaction coordinate diagram that represents the reaction reaction shown below. (5 points) SH OH DMSO OH SH E E Reaction Coord. Reaction Coord. E E Reaction Coord. Reaction Coord. h) Consider a compound known as allene: CH 2 =C=CH 2. (4 points) (i) How many carbon-carbon s-bonds does this molecule contain? (ii) How many p-bonds does this molecule contain? (iii) What is the hybridization of the central carbon of allene? (iv) What are the H-C-H angles in allene? 4
i) Which of the following compounds would react most rapidly with NaCN in an S N 2 reaction? (5 points) (1) iodocyclopentane. (2) 1-iodo-1-pentene. (3) 1-flouropentane. (4) ) 1-iodobutane. (5) Neopentyl iodide. j) Which of the following IS an nucleophile? (4 points) (1) BF 3 (2) O (3) Br 2 (4) All are electrophiles. (5) None is an electrophile.. k) Identify the following as either Z or E configurations. (4 points each) Br OH OH OH O 5
2. Designate the mechanism (S N 1, S N 2, E1, E2) that is most consistent with the following statements. Some of these statements are consistent with more that one symbol! Indicate so! (14 points) (i) Alkyl bromides react slower than alkyl iodide in reactions that proceed by these mechanisms. (ii) Benzyl halides react with sodium ethoxide in ethanol by this mechanism. (iii) These reaction mechanisms cannot involve carbocation intermediates. (iv) Rearrangement reactions are possible for reactions proceeding by these mechanisms. (v) Very bulky bases and sterically hindered sites with leaving groups attached favor this type of reaction. (vi) These reactions are often run in polar aprotic solvents. (vii) These reactions have second order kinetics. 6
3(a) For the molecule allyl anion,shown below, on the lef, write another resonance form in the box provided. (3 points) (b) The normal carbon-carbon single bond length between two sp 2 carbons (for example between carbons 2 and 3 in butadiene) is 1.46Å and the normal carbon-carbon double bond length between to sp 2 carbons is 1.34Å. What would you expect the carbon-carbon bond length between the carbons to be in allyl anion shown above? (3 points) 7
4. For the reaction shown below fill in the boxes with either the product or reagents as required. (4 points for each answer) Br 1. Mg, ether 2. O H 3 O + OH Na I OH OTs CN 1. di-isoamylborane 2. H 2 O 2, OH 8
5. Consider the following five pairs of structural formulas. For each pair, indicate whether the structures represent identical molecules, constitutional isomers, enantiomers, or diastereomers. (20 points) (a) H Br H Br H Br and H 3 C H 3 C H Br (b) H H 3 C H and H H HO H H OH (c) CH 2 CH 2 and Br Br (d) O O and H 3 C CH 2 H 2 C (e) H H 3 C H and \ 9
6. Provide a curved arrow mechanism (3 steps) for the following reaction. (10 points) HO I 2 O I HI 7. Show a complete mechanism for the reaction shown below using the curved arrow formalism. (10 points) 10
8. Provide a synthesis to get from the starting material on the left to the product on the right. To receive full (or partial credit) show the product of each step of the synthesis with the reagents required to effect each step. (10 points each) (a) S (b) 11
(c) OH CN plus enantiomer Note trans stereochemistry between methyl and CN!! 12
Extra Credit. Using resonance structures, explain why benzylbromide can undergo an S N 1 reaction even though it is a primary halide (hint: think about why there can be an unusually stable carbocation form as an intermediate). (10 points) Additional Problems. (a) Outline a sequence of reactions that is suitable for the preparation of the following compound from the indicated starting material and any needed organic or inorganic reagents. The synthesis will require at least three steps. (15 points) S + enantiomer O (b) Outline a sequence of reactions that is suitable for the preparation of the following compound from the indicated starting material and any needed organic or inorganic reagents. The synthesis will require at least two steps. (10 points) 13
CN. Draw the product for the reaction of propene and each of the reagents shown below (3 points for each reagent). For each of the following reactions draw in the major products that will be formed in the boxes. Be clear to indicate both the regiochemistry and expected stereochemistry. If more than one product is expected indicate the stereochemistry of each using dashes and wedges. For each chiral carbon in the molecule (whether or not it is created by the addition to the alkene), assign the carbon as being R or S. Then, determine the relationships between the molecules that have been created. Are they enantiomers, diastereomers, or identical. If a compound is meso, label it as meso. Note. There will not necessarily be four products formed. There should be no rearrangements necessary. (15 points) Example: 14
HBr R Br diastereomers S Br S S 15
a. HOBr Work space (no information here will be graded). 16
b. HBr, peroxides (trace) Work space (no information here will be graded). 17
9. For the reactions below draw: i) the initial carbocation intermediate formed, ii) the most stable intermediate formed (if a rearrangement is likely to occur and, use the curved arrow formalism to show how the rearrangement occurs) and iii) the product(s) of the reaction. If more than one product is expected to be formed in significant yield indicate which you expect to be the major product and why. (12 points each) HBr CH 2 Br H + /H 2 O 18
Extra Credit (4 points). Do this problem only if you have completed and checked the rest of the exam. Label each asymmetric center clearly and unambiguously as either R or S (10 pts). O H 3 C H H H 3 C N O H HN O HBr, peroxides(trace) 1. Hg ( COO) 2, OH 2. NaBH 4 Br 2, H 2 O 1. BH 3 2. H 2 O 2, OH H + H 2 O 19
Chart of Useful Information Polar Aprotic Solvents Polar Protic Solvents Dimethyl formamide (DMF; ( ) 2 NCHO) Water (H 2 O) Dimethyl sulfoxide (DMSO; ( ) 2 SO) Acetonitrile ( CN) Methanol (MeOH; OH) Ethanol (EtOH; CH 2 OH) Acetone ( CO ) Acetic Acid ( CO 2 H) 20