CHEMISTRY MIDTERM # 1 answer key February 12, 2009

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1 CEMSTRY MDTERM # 1 answer key February 12, 2009 Statistics: Average: 78 pts (78%); ighest: 97 pts (97%); Lowest: 43 pts (43%) umber of students performing at or above average: 28 (62%) umber of students performing at or below 55%: 2 (4%) 1. (10 pts) Mark as true (T) or false (F) the following ments. Do not explain! (T) σ bonds are stronger than π-bonds; (T) Single bonds are always σ-bonds; (F) The gauche conformation of butane is a global minimum; (F) The chair conformation of cyclohexane is a local minimum; (T) All molecules with angle strain also have ring strain; (F) Molecules with polar bonds always have non-zero dipole moments; (F) Stronger acids have higher pk a values; (F) Lewis acids are proton donors; (F) ønsted acids are proton acceptors; (T) ncreasing oxidation number indicates an oxidation process; 2. (5 pts) Provide an acceptable name for each of the following molecules: C 3 3 C trans-1,3-dimethylcyclopentane Cl F 2-chlorobutane piperidine 3-fluorocyclohexanol 3. (5 pts) Provide the structural formula for each of the following molecules: isopropylcyclobutane t-butyl bromide sobutyl alcohol cis-2-methylcyclopropanol tetrahydrofuran 4. (6 pts) For each of the following pairs, determine whether they represent the same compound, constitutional isomers or cis-trans isomers. same Cl C 3 C 3 cis-trans Cl F constitutional F constitutional 5. (6 pts) Draw all isomers (constitutional + stereo) of dimethylcyclobutane.

2 6. (4 pts) Draw structures of the following: A. A secondary alcohol; B. A nitrile with formula C 4 7 ; C C. An ester with formula C ; 7. (4 pts) dentify the types of carbons (1 o, 2 o, 3 o, 4 o ) in each of the following molecules (Please make assignment for ALL carbon atoms!). 2 o 1 o 1 o 1 o 2 o 3 o 4o 3 o 1 o 3 o 8. (6 pts) Circle name the functional groups in the following molecules: benzene ring amide amide Penicillin G sulfide S carboxylic acid 2 o alcohol 1 o alkyl halide alkyl halide benzene ring ether benzene ring Thyroxine 2 alkyl halide 9. (6 pts) ndicate which of the following pairs of structures represent resonance forms. Do not explain! amine carboxylic acid 3 C C 3 C C (6 pts) Complete the following Lewis structures, then identify the hybridization of each non-hydrogen atom: sp 2 2 C C sp sp 2 sp2 sp 2 sp 3 sp 3 sp 3 3 C Si 3 sp 3 sp 2 sp 2

3 11. (4 pts) Complete the following Lewis structure, then write two additional valid resonance structures. Rank the resonance structures (6 pts) dentify each of the following processes as oxidation or reduction, with respect to the organic substrate. + Zn(g)/Cl reduction oxidation Li reduction Li + Li 13. (4 pts) Arrange the following substances in order of increasing acidity. 1-butyne 2-butanol dibutylamine butanoic acid 1 (least acidic) (2 pts) Circle the most acidic hydrogen(s) in the following molecules: S 15. (2 pts) Write the structures of the following: A. The conjugate base of water; B. The conjugate acid of ammonia; (8 pts) Predict the direction of equilibrium shift in the following ønsted acid-base reactions: 2 S 4 + C 3 C S 4 + C 3 C C 3 C + (C 3 ) 2 C 3 C + (C 3 ) 2 2 C + C (6 pts) dentify the Lewis acid (electrophile) Lewis base (nucleophile) in each of the following reactions:

4 C 3 + C 3 Cl C 3 C 3 + Cl Lewis Base Lewis Acid (nucleophile) (electrophile) BF 3 2 C C 2 + BF 3 2 C C 2 Lewis Base (nucleophile) Lewis Acid (electrophile) 3 C C + C C C C 3 C 3 2 Lewis Base 3 C Lewis Acid (electrophile) (nucleophile)

5 18. (6 pts) Consider the molecule of 1,1,2-tribromoethane. Using ewman projections, draw the conformations arising upon a full 360 o turn around the C C bond, in 60 o steps. Represent the energy changes on a qualitative energy/dihedral angle diagram (int: A bromine atom is considerably larger than a hydrogen atom). Assign the proper term (i.e. minimum, s, etc.) to each conformation. C C θ = 0 o θ = 60 o θ = 120 o θ = 180 o θ = 240 o θ = 300o θ = 360 o = 0 o one --- staggered, one --- gauche zero --- s staggered, one --- gauche one --- staggered, two --- gauche s one --- E global minimum local minimum 0 o 60 o 120 o 180 o 240 o 300 o 360 o θ

6 19. (4 pts) Which of the two isomers below is more stable? Support your answer with detailed structural analysis. 3 C C 3 3 C C 3 or C 3 C 3 C 3 C 3 C 3 3 C C 3 a,a,a C 3 e,e,e First isomer: more stable conformation C 3 C 3 C 3 3 C a,a,e C 3 3 C e,e,a Second isomer: more stable conformation The first isomer (on the left) has a conformation with all groups equatorial. For the second, even in the lowest energy conformation one of the groups is axial. The isomer on the left is the more stable one. 20. (2 pts) BUS PRBLEM (n order to receive credit for this problem, it has to be solved entirely!!). Represent the following structure using chair conformations for the six-membered rings accurately show the stereochemistry at each ring junction.