ch 231 old exams 1 and 2

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Old CH 231 ExamsThe questions on these old exams are typical of the kinds of questions you can expect on the exams in CH 231 this semester. For the midterm exam, you should know all of the material in chapters 1-5, with one exception: you will not be tested on Fischer projections (section 5-4). The absence of questions about any particular topics on the old exams does not imply that questions on those topics will also be absent on the midterm exam this fall. The overall degree of difficulty of the midterm exam this fall will be comparable to that on the old exams, although there will undoubtedly be some variations in the degree of difficulty of individual questions on specific topics. You should be able to answer all of the questions on old exam 1 and all of the questions except 9, 12, 13, and the bonus on old exam 2.

L. T. Scott10/12/2004

B OS TON C OL L E GEChem 231 - Organic Chemistry I Exam I - September 27, 2000

name

Please write your answers on the exam. Do not spend too much time on any one question.

question 1. (06 points) 2. (06 points) 3. (04 points) 4. (08 points) 5. (05 points) 6. (06 points) 7. (04 points) 8. (06 points) 9. (06 points) 10. (06 points) 11. (06 points) 12. (10 points) 13. (10 points) 14. (06 points) 15. (05 points) 16. (06 points) total

points

Chem 231

Exam I - September 27, 2000

1. (6 points) Draw a molecular structure for each of the following compounds: (a) t-butylcyclopentane (b) 3-(1-chloroethyl)heptane

2. (6 points) Write the IUPAC name for each of the compounds shown below.

H F H

F

3. (4 points) In the drawing to the right, identify each of the labeled carbon atoms as primary, secondary, tertiary, or quaternary.

4. (8 points) For each of the organic species below, (a) indicate the hybridization of the atoms marked with an arrow and (b) indicate the best description of the geometry at each of the atoms marked with an arrow, choosing from the following list of descriptors: linear, bent, planar, pyramidal, or tetrahedral.

Br H3C N C O

(CH3)2CCH2CH3

5. (5 points) Circle the most acidic proton of those shown explicitly in the molecular structure to the right.

H H N H

O

H

H H

- 2 of 5 -

Chem 231

Exam I - September 27, 2000

6. (6 points) Every covalent bond in the molecule shown to the right can be described as resulting from the overlap of an atomic orbital on N one atom with an atomic orbital on the attached atom. For the bonds marked by arrows, indicate C what atomic orbitals are overlapping to make the bond.

7. (4 points) In the group of five compounds shown below, (a) circle the compound that should have the lowest boiling point and (b) draw a box around the compound that should have the highest boiling point.

8. (6 points) The C2 H4 Br cation is known to have a cyclic structure (3-membered ring). Draw the most reasonable Lewis structure for this cation. Show all the valence electrons around every atom, and be sure to indicate which atoms bear formal charges. 9. (6 points) The heat of combustion of methylcyclopentane is 942.1 kcal/mol, and the heat of combustion of cyclohexane is 937.7 kcal/mol. Which of these two C6 H1 2 hydrocarbons is the thermodynamically more stable isomer? Draw a potential-energy diagram that shows the potentialenergies of each of these two hydrocarbons plus oxygen relative to the potential-energies of their combustion products. 10. (6 points) The halogenation of methane under the influence of heat or light is actually a reversible reaction (equation shown below). When X=Br, the equilibrium constant is greater than 1.0 (Keq > 1.0). When X=I, however, Keq < 1.0. Is the change in Gibbs standard free energy (G) for the bromination of methane positive or negative? What is the equation that relates Keq to G?

CH4

X2

CH3-X

H-X

- 3 of 5 -

Chem 231

Exam I - September 27, 2000

11. (6 points) Which of the following four reactions will be the FASTEST? (a) G = +4.9 kcal/mol and Ea = +2.3 kcal/mol. (b) G = +4.1 kcal/mol and Ea = +8.7 kcal/mol. (c) G = -6.8 kcal/mol and Ea = +4.2 kcal/mol. (d) G = 0.0 kcal/mol and Ea = +5.7 kcal/mol. 12. (10 points) Free radical chlorination of 2-methylpropane (isobutane) gives two products with the molecular formula C4 H9 Cl in a ratio of about 2:1. If that mixture of products is then irradiated (h) in the presence of elemental bromine, those two compounds are converted to a mixture of four products with the molecular formula C 4 H8 BrCl. (a) Draw bond-line formulas for the two compounds formed with molecular formula C 4 H9 Cl.

(b) Draw bond-line formulas for the four compounds formed with molecular formula C 4 H8 BrCl.

(c) Two of the four C4 H8 BrCl compounds make up more than 99% of the product mixture formed by this sequential halogenation procedure. Circle them. (d) When the order of the halogenation steps is reversed, i.e., the bromination is performed before the chlorination, a single compound of formula C4 H8 BrCl makes up more than 99% of the product mixture. Draw a box around it. [Hint: It might or might not be the same as one of the compounds you have already circled.]

- 4 of 5 -

Chem 231

Exam I - September 27, 2000

13. (10 points) Draw a Newman projection for the lowest energy conformation of 2bromobutane (looking down the C2-C3 bond). Then draw a plot of potential energy vs torsional angle from 0 to 360 around the C2-C3 bond, with the torsional angle defined so that the lowest energy conformation corresponds to 0. You do not have to draw Newman projections for any of the other conformations, but be careful to show which energy maxima are higher than others, which energy minima are lower than others, and which conformations have the same energy.

14. (6 points) A Lewis structure for the cyanate ion is shown below. (a) Draw a second reasonable Lewis structure for the cyanate ion, being sure to indicate which atoms bear formal charges. (b) Circle whichever Lewis structure contributes more to the actual structure of the cyanate ion. (c) Briefly justify your answer to part (b).

O C N

15. (5 points) Which of the compounds below will have the largest dipole moment? Briefly show why.

H Cl-C C-Cl H

H CH2Br2 H CS2

16. (6 points) The bond-dissociation energy of H2 O is 119 kcal/mol, whereas the bonddissociation energy of HCl is 103 kcal/mol. When the Cl radical abstracts a hydrogen atom from an alkane, will it exhibit higher or lower selectivity than the OH radical in choosing tertiary vs secondary vs primary hydrogen atoms? Briefly tell why.

- 5 of 5 -

B OS TON C OL L E GEChem 231 - Organic Chemistry I Exam 2 October 18, 2000

name discussion section (time or TA) Please write your answers on the exam. Do not spend too much time on any one question.

question 1. (04 points) 2. (04 points) 3. (04 points) 4. (06 points) 5. (08 points) 6. (06 points) 7. (06 points) 8. (06 points) 9. (04 points) 10. (12 points) 11. (06 points) 12. (12 points) 13. (14 points) 14. (08 points) bonus (10 points) total

points

Chem 231

Exam 2 October 18, 2000

1. (4 points) Draw a molecular structure for each of the following compounds: (a) trans-1-chloro-3-isopropylcyclobutane (b) 4,4-bis(fluoromethyl)heptane

2. (4 points) Write the IUPAC name for each of the compounds shown below.

Br

Cl

3. (4 points) In the group of five compounds shown below, (a) circle the compound that should have the highest boiling point and (b) draw a box around the compound that should have the lowest boiling point.

Cl

Cl Cl

I

CF4

Br

4. (6 points) The heat of combustion for methylspiropentane (compound 1 below) is 782.0 kcal/mol, and the heat of combustion for cyclopropylcyclopropane (compound 2 below) is 778.3 kcal/mol. (a) Which of these two C6 H1 0 hydrocarbons is the more stable one? (b) What aspect of the structure of the less stable isomer would you conclude is causing that compound to be less stable than the other isomer?

1

2

- 2 of 7 -

Chem 231 5. (8 points) For each of the cyclohexane derivatives below, (a) Tell whether the groups are cis or trans, (b) flip the ring and draw the other chair conformation, and

Exam 2 October 18, 2000

(c) circle the more stable conformation for each compound (i.e., the original drawing vs yours).

Cl OH

CH3

6. (6 points) A colorless crystalline compound isolated from guaiac wood has been given the name guaiol. Its molecular formula (C1 5H2 6O) and structure strongly suggest that guaiol is a sesquiterpene. Given this information, which of the following structures is most likely correct for guaiol? Show your reasoning.

HO

HO

OH

7. (6 points) The groups on each stereogenic carbon atom below are labeled in order of decreasing priority according to the Cahn-Ingold-Prelog system (i.e., highest priority = a ... lowest priority = d). Indicate for each center whether it has the R or the S configuration.

b d c a a

b d c b

a c d

- 3 of 7 -

Chem 231

Exam 2 October 18, 2000

8. (6 points) Circle the substituent in each set below that has the highest priority in the Cahn-Ingold-Prelog system for assigning stereochemistry (R vs. S), and draw a square box around the substituent in each set that has the lowest priority.O CH2CH3 O CH2CH2CF3 C OOH

CH2OH

C OOH

N(CH3)2 N

CN

S

N CH3

9. (4 points) Fischer projections are shown below for four of the stereoisomers of 2,3,4trichloropentane.

CH3 H Cl H Cl H Cl CH3 A

CH3 Cl H H

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