practice 4c

7/29/2019 Practice 4C

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Chem 210 Practice Exam 4C

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Organic Chemist ry: Chem 210

Practice Exam 4C

There are34questions on this exam. Check that you have done all of the problems andfilled in the first34bubbles on the scantron. Most questions are worth 4 points; there areseveral 1 and 2-point questions clearly labeled in the text.

InstructionsAnswer sheet

1) On the scantron, you need to clearly fill: your nameand yourstudent number,section number

Section 001, 12:20 pm lectureSection 002, 3:35 pm lecture

test form (white =test form A; yellow =test form B; blue =test form C, green

=test form D).2) Use a #2 pencil

Exam policy

1)No electronic devices of any kind, such as calculators, cell phones, or even moreadvanced digital watches, are allowed. Possession of such devices during theexam, whether in use or not, is grounds for awarding a zero on the exam.

2)Molecular models are allowed (no instruction pages are permitted, however).3)There are some blank pages at the end of the test that can be used as scratch paper.4)Relevant tables, including the periodic table, are attached at the end of this exam.5)Numerical values given in one question apply only to that question, and should not

be used in other questions, unless there is a specific instruction to do so. Ifnecessary, the values from the provided tables should be used, even if they differfrom values that you may remember from different sources.

6)The exam results are based strictly on scantrons marks. No extraneousinformation is used to adjust the scores. Mark your choices with extra care.

7)You may not take the exam with you. Y ou may copy your answers on theprovided page and compare them with the answer key. The answer key will beposted on the web after the exam (under "News").

Hints

1) As you read the question, underline or circle key words to highlight them foryourself.

2) Questions have only one correct answer. No partial credit will be given.3) There is no penalty for guessing.


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Notes: I. When listing reagents, a slash (/) is used between reagents that are employedsimultaneously, and a semicolon (;) is used between reagents that are employed in astepwise fashion.

II. The stereochemistry conventions are illustrated below:

Br

OH

Br

OH

Br

OH

Br

OHNo stereochemistry shown(all stereoisomers possible)

Absolute stereochemistryshown

Relative stereochemistryshown (racemic mixture)

Absolute stereochemistry onone center, unspecified

stereochemistry on the other

III. Abbreviations: Me =methyl, Et =ethyl, Pr =propyl, Bu =butyl, Pe =pentyl, Ph =phenyl (i.e. benzene as a substituent), AcO =acetate (H3CCOO), MCPBA =m-chloroperbenzoic acid., TosO =TsO =tosylate

1. (4 pts) One can exchange127I with128I in some iodo compounds. When thereaction is run on optically active (S)-2-iodobutane, the rate of racemization (i.e.the rate of loss of optical activity) is found to be exactly twice the initial rate ofexchange (128I for 127I). Which reaction mechanism is most consistent with thisobservation?

NOTE: Optical rotation does not depend on the isotope present, i.e. (S)-2-iodobutane with127I has the same rotation as (S)-2-iodobutane with128I.

127

S-2-Iodobutane

+ CH3CH2CHCH3

I128

I

a) SN1 (free ions) b) SN2 c) E1 e) E2

2. (4 pts) Addition of Br2 tocis-2-hexene gives racemic products, even though attackof the bromide ion on the unsymmetrical bromonium ion intermediate is notequally likely at both carbons. The reasons for the racemic products are:

a) The reaction is reversible, after a while both enantiomers are produced inequal amounts

b) The product is a meso compoundc) The bromonium ion formed has a plane of symmetryd) Two enantiomeric bromonium ions are formed in equal amounts

e) The Br attack is equally likely from both sides of the bromonium ionf) Four different stereoisomers are formed, but all in equal amounts


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3. (4 pts) Reaction of HBr with 3-bromocyclohexene (A) yields only trans-1,2-dibromo-cyclohexane. Reaction of 3-methylcyclohexene (B) with HBr gives amixture ofcis- andtrans-1-bromo-3-methylcyclohexane andcis- andtrans-1-bromo-2-methylcyclohexane. Which statement explains the reactivity differencesbetweenA andB?

a) There is no difference in reactivity. In the case ofA the 1,2 and 1,3dibromo products cannot be distinguished.

b) The reactions ofA andB have totally different mechanisms: one is radicalthe other ionic.

c) A bromonium ion formed in the case ofA can be attacked only in the antimanner

d) A has a chiral center andB has a plane of symmetrye) In the case ofB, there is a hydride shift following the protonation

4. (4 pts) What is the structure of the species formed by removing hydride frommethane?a) pyramidal, carbon is sp3 hybridizedb) planar, carbon is sp2 hybridizedc) tetrahedrald) T-shaped, with angles between C-H bonds of 90 degrees

5. (4 pts) Two samples, A andB, are isomers and have identical physical properties

except for optical rotation. SampleA has []D =- 17o. SampleB has []D =+17

oWhatmust betrue aboutA andB?

a) A andB are diastereoisomersb) A andB have at least one chiral center eachc) A andB are enantiomersd) A hasRandB hasSabsolute configuratione) A andB are structurally unrelatedf) A can be converted toB by an SN2 process

6. (4 pts) The best way to accomplish the following transformation

Br O CH2CH3

is to use the following conditions:

a) CH3CH2O Na+in ethanol

b) CH3CH2OH and heatc) CH3CH2OH in water

d) CH3CH2O Na+in DMSO


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7. (4 pts) For the following series of reactions, what is the absolute configuration ofthe chiral centers produced?

catalyst

C C CH3H3C Product(s)H2

Lindlar's

Br2

CCl4

a) 2S, 3Sb) 2R, 3Rc) 2S, 3Rd) 2R, 3Se) 50% (a) and 50% (b)f) 50% (c) and 50% (d)g) 25% (a), 25% (b), 25% (c) and 25% (d)h) a meso form and 50% (a) and (b)i) a meso form and 50% (c) and (d)j) only a meso form is produced

8. (2 pts) Which series of reactions will achieve the following transformation?

Br

Cl

Cl

1. 2.

1. 2.

a) HBr Cl2b) Cl2 HBr

c) Cl2 Br2d) HBr Cl2

e) Cl2 NBS/h

f) NBS/h Cl2g) HCl HBrh) HBr HCli) Br2 @ -40

o C HCl


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9-12. The seven lowest energy molecular orbitals for ethylene (on the left) andformaldehyde (on the right) are shown below.

9. What is the HOMO for ethylene ? (2 pts)

10. What is the LUMO for ethylene ? (2 pts)

11. What is the HOMO for formaldehyde ? (2 pts)

12. What is the LUMO for formaldehyde ? (2 pts)


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13. (2 pts) Which of the following will show optical activity?

CH3C

H

CH3

H H H

H3C CH3

Cl

Br

Br

CH3

H

H

H3C

A B C D

a) A andB b) A andC c) B andC d) B andD e) C andD

14. (4 pts) (1S, 2S)-1-Bromo-1-deutero-2-methylbutane is converted to 1,2-dibromo-1-deutero-2-methylbutane in a two step process. What is the stereochemistry of themajor product(s)? Note: D =2H.

a. (1S,2R) e. (1S, 2R) and (1R, 2S)b. (1R, 2S) f. (1S, 2R) and (1R, 2R)c. (1S, 2S) g. (1S, 2R) and (1S, 2S)d. (1R, 2R) h. (1S,2S) and (1R, 2R)

15 (4 pts) Which of the listed molecules is the product of the following reaction?

O3

Zn/H3O+

1)

2)?

(a) OH

OH

(b)O

(c)H

OO

(d)OO

(e) HO OH

O

O

(f) H

O

O

Br

Br

H

D

tBuOK

Br

H

D tBuOH

Br2

CCl4


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What reagents are necessary to carry out reaction 16-19(2 pts each)?

(a) OsO4/NaHSO3 (b) BH3/THF; H2O2/OH (c) Br2/H2O

(d) KMnO4/H3O+ (e) NBS/CCl4/h (f) Hg(OAc)2/H2O; NaBH4

(g) HBr/ether (h) O3; Zn/HOAc (i) KMnO4/OH

(j) BH3/THF; H2O2/OH

PBr3

16

O

O

OH

17

Br

18

Br

19OH

20. (4 pts) What is the correct mechanism of reaction between formaldehyde and

boron trifluoride?

(a)

(b)

(c)

(d)

C

O

H H

+ BF3 CH H

O

BF3

C

O

H H

+ BF3 CH H

OBF3

C

O

H H

+ BF3 CH H

OBF3

C

O

H H

+ BF3 CH H

O

BF3


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21. (4 pts) The major product of the following sequence of reactions is:

NaOH

CH3CH2OH

NBS, H2O

DMSO

(a) (e)

O

CH3H3C

HH(c)

O

HH3C

CH3H

(b)

OH

Br H

H3C

CH3

H (f) CH3C C CH3(d)

OH

OH

H3C

HCH3H

22. (4 pts) The lowest energy conformation of (1S)(2R)-1-bromo-2-methylcyclohexane is best represented by:

BrBr

BrBr

BrBr

(a)

(b)

(c)

(d)

(e)

(f)


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23. (4 pts)Which of the following compounds is likely to form during the NBSbromination of 4-methylcyclopentene?

24. (4 pts) The major product of the following reaction is:

OH Br2H2O

?

(a)

(d)

OH

OH

Br OH

OH

Br OH

Br

HO

OH

Br

HO

OCH2Br

H OH

CH2Br

(b) (c)

(f)(e)

(g) (a) and (b) (h) (c) and (d) (i) (e) and (f) (j) none of the above

25. (2 pts) Which of the following compounds is (are) not aromatic:

f) a and b g) c and d h) b and c i) b and d

N

N H H

H H Na

a) b) c) d) e)

NBS

h, CCl4

Br

Br

Br

Br

Br Br

Br

a) b) c)

d) e) f)


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26. (4 pts) For the following reaction given below:

OCH3

OTos

CH3CO2H

(A)

OCH3

O

OCCH3

(RandS)

I . What type of mechanism is most likely to be involved?II . What happens to the rate of the reaction if the concentration of (A) isdoubled?

I. I I.a) SN1 no changeb) SN1 doublesc) SN1 quadruplesd) SN2 doublese) SN2 quadruplesf) E1 no changeg) E1 doublesh) E2 no changei) E2 doubles


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27. (4 pts) What is the most plausible mechanism for the following reaction ?

Na+ ?

CH3CHCH3

Br

+CH3C C

(a)

(b)

(c)

(d)

(e)

(f)

+CH3C CH2C CH CH3 + NaBr

+ CH3C CH

CH3C C CH3CHCH3

Br

CH3C CCH(CH3)2 + NaBr

CH3C C CH3CHCH3

Br

+ CH3CHCH3 CH3C CCH(CH3)2

CH3C C

HCH3

HH

BrHH2C CHCH3 + CH3C CH

CH3C C CH3CHCH3

Br

+ CH3C CH + CH3CH CH2

CH3C C CH3CHCH3

Br

+ CH3C CH

Br

CH3CHCH3

CH3CCH2

Br

CHH3C CH3

Na+

CH2CHCH3

Br

H

Na+ +

Na+

- NaBr

Na+ +

Na+

+ NaBr

+ NaBr

Na+

Na+


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NOTE: The following statement corresponds to questions28, 29, and 30.

HydrocarbonA (C7H12) was treated with BH3followed by H2O2/NaOH toprovideB (C7H14O) as theonly product. Reaction ofB with TsCl/pyridinefollowed by KOH givesC (isomeric withA) in addition to other olefin(s).

Treatment ofC with ozone, followed by Zn/AcOH produces only the compoundshown below:

H3CCHO

CHO

28. What is the correct structure for A ? (4 pts)

(a) (d)

(g)

(b)

(e)

(c)

(f)

29. What is the correct structure for B ? (2 pts)

OH

HO OH(e) (f) (g)

OH

OHOH

HO

(a) (b) (c) (d)


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30. What is the correct structure for C ? (2 pts)

(a) (b) (c) (d)

(e) (f) (g)

31-34. (1 pts each) Questions31-34 involve reactions of an alkyl bromide and twopossible reagents. Which reaction in each pair will show the faster rate ofdisappearance (consumption) of the alkyl bromide?

31.

32.

33.

34.

end of the exam

CH3

CH3

Br +

a. CN-

b. HS-

c. The reaction rates would be the same.

Br+


a. (CH3)3B

(CH3)3Nb.

BrH3C +

a. CH3O-

(assume protic solvent)

CH3S-

(assume protic solvent)b.


Br +


b. EtOH

EtOH in H2Oa.


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Total Strain Energies in Cycloalkanes

ring size kJ/mol kcal/mol

3 115 27.64 110 26.45 27 6.56 0 07 26 6.38 40 8.6

Energy Costs for Interactions in AlkaneConformers

Interaction kJ/mole kcal/mole

H-H eclipsed 4.0 1.0

H-CH3 eclipsed 6.0 1.4

CH3-CH3 eclipsed 11.0 2.6

CH3-CH3 gauche 3.8 0.9


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Selectivity of halogen radicals (perH)

3o 2o 1oF 2 1 1Cl 5 3.5 1Br 1600 80 1I 97000 1100 1


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Key4C Answers

1 B 4p

2 D 4p

3 C4p

4 B 4p5 C4p

6 B 4p

7 E2p

8 F4p

9 B 4p

10 A 2p

11 B 2p

12 A 2p

13 B 4p

14 E4p;

15 F2p16 D 2p

17 J2p

18 E2p

19 B 4p

20 B 4p

21 E4p

22 C4p

23 B 4p

24 I4p

25 H2p

26 B 4p27 D 4p

28 A 4p

29 E2p

30 G 2p

31 B 1p

32 B 1p

33 A 1p

34 C1p

practice 4c

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