practice 2b

7/29/2019 Practice 2B

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Chem 210 Practice Exam 2B

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Organic Chemistry: Chem 210

Practice Exam 2B

There are 33 questions on this exam. Check that you have done all of the problems and filled in the first33bubbles on the scantron. Most questions are worth 4 points; there are several two-point and one-pointquestions clearly labeled in the text. The maximum score on this exam is 100 points.

InstructionsAnswer sheet

1) On the scantron, you need to clearly fill:

yournameand yourstudent number, section number: (it is 001) test form(white = test form A; yellow = test form B).

2) Use a #2 pencil

Exam policy

1)No electronic devices of any kind, such as calculators, cell phones, or even more advanceddigital watches, are allowed. Possession of such devices during the exam, 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. If necessary, the values

from the provided tables should be used, even if they differ from values that you may

remember from different sources.

6)The exam results are based strictly on scantrons marks. No extraneous information is usedto adjust the scores. Mark your choices with extra care.

7)Feel free to take this copy of the exam with you. The answer key will be posted on the webafter the exam (under "News").

Hints

1)As you read the question, underline or circle key words to highlight them for yourself.2)Questions have only one correct answer, unless indicated otherwise. No partial credit will

be given.

3)There is no penalty for guessing.


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Notes: I. The stereochemistry conventions are illustrated below:

Br

OH

Br

OH

Br

OH

Br

OH

No stereochemistry shown(all stereoisomers possible)

Absolute stereochemistryshown

Relative stereochemistryshown (racemic mixture)

Absolute stereochemistry onone center, unspecified

stereochemistry on the other

II. Abbreviations: Me = methyl, Et = ethyl, Pr = propyl, Bu = butyl, Pe = pentyl, Ph = phenyl

(i.e. benzene as a substituent), AcO = acetate (H3CCOO).

*************************************************************************************

1. (4 pts) A radical-chain reaction of ethane with chlorine may proceed according to one of the followingmechanisms. Assuming that all initiation and termination steps are equally efficient, which of the following

propagation-step sequences is the most likely?

a) Cl + CH3CH3 HCl + CH3CH2CH3CH2 + Cl2 CH3CH2Cl + Cl

b) Cl + CH3CH3 CH3CH2Cl + HH + Cl2 HCl + Cl

c) Cl + CH3CH3 CH3Cl + H3CH3C + Cl2 CH3Cl + Cl

d) Cl + CH3CH3 HCl + CH3CH2CH3CH2 H + H2C=CH2H + Cl2 HCl + ClH2C=CH2 + HCl CH3CH2Cl

2. (4 pts) Which of the following is the most stabilized carbocation?

OMe OMe OMe

OMeOMe

a) b) c)

d) e)


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3. (4 pts) Consider the following energy profiles where A reacts with reagent R and gets converted to C, and Breacts with the same reagent (R) under identical conditions and gets converted to D. Compounds A and B areenantiomers of each other. Which statement about the transition statesfor these transformations must betrue?

a) The two transition states are enantiomers.

b) The two transition states are diastereomers

c) The two transition states are E/Z isomers.d) The two transition states are constitutional isomers.

e) The two transition states are superimposable mirror images of each other.

4. (4 pts) Which alkene will yield the following alkyl bromide as one of the major products of reaction with HBr?

Br

a) b) c)

d) e)

C

D

A+R

B+R


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5. (4 pts) Which of the following structures represents the lowest energy form of (1R, 2S, 4R)-4-t-butyl-1-chloro-2-methylcyclohexane?

Cl

ClCl

Cl

Cl

a) b) c)

d) e)

6-13. (1 pt each) Fill in the missing words in the passage below with words from the provided list to preserve bestthe meaning of the paragraph. Not all words have to be used. Some words may be used more than once.

Compounds that have the same atom connectivity, but differ from each other only in how their atoms are

arranged in space are known as ___(6)___ . These isomers may be classified into ___(7)___ isomers,which are interconvertible by rotation around single bonds, and ___(8)___ isomers, that have stereogeniccenters and are not convertible by rotations around single bonds. The isomers that belong to the latter

group can be either ___(9)___, which are related to each other as nonsuperimposable mirror images, or___(10)___ , which include all other ___(11)___. The last subgroup also includes E/Z isomers. Exceptfor optical activity, ___(12)___ have identical physical and chemical properties. In contrast, ___(13)___have different properties, and may not show optical activity, even if pure.

a) enantiomers d) racemic g) configurationalb) constitutional e) conformational h) isomers

c) diastereomers f) chiral i) stereoisomers

14. (4 pts) Which of the following best describes the electronic structure of the cation formed by protonation ofpropyne (H3CCCH)?

a) It is an allylic carbocation with sp2-hybridized cationic center and stabilized mainly by resonance

b) It is a vinylic carbocation with sp2-hybridized cationic center and stabilized by resonance and

hyperconjugationc) It is a 3o alkyl carbocation with sp2-hybridized cationic center and stabilized mainly by

hyperconjugation.

d) It is a 2o alkyl carbocation with sp3-hybridized cationic center and stabilized by hyperconjugation and

resonancee) It is a vinylic carbocation with sp-hybridized cationic center and stabilized mainly by hyperconjugation


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15. (4 pts) Racemic 3-methylhexane yields all possible monochloro derivatives when allowed to react with Cl2 ina radical chain reaction. How many different stereoisomers of 2-chloro-3-methylhexane are going to be

found in the mixture of products?

a) 2 b) 3 c) 4 d) 5 e) 6

16. (4 pts) 1,3-Butadiene (H2C=CH-CH=CH2) reacts with one equivalent of HCl (only mono addition takesplace). How many different structures (including stereoisomers) will form?

a) 1 b) 2 c) 3 d) 4 e) 5 f) 6

17. (4 pts) Allegra, a common prescription drug with the structure shown below, is given for the treatment ofseasonal allergies. How many stereogenic carbons does Allegra possess?

N

HO

H

CH3

CH3

COOH

OH

a) 1 b) 2 c) 3 d) 4 e) 5

18. (4 pts) Consider the addition of HCl to the following compounds. Which compound would be expected toreact the fastest?

H H

HH

H H

CH3H

H

OCH3H

H

Cl

HH

H

H

CH3H3C

H

a) b) c)

d) e)


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19. (4 pts) Taking into account the stability of various cycloalkanes and carbocations, as well as the rulesgoverning mechanisms of carbocation rearrangements, which reaction is most likely to occur?

H-Br ?

Br

Bra)

Br

Br

b)

Br

c) Br

BrBr

d)

Bre)

Br

20. (4 pts) What is the IUPAC name of the following molecule?

a) (2Z, 5Z, 7E, 4R)-4,6,9-trimethyl-2,5,7-decatrieneb) (3E, 5Z, 8E, 7R)-2,5,7-trimethyl-3,5,8-decatrienec) (2E, 5E, 7Z, 4S)-4,6,9-trimethyl-2,5,7-decatriened) (2Z, 5Z, 7Z, 4S)-4,6,9-trimethyl-2,5,7-decatrienee) (3Z, 5E, 8Z, 7R)-2,5,7-trimethyl-3,5,8-decatriene


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21. (4 pts) Consider the following addition reaction where the overall reaction is exergonic.

HH Br

H

H

BrH

HBr

Which of the following energy profiles best represents the thermodynamics of the overall reaction?

a) b) c)

d) e)

22. (4 pts) Which compound is the enantiomer of1?

ClH

H

Cl H

H

1a) b) c) d)

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl


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23. (4 pts) Hydride acts as a nucleophile in the following reaction with formaldehyde. Based on your knowledgeof the electronic structures of these compounds, what are the critical HOMO and LUMO of the reaction?

H

O

HH +

H

O

HH

formaldehyde

HOMO LUMO

a) sorbital of hydride orbital of formaldehyde

b) sorbital of hydride orbital of formaldehyde

c) porbital of hydride orbital of formaldehyde

d) orbital of formaldehyde sorbital of hydride

e) orbital of formaldehyde with * orbital of hydride

non-bonding electrons

24. (4 pts) An aqueous solution containing compounds A and B shows optical activity. A and B are

stereoisomers. Which of the following possibilities cannotbecorrect?

a) A has a plane of symmetry and B is a meso isomer.b) Both A and B contain at least one chiral center.c) A and B are enantiomers.d) A and B are diastereomers.e) A and B are not present in equal amounts.

25. (4 pts) How many stereoisomersare possible for the following molecule?

H3CH CHCH=CHCOOH

a) 1 b) 2 c) 3 d) 4 e) 5 f) 6 g) 7 h) 8

26. (4 pts) Which of the following orbital pictures is the best representation of the critical LUMO for the

nucleophilic "back-side" substitution by the hydroxide ion (HO) on methyl iodide (CH3I)?

f)e)d)

c)b)a)


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27-30.Use the reaction profile below to answer the following questions.

27. (2 pts.) What is the value ofG (in kcal/mol) for the transformation ofV to VI I?

a) 2 b) 1 c) 0 d) +1 e) +5

28. (2 pts.) What is the free energy (in kcal/mol) of the highest-energy transition state?

a) 3 b) 5 c) 6 d) 7 e) 9

29. (2 pts.) Which of the following statements is false?

a) IV represents the highest energy intermediateb) I I is more stable than VI c) I I represents the most stable transition stated) the overall reaction from VI I to I is exergonic

30. (2 pts.) What is the fastest step in either direction?

a) I to II I b) II I to V c) V to VI I d) VI I to V e) V to I I I f) I II to I

31. (4 pts) Which of the following molecules is/are chiral?

Cl

Br BrH H OH

OH

H

CH3

Cl

H

CH3

Cl

A B C

a) A b) B c) C d) A, B e) A, C f) B, C g) A, B, and C

-10123456789

10

reaction progress

freeenergy(kcal/mol)

I

II

I II

IV

V

VI

VII


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32. (4 pts) A compound was synthesized by a student in Chem 213, but its structure was not identified. However,his wonderfully helpful instructor told him that it was a mesocompound with 5 carbons. Which of thefollowing structures should the student consider as possibilities for his compound?

BrBr HO2C

CO2HOH

OH

Br

Br

OHOHBrBr

A BC

D E

a) A, B, D b) B, D c) A, C, E d) B, D, E e) A, B, D, E

33. (4 pts) The heats of formation (Hf) and the heats of hydrogenation (Hh) of several isomeric pentenes aregiven below. By how much (in kcal/mol) is 2-methylbutane more stable than pentane?

a) 1.0 b) 1.5 c) 2.0 d) 2.5 e) 3.0 f) 3.5

-- End of the exam --

Hf(kcal/mol) 5.0 7.5 6.5 9.0 10.0

Hh (kcal/mol) 30.0 27.5 28.5 28.0 27.0


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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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Answer key: 2B:

1. A2. D3. B4. B5. D6. I (1 p)7. E (1 p)8. G (1 p)9. A (1 p)10. C (1 p)11. I (1 p)12. A (1 p)13. C (1 p)14. E15. C16. D17. A18. C

19. A20. A21. E22. B23. B24. A25. D26. F27. B (2 p)28. E (2 p)29. A (2 p)30. F (2 p)31. D

32. A33. C

practice 2b

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