Midterm exam November 2015, questions and answers PDF

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Student number:__________________________

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Queen’s University, Faculty of Arts and Science Department of Chemistry CHEMISTRY 281 (all sections) – General Organic Chemistry I

MIDTERM 2 SOLUTIONS Friday November 13th, 2015 7:00 to 9:00 pm Duration: 2 hrs Instructors: Profs. David L. Zechel and John Carran INSTRUCTIONS Please write your student number clearly on each page of the exam. Hand in all pages of the exam when complete. Part I: Multiple choice (20 questions, 3 marks each) • Mark your answers on the Scantron sheet using a soft-lead pencil AND on the exam (for backup). The scanner will not read ink. • Write and code your name and student number on the Scantron • Do not bend or fold the answer sheet in any way. Parts II: Lab Exam (10 questions, 1 mark each). Use the same Scantron sheet. Remember to code in your name and student number. Part III: Short answer questions (6 questions, 5 marks each). • Answer all questions on the exam paper. Reagents, solvents, periodic, and pKa tables are provided at the end of the exam. Molecular models and calculators are allowed as aids in this exam.

GOOD LUCK!! Part I #1-20

Part II Lab Exam #21-30

Part III Short answer #31

Total Lecture Component

60

10

30

90

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Part I: Lecture Multiple Choice (20 questions, 3 marks each)

1) The most stable conformation of 2,3-dibromobutane, viewed through the C-2—C-3 bond, is: CH3 Br

H

Br

H

H

H H Br Br

H3C H3C

CH3

I

H3C

H3C

CH3 H

III Br

Br

a) I

Br

II

H3C

Br

H

H

H

H3C

Br H

Br

CH3

IV

V

b) II

d) IV

e) V

2) Cis-trans isomers are: a) diastereomers. b) enantiomers. c) stereoisomers. d) constitutional isomers.

3) What is the most thermodynamically stable chair conformation for the substituents on cis-4-tertbutylcyclohexanol? a) both groups equatorial b) both groups axial d) tert-butyl group axial and OH group equitorial e) a boat conformation is adopted due to steric strain

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4) Which pair of structures represents the same compound? H H H

CH3 OH OH OH CH3

H HO H

CH3 OH H OH CH3

H HO HO

II

I

CH3 OH H H CH3

HO HO H

III

CH3 H H OH CH3

H H HO

IV

CH3 OH OH H CH3

V

A) I and II B) II and III C) III and IV E) IV and V

5) What can be said with certainty if a compound has [α] A) The compound has the (S) configuration. B) The compound has the (R) configuration. D) The compound possesses only one stereogenic center. E) The compound has an optical purity of less than 100%.

6) Name the following compound: OCH3

A) (cis)-3-methoxyhex-3-ene B) (Z)-4-methoxyhex-4-ene D) (E)-3-methoxyhex-3-ene E) 3-methoxyhex-3-ene

25 D

= -9.25 ?

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7) The preferred conformation of cis-3-tert-butyl-1-methylcyclohexane is the one in which: A) the tert-butyl group is axial and the methyl group is equatorial. B) the methyl group is axial and the tert-butyl group is equatorial. C) both groups are axial. E) the molecule exists in a boat conformation.

8) Select the structure for cis-3-methyl-6-vinylcyclohexene.

A) I C) III D) IV E) V

9) Enantiomers are: A) molecules that have a mirror image. B) molecules that have at least one stereogenic center. C) non-superposable molecules. D) non-superposable constitutional isomers.

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10) (R)-2-Chlorobutane is represented by: CH3 H3 C Cl

H

CH3 Cl

H3C

CH3

Cl

H3C

CH2CH3 Cl

H

CH2Cl H

CH2CH3

CH2CH3

CH2CH3

H

CH2CH3

I

II

III

IV

V

A) B)

I II

D) E)

IV V

11) I and II are: OH

HO Br

Br

I

II

A) B) C)

constitutional isomers. enantiomers. identical.

E)

not isomeric.

12) What is the percent composition of a mixture of (S)-(+)-2-butanol, [α] and (R)-(-)-2-butanol, [α] A) 75%(R) 25%(S) C) D) E)

50%(R) 50%(S) 67%(R) 33%(S) 33%(R) 67%(S)

25 D

= -13.52º, with a specific rotation [α]

25 D

= +13.52º, = +6.76º?

25 D

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13) Which structure represents (R,Z)-5-methylhept-2-ene? A)

B) C)

E) None of the above

14) How many stereogenic centers are there in Lovastatin (Mevacor®, a cholesterollowering drug) ? HO

O O

O

A) B) C) D)

(Lovastatin) 4 5 6 7

15) Which alkyl halide below will react most rapidly with NaCN in acetone?

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16) Which halide would be most reactive in the solvent HMPA? O P N N N hexamethylphosphoramide (HMPA)

(b)

Cl-

Br-

(c)

(d)

I-

17) What is the major product of the following reaction?

O

MeOH, cat. H2SO4

HO

?

OH OMe A

MeO

OMe MeO

C

D

18) According to molecular orbital theory, why does an SN2 reaction on an alkyl halide occur with inversion of stereochemical configuration? a) Because of repulsion from lone pair electrons on the leaving group. b) Because the maximum contribution of the filled C-X σ orbital is opposite of the leaving group. d) Because of steric crowding in the transition state.

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19) What are reactant X and product Y in the following sequence of reactions? O S Cl O reactant X

O O S O

pyridine

reactant X

NaN3 EtOH / H2O

product Y

A

OH

N3

C

OH

N3

D

OH

N3

20) What is the product of the following reaction?

product Y

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Part II: Virtual Lab Questions (10 questions, 1 mark each) NOTE: Answer these questions on the same scantron sheet (#21-30).

21) Which one of these solvents is heavier than water and could be used in a biphasic (two immiscible layers) extraction? a) Diethyl ether b) Dilute NaOH

d) Hexane e) Acetone

22) Compound A and compound B are dissolved in a methylene chloride solution, how would you separate the two? O

B

O

A

CH3 O

CH3 O

N H

OH

a) Filter the methylene chloride solution

c) Extract the methylene chloride solution with 1M NH4Cl d) Extract the methylene chloride solution with 1M HCl e) Extract with water

N H

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23) What procedure would you use to separate compound A and compound B into a solution of methylene chloride?

Compound A

Compound B

COOH Br

a) Extract with water b) Extract with water and NaCl c) Extract with dilute NH4Cl d) Extract with dilute HCl

24) When using a separatory funnel, what is the purpose of venting? a) To release unwanted gas b) To ensure the funnel does not explode c) So that you allow air into the mixture

e) a) and c)

25) Which compound is soluble in both hydrochloric acid and sodium hydroxide solutions?

b)

c)

COOH

COOH CH3

d)

e)

NH2 COOH

COOH NH2

CH3

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26) Where would you expect the following molecule to show an absorption peak in the IR spectrum? (Ignoring C–H absorptions)

-1

a) 3600 cm

-1

b) 3300 cm

-1

c) 2200 cm

-1

e) 1050 cm

27) Based on the following spectrum which is the most likely compound?

Both B and E are acceptable

A"

"

C"

"

D"

"

E"

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28) An unknown compound which contains oxygen shows absorption peaks at 1630–1750 cm-1 and 3200–3550 cm-1. The unknown most likely contains which functional group? a) An alcohol b) An aldehyde

d) A nitrile e) None of the above

29) Which of the following will not absorb infrared radiation? a) Quartz

c) Plastic d) Glass e) They will all absorb infrared radiation

30) Which is the appropriate method to clean the IR cells? a) Clean with Water

c) Clean with Methanol, Ethanol or Isopropanol d) Wash the cells in an acid bath and then base bath e) Just allow the cells to air dry in the fumehood

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Part III: Short Answer 31) What are the products of the following reactions? If more than one product is possible, provide the MAJOR product if appropriate. Racemic mixtures might be possible. Indicate explicit stereochemistry where necessary. (5 marks each, 30 marks total) a) NaOEt

I

OEt

EtOH 55 oC

I

3.5 marks substitution

5 marks

2.5 marks

-0.5 marks for minor structural or formal charge errors

b) N OTs H3 C

CH3

CH3

N3

NaN3 H3 C

EtOH

CH3

+

H3 C

N3

5 marks, SN1 products

H3 C

H3 C

CH3

Also OK for 5 marks (if other isomer given as well)

4 marks if only 1 isomer shown or "plus enantiomer" stated

CH3

OEt CH3

+

H3 C

OEt

4 marks, solvolysis products 3.5 marks if only one isomer shown or "plus enantiomer" stated

1 mark elimination

c)

NaCN Cl

Cl

Cl

acetone

CN

NC

Cl

NC

N C

or or Cl

C N

or

NC

CN

3 marks

Cl

N 5 marks

deduct 0.5 marks for minor structural or formal charge errors.

NC

N

N

N 2.5 marks

C

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d)

NaSCH3 O

CH3

CH3

H3CS

Et2O

H

O

H3CS

CH3

OH

H

ONa

H

H3CS

OH CH3

'edge-on' view =

CH3

H3CS

CH3

H3CS

H

H

ONa

'top' view 5 marks trans ring opening

CH3

H3CS

OH H 4.5 marks enantiomer

OH

H H3CS

CH3

HO

SCH3

H

missing Me group 4.5 marks

4 marks: incorrect regio, but still trans

SCH3

HO

CH3

H

3 marks: syn ring opening, either regioisomer

Additionally deduct 0.5 marks for minor structural or formal charge errors.

e) OH

HBr

ONa

Br

tBuOH, 60 oC 5 marks

5 marks H O

3 marks

O

H 3 marks

deduct 0.5 marks for minor structural or formal charge errors.

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Potentially useful reagents and solvents NaOMe / MeOH, Δ NaOEt / EtOH, Δ

O S Cl O tosyl chloride (TsCl)

O H3C S Cl O mesyl chloride (MsCl)

O F3C S Cl O triflic chloride (TfCl)

ONa sodium t-butoxide (t-BuONa)

Na+ C N (NaCN) Na+

t-BuONa / t-BuOH, Δ N

conc. HX (HCl, HBr, HI) pyridine

Solvent

Structure

O HO S OH O (H2SO4)

O HO P OH OH (H3PO4)

Abbreviation

Dielectric Constant (εε , at 25oC) 6

acetic acid

AcOH

acetonitrile acetone

MeCN or ACN -

benzene

-

2.3

tBuOH Et2O

11 9.1 4.3

dimethyl sulfoxide

DMSO

47

dimethyformamide

DMF

37

ethyl acetate

EtOAc

6

EtOH -

25 59

MeOH THF

1.9 33 7.6

tert-Butyl alcohol dichloromethane diethyl ether

ethanol formic acid hexane methanol tetrahydrofuran water

(CH3)3COH CH2Cl2

CH3CH2OH

CH3OH

H2 O

-

N N N (NaN3)

38 21

80

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pKa values

Note: pKa of an alkyl thiol, RCH2SH, is approximately 8 IR Frequencies

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