Chem 2312-003 Midterm I F‘13  

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Chemistry 2312-003

Organic Chemistry I

Midterm Examination I 11:00 am – 12:20 pm

September 26, 2013

J. Jeon

Name: ___________Answer Key______________ Total /105 please print (last, first)

ID #: ______________________________________

• There are 102 points on this exam and the four exam pages. • For the multiple choice questions 1-15 (each 7 pts), select the best answer and darken

completely with a pencil the space corresponding to that answer in scantron (form # 882-E).

• You are free to use molecular models during the exam.

Have fun!

Chem 2312-003 Midterm I F‘13  

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1. (7 pts) Select the major resonance contributor.

NH3C NH3CH

1a 1bH

a) 1a

b) 1b

c) same

2. (7 pts) Select the best explanation regarding your choice above (1a vs. 1b in question 1).

a) charges b) octet rule c) atom size d) electronegativity e) molecular geometry

3-4. Questions 3 and 4 are about isomerism.

3. (7 pts) What is the relationship of the following pairs (3a and 3b)?

H

Et

H

OH

OH

HEt

H

3a 3b(1R,3S)-3-ethylcyclohexanol (1R,3R)-3-ethylcyclohexanol

Et OH

3a 3b

OH

Et

a) constitutional isomers b) conformational

isomers c) enantiomers d) diastereomers e) identical

◆ these molecules have same connectivity (constitution). ◆ conformational isomerism occurs within the same molecule, which has a temporarily different three-dimensional

shape. ◆ enantiomers are nonsuperimposable mirror images. ◆ diastereomers are nonsuperimposable and not mirror images. ◆ example of enantiomers

Et OH HO Et

H

Et

H

OH

H

HOEt

H

OHEt

4. (7 pts) What is the relationship of the following pairs (4a and 4b)?

Me

OH

OH

MeMe

OH

Me

HO

4a 4b

90°rotation Me

OH

OH

Me

4a

120°rotation

identical but conformational isomer

Me

OH

Me

OH

meso

a) constitutional isomers b) conformational isomers c) enantiomers d) diastereomers e) identical (Also I give a credit for this.)

Chem 2312-003 Midterm I F‘13  

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5. (7 pts) What is the total number of stereoisomers possible for the compound 5?

MeMe OH

Me

5* *

Me

Me OH

Me MeMe OH

Me MeMe OH

Me MeMe OH

Me

2n = possible stereoisomers (n = chirality center), 22 = 4

a) 3 b) 4 c) 5 d) 6 e) 7

6. (7 pts) How many π bonds are in acetylene 6?

HH

6

1 σ and 2 π bonds in a triple bond

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

7. (7 pts) This problem is about the IUPAC (International Union of Pure and Applied Chemistry) nomenclature of organic compounds. Select the best name for the following molecule 7.

Me

Me Me Et

Me

7

H3C

Me Me

CH37

CH3

1

8

H

(R)

a) (4S,6R)-2,4,6-Trimethyloctane

b) (4R,6S)-2,4,6-Trimethyloctane

c) (2S,4R)-2-Ethyl-4,6-diimethylheptane

d) (2R,6S)-6-Ethyl-2,4-diimethylheptane

e) (2R,4R,6R)-2-Ethyl-4,6-diimethylheptane

8. (7 pts) The following molecules, 8a and 8b, contain chirality centers. Which of the following compound(s) is (are) optically inactive? (find meso compounds)

Cl

OHH

OH

HClH

Cl

OH

HCl

OH

2b

OHHO

H

Cl

OH

HCl

OH

2b same as2a 2bsame as

then

Cl

OHH

Cl

OHHH

Cl

OH

OHH

Cl

2b

OHHO

H

Cl

OH

OHH

Cl

2b same as2a 2bsame as

a) 8a

b) 8b

c) all

d) none

Chem 2312-003 Midterm I F‘13  

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9-11. Consider the molecule “butane,” viewing the C2-C3 bond.

H

H

Me

HH

Me

Me

H H

HH

Me

H

Me

H

HH

Me

H

Me H

HH

Me

9c9b 9d9a

2

3

2 22 2

MeMe

H HH H

MeMe

HH

H

HHMe

HH

Me

HHMe

H MeH H

9g9f 9h9e

Interaction

methyl/methyl gauchemethyl/H eclipse

methyl/methyl eclipseH/H eclipse

Energy cost (KJ/mol)

3.86.011.0 4.0

2 22 2

antimost stable

eclipse gauche eclipse

6 kJ/mol

6 kJ/mol4 kJ/mol

3.8 kJ/mol11 kJ/mol

4 kJ/mol

4 kJ/mol

Me

H

H

HH

Me

H

H Me

HH

Me

22

same

9. (7 pts) Which of the following conformers (i.e. conformational isomers), 9a to 9d, is in the highest energy?

a) 9a b) 9b c) 9c d) 9d

Based on the table, two eclipsed conformations. But, 9d is in a higher energy level.

10. (7 pts) Based on the table regarding energy costs, what is the total energy cost of the conformation 9f?

a) 0.0 KJ/mol b) 9.8 KJ/mol c) 3.8 KJ/mol d) 19 KJ/mol e) 16 KJ/mol

11. (7 pts) Match the same conformers (i.e. conformational isomers)?

a) 9a and 9h b) 9b and 9g c) 9c and 9e d) 9d and 9f e) none

Chem 2312-003 Midterm I F‘13  

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12. (7 pts) The following acid-base (proton transfer) reaction is in equilibrium. Which of the following statements is correct?

O NH O NHHa Hb

12a 12b

Me Me

a) Ha in 12a is less acidic than Hb in 12b.

Oxygen stabilizes anion better than nitrogen due to electronegativity of atoms; the corresponding acid (Ha) is more acidic. (H2O vs NH3)

b) Keq is [12a]/[12b]. Keq = [12b]/[12a].

c) None of 12a and 12b contains a chirality center. one each

d) This is an intermolecular proton transfer reaction.

intramolecular

e) The reactants 12a will be favored at equilibrium (Keq > 1).

equilibrium rises toward more stable anion.

13. (7 pts) Chair conformations of dimethylcyclohexanes, 13a to 13d are presented below as Newman projections. Select the most stable conformation.

H

Me

H

H

H

Me

H

H

Me

H

H

H

H

Me

H

H

13b13a

Me

H

H

H

Me

H

H

H

Me

H

H

H

H

H

H

Me

13d13c

1

2

equitorial equitorial equitorial

axial axial axial

axial

1,3-diaxial interaction 1,3-diaxial interaction

1,3-diaxial interaction

1,3-diaxial interaction

a) 13a b) 13b c) 13d d) 13d

14. (7 pts) Consider two bolded hydrogens, Ha and Hb in compound 14. Select the hydrogen with the lower pKa (i.e. more acidic).

Ha

CF3

Hb

CH3

14 Ha is closer to electronegative atoms (CF3) than Hb.

a) Ha

b) Hb

c) same

d) none

15. (7 pts) Select the best explanation regarding your choice above (in question 5).

a) atoms (bearing the charge) b) atom size c) resonance d) inductive effect (through σ bond) e) orbitals

***** End of Exam *****