Prof. J. Sivaguru
Chem341 - Exam 1- [Green]
Exam 1: CHEM 341 Organic Chemistry I
September 29, 2010 – 8.00 pm to 8.50 AM
Answer Key
Name: _______________________________
student id:_____________________
There are 10 questions and each carry 10 points. The 11th Question is a bonus
question.
Question
1 – True / False
Points – allotted
5 questions x 2 points = 10
2 – Concept question 1
10
3 – Concept question 2
10
4 – Concept question 3
10
5 – Concept question 4
10
6 – Choose the best answer
5 questions x 2 points = 10
7 – Fill up
5 questions x 2 points = 10
8 – Bonus question
2
Total (70)
1/6
Points - earned
Prof. J. Sivaguru
Chem341 - Exam 1- [Green]
1) Answer True (T) or False (F) in the box provided for the statements below (10 points)
1-a)
CH3CH2CH2CH2CH2OH
T
is a polar molecule.
T
1-b) Lewis acids accept electrons when reacting.
F
1-c) The stronger acid has the larger (more positive) pKa.
1-d) The following structure represents the same molecule
T
1-e) Electronegativity is the ability of the atom to attract the proton.
F
2) Concept question 1 (10 points)
Write a more stable contributing structure for each of the following. Use curved arrows to show
how to transform the original Lewis formula to the new one. Be sure to specify formal charges if
any.
Ans:
(A) H
3C
N N
H3C N N
O
H
H
(B)
O H
O H
(C)
O
O
O
H 2C
O
(D)
O
H 2C
2/6
Prof. J. Sivaguru
Chem341 - Exam 1- [Green]
3) Concept question 2 (10 points)
Identify the more stable stereoisomer in the following pairs and give reason for your choice.
a) cis-1-Isopropyl-2-methylcyclohexane or trans-1-Isopropyl-2-methylcyclohexane
H
H
H
H
H
H
H
Ans:
H
CH3
H
H
CH3
cis
H
CH3
H
H
H CH3
H
H
H
H
CH3
H
trans
CH3
H
Trans isomer (stable) Isopropyl group and methyl group equatorial position.
Hence the groups will not have any steric strain (1,3-diaxial strain).
b)
t-Bu
t-Bu
t-Bu
= tert-butyl
Et = ethyl
Me = Methyl
or
Me
Ans:
H
H
H
H
(i)
CH3
H
C2H5
H
H
H
H H
H
H3C
Et
(ii)
H
H3C
H3C
Me
Et
(i)
H3C H
H3C
H3C
H
H
CH3
H
C2H5
H
H
(ii)
(ii) is more stable than (i) t-Bu, Et, Me groups are in equatorial position. Hence in
two there will not be any steric strain (1,3-diaxal interaction). On the other hand in
compound (i), there are two axial subsitutent making them less stable due to severe
steric strain (1,3-diaxal interaction).
4) Concept question 3 (10 points)
Write a structural formula for each of the following compounds:
a) 6-Isopropyl-2,3-dimethylnonane
b) 4-tert-Butyl-3-methylheptane
(note: Isopropyl group is 1-methylethyl group)
(note:tert-butyl group is 1,1-dimethylethyl group)
c) 4-Isobutyl-1,1-dimethylcyclohexane
(note: iso-butyl group is 2-methylpropyl group)
d) sec-butylcycloheptane
(note: sec-butyl group is 1-methylpropyl group)
3/6
Prof. J. Sivaguru
Chem341 - Exam 1- [Green]
5) Concept question 4 (10 points)
Assume the following reaction below
CH3
H3C
+ H-Br
H3C
H3C
CH3
H3C
Br
heat
OH
+
H2O
a) Will it undergo SN1 or SN2 reaction and reason why? (2 points)
The alcohol will undergo SN1 reaction as it is a tertiary alcohol. Hence an SN2 attack is not
feasible. Additionally, the carbocation formed will be a tertiary carbocation as shown in the
reaction sequence below..
b) Write a chemical equation. Use curved arrows to show the follow of electrons (5 points)
CH3
CH3
OH
H 3C
(A)
H
Br
OH2
H3C
Step - 1
CH3
CH3
CH3
CH3
H2O
OH2
H 3C
Br
Step - 2
H3C
CH3
CH3
CH3
CH3
Br
H 3C
Step - 3
Br
H3C
CH3
CH3
c) Draw a potential energy diagram (3 points)
Potential Energy Diagram
CH3
Energy
H3C
CH3
CH3
(A)
H3C
OH2
CH3
Br
Reaction Coordinate
6) Choose the best answer (10 points)
Which of the following best describes the relationship between the following two
6-a) structures?
A) identical compounds
B) resonance structures
C) constitutional isomers
D) different compounds with
different constitutions
Ans: A
4/6
Prof. J. Sivaguru
Chem341 - Exam 1- [Green]
6-b) The correct IUPAC name of the following is
A)
B)
C)
D)
Ans:
2-ethyl-5,7-dimethyloctane.
3,6,8-trimethylnonane.
7-ethyl-2,4-dimethyloctane.
2,4,7-trimethylnonane.
D
6-c) Which statement is correct concerning the relative stabilities of the two conformations, A
and B, below?
A)
B)
C)
D)
A is more stable.
B is more stable.
A and B are equal in stabilities.
A and B are not equal in stability, but the preferred
conformation cannot be determined
by inspection.
Ans: B
6-d) Which of the following mechanistically depicts the protonation of tert-butyl alcohol by
hydrogen bromide?
Ans: A
6-e) What is the nucleophile in the following substitution reaction?
Ans: C
5/6
Prof. J. Sivaguru
Chem341 - Exam 1- [Green]
7) Fill up the needed answer in the questions below Choose the best answer (10 points)
7-a) The formal charge on the carbon atom in the molecule below is ____-1______
7-b) Pka is defined as ___- log Ka_________________
7-c) The sawhorse drawing of butane below is a __gauche___ conformation.
7-d) The IUPAC name of the compound below is 2-chloro-6-isopropyl-3,6dimethylnonane_or 2-chloro-6-(1-methylethyl)-3,6-dimethylnonane
7-e) Fill in the substituents for of trans-1-ethyl-3-methylcyclohexane. It must be the most
stable conformation. You must also fill in all the hydrogen’s appropriately.
H
H
H
CH3
H
H
H
CH3
H
C
H2
H
H
H
8) Bonus question (2 points)
8-a) Which one of the following is the strongest acid?
A) FCH2CO2H B) ClCH2CO2H C) BrCH2CO2H
Ans: A
---- Best of luck ----
6/6
D) ICH2CO2H