Prof. J. Sivaguru
Chem341 - Exam 3- [White]
Exam 3: CHEM 341 Organic Chemistry I
November 22, 2010 – 8.00 AM to 8.50 AM
Name: _______________________________
Student id:_____________________
Question
Points – allotted
Points - earned
Answer only 3 concept questions (There are total of 4). Do not try all 4 as you might
not have enough time.
1 – Concept question 1
20
2 – Concept question 2
20
3 – Concept question 3
20
4 – Concept question 4
20
5 – True / False
10
6 – Choose the best answer
15
7 – Fill up the blanks/box
10
8 – Bonus question
10
Total (95)
Chemistry and Biochemistry
1/7
North Dakota State University, Fargo, ND. USA.
Prof. J. Sivaguru
Chem341 - Exam 3- [White]
Concept Questions (Answer any 3 questions)
1) Concept Question – 1 (20 points)
Give the correct reagents for the transformations below
(A) Ph
C
Ph
CH
Ph
C
CH
(C) Ph
C
C
CH3
(D) Ph
C
C
CH3
C
Ph
intermediate
(B) Ph
C
C
CH3
O
O
Ph
O
OH
HO
CH3
H
H
C
Ph
+
C
CH3
2) Concept Question - 2 (20 points)
Consider the Diels-Alder reaction of 1,3-butadiene and ethene below.
(A) Fill the transition state and the product below (5 points)
+
Transition state
Product
(B) Fill the molecular orbital of 1,3-butadiene and ethene. For clarity the p-orbital lobes are provided
below. Shade them correctly and indicate the HOMO and LUMO levels. (7 points)
Chemistry and Biochemistry
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North Dakota State University, Fargo, ND. USA.
Prof. J. Sivaguru
Chem341 - Exam 3- [White]
1,3-butadiene
ethene
(C) Write the structure of the product in the Diels alder reaction. If there is a possibility of
stereoisomers (write both of them) and indicate the relation between them (enantiomers;
diastereomers or meso) (7 points)
CN
+
CN
CN
+
NC
Chemistry and Biochemistry
3/7
North Dakota State University, Fargo, ND. USA.
Prof. J. Sivaguru
Chem341 - Exam 3- [White]
3) Concept Question - 3 (20 points)
Consider the SN1 reaction of (1S,2R)-1-Bromo,2,3-dimethylcyclohexane with NaSH (structure
given below).
CH3
Br
CH3
Na SH
SN1
(A) Draw the correct stereochemistry (1S, 2R isomer) of the reactant and correct
stereochemistry of products (indicate the R and S configuration in the product). Label the nature
of products (enantiomers, meso, diastereomers). Reason your product stereochemistry
(inversion/retention etc) for the reaction sequence [9 points]
(B): Write the reaction mechanism the intermediate with arrow pushing. (You must show the
correct way of arrow pushing) [8 points].
(C): Draw the Potential energy reaction diagram showing for the above reaction showing the
reactant, product, intermediate [3 points]
Chemistry and Biochemistry
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North Dakota State University, Fargo, ND. USA.
Prof. J. Sivaguru
Chem341 - Exam 3- [White]
4) Concept Question - 4 (20 points)
Fill in the appropriate products in the box
(A) Reaction sequence (8 points)
Na
NH3
H+/ H2O
O
S Cl
O
TsCl =
NaSH
(B) Reaction sequence 2 (12 points)
HBr
-80 oC
NaCN
Kinetic controlled
product
Br2
1) 3 NaNH2, NH3
H2O, H2SO4
2) H2O
HgSO4
Keto-enol tautomers
5) True or False (5 x 2 points: 10 points)
5-a) In the SN2 reaction, the "2" stands for two steps in the reaction.
5-b) A good leaving group has to be a very strong base.
Chemistry and Biochemistry
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North Dakota State University, Fargo, ND. USA.
Prof. J. Sivaguru
Chem341 - Exam 3- [White]
5-c) Polar aprotic solvents like DMSO favor SN2 reactions.
5-d) OH – is a very good leaving group.
5-e) In a solvolysis reaction, solvent can act as a nucleophile.
6) Choose the best answer (5 x 3 points = 15 points)
6-a. Which of the following is not true concerning the structure of benzene?
A)
All C-C-C bond angles are exactly 120o.
B)
The six hydrogens are equivalent.
C)
The carbon-carbon bonds rapidly alternate between single and triple bonds.
D)
The p bonds are completely conjugated.
6-b. Which of the following are conjugated dienes?
I. 1,2-octadiene II. 1,3-octadiene III. 2,5-octadiene
A) only I B) I and II C) II and III D) only II
IV. 1,7-octadiene
6-c. Which of the following is not true concerning the addition of HBr to 1,3-butadiene?
A)
The intermediate is an allylic carbocation.
B)
A carbocation rearrangement leads to the 1,4-addition product.
C)
The 1,4-addition product is the thermodynamically controlled product.
D)
The reaction mechanism has two steps.
6-d. Which one of the following describes the C(2)-C(3) sigma bond in the structure below?
A) sp-sp
B) sp2-sp2
C) sp3-sp3
D) 2p-2p
6-e. For nucleophilic substitution involving alcohols, the OH group is quite often converted to
Tosylates (OTs) as:
A) Tosylates are easy to make
B) Tosylates are more stable than alcohol
C) Tosylates excellent leaving group.
D) Tosylates are excellent nucleophiles.
7) Fill up the blanks (5 x 2 points: 10 points)
7-a) The number of vinylic hydrogens in the compound below is _____________
7-b) The product (1,2-addition product) that is formed faster in the addition of HBr to a diene is called
________________ product.
7-c) LUMO stands for ___________________________________________.
Chemistry and Biochemistry
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North Dakota State University, Fargo, ND. USA.
Prof. J. Sivaguru
Chem341 - Exam 3- [White]
7-d) Benzene is an example of ______________ compound
7-e) The special stability of aromatic compound is due to ________________.
8) Bonus question – (10 points)
Bq-1. Which halide ion reacts the fastest with cyclopentyl p-toluenesulfonate in ethanol/water?
A) F
-
B) Cl
-
C) Br
-
D) I
-
Bq-2. Which of the following mechanistic steps is the most likely route for the formation of the cyclic
ether shown?
Bq-3. What major product results from this reaction?
O
O
O3
A
B
?
H2O
CO2H
CO2H
C
O
D
**** Best of Luck ***
Chemistry and Biochemistry
7/7
North Dakota State University, Fargo, ND. USA.