2010 che 230 exam 3 solutions - Department of Chemistry - Illinois ...

ILLINOIS STATE UNIVERSITY
Department of Chemistry, Fall Semester 2010
Organic Chemistry I, CHE 230, Exam 3
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_____/140 pts
_____/180 pts
_____/220 pts
_____/240 pts
_____/220 pts
Bonus
______/20 pts
Total
/1000 pts
First letter of last
name
Name (Print):____________________
This solution set should be used only for studying purposes and should not be considered as a stand-alone
tool for doing well on the exam.
© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-4160 0

1. Which of the following cycloalkanes is the most stable (20 pts)
(a)
(b)
(c)
(d)
cyclopropane
cyclobutane
cyclohexane
cyclodecane
C
2. Which definition presented here is best for enantiomers (20 pts)
(a)
(b)
(c)
(d)
The exact opposite of diastereomers
Stereoisomers that cannot be related by a mirror plane
Non-superimposable mirror image molecules
Molecules that are not superimposable and have optical activity
C
3. Which definition presented here is best for diastereomers (20 pts)
(a)
(b)
(c)
(d)
The exact opposite of enantiomers
Stereoisomers that cannot be related by a mirror plane
Non-superimposable mirror image molecules
Molecules that are not superimposable and have optical activity
B
4. Which is a true statement concerning the transition state of an S N 2 reaction (20 pts)
(a)
(b)
(c)
(d)
Involves only the electrophile
Involves only the nucleophile
Involves both the nucleophile and electrophile
Involves both the nucleophile and electrophile and the reaction solvent
C
5. What is the rate expression used for the S N 1 reaction. (20 pts)
(a)
(b)
(c)
(d)
Rate = k[nucleophile]
Rate = k[electrophile]
Rate = k[nucleophile][electrophile]
Rate = k[nucleophile][nucleophile]
B
6. When a chiral substrate undergoes S N 1 reaction at the asymmetric carbon, the final product has (20 pts)
(a)
(b)
(c)
(d)
retention of stereochemical configuration
stereochemical scrambling
a change from (S) to (R) stereochemistry
inversion of stereochemical configuration
B
7. Increasing the concentration of an electrophile in a typical S N 1 reaction by a factor of 3 and the
concentration of the nucleophile by a factor of 3 will change the reaction rate: (20 pts)
(a) increase by a factor of 3
(b) increase by a factor of 6
(c) increase by a factor of 9
(d) increase by a factor of 12
A
© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-4160 1

8. Provide a reasonable name (IUPAC or common) for the following molecules. (20 pts each/40 pts total)
Br
trans-1-bromo-2-tert-butylcyclodecane
trans-1-bromo-2-(1,1-dimethylethyl)cyclodecane
Bicyclo[3.3.0]octane
9. Using any appropriate cycloalkanes, illustrate what is meant by (a) angle strain (5 pts), (b) torsional
strain (5 pts), and trans-annular strain (10 pts).
(20 pts total)
H
H
H
H
H
H
The problems associated with cyclopropane are angle strain (60 o vs.
normal 109.5 o . There is also a problem with torsional strain from
eclipsing hydrogens. There is no other conformation that cyclopropane
can adopt to alleviate these problems.
10. Consider the combustion reaction of cycloalkanes. Which compound would give off the most heat, cis-1,2-
dimethylcyclopropane or trans-1,2-dimethylcyclopropane Defend your answer. (20 pts)
Hint: Draw the structures and compare them.
© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-4160 2

11. Draw the equilibrating chair conformations of trans-1-chloro-3-isopropylcyclohexane. Circle the least
stable conformation. (60 pts)
12. Draw all of the isomers of 1,3-dimethylcylcopentane and identify which compounds cannot rotate the
plane of polarized light. (40 pts)
CH 3
CH 3
CH 3
same molecule
achiral
CH 3
CH 3
chi
ral
CH 3
CH 3
CH 3
chiral
13. Determine if the following molecules are either chiral (Χ) or achiral (A). (20 pts)
14. Determine the stereochemical assignment (R or S) for the following. (60 pts total)
R R S R
15. Determine if the following molecules are (E) enantiomers, (D) diastereomers, (M) meso, or (I) identical.
(40 pts total)
© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-4160 3

16. The specific rotation of an impure solution is [α] D -200 o . The enantiomerically pure (S)-enantiomer
related to this solution rotates light with an [α] D +500 o . What is the percent composition of the mixture
(40 pts) There is no partial credit for this problem.
70%(R)
30%(S)
17. Consider the following molecules.
1 2 3 4
1 and 2 and 1 and 4 are diastereomers (20 pts)
1 and 3 are enantiomers (20 pts)
2 and 4 represent a meso-compound (20 pts)
18. Show the reaction products, the complete reaction mechanism, and postulated transition state for the
following reaction. You must address the stereochemical consequences of the reaction. (80 pts)
See class notes for this SN2 reaction mechanism.
19. Provide the major substitution products and detailed reaction mechanisms for the reactions that are listed
below. The mechanism must be drawn out completely. (80 pts)
See class notes for this SN1 reaction mechanism.
20. Describe in detail the two forces that stabilize a carbocation. Use the molecular structure that is drawn
below to help your explanation. (20 pts)
© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-4160 4

Hyperconjugation
H
inductive ef fect
inductive eff ect
H 2 C
C
CH 3
CH 3
inductive ef fect
Hyperconjugation is a result of the empty p-orbital draining electron density from
the nearby CH bond. The inductive effect which results from a difference in
electronegativity draws electron density to the central carbon. There is still debate as
to which effect is most responsible for the observed behavior of cations.
21. Rank the following substrates for their ability to participate in the S N 2 reaction. (1 = best; 4 = worst) (20 pts)
22. Which of the following alkyl halides would react most quickly in the S N 1 reaction (20 pts)
(a) 2-chloro-2-methylpropane (b) chlorocyclopentane
(b) chloropentane (d) chloromethane
A
23. Which of the following molecules would have a carbon halogen bond most susceptible to nucleophilic
substitution Write the best answer in the box. (20 pts)
(a) 2-fluoropentane (b) 2-chloropentane
(c) 2-bromopentane (d) 2-iodobutane
D
24. Complete the following reactions. If there is stereochemistry, then you must address it. If there is nor
reaction, then write NR. (160 pts)
© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-4160 5

26. Draw out the major product for the following reactions and state which reaction would proceed most
quickly Provide a rationale for your answer. (20 pts)
27. What would be the product of the following reaction Draw this product and show arrows to show how the
product formed. (20 pts)
28. Show the reagents necessary to induce the following transformation. This transformation may require more
than one step. (20 pts)
© 2010 Prof. S. R. Hitchcock, Department of Chemistry, Illinois State University, Normal, IL 61790-4160 6