1. No category

Fall 2014
MASC Chem 261 Review Questions
Dr. Bryan Rowsell
7. For the following acid/base reaction, which side of the equilibrium is favoured? Fully explain your
answer using words and/or calculations:
The strongest acid is the alcohol on the product side, thus this reaction
should favour reactants
pK eq  pK a (stronger acid)  pK a (weaker acid)  21  15  6
 K eq  106
8. George Wittig won the 1979 Nobel Prize in chemistry for his discovery of the ylid zwitterion, which is
a two−step process: a Lewis acid/base reaction and a Brønsted acid/base reaction:
Ph
Ph
CH2CH3
P
Ph
+
Ph
C
H
CH2CH3
CH2CH3
Ph
Ph
P
C
CH2CH3
H
Li
CH2(CH2)2CH3
Ph
Ph
CH2CH3
P
C
CH2CH3
Ph
ylid zwitterion
Ph = C6H5
(a) Identify any and all acid/base species (Lewis and Bronsted) and complete the structures above with
formal charges (if any), lone pairs (if any) and draw a resonance form of the ylid.
(b) Show on the reaction above, using arrows of appropriate headedness, how the two transformations
occur.
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Fall 2014
MASC Chem 261 Review Questions
Dr. Bryan Rowsell
10. There are 3 constitutional isomers with the molecular formula C3H6O. What are the structures of
these 3 isomers and what IR absorptions would you expect to see (or not to see) that would allow
you to distinguish between the three.
Isomer
Isomer
Isomer
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Fall 2014
MASC Chem 261 Review Questions
IR info:
C=O
stretch
observed
Dr. Bryan Rowsell
IR info:
is
IR info:
C=O
stretch
observed
C−H aldehyde str.
(2700 cm−1)
12. Draw structures for the following molecules.
(a) 3−cyclopropyl−5−propyldecane
13. Write IUPAC names for the following molecules.
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is
C=C
stretch
observed
O−H alcohol str.
(3300 cm−1)
(b) trans−1,2−diethylcyclohexane
is
Fall 2014
MASC Chem 261 Review Questions
Dr. Bryan Rowsell
14. Identify the relationship between the following pairs of molecules as enantiomers, diastereomers,
geometric (configurational) isomers, conformational isomers, meso compounds or same
molecule. Put in any and all that apply.
16. A chemist performs a synthesis obtaining a crystalline product (the nature of the synthesis is not
relevant to the question). After recrystallization, the chemist observes the specific rotation of a 0.1
M aqueous solution of the compound to be −13.2°. What can or can’t the chemist infer about the
nature of the product? BE SPECIFIC!!! Think of what that piece of data tells you.
This data tells us two major things:
1. The compound prepared is chiral (optically active) and not a
meso compound
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Fall 2014
MASC Chem 261 Review Questions
Dr. Bryan Rowsell
2. One of the enantiomers prepared in the synthesis is in excess (i.e.
it is not a racemic mixture).
What we don’t know is the enantiomer that is in excess...remember
there is no correlation between +/− and R/S. This mixture might be
pure R or pure S, but we can’t say for certain.
17. Explain why 1,3−cyclohexadiene is a stable compound, yet 1,2−cyclohexadiene does not exist.
Can you think of a cyclic 1,2–diene compound that might?
Too much ring strain in the 1,2−cyclohexadiene, as the
bond angle at the sp carbon in the middle of the two C=C
bonds should be close to 180°. The ring is not large
enough to accommodate such a requirement.
If the ring were large enough, that angle could, in
theory, be reduced to (nearly) linear:
18. Fill in the blanks for the following table. For any chiral centres, label them with the appropriate
absolute configuration.
Line Angle Diagram (or Newman Projection)
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Fischer Projection
Fall 2014
MASC Chem 261 Review Questions
Dr. Bryan Rowsell
19. Draw a fully−labeled reaction coordinate diagram for the SN2 reaction.
Things to note about an SN2 mechanism:
1. geometry of transition state? trigonal bipyramidal
2. rate law? rate = k[Nu:–][sub]
bimolecular
3. stereochemical change if the attacked C is chiral? Yes!
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Fall 2014
MASC Chem 261 Review Questions
20. Why do SN2 reactions invert stereochemistry?
Back−side attack is favoured due to electronic effects.
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Dr. Bryan Rowsell
Fall 2014
MASC Chem 261 Review Questions
Dr. Bryan Rowsell
21. Predict the structure of the SN2 products for the following reactions:
a. (R)-2-bromopentane + KCN
(b) (2R,3R)-2-bromo-3-methylpentane
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