Chem 241
Fall 2008
Practice Problems For Chapters 1-4
Question 1. Nitrate (NO3–) is a common inorganic anion that you have encountered in
general chemistry. The structure of the nitrate anion is shown below:
O
Overall charge: –1
N
O
O
a) On the structure above, draw in the nonbonding valence electrons for each atom.
b) On the structure above, provide formal charges for each atom that bears a formal
charge.
c) What is the hybridization state of the nitrogen atom?
sp2
d) Draw two additional resonance structures for the nitrate anion.
O
O
N
N
O
O
O
O
e) Draw a resonance structure for the nitrate anion that has a positive charge on one
of the oxygen atoms (it is OK if this is a duplicate structure from part d). Is this a
"major" or "minor" resonance form? Provide two justifications for your answer.
O
N
O
O
This is a "minor" resonance structure
1. The positively-charged oxygen atom does not have a full octet.
2. Oxygen is electronegative (3.0) and does not easily support a positive charge.
f) Although nitrate appears to have one N–O double bond and two N–O single
bonds, all three bonds are actually the same length. Explain.
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Chem 241
Fall 2008
O
O
N
O
O
N
O
N
O
O
O
O
The 3 above resonance structures, when combined, give an accurate depiction of the nitrate anion.
A more accurate single structure is shown below:
–2/3
O
1/3 of a double bond
N
–2/3
O
O
–2/3
Question 2. Translate the following Newman projections into Lewis structures (line
structures are OK).
a)
H
Me
Me
Me
b)
CH3
H3C
Cl
H
CH3
c)
Et
H
Me
Et
Me
Me
C2H6
OH
F
OH
F
Cl
Question 3. Translate the following compounds into staggered Newman projections.
a) 1,2-dichloroethane
H
Cl
H
c) H2NNH2
b)
H
H
Cl
Et
Me
or
H
Cl
H
H
H
Et
H
N
H
H
Me
H
or
Cl
H
H
H
Me
or
H
H
N
H
Me
H
H
H
H
The conforational isomers in boxes for parts a and b above are the lowest energy conformations. Why might
this be? (Part c is much more complicated)
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Chem 241
Fall 2008
Question 4. The chemical structure for vitamin A, a highly unsaturated hydrocarbon
(with one oxygen atom) is shown below. Use this structure to answer the following
questions.
H3C
CH3
CH3
CH3
OH
CH3
Vitamin A
a) How many degrees of unsaturation are there in vitamin A?
one ring and 5 double bonds = six degrees of unsaturation
b) How many atoms are sp3-hybridized?
12 total: 11 carbon atoms and one oxygen atom
1
2
H3C
CH3
7
8
CH3
CH3
11
9
3
OH
10
4
CH3
5
6
c) How many atoms are sp2-hybridized?
10 total: all carbon atoms
H3C
CH3
CH3
2
7
3
4
1
CH3
5
OH
9
6
8
10
CH3
d) How many atoms are sp-hybridized?
none
e) Label each double bond in vitamin A as (E), (Z), or "not applicable."
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Chem 241
Fall 2008
H3C
CH3
CH3
CH3
E
E
E
E
OH
Z
CH3
Question 5. For each compound below, briefly describe why the structure is unstable or
cannot exist.
Triple bonds are only stable in rings of 8 carbons or greater
a)
Me
b)
The central carbon atom has 5 bonds (impossible)
This structure diobeys Bredt's Rule
c)
d)
H
C
C
H
4
The circled carbon has 6 bonds (impossible)
Chem 241
Fall 2008
Question 6. For the following pairs of compounds, circle the molecule that is more
stable.
Increased olefin substitution
a)
This structure disobeys Bredt's Rule
b)
Increased olefin substitution
c)
Question 7. Label each of the following molecules as chiral or achiral.
a. CHFBrCl
chiral
b. CHF2Br
achiral
c. 2-methylheptane
achiral
d. 3-methylheptane
chiral
e. 4-methylheptane
achiral
f. cis-2-butene
achiral
g. trans-2-butene
achiral
h. cis-1,2-dimethylcyclobutane
achiral
i. trans-1,2-dimethylcyclobutane
chiral
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Chem 241
Fall 2008
Question 8. Assign the stereochemistry (R or S) for each stereocenter in the following
molecules. No, you do not have to fully name the compounds.
Cl
3
S
a)
Me
H
Me
Br
H
4
O
b)
Cl
2
Br
1
O
2
R
HO
OH
H3C
H
Me
H
1
HO
OH
4
H
H3C
3
3
4
H
Me
c)
3
Me
R
F
Me
3
Me
4
F1
4
OH
Me
H
2
1
S
Me
OH
2
HO
2
F
1
Me
Me
Question 9. The common non-steroidal analgesic, ibuprofen, has the structure shown
below.
CH3
OH
O
ibuprofen
a) How many degrees of unsaturation does ibuprofen have?
Five. Four double bonds and one ring
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Chem 241
Fall 2008
b) Only the (S)-enantiomer of ibuprofen helps relieve pain. Draw the (S)enantiomer of ibuprofen.
H
CH3
OH
O
Question 10. The following compound is a bit of an oddity. Is it chiral or achiral? How
many stereocenters does it contain? Does it have an internal mirror plane? Does this
change our "rules" for chirality at all?
Br
F
F
Br
Br
F
*
F
Br
This molecule is achiral (mirror images are superimposable)
It contains zero stereocenters (wierd - I know)
It does not have a mirror plane – but it does contain a type of symmetry. It contains a
point of inversion (all aspects of the molecule are reflected through this point - marked
with a *). Molecules with points of inversion are rare.
This does not change our rules for chirality - whether a molecule is or is not
superimposable with its mirror image. But some "strange" molecules may force us to
look very closesly!
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