Name:
TF Name:
LS1a Fall 06
Problem Set #1
Due Friday 9/29 at noon in your TF’s drop box on the 2 nd floor of Science Center
(100 points total)
1. (6 points) Draw a standard line bond structure for compounds of the following
molecular formulas:
(a) CH3CH2CH2OH
(b) NH2CH2COOH
(c) (C6H5)CN
N
O
OH
NH2
OH
Note: the triple bond should be drawn with linear geometry to be correct. We
did not take off points for this since it was drawn bent in 2c.
2. (9 points) Provide a molecular formula for each of the following molecules. Describe
the geometry at the circled carbon atoms.
(a) caffeine
O
(b) THG (“The Clear”)
(c) DEET
HO
N
O
N
N
N
N
O
HO
C8H10N4O2
trigonal planar
tetrahedral
C22H30O2
C12H17NO
tetrahedral
trigonal planar
linear
tetrahedral
2 points for each formula. 0.5 points for each geometry
3. (12 points) Shown below are four structures. For each possible pair of structures
(a-b, a-c, a-d, b-c, b-d, and c-d), indicate whether the two structures represent the
same molecule (including different conformations of the same molecule), or whether
they represent enantiomers.
(a)
(b)
O
HO
OH
OH
HO
HO
OH
HO
O
(c)
O
HO
(d)
OH
O
HO
OH
OH
(a-b), (a-d), (b-c) and (c-d) are enantiomers; (a-c) and (b-d) are the same.
Name:
TF Name:
4. (20 points)
HO
HO
OH
O
O
O
O
O
OH
HO
OH
OH
OH
O
(a) Draw one possible wedge and dash structure of the molecule above, showing the
hydrogens at chiral centers.
(5.5 points) The 11 chiral carbons should have wedge/dash notation-one
possibility is shown below.
HO
HO
H
HO
H
HO
H
O
H
O
HO
O
H
HO
O
O
H
H
OH
H
HO
H
OH
(b) What is the total number of potential stereoisomers for this molecule? Draw one
additional stereoisomer.
11
(3.5 points) 11 stereocenters = 2 =2048
(3 points) One possibility out of 2047 shown below
HO
HO
H
HO
H
O
H
HO H O
H
H
HO
H
HO
O
O
H
O
H
H
OH
O
OH
Name:
TF Name:
(c) Naringin, one of the stereoisomers of this molecule, gives grapefruit its bitter taste.
However, other stereoisomers of this molecule do not taste bitter at all. Why might this
be?
(8 points) We perceive taste when molecules interact with our taste
receptors. These receptors are chiral, like almost all the molecules of life.
Different stereoisomers of naringin interact differently with our taste
receptors, and only one of these interacts in such a way as to produce the
bitter taste.
5. (20 points)
(a) Can two molecules of ethanol (CH3CH2OH) form hydrogen bonds with each other? If
so, draw a hydrogen bonding interaction between two molecules of ethanol and label
the donor and acceptor. If not, explain why not.
(4 points) Yes, two molecules of ethanol can form hydrogen bonds with one
another. [Note: H bonds should have a linear geometry between the donor
and acceptor]
donor
acceptor
H
O
O
H
(b) Draw all of the hydrogen bonds you would expect a molecule of ethanol to form
when a small amount of ethanol is added to a large amount of water.
(6 points)
H O
H
O
H
O
H
H
H
O
H
(c) Can two molecules of ethyl ether (CH3CH2OCH2CH3) form hydrogen bonds with each
other? If so, draw a hydrogen bonding interaction between two molecules of ethyl ether
and label the donor and acceptor. If not, explain why not.
(6 points) No, two molecules of ethyl ether cannot form hydrogen bonds with
one another. In order for a hydrogen to act as a “donor” it must be bonded to
a very electronegative atom (F, O, N, Cl). Therefore, none of the hydrogens in
ethyl ether can act as “donors.”
(b) Draw all of the hydrogen bonds you would expect a molecule of ethyl ether to form
when a small amount of ethyl ether is added to a large amount of water.
(4 points)
H O
H
H
O
O
H
Name:
TF Name:
6. (16 points) Shown below is part of a protein interacting with part of a DNA strand.
protein
O
DNA
NH
NH2
O
O
P
HN
O
NH2
O
DNA
(a) Draw all lone pairs on all the nitrogen and oxygen atoms, assuming that all nitrogen
and oxygen atoms fulfill the octet rule.
(3.5 points-14 pairs, 0.25 each)
protein
O
+1 charge
NH
NH2
O
NH2
O
HN
DNA
O
P
O
DNA
-1 charge
(b) Calculate the formal charge on all the nitrogen and oxygen atoms.
(4.5 points-9 atoms, 0.5 each) All charges are 0, except the ones shown
above.
(c) What types interactions between the protein and DNA would you expect to be
present in the region shown above?
(8 points) An ionic bond (ionic interaction or electrostatic attraction) because
the fully positively charged protein side chain (Arg) is electrostatically
attracted to the fully negatively charged phosphate group.
The phosphate O is also a good H-bond acceptor, and the NH group of the
protein is a good H-bond donor, so H-bonding is also a factor.
For full credit on an exam you would need to identify both interactions. Here
we gave full credit for either answer (H- bonding or ionic) since some
students were confused about the difference between ionic bond and ionic
interaction. There is no difference they are the same.
7. (17 points) Fats and oils are composed of compounds called triacylglycerides. The
general structure of a triacylglyceride is shown below. R1, R2 and R3 are different fatty
acids (Note: Chemists use "R" groups to represent a portion of the molecule that is not
drawn out. R groups with different subscripted numbers are used to differentiate
between the different "R" groups in a molecule.)
Name:
TF Name:
H2C
R1
HC
R2
H2C
R3
(a) If R1, R2 and R3 are all different, is the triacylglyceride chiral? If so, circle the chiral
center(s).
(1 point) Yes, chiral center is carbon connected to R2.
(b) When you consume oil or fat, your body releases the individual fatty acids from the
triacylglyceride. Structures of two fatty acids are shown below. Indicate whether the
carbon-carbon double bonds in each are in cis or trans conformations.
(4 points)
O
HO
cis
O
trans
HO
(*extra*) These fatty acids are geometric isomers of each other. One of them is a
component of the "good fat" in olive oil, while the other is present in margarine and
processed foods, and is thought to be detrimental to cardiovascular health. Can you
guess which is which?
The second molecule is eladic acid, a component of the "trans-fats" (so
named because they contain fatty acids with trans double bonds) found in the
hydrogenated oil of processed foods. Trans-fats have been implicated as
being detrimental to cardiovascular health because they behave like
saturated fats and raise the level of LDL or bad cholesterol in the blood. The
first molecule is oleic acid, the primary fatty acid in olive oil.
(c) To release the fatty acids from the triacylglyceride, your body uses protein enzymes
called lipases. One step of the lipase-catalyzed reaction is shown below using arrowpushing formalism.
H2C R1
HC R2
O
O
H2C R1
HC R2
O
C17H33
OO
lipase
O
lipase
C17H33
How many electrons move during this process? In sentences, describe the movement of
electrons; indicate where the electrons start out and where they end up.
(12 points) 4 electrons (2 pairs) move. Lipase oxygen's lone pair moves to
form a bond with the triacylglyceride carbon. A pair of electrons from the
Name:
TF Name:
carbon-oxygen double bond moves to form a lone pair on the triacylglyceride
oxygen.
(*extra*) Why do you think the lipase oxygen forms a bond with the carbon atom
shown? Why does it not form a bond with one of the oxygen atoms of the
triacylglyceride? Why does it not form a bond with one of the other carbon atoms of the
triacylglyceride?
The electrons from the electron-rich lipase oxygen will form a bond with the
most electron-poor atom possible. In this case, the most electron-poor atom
is the carbonyl carbon of the triacylglyceride because it is bonded to two
electronegative oxygens (these are polar covalent bonds, which give the
carbon a partial positive charge). The lipase oxygen does not form a bond
with another oxygen atom because oxygen atoms are electron-rich, not
electron poor. It does not form a bond with one of the other carbon atoms
because the other carbon atoms are not as electron-poor as the carbonyl
carbon; the carbonyl carbon is the only one bonded to two electronegative
oxygen atoms.