Chemistry 14C
Spring 2017
First Midterm Exam
Page 1
Ascorbic acid (vitamin C) is a widespread hydrophilic (water-soluble) antioxidant
vitamin found in oranges and other fruits. On this exam page we explore various aspects
of this molecule's structure.
Ascorbic acid
1.
(3) Using ten words or less, complete this explanation of the meaning of the solid wedge symbols (
ascorbic acid structure shown above: The solid wedge indicates...
) in the
2.
(2) Many properties of ascorbic acid are controlled by the electronegativities of the elements that compose it. Rank
the electronegativities of these three elements by writing their symbols in the corresponding blanks below. In the
event of an electronegativity tie, write more than one element symbol in the blank, and leave one (or two) blanks
empty.
Most electronegative: __________ > __________ > __________ Least electronegative
3.
(2) Write the name of the one functional group that appears the most times in ascorbic acid: ___________________
4.
(2) Write the name of one functional group that contains one or more oxygen atoms, and is not present in ascorbic
acid.
5.
(10) How many of each of the following is present in ascorbic acid? Write one number in each blank.
Lone pairs: _______
sp2 atoms: _______
3
Total number of sp stereocenters: _______
6.
pi electrons: _______
3
Total number of sp stereocenters with R configuration: ______
(6) Complete each structure drawing in boxes below. If the structure is not possible write "not possible" in the box.
Ascorbic acid enantiomer.
Ascorbic acid diastereomer.
Optically active? _____
Optically active? _____
7.
(2) In each "optically active?" blank in question 6 write "Y" (for yes) if the molecule is optically active or "N" (for
no) if the molecule is optically inactive. If you wrote "not possible" in the structure box write "N/A" (for not
applicable) in the optical activity blank.
8.
(1) Complete the following statement by writing "Y" (for yes) or "N" (for no) in the blank. Does ascorbic acid have
a meso stereoisomer? _______
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Chemistry 14C
Spring 2017
First Midterm Exam
Page 2
Under physiological conditions, ascorbic acid ionizes to form ascorbate. Ascorbate is the
active, antioxidant form of vitamin C. It can protect cellular structures from attack by
reactive oxygen species such as superoxide.
Ascorbate
9.
(2) Despite its potential for damage to biological systems, superoxide has a very simple
molecular structure (shown here). All electrons are shown, but formal charges need to be
assigned. In the box next to each oxygen atom write that atom's formal charge (-2, +6, etc.).
10. (4) Is ascorbate an aromatic molecule? Write "is" in the answer blank if ascorbate is aromatic, or "is not" if it is not
aromatic. If you write "is" in the blank you are done with this question. If you wrote "is not" then complete the
explanation by adding no more than ten words of explanation.
Ascorbate ____________ aromatic because...
11. (6) In the boxes below draw two additional ascorbate resonance contributors. Make these contributors as significant
as possible. Less significant contributors will receive less credit. Clearly include all lone pairs and formal charges
for the contributors you draw.
Contributor A
Contributor B
Contributor C
12. (2) Complete this sentence by writing one or more letters (B, C, or "N" if none) of a contributor from question 11:
The ascorbate resonance contributor(s) that is/are at least as significant as contributor A is/are contributor(s) ______.
13. (3) Circle all ascorbate atoms having a p orbital. Make your circles neat and
precise. Do not circle any atoms lacking a p orbital. You may need to consider
some resonance contributors that you did not draw in question 11.
14. (3) Complete this drawing of the ascorbate resonance hybrid by adding all partial
pi bonds and lone pairs. Do not include any partial or full formal charges.
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Chemistry 14C
Spring 2017
First Midterm Exam
Page 3
15. (2) Complete this sentence by writing 0, +1, δ+, -1, or δ- in the blank: The charge on the
indicated oxygen atom in the ascorbate resonance hybrid is ________.
Questions 16 – 21 concern the common plant pigments shown below.
α-Carotene
(Pigment A)
β-Carotene
(Pigment B)
Canthaxanthin
(Pigment C)
16. (2) Complete this sentence by writing a number in the blank: When in its most stable conformation, α-carotene has
________ atoms which all lie in the same plane.
17. (2) Complete this sentence by writing a number (1, 2, or 3) in the blank: The bond
in the β-carotene partial structure shown here which has the lowest barrier to
rotation is bond B________.
18. (4) Complete this sentence by writing a pigment letter (A, B, or C) in the first blank, and then a number in the
second blank. The greatest number of conjugated p orbitals is present in pigment ________, which has ________
conjugated p orbitals.
19. (2) λmax is the photon wavelength at which the molecule absorbs the most photons. This wavelength is used to
predict color. Recall the relationship of photon wavelength and energy: higher wavelength = lower energy. For αcarotene λmax = 450 nm and for β-carotene λmax = 467 nm. Answer choices: 440, 450, 458, 467, or 477. Complete
this sentence by writing a number in the blank: Therefore a likely λmax value for canthaxanthin is ___________ nm.
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Chemistry 14C
Spring 2017
First Midterm Exam
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20. (2) These three plant pigments have similar, but not quite the same red-orange-yellow colors. Assign the most likely
colors to the pigments by writing a pigment letter (A, B, or C) next to the most likely color for that pigment:
Red: __________
Orange: ___________
Yellow: ___________
21. (2) Lycopene is just one pigment that contributes to the red color of tomatoes. Oxidation by atmospheric ozone (O3)
causes lycopene to become colorless. Shown below is a figure showing this reaction, taken from an Internet source:
The error is not related by something we haven't covered in Chem 14C. Complete this sentence by adding no more
than ten words: One or more structure(s) in this reaction scheme is/are clearly wrong because...
Oclacitinib (brand name Apoquel) is a janus kinase inhibitor used in
the control of pruritus (itching) associated with allergic dermatitis
and the control of atopic dermatitis in dogs.
Oclacitinib (Apoquel)
22. (2) Complete the following statement by writing a number in the blank: Including the oclacitinib stereoisomer
shown above, and considering only sp3 stereocenters, there are a total of ________ oclacitinib stereoisomers. Hint:
This is the "inclusive" version of the standard "how many stereoisomers" question.
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Chemistry 14C
Spring 2017
First Midterm Exam
Page 5
23. (8) Let's determine if the oclacitinib core structure shown here is aromatic. Complete each
question below by drawing a series of circles, writing a number, or by writing "Y" for yes or
"N" for no, as needed.
(a) On the structure above circle all of the atoms having p orbitals. Do not circle any atoms without p orbitals.
(b) The total number of pi electrons contained in the entire oclacitinib core structure is ________.
(c) The molecule has a closed p orbital loop? ________
(d) Does the molecule obey Hückel's rule? ________
(e) Is the molecule aromatic? ________
24. (6) Metabolism of oclacitinib might involve oxidation in the liver to produce one of the metabolite molecules shown
below. R is an abbreviation for a portion of the molecule not changed by metabolism.
or
Metabolite D
or
Metabolite E
Metabolite F
Complete this sentence by writing a letter (D, E, or F) in the blank, followed by no more than fifteen words of
explanation: The least likely oclacitinib metabolite is metabolite _______ because this metabolite...
Questions 25 – 28 concern the molecular structure of idose, a monosaccharide that differs from glucose in the
configuration of all stereocenters, except for the stereocenter as C5.
25. (2) Complete this sentence by writing no more than three words in the blank: A monosaccharide is a carbohydrate
that _________________________________ be decomposed into two or more simpler carbohydrates.
26. (4) Complete this drawing of D-idose. Neatness and precision count.
27. (2) In the space below write all of the words that describe the structure of acyclic D-idose. Answer choices: Aldose,
ketose, triose, tetrose, pentose, hexose.
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Chemistry 14C
Spring 2017
First Midterm Exam
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28. (2) Complete this sentence by writing no more than three words in the blank: In water the structure of Didosopyranose is a mixture of its α and β forms, whereas the structure of D-sucrose is locked as the α form because
sucrose lacks a(n) ____________________.
29. (4) Write the name of the monosaccharide found in cellulose. Be very specific. As an alternate answer, you many
draw the structure of this monosaccharide. Neatness and precision count. Please write just one answer.
30. (2) Complete the following statement by writing "T" in the blank if the statement is true, "F" if the statement is false,
or "C" if we cannot determine from the given information: L-sucrose is sweet, just like D-sucrose. ______
31. (4) Complete following statement by writing no more than one word in each blank: Before Pasteur separated (+)tartaric acid and (-)-tartaric acid from each other, they existed as a 1:1 ____________________ mixture that was
optically inactive. The process of separating a substance into its constituent enantiomers is called
_____________________.
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