Download Final Part B - UCLA Chemistry and Biochemistry

Chemistry 14C Winter 2016
Final Exam Part B
Page 1
O
O
N
The questions on this exam page concern DNA bases
guanine and thymine, whose structures are shown here.
N
H
Bond 1
N
H3C
Bond 2
N
N
NH2
Guanine
1.
H
N
O
Thymine
(2) In the box below draw another significant resonance contributor for thymine resonance contributor A. Less
significant contributors will earn less credit. Include all lone pairs for contributor B.
H
O
H3C
H
H
N
O
C
H
N
H
N
N
O
H
H
Thymine contributor A
O
Thymine contributor B
Thymine resonance hybrid
(question 3)
2.
(2) Complete this statement by writing 'contributor A', 'contributor B', or 'equally significant' in the space
below: Between thymine resonance contributor A and thymine resonance contributor B, the most significant
contributor is...
3.
(2) In the thymine resonance contributor box above complete the drawing the thymine resonance contributor by
adding all necessary partial bonds and lone pairs. Do not include formal charges or δ+/δ- charges.
4.
(2) Write a number in the blank. Guanine has _____________ conjugated atoms.
5.
(2) In the space below write the name of the molecule that absorbs the lowest energy photons. Answer choices:
guanine, thymine, about the same, cannot determine.
6.
(3) Complete the following statement by writing ‘higher’ or ‘lower’ in the blank, then adding no more than
twenty words: The barrier to rotation around bond 1 of guanine is ___________________ than the barrier to
rotation around bond 2 of thymine because...
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Chemistry 14C Winter 2016
7.
Final Exam Part B
Page 2
(2) In the answer box write the letter(s) of the molecule(s) that are not aromatic. If all are aromatic, write 'all
aromatic' in the answer box.
Answers box
O
H3C
N
H
H
N
N
N
H
O
A
8.
B
C
O
D
E
(2) Complete each sentence by adding five words or less in each case.
(a) In addition to planarity, aromaticity provides a molecule with...
(b) In addition to planarity, aromaticity provides a molecule with...
9.
(2) Draw in the box an aromatic molecule that has exactly seven carbon atoms,
exactly one oxygen atom, any number of atoms of any other elements, no nonzero
formal charges, and is not completely planar. If this is not possible write “not
possible.”
10. (2) Write numbers in the blanks. (-)-Nicotine has ________ stereocenter(s) with R
H
configuration and ________ stereocenter(s) with S configuration.
N
N
H
(-)-Nicotine
11. (3) Draw in each box the requested
structure. If the structure is not possible
write 'not possible' in the box.
Enantiomer of (-)-nicotine
Diastereomer of (-)-nicotine
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Chemistry 14C Winter 2016
Final Exam Part B
Page 3
12. (2) Tobacco plants produce (-)-nicotine to repel insects that might feed on their leaves. Write
in the box the one letter of the best conclusion concerning the effect of (+)-nicotine on insects.
Answer box
(a) (+)-Nicotine and (-)-nicotine repels insects with equal effectiveness.
(b) (+)-Nicotine also repels insects but not a strongly as (-)-nicotine.
(c) (+)-Nicotine does not repel or attract insects.
(d) (+)-Nicotine attracts insects.
(e) (+)-Nicotine has no effect on insects.
(f) None of these conclusions are valid.
13. (8) In addition to nicotine and other nasty chemicals, cured tobacco also contains this carbohydrate:
CH2OH
CH2OH
O
CH2OH
O
O
O
HO
O HO
HO
OH
OH
O
O
OH
Write in the blank one adjective from each set that describe all or part of this carbohydrate.
(a) Amylopectin
Amylose
(b) Galactose
Glucose
(c) Furanose
Fyranose
(d) α-1,3-glycoside
Cellulose
Mannose
Ribose
Puranose
α-1,4-glycoside
Starch
_________________________
Sucrose
_________________________
Pyranose
β-1,3-glycoside
_________________________
β-1,4-glycoside _________________________
14. (8) Write in the blank the letter of the requested molecule in each set.
(a) The molecule requiring the most energy per mole for evaporation: ______________
CH3(CH2)6CH3
(CH3)2NCH2CH2N(CH3)2
CH3CH2CH2OCH2CH2CH2CH3
CH3CH2OCH2CH2OCH2CH3
A
B
C
D
(b) The molecule with the lowest boiling point: __________
O
A
B
C
D
(c) The molecule most soluble in water: __________
OH
CH3
O
O
O
A
B
C
D
CF3CH2OH
CH3CH2S-
C
D
(d) The strongest base: __________
CH3CH2OA
CF3CH2OB
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Chemistry 14C Winter 2016
Final Exam Part B
Page 4
15. (4) Write in the blank the letter of the requested molecule in each set.
(a) The most acidic molecule: __________
OH
OH
NH2
OH
H
O
A
B
C
D
(b) The molecule with the lowest pKa: __________
OH
OH
NH2
OH
H
O
A
B
C
D
16. (2) Using the strongest base circled in question 14(d) and the most acidic molecule circled in question 15(a)
write a proton transfer equilibrium reaction showing the formation of the conjugate base and conjugate acid of
these molecules. Do not include any curved arrows.
17. (1) Complete this statement concerning your equilibrium expression in question 16 by writing 'greater than',
'equal to', or 'less than' on the blank:
For the acid/base reaction of question 16, Keq is _______________________ 1.
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Chemistry 14C Winter 2016
Final Exam Part B
Page 5
18. (6) Chocolate is a complex mixture of chemicals, including the three shown below. In the box
next to each structure write the name of the lipid category to which it belongs. If the molecule is
not a lipid write “not a lipid.”
O
H3C
O
(CH2)16 CH3
H3C
O
H
(CH2)16 CH3
CH3(CH2)16
H
O
O
O
HO
O
CH3
H
N
N
O
H
N
N
CH3
19. (4) Complete the following drawing of the tripeptide Cys-Ser-Ala. Serine’s side chain is –CH2OH and the side
chain of alanine is a methyl group. Cysteine (R = –CH2SH) is at the N-terminus. The tripeptide is in the
zwitterionic form.
O
N
O
20. (2) In the space below write the names of the amino acids of the question 19 tripeptide having hydrophobic side
chains. Answer choices: alanine, cysteine, serine, or none of these.
21. (6) The following DNA segment and accompanying text may have one or more significant errors. Clearly
indicate (circle or arrow) and describe each error (ten words or less). If there are no errors write “no errors.”
Accepts H-bond from cytosine
O
N
N
O
P
O
Donates H-bond to cytosine
H
Accepts H-bond from cytosine
O
N
O
NH2
O
O
OH
The heterocyclic base is guanine (a purine).
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Chemistry 14C Winter 2016
Final Exam Part B
Page 6
Final Structure Box
22. (33 points) Deduce the structure that corresponds to the
spectral data on pages 6–8. Write your final answer in the
Final Structure Box. A correct answer is worth full credit.
If the answer is incorrect, your analysis of the spectra can
be worth significant partial credit, so show your work
clearly in the space below each set of data only. Answers
outside of these places will be ignored.
(4) Mass spectrum: m/z = 226 (M; 100%), m/z = 227 (13.7%), and m/z = 228 (32.8%). No fluorine or iodine.
Mass spectrometry workspace:
Write in the box the one formula that is consistent with the mass spectrum, and is not
rejected due to other reasons.
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Chemistry 14C Winter 2016
Final Exam Part B
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(13) IR:
IR workspace:
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Chemistry 14C Winter 2016
Final Exam Part B
Page 8
1
Anything written outside the boxes on this page will be ignored. Write only H-NMR implications and 13C-NMR
conclusions in the corresponding boxes.
(12) 1H-NMR:
Chemical shift
Splitting
Integral
7.23-7.19 ppm
multiplet
5
4.62 ppm
singlet
1
2.94 ppm
singlet
2
2.85 ppm
triplet
2
2.75 ppm
triplet
2
1.84 ppm
singlet
3
#H
Implications
(2) 13C-NMR: 201.8 ppm (singlet), 141.3 ppm (singlet), 128.6 ppm (doublet), 127.7 ppm (doublet), 125.9 ppm
(doublet), 87.2 ppm (singlet), 57.4 ppm (triplet), 44.9 ppm (triplet), 35.2 ppm (quartet), and 20.0 ppm (triplet).
13
C-NMR conclusions:
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