Download Chemistry 14C Winter 2017 Final Exam Part A Solutions Page 1

Chemistry 14C
Winter 2017
Final Exam Part A Solutions
Page 1
Statistics: High score, average, and low score will be posted on the course web site after exam grading is complete.
Some questions have more than one answer, even though only one answer may be listed here.
To see the final projected course grade cutoffs, consult the grading scale on the Chemistry 14C course web page.
1.
A standard amino acid is an amino acid that is directly coded by DNA.
H
2.
CH2SH
OH
H2 N
O
3.
Cysteine is unusual among the twenty standard amino acids because cysteine contains a sulfur atom (only two of the
twenty have sulfur) --or-- because cysteine can form a disulfide bridge (cysteine is the only amino acid than do this)
--or-- because it is one of just two amino acids (cysteine and tyrosine) with a hydrophobic, acidic side chain.
4.
(a) Tyrosine. Tyrosine has seven signals, lysine has six, and serine and cysteine each have three.
(b) Cysteine, tyrosine, serine, and lysine. Each has a carbonyl carbon.
(c) Lysine. The given C-NMR data has six signals.
5.
(a) Yes (b) No (c) Yes (d) Yes (e) No. X-ray crystallography reveals the positions of atoms in space. Anything
which can be proven from these positions can be proven/verified. X-ray crystallography cannot 'see' or count
electrons, so anything dependent on knowing electron locations cannot be proven. (Perhaps implied, but not
proven.)
6.
(a) Cysteine: London forces only. The cysteine side chain has no polar bonds and no aromaticity.
(b) Tyrosine: Dipole-dipole, hydrogen bonding, pi stacking, London forces. Tyrosine and phenol both lack ions, so
anion-cation, ion-dipole, and cation-pi interactions cannot occur.
7.
(a) Amino acid cysteine cannot be a hydrogen bond donor because the S–H bond isn't polar or because the S–Hδ+
isn't large enough (S–H ΔEN < 0.5).
(b) Amino acid cysteine cannot be a hydrogen bond acceptor because neutral sulfur is too large.
8.
Most soluble in water: Serine. Least soluble in water: Tyrosine or cysteine. Serine has the largest polar-to-nonpolar
bond ratio in its side chain. Tyrosine has a polar OH group, which is offset by many nonpolar bonds. Cysteine has
no polar bonds in its side chain.
9.
Resonance: Tyrosine. (Tyrosine's conjugate base enjoys resonance stabilization, whereas the other three conjugate
bases to not.) Atomic radius: Cysteine. (Sulfur is a third row element, whereas oxygen, nitrogen, and carbon are all
second row elements.) Electronegativity: Tyrosine or serine. (Oxygen EN = 3.5; nitrogen EN = 3.0; sulfur EN =
2.5.) Recall that any factor making the conjugate base more stable makes the corresponding acid less stable (more
acidic). Thus a factor that better accommodates the electron pair used to form the new base–H bond makes the acid
more acidic.
10. Inductive effects and formal charge.
11. Most acidic: Tyrosine (resonance and highest electronegativity). Least acidic: Lysine.
12. Tyrosine and cysteine. An amino acid's side chain pKa is less than the pKa of water when the amino acid side chain's
conjugate base is more stable than the conjugate base of water (HO-). Tyrosine's conjugate base enjoys resonance
stabilization (HO- does not). Cysteine's conjugate base is stabilized by atomic radius (sulfur is larger than oxygen).
Chemistry 14C
Winter 2017
Final Exam Part A Solutions
Page 2
13. Hydrophobic side chain: Tyrosine and cysteine. Basic side chain: Lysine.
14. NFEPP is a weaker base because of the fluorine atom's electron-withdrawing inductive effect.
15. Anion-cation, cation-pi, and ion-dipole. NFEPP lacks a cation or anion, so ion-based noncovalent forces are not
possible with NFEPP.
16. Lipid: A hydrophobic or amphiphilic molecule of biological origin.
17. Triacylglyceride or triacylglycerol or triglyceride. These terms are interchangeable.
18. (a) The Lorenzo's oil lipid is not an example of a fatty acid because the Lorenzo's oil lipid does not contain a
carboxylic acid group.
(b) The Lorenzo's oil lipid is not an example of a wax because the Lorenzo's oil lipid contains more than one ester
group.
Other answers are possible.
19. The alkenes present in the Lorenzo's oil lipid are mostly likely cis. Cis fatty acids are common in nature, whereas
trans fatty acids are rare in nature.
20. 57 oxygens. Lipids are amphiphilic or nonpolar, so less oxygen atoms are more likely than more oxygen atoms.
21. (a) LiF is the only ionic answer choice.
(b) FI has the most polar bonds and the larges London forces.
(c) HOCH2CH2OH has the most hydrogen bond donors and the most hydrogen bond acceptors.
(d) I2 has the largest London forces.
O
22. (a)
N
OH
This molecule's conjugate base has the most resonance contributors.
O
(b) HI because iodine has the largest atomic radius.
(c) HOF because fluorine provides the strongest electron-withdrawing inductive effect. Atomic radius and electronegativity are not factors here because in each case the conjugate base's negative formal charge resides on an
oxygen atom: -OF, -OCl, -OBr, and -OI.
(d) HF because fluorine is more electronegative than oxygen, nitrogen, or carbon.
O
23. (d)
H
H3C
O
The conjugate base in the case has two resonance contributors, each having the negative
formal charge on an oxygen atom. Water's conjugate base has no resonance
stabilization. In the other two conjugate bases an oxygen atom and a carbon atom share
the negative formal charge.
24. This nucleobase is part of G. It is a purine (two rings), and it is aromatic (contains ten pi electrons in the closed porbital loop).
Chemistry 14C
Winter 2017
Final Exam Part A Solutions
H
N
25. Three hydrogen bonds.
H
O
deoxyribose
N
deoxyribose
N
N
N
N
H
N
O
H
N
H
Page 3