Chemistry 14C Spring 2016
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.
A note about exam keys: The answers presented here may be significantly longer than expected from a student
taking the exam. An exam key serves not only to reveal what was expected, but to instruct you as well.
To see the projected course grade cutoffs, consult the grading scale on the Chem 14C course web page.
1.
A hydrogen bond donor requires a hydrogen atom with a large δ+ (or bonded to a high EN atom), whereas all
hydrogen bond acceptor atoms have a lone pair.
H
N
H
2.
O
H
N
N
N
H
H
N
H
N
O
N
N
H
O
H
3.
Anion-cation, ion-dipole, cation-pi, and pi stacking. Melamine and barbituric acid are not ionic. Barbituric
acid is not aromatic.
4.
(c) Water. This molecule has the smallest surface area.
5.
Barbituric acid is most soluble in water and least soluble in propanol. Like dissolves like. Barbituric acid is a
polar molecule. Of the solvents list water is the most polar and propanol the least polar.
6.
(a) Melamine and barbituric acid have nearly identical solubility properties. Both are highly polar molecules.
7.
Barbituric acid is not a lipid because it is soluble in polar solvents (such as water).
8.
Hydrophobic can be used interchangeably with lipophilic.
9.
Water: Middle. Urea: Least. Carbon dioxide: Most. Carbon dioxide is a gas a room temperature; the others are
not. Urea has more hydrogen bonding, dipole-dipole, and London force attractive interactions than does water.
10. Many answers are possible. Argon and methane are perhaps the most obvious.
11. The answer depends upon your molecule in question 10. In this case the correct answer is dipole-dipole. All
molecules experience London forces to some degree, so this cannot be a correct answer.
12. (a) The acidity of melamine's NH bond is greater than the acidity of aniline's NH bond due to electronegativity
and inductive effects of the nitrogen atoms in the ring.
(b) Ka for melamine is greater than Ka for aniline because melamine is a stronger acid than aniline.
(c) The pKa of melamine's NH bond is less than the pKa of aniline's NH bond because melamine is a
stronger acid than aniline.
13. When melamine is protonated, the proton becomes bonded to the ring nitrogen because the NH2 nitrogen lone
pair is delocalized by resonance whereas the ring nitrogen lone pair is not. Or: ...because the positive formal
charge resulting from ring nitrogen protonation enjoys resonance stabilization whereas the positive formal
charge resulting from NH2 nitrogen protonation does not. Other answers may be possible.
Chemistry 14C Spring 2016
Final Exam Part A Solutions
Page 2
14. Melamine is a stronger base than barbituric acid because melamine can accept a proton at a nitrogen lone pair
that is not delocalized by resonance whereas barbituric acid cannot. Barbituric acid has oxygen lone pairs that
are not delocalized by resonance, but oxygen is more electronegative than nitrogen.
15. For the equilibrium shown Keq is less than 1 because an acid-base equilibrium favors the weakest acid (pKa
barbituric acid > pKa H3O+). The pKa values of water and barbituric acid are irrelevant because these
molecules are not acids in this reaction (they are bases).
O
H
H
N
16. Many answers are possible. For example we could replace a hydrogen atom with a
fluorine atom and make the molecule more acidic via an inductive effect.
N
O
O
F
17. HOF because fluorine has the strongest electron-withdrawing inductive effect. Note that the conjugate bases
(-OF, -OCl, -OBr, -OH, and -OCH3) have the same formal charge, the same electronegativity effect, and the same
atomic radius effect.
18. Solubility or hydrophobicity (soluble in nonpolar solvents and insoluble in polar solvents) and origin
(biological origin).
19. Prostaglandin.
20.
21. Fatty acid, wax, triglyceride (or triacylglyceride), phospholipid, and prostaglandin.
22. B. A is a phospholipid bilayer, C is soap molecules, and D is soap molecules with tails dissolved in greasy dirt.
CH3
23.
H
C
H2 N
OH
C
Alanine is the only acceptable answer. Other one-carbon side chains are either acidic
(cysteine) or hydrophilic (serine).
O
24. A tripeptide contains three amino acids and two peptide bonds.
25. Cysteine is the only amino acid that can form a disulfide bridge.
26.
S
S
27. A disulfide bridge is an example of the tertiary level of protein structure.
28. N
Chemistry 14C Spring 2016
Final Exam Part A Solutions
Page 3
29. (a) One DNA base pair occurs between nucleobases G and C, and it involves three hydrogen bonds.
(b) The other DNA base pair occurs between nucleobases A and T, and it involves two hydrogen bonds.
30. In DNA, phosphorus is bonded to O.
31. Pi stacking or aromatic stacking.