CHEM 203
HOMEWORK 1
Review of CHEM 123
1. Experiments indicate that the molecule of PCl3 has a dipole moment, µ, equal to ca. 1 D, but AlCl3 has very
nearly µ = 0 D. Propose a molecular geometry for PCl3 and AlCl3.
2. Each atom in the following molecules has a full complement of valence electrons:
H
C N O
O O
H
2
1
H
H Al H
H
3
H
Cl
H
H
H
4
For each molecule:
a. Calculate the formal charge on each non-hydrogen atom and determine the total charge on the molecule.
b. Specify hybridization states of non-hydrogen atoms and propose a reasonable geometry for the molecule
on the basis of VSEPR (Valence Shell Electron Pair Repulsion).
c. Indicate whether individual bonds are σ-type or π-type.
d. Predict whether the molecule will exhibit a permanent dipole moment.
e. Indicate whether the molecule may be regarded as a Lewis acid, a Lewis base, or neither.
3. Certain poisonous mushrooms contain a toxin that exhibits a structural motif termed a "pyridine N-oxide."
This subunit is variously represented as either structure A or B below. In your opinion, which structure, A or
B, best depicts pyridine N-oxide? Why?
H
H
C
C
A
H
C
C
H
C
N
H
H
O
H
C
C
H
C
C
H
C
N
H
O
B
4. Draw a complete (electron dots, formal charges, ...) Lewis structure of:
a. a molecule that is likely to behave as a Lewis base;
b. a Lewis acid that is not a Bronsted acid;
c. a Bronsted acid that is also a Lewis base;
d. a molecule that is neither a Lewis acid nor a Lewis base;
e. an anion that exhibits Bronsted acidity.
CHEM 203
HOMEWORK 1
p. 2
5. The table below lists the standard bond dissociation enthalpies (ΔHo, kcal/mol) for all possible diatomic
molecules formed by univalent atoms A, B, C, and D.
A-A + 50
A-B + 100
A-C + 85
A-D + 70
B-B + 65
B-C + 80
B-D + 90
C-C + 45
C-D + 95
D-D + 105
a. Which molecule possesses the strongest bond? The weakest bond?
b. Estimate ΔHo for the reactions shown below and state whether each reaction is enthalpically favorable or
unfavorable:
a. A–A + B–B
2 A–B
c. A–D + C–D
A–C + D–D
b. A–B + D–D
A–D + B–D
d. A–B + C–C
A–C + B–C
6. Rotation about the CH2–CH2 bond of the molecule of 1-propylamine, CH3–CH2–CH2–NH2, produces four
limiting conformers. One of these is particularly stable. The remaining three limiting conformers may be
estimated to contain 0.7, 3.0, and 5.0 kcal/mol more energy than the most stable conformer.
a. Define two reference atoms in the molecule of 1-propylamine.
b. Clearly label the dihedral angle, θ, between them.
c. Draw a Newman projection of each limiting conformer and assign their relative energy content.
d. Construct a conformational energy diagram of the molecule for 0 ≤ θ ≤ 180°.
7. Internal rotation about the CH–CH bond in the molecule of 2,3-dimethylbutane produces two gauche-type
conformers shown below as A and B. On purely enthalpic (ΔH) grounds, which conformer, A or B, would
be anticipated to be more stable (= to contain less energy)? Why?
CH3
CH3
H
C
C CH3
H
CH3
H
CH3
CH3
CH3
H
CH3 A
H
H
CH3
CH3
CH3
CH3
B
2,3-dimethylbutane
(note: an accurate conformational analysis needs to take into account entropic effects as well, because what really counts
is the ΔG between A and B)
8. Draw a clear structure of:
a. A chiral, acyclic organic molecule, A, which contains two stereogenic carbons
b. An achiral, acyclic organic molecule, B, which contains two stereogenic carbons
c. The enantiomer, C, of molecule A above
d. A diastereomer, D, of molecule B above
e. Fischer projections of molecules A - D