Chemistry 217
Problem Set 2
Recommended Problems from the Book: 1.10-1.15, 1.26-1.29, 1.30-1.35, 1.43-1.52,
1.63-1.67, 1.76-1.76, 1.80, 1.82-1.83, 3.2-3.3, 3.15-3.17, 3.41 (a, b, e)
1st ed: 1.9-1.14, 1.24-1.27, 1.28-1.33, 1.41-1.50, 1.59-1.63, 1.72-1.76, 1.78, 1.79, 3.2, 3.3,
3.14-3.16, 3.4
Resonance: Weeks, Ch. 2; Klein, Ch. 2
(1) The structure of acetylene is shown here:
H
C
C
H
(a) What is the hybridization of the carbons in acetylene?
(b) Using valence bond theory, draw an energy diagram of the orbitals in acetylene
similar to the one we drew for ethylene in class. Sketch each molecular orbital.
(c) Do you expect the carbon-carbon bond in acetylene to be stronger or weaker than in
acetylene? Do you expect the bond to be longer or shorter than acetylene?
(2) Repeat parts (a) and (b) from problem 1 for allene:
H2C
C
CH2
(3) Determine the hybridization of the carbons, oxygens, and nitrogens in the following
molecules. Unless otherwise noted, assume that lone pairs are in hybridized orbitals.
O
OH
N+
OH
(a)
two lone pairs in hydbridized,
one lone pair in unhybridized.
O
O
O-
OH
nitroglycerine
(b)
O
AL-2 (an anti-cancer compound
I study in my research)
(4) For the following two compounds, the π-molecular orbitals are shown.
(a) Determine if each molecular orbital is bonding, non-bonding, or anti-bonding (an
orbital is bonding if there is net bonding interactions between adjacent carbons, antibonding if there is net anti-bonding, and non-bonding if there is no net bonding or antibonding). Then, fill in the appropriate number of electrons into the orbitals. Assume that
lone pairs are in unhybridized orbitals.
(b) Draw all resonance forms of the molecules from part (a). Are the resonance forms
you drew consistent with the molecular orbital diagrams?
(5) An energy diagram of the π-molecular orbitals for three cyclic molecules is shown
below. Note that some orbitals are degenerate (have the same energy).
(a) For each one, fill in the appropriate number of electrons into the orbitals. Assume that
lone pairs are in unhybridized orbitals.
(b) Rank the three compound(s) from least to most stable.
A
B
C
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E
!n.b.
!n.b.
!
!n.b.
!
!
!
!
(6)
Benzene is a special type of molecule called aromatic. It has special qualities and
reactivities that we will study in detail during second semester.
H
H
H
C
C
C
C
C
C
H
H
H
benzene
(a)
Fill in the missing blocks on the following table describing the molecular orbitals
of benzene. For reference, the first row is provided for you.
What is the type of
bond?
Made from which atomic
orbitals?
σC-H
C sp2, H 1s
Sketch the bond.
C sp2, C sp2
σ∗C-H
C 2p, C 2p
(b) Compound A in problem 5 is called anti-aromatic, while compund C is called
aromatic. Benzene is also aromatic, and has three bonding and three anti-bonding πmolecular orbitals. There are several factors that we will study later that make a
compound aromatic. Based on the molecular orbitals of A, C, and benzene, predict one
of these propoerties.
(7) Make sure you can do problems 1.43-1.52 (for more practice, there are 75 problems
in Klein, Ch. 2).
(8) Make sure you do problems 3.2-3.3, 3.15-3.16. 3.41 (a,b)