Midterm Exam 2
CHEM 181: Introduction to Chemical Principles
October 13, 2015
Solutions
1. Draw Lewis structures for the isomers with the molecular formula C3 H3 NO2 that
are shown on the following pages. For each isomer shown, do the following:
• Label non-zero formal charges.
• Include all resonance structures that follow the octet rule.
• Label each resonance structure as either major, minor, very minor, or
not valid (i.e., should not be considered).
• Rank the resonance structures in numerical order (1=best, 2=next best,
etc.) If two structures are exactly equivalent, give them the same number.
(a) Methyl cyanoformate is an isomer of C3 H3 NO2 , with the connectivity
shown below:
H
H
O
C
O
C
C
N
H
Provide a Lewis structure for methyl cyanoformate.
H
H
O
C
O
C
H
major (1)
H
C
N
H
C
H
1
O
O
C
minor (2)
C
N
(b) Acetyl isocyanate is an isomer of C3 H3 NO2 , with the connectivity shown
below:
H
H
O
C
C
N
C
O
H
Provide a Lewis structure for acetyl isocyanate.
H
H
O
C
C
H
H
C
O
H
O
C
C
N
C
O
H
C
C
H
O
C
C
H
N
N
C
O
minor (2)
H
O
C
C
H
minor (3)
H
O
H
major (1)
H
H
N
H
N
C
O
minor (4)
C
O
minor (5)
#2 and #3 are very close, and the order could be swapped, or they could
be listed as (almost) equivalent. #4 and #5 could also be called “very
minor.”
2
(c) Oxazolone is an isomer of C3 H3 NO2 , with the connectivity shown below:
O
C
H
C
H
O
N
C
H
Provide a Lewis structure for oxazolone.
O
O
C
H
H
C
O
N
C
H
H
C
O
N
C
C
H
H
major (1)
minor (2)
O
O
C
H
H
C
O
H
N
C
H
C
C
O
N
C
H
minor (3)
H
not valid
The third structure could also reasonably be called “very minor.”
3
2. Here is the best single electron-dot structure for acetyl azide (C2 H3 N3 O):
H
H
O
C
C
N
N
N
H
Consider the following proposed (but not necessarily correct) resonance structures, labeled A through E. Categorize each one according to the following.
(You will not receive credit for categorizing a structure if you do not provide an
explanation.)
A
H
H
O
C
C
B
N
N
H
N
C
H
O
H
C
C
N
N
N
H
H
H
O
C
C
N
N
H
H
H
O
C
C
N
N
N
H
H
H
O
C
C
N
N
N
H
D
E
(a) Which structures (if any) must be included as resonance structures because
they tell you something new about the molecule? Identify each structure
by its label, and explain (one sentence each) what is important about its
inclusion (i.e., its effect on bond order, formal charge, and/or hybridization).
Structure E tells you that negative charge is distributed between the oxygen and that nitrogen, that the C–O bond order is greater than 1, the
C–N less than 2, and the N–N bonds greater than 1 and less than 3—it’s
not a bad structure (negative formal charge on the oxygen instead of the
nitrogen).
(b) Which structures (if any) could be included, but are very minor and their
inclusion would not change bond order, formal charge, or hybridization in
4
N
any significant way. Identify each structure by its label, and explain (one
sentence each) what makes the structure unimportant.
No structures fall into this category.
(c) Which structures (if any), are bad structures that can not be included.
Identify each structure by its label, and explain (once sentence each) the
problem or problems with it.
Structure A and structure C have large formal charges, and they are much
worse than structure E. Since their inclusion would require the C–N nitrogen to be sp-hybridized, they should be left out; they also don’t really
tell you anything new about bond order or formal charge that the main
structure and structure E don’t.
(Also worth full credit: put A in the “must be included” category and C in
the “could be included” category, and say that the structures are important
enough that the sp hybridization is real; while this is not what happens,
it’s not a bad judgement call based on what you know.)
Structure B moves an H atom, and structure D violates the octet rule, so
neither should be included.
5
3. Draw 4 isomers that have the molecular formula C2 H3 NO. Do not use rings.
(a) For each isomer, provide a Lewis structure with major and minor resonance
structures and non-zero formal charges (if present).
(b) Label which of the four isomers you expect to be the most stable compound
(“most” or “most stable”) and explain (1 sentence):
(c) Label which of the four isomers you expect to be the second most stable
compound (“second most” or “second”) and explain (1 sentence):
(d) Label which of the four isomers you expect to be the third most stable
compound (“third most” or “third”) and explain (1 sentence):
(e) Label which of the four isomers you expect to be the least stable compound
(“least” or “least stable”) and explain (1 sentence):
There are a lot more than four isomers. Some of them are listed on the next page.
6
1
H
H
C
H
N
C
O
2
H
C
H
H
H
H
C
N
C
O
H
3
H
O
H
H
H
N
H
C
C
N
O
H
C
N
O
H
C
N
O
H
C
N
O
H
C
N
O
H
H
H
C
H
C
H
O
C
C
N
H
H
C
H
H
C
N
C
O
H
C
O
C
N
H
H
4
H
H
1
H
C
C
N
O
H
5
H
H
H
O
C
H
C
C
N
O
H
C
O
N
5'
O
H
H
C
C
9'
H
H
N
H
H
C
C
N
O
C
N
O
H
H
C
C
N
O
C
H
11
O
N
H
7'
O
H
C
C
O
N
C
H
H
H
H
C
N
C
N
H
H
C
N
H
H
C
H
O
12
H
O
C
C
H
H
C
H
N
H
C
C
H
H
H
H
9
H
H
7
C
H
H
H
C
H
O
C
H
N
C