CHEM 233, Fall 2016
Ian R. Gould
Midterm #2
COMPLETE THIS SECTION : Up to TWO POINTS will be removed for incorrect/missing information!
Answer Key
PRINTED FIRST NAME
PRINTED LAST NAME
Person on your LEFT (or Empty or Aisle)
Person on your RIGHT (or Empty or Aisle)
Class you are REGISTERED FOR (onground or hybrid)
The room where most students will take the test for your
class, i.e. LS A-191 for onground and PS H-152 for hybrid)
**YOU ARE NOT ALLOWED TO TAKE SPARE COPIES OF THIS EXAM FROM THE TESTING ROOM**
• PRINT YOUR NAME ON EACH PAGE!
• WRITE CLEARLY!
• READ THE DIRECTIONS CAREFULLY!
• MOLECULAR MODELS ARE ALLOWED
• USE BLANK PAGES AS SCRATCH PAPER
• DO NOT USE RED INK
work on blank pages will not be graded...
• DON'T CHEAT, USE COMMON SENSE!
H
He
Li Be
B
Na Mg
K
Ca
Sc Ti V
Cr Mn Fe Co Ni Cu Zn
Rb Sr
Y
Zr Nb Mo Tc Ru Rh Pd Ag Cd
Cs Ba
Lu Hf Ta W
Re Os Ir Pt Au Hg
small range
range of values
broad peak
C
N
O
F
Ne
Al Si P
S
Cl
Ar
Ga Ge As Se Br
Kr
In Sn Sb Te I
Xe
Tl Pb Bi Po At
Rn
O H
C N
N H
C O
C
usually
strong
C
O
N
C
R
R
C
O
C O H
O
CH
O
C
2200
R C OH
(δ, ppm)
11
220
10
200
9
180
–OCH2–
6
120
R C
C
R2C
Aromatic
1650 1500
NR2
O
C CH3
–H2C NR2
7
140
O
X
–H2C
C CH2
O
R C OH
C
1710
NMR Correlation Charts
8
160
O
2000
2500
Aromatic Ar H
mainly 8 - 6.5
H
1600
C
3000
NH2 variable and condition dependent,
ca. 2 - 6 δ
OH
O
1680
1735
2850–2960
3500
OR
2200
H
broad ~3000
(cm-1)
N
C
C
broad with spikes ~3300
broad ~3300
1600–1660
O
2720–2820
2 peaks
C
O H
C
C
H
H
H
3000–
3100
N H
Infrared Correlation Chart
O
C H
3300
Interaction Energies, kcal/mol
Gauche
Eclipsing
~1.0
~0.9
H/H
Me/Me
~1.4
~0.95
H/Me
Et/Me
Me/Me ~2.6
i-Pr/Me ~1.1
Et/Et ~3.1
t-Bu/Me ~2.7
CR2
5
100
N
4
80
RC
C CH
3
60
–OCH2–
CR
Alkyl
3° > 2° > 1°
2
40
C
C NR2
1
20
Alkyl
X
3° > 2° > 1°
0
0
CHEM 233, Fall 2016
Midterm #2
Ian R. Gould
YOU MUST COMPLETE THIS PAGE WITH YOUR NAME
(EVEN THOUGH YOU ALREADY DID THIS ON THE COVER PAGE)
AND ALSO GIVE YOUR ASU OR POSTING ID NUMBER
WE NEED THIS NUMBER BECAUSE YOU WOULDN'T BELIEVE THE NUMBER OF
STUDENTS WHOSE NAMES WE CAN'T READ!
PRINTED
FIRST NAME
ANSWER
PRINTED
LAST NAME
KEY
ASU ID or
Posting ID
Points by question
nomen
1_____________/13
chairs1
2_____________/24
newman
3_____________/26
newman2
4_____________/18
IR spectra
5_____________/18
res1
6_____________/24
res2
7_____________/18
res3
8_____________/24
IR
explain
9_____________/10
Points Removed for cover errors ____/2
Extra Credit_____/5
Total (incl Extra)________/175+5
**YOU ARE NOT ALLOWED TO TAKE SPARE COPIES OF THIS EXAM FROM THE TESTING ROOM**
-2-
CHEMISTRY 233, Fall 2016, MIDTERM 2
NAME
Question 1 (13 pts.) Give the IUPAC name for the following structure.
1
2
9
3 4 5
8
6
longest chain with max # of substituents
7
4-cyclopentyl-3-ethyl-2,7,8-trimethylnonane
Question 2 (24 pts.)
a) Draw both chair conformations for the provided cyclohexane and calculate the energy difference
between the chairs. Indicate CLEARLY which energy factors you are taking into account for each
chair. INDICATE WHICH IS THE LOWEST ENERGY CHAIR
b) Each carbon in the provided cyclohexane is labelled a, b, c, d etc. Label the carbon atoms on
your chairs with the letters a, b, c etc. to identify exactly which carbons in your chairs correspond to
which carbons in the provided structure
Me
f
e
lowest energy
e
i-Pr
d
f
c
a
d
Me
b
c
i-Pr
i-Pr
a
b
Me
d
ΔE = 2.2 kcal/mol
Me
2 x Me/Me gauche interactions here = 1.8
1 x Me/Me gauche interaction here = 0.9
TOTAL = 2.7 kcal/mol
f
e
c
b
a
Me
Me
2 x i-Pr/Me gauche interactions here = 2.2
2 x Me/Me gauche interactions = 1.8
1 x Me/Me gauche interaction here = 0.9
TOTAL = 4.9 kcal/mol
EXTRA CREDIT (5 pts) Which types of structures are the primary constituent of gasoline?
alkenes
ketones
alkanes
aromatics
-3-
CHEMISTRY 233, FALL 2016 MIDTERM #2
NAME
Question 3 (26 pts.) For 3,3-dimethylhexane:
a) Draw a line-angle structure
b) For rotation around the C3-C4 bond, draw Newman projections AND 3D (sawhorse)
structures for BOTH the HIGHEST and the LOWEST energy conformations, (clearly indicate
which is which). Your Newman projections should look FROM carbon 3 TO carbon 4.
4
3
Et
H
H
Me
lowest
Me
Et
EtEt
H H
Et
Me
Me
H
Me
highest
Et
Et
Et
Me
H
Me
Me
H H
c) Determine the energy difference between these two conformations. CLEARLY indicate the
energy factors that you included in your calculation of this energy difference
lowest E conformation:
2 Et/Me gauche = 2 x 0.95 = 1.9
Total = 1.9 kcal/mol
highest E conformation: 2 Me/H eclipse = 2 x 1.4 = 2.8
+ 1 Et/Et eclipse = 3.1
Total = 5.9 kcal/mol
ΔE = 5.9 - 1.9 = 4.0 kcal/mol
Question 4 (18 pts.) FOUR Newman projections are provided for rotation around the C2-C3 bond
in 3-methylbutane. Identify which Newman projection corresponds to which position on the energy
diagram (they are NOT necessarily in order!). For instance, if you think that Newman B
corresponds to position 1, draw an arrow from B to the number 1, if you think that B corresponds
to position 4, draw an arrow from B to the number 4 etc. You do not have to give any energies,
but you will need to calculate the energies to distinguish the two eclipsed conformations.
Me/H eclipse
Me/Me eclipse
1
x
Me/Me
gauche
3 x Me/H eclipse
2 x Me/Me gauche
H/H eclipse
= 0.9
= 4.2
= 1.8
= 5.0
Me
Me H
Me
MeH
H
H
H
Me
H
Me
Me
A
H
Me
H
H
H
B
Me
Me
Me
H
Me
D
C
2
4
Energy
H
1
3
dihedral angle
-4-
CHEMISTRY 233, Fall 2016, MIDTERM 2
NAME
Question 5 (18 pts.) Assign the provided IR spectra to ONE of the FOUR provided structures A D (two of the structures do not have a provided spectrum). On each spectrum, identify the peaks
that are associated with a specific functional group or type of C-H bond by drawing the
functional group or bond and drawing an arrow from the specific bond in the functional
group that vibrates to the absorption peak, as appropriate.
O
O
O
O
H
HO
B
A
D
C
3346
3009
H
H
C
C
sp3
2901
sp2
1279
1470
1639
O
3004
O
1171
975
1386
this spectrum
A
corresponds to structure _____
1692
3389
1260
1134
1036
O
C
2820
2725
1380
H
947
1467
2876
2935
H
C
O
sp3
H
2963
1708
this spectrum
D
corresponds to structure _____
-5-
CHEMISTRY 233, Fall 2016, MIDTERM 2
NAME
Question 6 (24 pts.) One of the ketones A and B below has a higher frequency C=O stretching
vibration than the other. Indicate which has the higher and which the lower vibration frequency, and
give a BRIEF EXPLANATION using drawings of minor resonance contributors. Include all curved
arrow pushing, resonance brackets and arrows. You do NOT need to draw the "actual" resonance
hybrid structures
O
O
O
higher frequency
H2N
A
H2N
H2N
O
O
O
NH2
NH2
B
O
NH2
NH2
lower frequency
structure B has one additional minor resonance contributor that has a single C-O bond,
AND it contributes more to the resonance mixture than the other minor contributors
because it has two formal pi-bonds, structure B this has more single bond character in
the C=O bond, the C=O bond is thus weaker and vibrates with a lower frequency
Question 7 (18 pts.)
a) Which of the anions A or B has the HIGHER energy non-bonding electrons? Give a BRIEF
explanation and use resonance contributors to support your argument.
b) Which of the anions A or B would be more CHEMICALLY REACTIVE, give a brief explanation
c) CIRCLE ALL of the sp2 hybridized atoms for each anion, take other resonance contributors
into account as appropriate
sp2
O
O
sp2
A
sp2
O
B
sp2
sp2
the electrons in A are resonance delocalized, and
thus stabilized (lowered in energy), the electrons in
B are thus higher and also thus MORE REACTIVE
-6-
CHEMISTRY 233, Fall 2016 MIDTERM #2
NAME
Question 8 (24 pts.) For the following structure:
a) Draw all reasonable resonance contributors, show how they are related using the curved arrowpushing formalism. Do not forget formal charges and non-bonding electrons as appropriate and
resonance arrows and brackets.
b) Indicate which is the MAJOR AND which is the MINOR resonance contributor (if there are
equal major or minor indicate so) and give a BRIEF explanation for your choices.
c) Draw the "actual" structure using the delta notation to indicate the positions of partial charges
N
O
H
C
C
C
H
C
N
O
H
C
H
C
C
C
N
O
H
H
C
C
C
H
C
H
H
H
H
H
H
H
major
the major resonance structure has the formal charge on the most electronegative atom
the minor resonance structure has the formal charge on the least electronegative atom
H minor H
H
H
δ
δ
O
H
C
δ
C
H
N
C
H
C
H
H
H
d) The structure is redrawn for you again below. Indicate the hybridization of all of the atoms
indicated by the arrows, AND, for each one, indicate whether it is part of the CONJUGATED
SYSTEM or not (use any method of indicating you like just so long as it is clear)
H sp2 YES
N
O
YES sp2
sp2 YES
H
C
C
YES sp2
sp3 NO
H
C
C
H
H
H
sp2
YES
Question 9 (10 pts.) In infrared spectroscopy, the absorption peak corresponding to the C=N
stretching vibration in an imine is usually much weaker then the corresponding absorption peak
corresponding to the C-N stretching vibration in a nitrile. Give a BRIEF explanation for this.
H
H3C
N
N
C
C
H3C
a nitrile
H
an imine
IR absorption strength depends on bond dipole moment, the C-N triple biond has a larger
dipole moment due to the 2 pi-bonds, pi-electrons are more polarizable