Chemistry& 241
Clark College
Homework 3 SOLUTION
Please put your answers on a separate sheet of paper. You may label the IR spectra directly on
this homework sheet.
Due Thursday, October 9, 2008 IN CLASS.
SDBS Spectral Database for Organic Compounds, http://www.aist.go.jp/RIODB/SDBS/cgibin/cre_index.cgi, (November 5, 2006).
1. Give all structural isomers for the following compounds. Draw them all as line structures!
a. C4H8 (disregard cis/trans, I found 5)
The formula indicates that there is either one double bond or one ring. By name, the five
structures are:
1-butene, 2-butene, 2-methylpropene, cyclobutane, and methylcyclopropane.
b. C4H10O (I found 7)
1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, diethyl ether, methyl propyl
ether, isopropyl methyl ether.
HO
OH
HO
OH
O
O
O
The alcohols
The ethers
2. More nomenclature.
a. Name the following compounds, using IUPAC or common schemes, where appropriate:
1
2
Br
3
I
4
5
HO
8
1
7
6
2 3
4
6
Priority is given to the double
bond- it should have the lowest
number possible. Since it is in
the middle of the chain, then the
chain is numbered to give the
lowest numbers to the side group.
4
5
Non-carbon functional
groups take precedence
over carbon functional
groups for numbering.
1
2
3
6
Cl
5
7
The chain should be
numbered to give the
triple bond the lowest
possible number.
i. 2-bromo-4-iodo-3-methyl-3-hexene
ii. 2-tert-butyl-6-methyl-4-cyclooctenol (or cyclooct-4-en-1-ol)
iii. 5-chloro-6-methyl-2-heptyne
b. Draw structures for the following molecules.
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Chemistry& 241
i.
Clark College
2,7-dibromo-4-isopropyl-3,6-octadiene This name is actually incorrect – I got it
from another book! It should be 2,7-dibromo-5-isopropyl-2,5-octadiene
Br
Br
ii. 4-sec-butyl-6-hepten-1-yne
iii. 1,1-diethylcycloheptane
3. Draw the structure of 2,3-dimethyl butane. Draw all 6 Newman Projections for 2,3dimethylbutane, as viewed between C2 and C3. Label the projections as staggered or
eclipsed. Rank the six projections in energy order, where 1 is the lowest energy conformer
and 6 is the highest energy conformer.
2,3-dimethylbutane
H 3C
H
CH3
H
CH3
CH3
Lowest Energy
CH
H 3
CH3
CH3
H3C
CH3
CH3
H
CH3
H
CH3
CH3
H3C
H
CH
CH3 3
Same energy, higher than previous
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H
Staggered Conformers
Same energy, higher than previous
CH3
H
CH3
CH3
H3C
H
H
CH3
CH3
H
H
Eclipsed Conformers
CH
CH3 3
Highest Energy
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Chemistry& 241
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4. Read Section 2.7 in your text. Draw line structures for the following constitutional isomers,
and rank them in order of increasing boiling points (1 = lowest bp, 4 = highest bp).
3,3-dimethylhexane; octane; 2,2,3-trimethylpentane; 3-ethylhexane
5. Read Section 2.8 in your text. For the same isomers, rank them in order of increasing
∆Hcombustion (1 = lowest ∆H, 4 = highest ∆H). Which molecule is the most stable?
Octane
2,2-dimethylhexane
3-ethylhexane
2,2,3-trimethylhexane
BPs
4
2
3
1
!Hcomb
4
2
3
1
Most stable
Both questions deal with analyzing molecules for their degrees of branching. With boiling
points, molecules with less branching have higher boiling points. Molecules with a greater
degree of branching have less surface area, therefore they have lower dispersion forces
between molecules because less of a molecule can interact with another. As for the enthalpy of
combustion, branching makes a molecule more stable, so it releases less energy upon
combustion and has a smaller ∆H.
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Chemistry& 241
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6. Draw both chair forms for the following cyclohexanes. Circle the more stable form.
H3C
CH3
CH3
H
a)
CH
H
H3C
H3C
CH3
CH3 CH
b)
H
CH
H3C
H
Cl
CN
H
NC
H
H
CH3
H
CH3
Cl
H
H
H
H
d)
Cl
H
H
H
e)
CH3
H
H
c)
CH3
H
H3C
CH3
H
CH
CN
H
Cl
CN
H
iPr
H
H3C
H
CN
H
H
iPr
CH3
Looking at Table 2.4 in the text, the
!G values show that the isopropyl
group should be equitorial.
H
CN
7. Name 6a and 6b.
a) cis-1-isopropyl-4-methylcyclohexane b) cis-1-isopropyl-3-methylcyclohexane
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Chemistry& 241
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8. All of the following molecules have the formula C6H10O2. Match each compound to its
corresponding spectrum, and give a reason/evidence for your choice.
O
O
Compound A
O
O
Compound B
O
HO
OH
O
Compound C
Compound D
This spectrum matches compound: Compound C
Reason: This spectrum only shows a carbonyl peak, at ~1720 cm-1. Most of the other
peaks are for sp3-C-H stretching and bending.
This spectrum matches compound: Compound A
Reason: This spectrum shows both a carbonyl peak around 1720 cm-1, but also an alkene
C=C peak at 1620 cm-1.
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Chemistry& 241
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This spectrum matches compound: Compound B
Reason: In addition to the carbonyl around 1720 cm-1, this spectrum shows a strong peak
below 1200 cm-1, which is consistent with the ester functional group and a C-O single
bond.
This spectrum matches compound: Compound D
Reason: The alcohol peak!
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Chemistry& 241
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9. Draw structures for the three isomers 1-butyne, 1,3-butadiene, and 2-butyne. Explain how
you would distinguish these compounds using IR spectroscopy. Give specific examples of
resonances that would illustrate the differences.
H C C
H3C C C CH3
The most diagnostic
peak is the sp C-H
peak around 3300
cm-1. The C-trip-C
peak is around 2200
cm-1.
To differentiate this alkyne from
the first, note that there is no sp
C-H on the molecule, so no
peak at 3300 cm-1. Also, since
the molecule is symmetric, the
C-trip-C stretch would be very
small, since their would be no
net change in molecular dipole.
Diagnostic peaks
include the sp2 C-H
peaks just above
3000 cm-1, and the
C=C around 1650
cm-1.
10. For each of the following IR spectra, select at least 3 IR peaks and label the functional
groups to which they correspond. Make sure that they are consistent with the given
molecular formula.
-OH acid
C=O
alkane
C-H
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C=C
C-O
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Chemistry& 241
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alcohol
-OH
alkane
C-H
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C-O
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