Chem 121 Lab
Clark College
Name:
Partner:
Experiment 4: Polarity and Electronegativity
Instructor initials:
Do bonds to electronegative atoms affect carbon atoms in a molecule?
Content Goals:
• Gain a working understanding of polarity in covalent bonds.
• Understand how electronegative atoms affect carbons in an organic molecule.
• Use a carbon NMR spectrum to locate electronegative atoms on a carbon
skeleton.
• Use an NMR spectrum to predict molecular polarity
Process Goals: Lab Skills
• Reading data from a graph (spectrum)
• Interpreting sets of data to establish trends
Required Reading:
Chapter 11, Smith 1/e
Bring your textbook to lab to help understand how to name and draw carbon chains in
organic molecules.
Purpose
The goal of the experiment is to introduce you to how chemists know what molecules
look like and how the atoms attached to carbon molecules affect their structure.
Introduction
From the models lab we explored the three-dimensional nature of carbon molecules
having shapes like tetrahedral, trigonal planar or linear. These molecules, built using a
carbon chain (or skeleton), are called organic molecules. Organic chemistry is the study
of carbon molecules. If we consider carbon, hydrogen, nitrogen and oxygen atoms as
our only building blocks, a wide variety of structures can be created - even with just a
few of atoms. Many organic molecules are made in our bodies, such as proteins from
amino-acids, or molecules like fatty acids that can be converted into things like
cholesterol and hormones. We will limit our discussion here to smaller molecules as
they present plenty of complexity for us in the context of this class.
Chemists use many tools to observe organic molecules, the most prominent is Nuclear
Magnetic Resonance, or NMR for short. We will use NMR in this lab and learn how to
read the data given by instrument and learn more about polarity in organic molecules.
Model 1. C2H6 (ethane)
Electron (Lewis) Dot Symbols
tetrahedral atom
each hydrogen atom
makes 1 bond!
H
H
H
C
C
H
H
H
tetrahedral atom
C
H
each carbon atom
needs to make 4 bonds!
Model 2. C2H5Cl (1-chloroethane)
Electron (Lewis) Dot Symbols
tetrahedral atom
each chlorine atom
makes 1 bond!
Cl
H
H
C
C
H
H
H
tetrahedral atom
C
each carbon atom
needs to make 4 bonds!
Cl
Critical Thinking Questions
1. With your lab partner(s), confirm that molecules in Model 1 and 2 are drawn
correctly.
2. Looking at Model 1, determine if ethane (C2H6) is a polar molecule.
Polar
or
Non-polar
(circle one)
2. Looking at Model 2, determine if 1-chloroethane (C2H6Cl) is a polar molecule.
Polar
or
Non-polar
(circle one)
3. Looking at bonds made by each carbon in Model 2, how different “types” of carbon exist
in 1-chloroethane? (i.e. are both carbons “the same” or are they “different”). Explain.
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Model 3. Carbon NMR Spectra for Two Molecules
A. The carbon-NMR spectrum of 1-chloroethane (C2H5Cl)
Cl
H
H
C
C
H
H
80
H
70
60
50
PPM
40
30
20
10
0
20
10
0
B. The carbon-NMR spectrum of 1-bromoethane (C2H5Br)
Br
80
H
H
C
C
H
H
70
H
60
50
PPM
40
30
Information
•
•
•
An NMR looks at the electronic (magnetic) properties of carbon atoms.
Each line (or peak) in an NMR spectrum represents a “different” type of carbon.
An NMR works exactly like an MRI in a hospital or radiography office (Magnetic
Resonance Imaging, or MRI, looks at magnetic properties of hydrogen, not carbon).
Critical Thinking Questions
5. How many different carbons are represented in the NMR spectrum for 1chloroethane?
6. How many different carbons are represented in the NMR spectrum for 1bromoethane?
7. Learning Check: How would the NMR spectrum of ethane (C2H6) be different from
the NMR of 1-chloroethane (C2H5Cl)? Explain. (NMR spectrum shown on the next
page!)
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C. The carbon-NMR spectrum for ethane (C2H6)
80
70
60
50
PPM
40
30
20
10
0
8. From Model 3, what number (shown in PPM on an NMR) is: (circle one)
a. a carbon atom in a C-H bond in 1-chloroethane: ~20ppm
~35ppm ~40ppm
b. a carbon atom in a C-Cl bond in 1-chloroethane: ~20ppm
~35ppm ~40ppm
c. a carbon atom in a C-H bond in 1-bromoethane:
~20ppm
~35ppm ~40ppm
d. a carbon atom in a C-Br bond in 1-bromoethane:
~20ppm
~35ppm ~40ppm
e. a carbon atom in a C-H bond in ethane:
~20ppm
~35ppm ~40ppm
9. Of the bonds given in the last question, which is the most polar:
(circle one)
C-H
C-Cl
C-Br
10. Based on Model 3, for the molecules shown, which bond type has the largest PPM in
an NMR spectrum:
(circle one)
C-H
C-Cl
C-Br
11.Based on Model 3, for the molecules shown, which bond type has about the same
PPM in an NMR spectrum:
(circle one)
Polarity and Electronegativity
C-H
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C-Cl
C-Br
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12. Use the blank spectrum below to sketch the predicted NMR spectrum you expect
for 1-fluoroethane (C2H5F).
80
70
60
50
PPM
40
30
20
10
0
13. The NMR spectrum below belongs to 1-fluoroethane (C2H5F). How does it compare
to your prediction? What factors in the structure did you account for when making
your prediction?
80
70
60
50
PPM
40
30
20
10
0
14.The NMR spectrum below belongs to one of the following molecules. Figure out
which molecule is represented in the spectrum and briefly explain your choice.
80
Cl
H
H
C
C
H
H
70
Cl
Polarity and Electronegativity
60
50
Br
PPM
40
H
H
C
C
H
H
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30
Cl
20
H
10
O
0
H
H
C
C
H
H
Cl
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Model 4. Carbon NMR Spectra for Isomers
A. The carbon-NMR spectrum of 1-chlorobutane (C4H9Cl)
80
70
60
50
PPM
40
30
20
10
0
20
10
0
B. The carbon-NMR spectrum of 2-chlorobutane (C4H9Cl)
80
70
60
50
PPM
40
30
Information
•
•
•
An isomer is a molecule that has the SAME chemical formula as another molecule, but
the atoms are connected differently in space.
1-chlorobutane and 2-chlorobutane are constitutional isomers - same 4 carbon chain, but
different in where the Cl atom is located on the chain.
Constitutional isomers have different chemical and physical properties and are therefore
different molecules.
Critical Thinking Questions
16. Draw the structures of 1-chlorobutane and 2-chlorobutane on their NMR spectra
above. You may want to do this first on scratch paper!
17. Number the carbons chains on each molecule, and assign a number to each line on
the spectrum (i.e. which line goes with carbon #1, carbon #2, etc.).
18.Looking at your numerical assignments, do the NMR spectra demonstrate the
structural differences between the two isomers? Explain why or why not.
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19. The spectrum below is for another constitutional isomer of chlorobutane (C4H9Cl).
Use the data in the spectrum to draw the correct chemical structure for this isomer.
Hint: Why are there only three peaks? And why is one peak taller than the other two?
80
70
60
50
PPM
40
30
20
10
0
If you get stuck, draw all the possible isomers for C4H9Cl and see which BEST matches
the NMR spectrum above!
Post-Lab Questions.
1. Did any NMR spectrum for a molecule in this lab show lines for atoms other than
carbon?
2. Without a molecular formula, what information could a carbon-NMR spectrum give
you about a molecule? Did you encounter any exceptions?
3. What structural feature of a molecule has the biggest effect on the PPM value for a
carbon atom in an NMR spectrum?
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4. When a doctor performs an MRI, they are doing the same kind of experiment on
patients that chemists do with carbon atoms in an NMR. While chemists want to look
at chemical structure, doctors want to image trauma in the body - and MRI allows
them to image areas in the body without being invasive (entering the body physically
with surgery or incision). In addition, MRI uses radio pulses instead of high-energy
X-rays, making exposure to high levels of radiation a very minimum by comparison.
MRI (Magnetic Resonance Imaging), like NMR (Nuclear Magnetic Resonance), works in
the principle that certain atoms have a magnetic moment. That is, when placed in a
magnetic field, some atoms become magnetic - and can be detected if the machine if
tuned to receive the signals. In addition to carbon atoms, hydrogen atoms are also
magnetically “active” in an MRI or NMR. A chemist would look at both carbon and
hydrogen NMR spectra for a molecule (we just looked at carbons in this lab!), while an
MRI looks only at hydrogen, specifically hydrogen on water molecules, to create an
image from the body. Use the following link to learn more about MRI and to answer the
following questions:
http://health.howstuffworks.com/medicine/tests-treatment/mri.htm
a. Explain why hydrogen is a better choice for MRI when imaging in the body (i.e. why
not use carbon)?
b. Why does MRI (and NMR) use magnets?
c. What do radio frequencies do during an MRI scan?
d. Why is there so much noise associated with an MRI scan?
e. What is the difference between an MRI and a CT scan?
f. Why does a patient have hold still during an MRI?
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