Carbonyl derivatives and IR Spectroscopy
Jolie DeForrest
Department of Chemistry, University of Pittsburgh
July, 2006
Learning Objectives:
 Demonstrate through modeling the relative geometry of an aldehyde, ketone,
ester, acid, and amide.
 Examine the correlation between bond length, and electronic environment of each
molecule.
 Help the students understand IR spectroscopy and learn how to calculate the IR
spectrum of an acetaldehyde, acetone, methyl acetate, acetic acid, and N,Ndimethylacetamide.
Learning Activities:
 Model acetaldehyde, acetone, methyl acetate, acetic acid, and N,Ndimethylacetamide using CAChe software
 Calculate various bond lengths of each molecule
 Calculate and analyze the IR spectrum for the five carbonyl compounds
Carbonyl derivatives and IR Spectroscopy
The carbonyl group is the most important functionality in organic chemistry. This group
is composed of a C=O double bond where both the carbon and oxygen atoms are sp2hybridized. The oxygen atom is more electronegative than the carbon atom causing the
carbon-oxygen double bond to be polarized. This polarization is rationalized by a
resonance contributor in which the carbon atom is electropositive and the oxygen atom an
electronegative (Figure 1).
Figure 1: Resonance Contributors for Acetone
O
O
Infrared (IR) spectroscopy is used by organic chemists to determine which
functionalities, like a carbonyl group, are present in a molecule. The carbonyl moiety is
present in many biologically active compounds such as ovalicin (anti-cancer agent),
guanacastepene A (antibiotic), and phorbol (protein kinase C activator) (Figure 2).
Infrared spectroscopy is especially useful for determining different types of carbonyl
groups in a compound including ketones, aldehydes, and esters (among others).
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Figure 2: Natural Products Containing a Carbonyl Group
OH
O
O
O
O
O
OH
OH
H
OH
O
OH O
H
O OH
Guanacastepene A
Ovalicin
Phorbol
OH
This computational exercise will use CAChe software package to computationally model
acetaldehyde, acetone, methyl acetate, acetic acid, and N,N-dimethylacetamide.
Figure 3: Carbonyl Derivatives
O
O
O
O
acetaldehyde
acetone
O
O
OH
methyl acetate
acetic acid
N
N,N-dimethylacetamide
Part 1: In the first series of experiments acetaldehyde, acetone, methyl acetate, acetic
acid, and N,N-dimethylacetamide will be modeled. From the optimized geometry, bond
lengths and angles will be measured, and the heat of formation (ΔfH) will be examined.
(If needed, hints for using the CAChe software package to solve these exercises can be
found in the Supplemental Information at the end of this document.)
Table 1: Calculated enthalpy, bond length and angle values
ΔfH
(kcal/mol)
sp3 C-H
(Å)
C=O
(Å)
sp3-sp2 C-C
(Å)
C-C-O
angle (°)
Acetaldehyde
Acetone
Methyl Acetate
Acetic Acid
N,N-dimethylacetamide
Questions
1) Is each molecule the shape that you would predict?
2) A carbonyl C=O double bond is typically around 1.22 Å. Do the measured values for
each compound agree with the average value? Which compound has the longest C=O
bond and why? (hint: answer question in terms of resonance)
.
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Part 2: The second set of experiments involves determining the IR frequency of
acetaldehyde, acetone, methyl acetate, acetic acid, and N,N-dimethylacetamide. The
carbonyl stretch C=O stretch will then be recorded and compared to the experimental
value. (hint: the carbonyl frequency should be between 1650-1850 cm-1)
Table 2: Calculated C=O stretch of acetone, methyl acetate, acetic acid, and N,Ndimethylacetamide
Calculated values
(cm-1)
Acetaldehyde
Acetone
Methyl Acetate
Acetic Acid
Experimental values
(cm-1)
1730
1715
1735
1710
N,N-dimethylacetamide
1650
Questions
3) Do the calculated wavenumbers for the C=O stretches of the different compounds
agree with the experimental values? If not, calculate the difference for each of the
values.
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4) Based on the data collected, label the carbonyl moieties epothilone C in order of
increasing wavenumber.
S
HO
N
O
O
OH
O
Epothilone C
5) Sp3 C-H stretches usually absorb in the range of 3000-2840 cm-1. Looking at the IR
spectrum of each molecule, what types of stretching do you see?
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Supplemental CAChe Instructional Guide For
Carbonyl derivatives and IR Spectroscopy
Jolie DeForrest
Department of Chemistry, University of Pittsburgh
July, 2006
Part 1: Constructing Acetaldehyde
1) Open CAChe Workspace (a blue screen should appear).
2) Select the drawing pencil tool (5th down on left side).
3) Choose C / sp3 / blank / single in the drop down windows at the top of the screen.
4) Click in workspace. A grey carbon atom should appear.
5) Select C / sp2 / blank / single.
6) Click on sp3 hybridized carbon and drag away, then release. You should have a
sp2-sp3 C-C single bond.
7) Now, Select O / sp2 / blank / single. Click on sp2 hybridized carbon atom, drag
away, and then release. A red oxygen atom should appear. Draw another bond
between the sp2 carbon and oxygen atoms so that there is a carbon-oxygen double
bond.
8) Click in workspace and all atoms present should appear in full color.
9) Clean up the structure by selecting Beautify / Comprehensive. CHOCH3 should
appear.
Optimize Molecule
1) Choose Experiment / New. Save as acetaldehydeDH.csf
2) Choose Property of: chemical sample, Property: optimized geometry,
Using: PM3.
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3) Click start. Record the calculated ΔfH value and close both the experiment status
and experiment windows.
Measure Properties
1) Click select tool (black arrow). Click on the sp3 carbon atom, shift and then on the
hydrogen atom.
2) Choose Adjust / Atom Distance and record the sp3C-H bond length. Repeat for
C=O, sp2-sp3 C-C.
3) Click on the sp2 oxygen atom, shift, and then select the sp2 C and sp3C atoms.
Chose Adjust / Bond Angle and record the calculated angle.
Now repeat process for acetone, methyl acetate, acetic acid, and N,Ndimethylacetamide
Part 2: Calculating the IR spectrum of an acetaldehyde
1)
2)
3)
4)
5)
6)
7)
8)
Build another model of acetaldehyde in CAChe.
Clean up the structure by selecting Beautify / Comprehensive.
Save as acetaldehydeIR.csf in your CAChe folder.
Optimize the geometry by selecting chemical sample / optimized geometry / B88LYP DFT geometry. Close the two windows.
Calculated the infrared spectrum by selecting Experiment / New. Then select
chemical sample / IR transitions / B88-LYP DFT IR spectrum. After the
calculation is complete, close the two windows.
When the calculation is complete, view the infrared spectrum by selecting
Analyze / IR Transitions. Fit both windows into the screen by choosing Windows
/ Tile.
On the IR spectrum, single click on a triangle and the type stretching associated
with each transition will appear. Double clicking on any given triangle will give
the receptive wavenumber for each stretch.
Record the wavenumber for the C=O stretch into the table below. Repeat for
acetone and methyl acetate.
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