Experiment 5: Synthesis and Modeling of a Lewis Acid-Base Adduct
CH3500: Inorganic Chemistry, Plymouth State University
Adapted from C Nataro, MA Ferguson, KM Bocage, BJ Hess, VJ. Ross, DT Swarr, “ Lewis Acid-Base, Molecular Modeling
and Isotopic Labeling in a Sophomore Inorganic Chemistry Laboratory ,” J Chem Educ, 81:722 (2004) ; CD Montgomery,
"Integrating Molecular Modeling into the Inorganic Chemistry Laboratory ," J Chem Educ, 78:840 (2001) and GS Girolami,
TB Rauchfuss, RJ Angelici, "The Borane-Amine Adduct BH3:NH2C(CH3)3," Synthesis and Technique in Inorganic
Chemistry, 3rd Ed, University Science Books, 1999; pp 47-54.
Introduction:
All chemical reactions can be classified as either ionic, oxidation-reduction, or acid-base, assuming
the use of the Lewis acid-base definition: an acid is an electron-pair acceptor, and a base is an electron
pair donor. This definition is particularly important in the world of inorganic synthesis, wherein bonds
are often formed between an electron-deficient Lewis bases and a electron-rich Lewis acids in the
absence of water or donate-able protons.
The chemistry of compounds containing boron and hydrogen is extremely different from the
chemistry of hydrocarbons. Such compounds, for example, exhibit two types of boron-hydrogen bonds:
classical and non-classical. The classical B-H bond is a 2-center-2-electron bond analogous to the C-H
bond. The non-classical bond, by contrast, involves a 2-electron formed between two boron atoms and
one hydrogen atom (3-center-2-electron bond ). This unusual mode of bonding was the primary focus
of the initial studies in boron hydrides, which also provided insight into the bonding of other molecules,
such as metal clusters. While the bonding is still a unique aspect, boron hydrides have also become
common reagents in organic synthesis. The simplest boron hydride imaginable would be BH3. It is
considered a Lewis acid (electron acceptor) because it has six electrons, leaving the boron two short of
an octet. Lewis bases (electron donors) will react with BH3 to form a complex known as an adduct. The
BH3 molecule is a very strong Lewis acid and actually exists in equilibrium with B2H6. The B2H6
molecule can be pictured as an adduct formed by the electrons in a B-H bond of one BH3 molecule
(Lewis base) and the empty orbital of the second BH3 molecule (Lewis acid).
Lewis base adducts of BH3 can be prepared from B2H6, but it is not an ideal compound for
laboratory use, as it spontaneously combusts in air, reacts with traces of water, and is toxic. An
alternative route has been developed using NaBH4. While this compound is reactive if exposed to
acidic solutions, it is air and water stable. The BH4- can act as a Brønsted-Lowry base towards acidic
protons under rather mild conditions (Equation 1). After this step, a Lewis acid-base reaction can occur
to give the desired adduct. In this lab, you will prepare the tert-butylamine (H2NC4H9) adduct of BH3
(Equation 2) and characterize it with infrared spectroscopy.
(Eqn 1) BH4- + H+ → BH3 + H2
(Eqn 2) BH3 + H2NC4H9 → H3B—NH2C4H9
In this experiment, a new bond is formed between N and B, which should result in new vibrational
modes in the molecule. Infrared spectroscopy, which uses light in the infrared region to analyze the
vibrations of molecules, can thus be used to analyze the product. Extensive investigation of
compounds using IR spectroscopy has led to knowledge of patterns that allow the assignment of certain
peaks to particular structural components of molecules (the carbonyl/C=O stretch at ~1700-1800 cm-1 is
one in particular with which you are likely familiar). However, with no other resources available, a
comparison of infrared spectra of the reactants and products of the reaction could only reveal that a
new bond was formed, but not which bond.
© Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only.
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The vibrational motions in a molecule can be approximated using Hooke's Law (Equation 2):
(Eqn 3) V(x) = ½kx2
where V is the potential energy as a function of the displacement from equilibrium (x), and k is the
force constant. For a molecular vibration, the force constant can be considered a combination of the
masses of the atoms and the strength of the bond between them. Thus, if these quantities are known, it
should be possible to calculate the energy of a particular vibration and subsequently the IR spectrum of
a compound. Indeed, Hooke's Law is the basis of a quantum mechanical treatment of molecules that
can be used by molecular modeling programs to predict such spectra. Therefore, in addition to the
laboratory work, a portion of the lab will involve the use of the quantum mechanical modeling package
Spartan to model the reactants and products and to simulate the IR spectrum of the Lewis Acid/Base
adduct, which is the product. Visual representations of the actual molecular motions can be viewed,
allowing a reasonable assignment of the infrared bands. The IR spectrum obtained through computation
methods can then be compared to the experimental spectrum. While the agreement is not bad,
differences of up to 60 cm-1 are not uncommon, largely due to the fact that the calculations are done in
the gas phase.
Safety Considerations:
• Sodium borohydride are flammable solids and corrosive.
• Tetrahydrofuran is flammable.
• Chloroform is a cancer suspect agent and appropriate precautions should be used.
• Hydrogen gas is generated in the synthesis of the adduct. The synthesis should be performed in
a well-ventilated hood away from any possible ignition sources.
Procedure
• Remember to record what you do, data collected, and observations at each step in the
procedure.
A. Pre-lab Work
1. Draw Lewis structures of starting materials and the product for Equation 2. Using VSEPR
theory, indicate the geometry of the B, N and C atoms.
2. Calculate the molecular masses of the starting materials and the product for Equation 2.
B. Synthesis of Lewis Acid-Base Adduct
• Be sure to record any and all observations at each step.
1. Add 1.3g of NH3(tBu)+Cl- and 15 mL of THF to a round bottom flask equipped with a stir bar.
Mount the flask on a ring stand and begin stirring.
2. To the stirred suspension, add 0.20g of powdered NaBH4. At this point, a gas will be evolved.
Add an additional 10-15 mL of THF and continue stirring the solution for about 2 hrs at room
temperature. Work on Part C will the reaction stirs.
3. Filter the solution using a vacuum filter apparatus, a small Buchner funnel, and filter paper.
Discard the solid into an appropriate waste container and keep the solution.
4. Using a rotary evaporator, evaporate the THF solution to dryness.
© Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only.
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5. Dissolve the solid in a minimal amount of toluene (~1-2 mL). Add 20 mL of hexanes and set
the flask in ice for five minutes to recrystallize the product. Recover the product by filtering
through a glass fritted filter funnel and rinsing twice with a few milliliters of cold hexanes.
Weigh the product.
6. Determine the melting point of the product using a melting point apparatus.
7. Measure the IR spectrum of the product. Label and note the frequencies of the major peaks.
Save an image of the spectrum with labeled peaks by printing it to a post-script file.
C. Computer Modeling of Lewis Acid-Base Adduct
• Calculations should be done on the following compounds from Equation 2:
º BH3
º NH2(C4H9)
º H3B-NH2(C4H9)
1. Turn on and log in to a computer. Run Spartan.
2. Draw each structure:
a) Under the "File" menu select "New".
b) A menu will appear on the right side of the screen that provides molecular fragments for you
to use in building molecules. If this menu ever disappears, and you want it back, choose
"Add Fragment" under the "Build" menu.
c) Select the atom type with geometry as close as possible to what you predicted from the
prelab. The "Organic" menu will supply all the 'C' and 'N' groups you should need. Use the
"Inorganic" group to select the 'B' groups you need. Focus on the central atoms first, then
add the H's later.
d) If you need to delete a bond or an atom, do so with the red 'star' button under the "Build"
menu.
e) Complete the structure by adding H's to the ends of the bonds that need them. The 'H' in the
terminal position can be found under the "Organic" menu.
f) The left mouse button will rotate your structure. The right mouse button will move it.
g) Holding shift while using the left or right mouse buttons will change their behavior. Play
with it to see what happens.
h) When you are done building the structure, do a quick energy minimization with the 'E'
(minimize) button under the "Display" menu.
i) SAVE the structure with a unique and useful name! If you have edited a previous structure
to get the current one, use "Save As" to make sure you do not overwrite the original.
3. Calculate and analyze the molecules. For all structures:
a) Open the previously saved file.
b) Under "Options" select "Monitor".
c) Under "Display" select "Output".
d) Go to "Setup -> Calculations.” Choose "Energy", "Hartree-Fock", and "STO-3G". Select
Calculate "Infrared Spectra". Select the Print "Orbitals & Energies" and "Vibrational
Modes" options. Do NOT click "Submit" here! Click "OK" instead.
e) Go to "Setup -> Submit".
f) Enter a name for the file. At this point, you should see the submitted job appear in the
"Monitor" window you opened previously.
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g) Once the calculation is complete, you will see some results appear in the "Output" window
you opened. Scroll through this and verify that the calculation completed successfully.
h) Right-click the molecule, select "Properties", and record the energy of the molecule.
i) Using the "Setup -> Surfaces" menu, add the HOMO and LUMO surfaces.
j) Display one surface at a time. Click on the molecule, and a menu will appear at the bottom
right corner that allows you to change the view of the surface. Select "Mesh". Save the
image to a .jpg using "Save As..." You may wish to change the background color to white
using "Option -> Colors". You should have a HOMO and a LUMO figure for both the acid
and the base reactants.
k) Determine the energies for the HOMO's and LUMO's by looking through the output. The
HOMO will be the negative value closest to 0 and the LUMO will be the positive value
closest to 0. Note: energies are given in electron-Volts (eV).
l) View the IR spectra with "Display -> Spectra". Select "Draw Calculated." Analyze the
spectrum by placing checks in the "Frequency" boxes in the "Spectra" window and/or by
clicking on specific peaks in the spectrum. Look for the vibrations involving the boron and
nitrogen and record these calculated frequencies. The bands of interest are:
- B-N stretch
- N-H scissor
- B-H scissor and wagging
- N-H stretch (symmetric and asymmetric)
- B-H stretch (symmetric and asymmetric)
m) Save a picture of the spectrum. Hint: zoom out to make the molecule smaller so you can
clearly see the spectrum.
Analysis (Lab Notebook)
The following must be completed in your lab notebook before you can turn in your Notebook Report:
1. Calculate the percent yields for product.
2. Compare the calculated energies of the molecules. Do these help explain why the reaction
occurs?
3. Identify any band(s) in the IR spectrum of the product that do(es) not appear in the reactant
spectra.
Lab Report including Conclusions and Discussion
Your lab report is due by lecture on Wednesday. Your report must be handed in BOTH
electronically and in hard copy form. See the document "InorgChem-LabReportGuide.pdf" on the
course website for guidelines on writing your report.
In addition to the Analysis performed in your notebook, include the following in your report (place
them in the most relevant sections, not as a series of questions and answers):
1. Include the IR spectrum of the adduct and a table of peaks with their assigned vibrational
modes.
2. Discuss the HOMOs and LUMOs of the reactants. Given the definition of Lewis acids and
bases based on electron donation, which orbital (HOMO or LUMO) of the acid is reacting with
which orbital (HOMO or LUMO) of the base? Compare the reactant orbitals that are
interacting with the HOMO of the product to explain why this reaction occurs.
3. Compare the experimental and computational IR spectra. Pay particular attention to the
computational B-H stretches in the labeled compounds. Describe and account for any observed
differences.
4. Include pictures of the LUMO of the Lewis acid and the HOMOs of the Lewis base and adduct.
© Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only.
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