CHEM 350
Principles of Organic Chemistry I Lab
Prof. T. Nalli, WSU
Experiment 6 - Computational study of carbocation
stability and SN1 reaction rates.
References
(1) Smith, Chapter 7. 14. (2) Pavia et. al., Expt 19B&D and pp 160-168. (3) Screttas, C. G.
J. Org. Chem. 1980, 45, 333-326.
OVERVIEW
We will carry out experiments 19B and 19D in Pavia using Hyperchem molecular
modeling software. The molecular orbital calculations being used make fewer
assumptions and provide more reliable molecular energies that the molecular
mechanics method used in previous modeling labs.
You will need to install Hyperchem on your computer: (You must be logged on to the
campus network to install and run Hyperchem.
1. Go to \\appsrv1\apps\hyper75install\install\Hyper75 and run the setup file. When
prompted enter your user name, “wsu” as your organization, and “edutechnology” as
the dealer. The serial number must be entered as 12-750-1601800013. Choose
“networked installation” and “software license key”.
2. Go up two folders (to \\Appsrv1\apps\hyper75install\install and run the
“HyperChem752Update” file followed by the “LSHOST check” file.
3. Go to \\Appsrv1\apps\Hyper75\Program and click and drag the CHEM (a green
beaker) on to your desktop. Double click on your new HyperChem shortcut and the
program should start. (You may be asked to once again enter the serial number, etc.
Part 1 – (Expt 19B, Part 1) Gas Phase SN1 (No Solvent)
General Instructions. Construct the compound of interest by using the “draw” tool (left
most icon in the toolbar at top) to click and drag C-C bonds just as you would in
Chem3D (one click gives you a single carbon). Attach Br by first selecting Br as the
“default element” under the “build” menu. (You will need to change this back to carbon
before building other molecules.) Then click, drag, and release from the carbon to which
the Br needs to be bonded. Finally, select “Add H and Model Build” under the “Build”
menu.
(There is no “undo” capability in Hyperchem, but if you make a mistake you can delete
the offending atoms by right clicking on them with the draw tool.)
As directed in Pavia, we will be doing semi-empirical MO calculations using the AM1
parameter set. Set up Hyperchem to do this by selecting “Semi-empirical” under the
“Setup” menu and then “AM1”.
To compute the energy of each molecule once built, select “Geometry Optimization”
under the “Compute” menu and then “OK”. (An RMS gradient of 0.01 and the PolakRibiere energy minimization algorithm should both work fine for our purposes.) The
energy displayed by the program once the calculation is completed represents the
Standard Enthalpy of Formation of the molecule in kcal/mol.
Suggested Procedures
1. Calculate the energy of each of the four alkyl bromides in turn. Save each molecule
once the calculation is complete (you will need them later) and then create the next one
by replacing one H with a CH3.
2. Now calculate the energy of the carbocations. Pull up each saved RBr file in turn and
first remove the bromine atom (right click on it). Under “Build”, choose “Explicit
Hydrogens” and “Allow Arbitrary Valence”. Then go to the “Setup” menu and select
“Semi-empirical”, “Options”. This is where you set the charge to +1. Also set the spin
multiplicity to 1. Calculate the energy of the carbocation using “Geometry
Optimization” as before. (Also note your observations on the geometry of the optimized
carbocation.) (Also save each carbocation file before going on to the next calculation!)
3. Lastly, compute the energy of a bromide ion. Set the charge to –1 in the same manner
as in step 2.
Part 2 – (Expt 19B, Part 2) Solution Phase SN1 (H2O Solvent)
For this part, we will need to repeat all of the calculations of part 1 but this time
including a surrounding matrix of water molecules. In Hyperchem, this is accomplished
by placing the molecule in a defined box. First reopen the file for the structure to be
calculated. Then under “Setup”, choose “Periodic Box”. Set the periodic box dimension
for all directions to nine angstroms. Now, as before compute the energy using
“Geometry Optimization”. Set the maximum number of cycles to 250. The calculations
will take some time- be patient.
Part 3 – (Expt 19D, Part 1) Carbocation Electrostatic Potential Maps
Unfortunately, Hyperchem 7.5 no longer allows the generation of electrostatic potential
maps that can be compared meaningfully from molecule to molecule. Specifically it will
not allow the setting of color values (as described in Pavia) to the same range for the
four carbocations to be examined. (Hyperchem now automatically determines the range
of color values to be used for each molecule). Suffice it to say the maps would come out
exactly as pictured on p 249, Fig 7.17 in Smith. No matter, what we will do is
better…obtain quantitative predictions of the charge on each carbon
Reopen each carbocation structure from part 1. Under the “Display” menu select
“Labels”. Under “Atoms” select “charge”. Record the charge of each carbon atom as
data in your notebook. Pay special attention to the charge on the carbocation carbon, i.e,
the carbon with formal charge = +1.
Part 4 – (Expt 19D, Part 2) Allyl Cation Electrostatic Potential Map
Build and optimize the allyl cation, CH2=CH-CH2+, using the techniques learned earlier
in the lab. Note your observations on the optimized structure and then display and
record the atomic charges using the same procedures as in part 3 above.