hyper chem car bocatio nex pt

7/28/2019 Hyper Chem Car Bocatio Nex Pt

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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 Hyperchemmolecular modeling software. The molecular orbital calculations being usedmake 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 loggedon 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, andedutechnology as the dealer. The serial number must be entered as 12-750-1601800013. Choose networked installation and software licensekey.2. Go up two folders (to \\Appsrv1\apps\hyper75install\install and run theHyperChem752Update file followed by the LSHOST check file.3. Go to \\Appsrv1\apps\Hyper75\Program and click and drag the CHEM (agreen beaker) on to your desktop. Double click on your new HyperChemshortcut and the program should start. (You may be asked to once againenter the serial number, etc.

Part 1 (Expt 19B, Part 1) Gas Phase SN1 (No Solvent)

General Instructions. Construct the compound of interest by using thedraw tool (left most icon in the toolbar at top) to click and drag C-C bondsjust as you would in Chem3D (one click gives you a single carbon). Attach Brby first selecting Br as the default element under the build menu. (Youwill 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 bebonded. Finally, select Add H and Model Build under the Build menu.

(There is no undo capability in Hyperchem, but if you make a mistake youcan delete the offending atoms by right clicking on them with the draw tool.)
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As directed in Pavia, we will be doing semi-empirical MO calculations usingthe 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 gradientof 0.01 and the Polak-Ribiere energy minimization algorithm should bothwork fine for our purposes.) The energy displayed by the program once thecalculation is completed represents the Standard Enthalpy of Formation ofthe molecule in kcal/mol.

Suggested Procedures1. Calculate the energy of each of the four alkyl bromides in turn. Save eachmolecule once the calculation is complete (you will need them later) andthen 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 theSetup menu and select Semi-empirical, Options. This is where you setthe charge to +1. Also set the spin multiplicity to 1. Calculate the energy ofthe carbocation using Geometry Optimization as before. (Also note yourobservations on the geometry of the optimized carbocation.) (Also save eachcarbocation file before going on to the next calculation!)3. Lastly, compute the energy of a bromide ion. Set the charge to 1 in thesame 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 thistime including a surrounding matrix of water molecules. In Hyperchem, thisis accomplished by placing the molecule in a defined box. First reopen thefile for the structure to be calculated. Then under Setup, choose PeriodicBox. Set the periodic box dimension for all directions to nine angstroms.Now, as before compute the energy using Geometry Optimization. Set themaximum number of cycles to 250. The calculations will take some time- bepatient.

Part 3 (Expt 19D, Part 1) Carbocation Electrostatic Potential MapsUnfortunately, Hyperchem 7.5 no longer allows the generation of

electrostatic potential maps that can be compared meaningfully frommolecule 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 beexamined. (Hyperchem now automatically determines the range of colorvalues to be used for each molecule). Suffice it to say the maps would comeout exactly as pictured on p 249, Fig 7.17 in Smith. No matter, what we willdo is betterobtain quantitative predictions of the charge on each carbon


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Reopen each carbocation structure from part 1. Under the Display menuselect Labels. Under Atoms select charge. Record the charge of eachcarbon atom as data in your notebook. Pay special attention to the charge onthe carbocation carbon, i.e, the carbon with formal charge = +1.


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Part 4 (Expt 19D, Part 2) Allyl Cation Electrostatic Potential Map

Build and optimize the allyl cation, CH2=CH-CH2+, using the techniqueslearned earlier in the lab. Note your observations on the optimized structureand then display and record the atomic charges using the same procedures

as in part 3 above.

hyper chem car bocatio nex pt

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