Chemistry 81 MtSAC Dr. V. Prutyanov Spring 2020 Mechanism of SN2 Reaction Student:______________________ Predicting SN2 reactivity by calculating transition states for RCl + Br - In this exercise you will consider the reactivity of alkyl chlorides towards SN2 reactions. Specifically, you will use Molecular Orbital calculations to explore the relative SN2 reactivity of the following compounds: CH3Cl, EtCl, i-PrCl and PhCH2Cl. The approach you will use is to calculate the energy of the SN2 transition state for the attack of Br - on the alkyl chloride, and subtract the energy of the reactants from it. That should allow to calculate the activation energy, Ea for the following process: RCl + Br- → RBr + Cl- The obtained data analysis should illustrate the impact of the structure of R on the rate of the SN2 reaction as larger activation energy corresponds to a slower reaction. Another variable in this work is the solvent that is going to be introduced later to model the reaction more realistically. Procedure 1. Choose the “5-bonded-carbon” structure from the Inorganic drawing template. This puts the five valences in the desired trigonal bipyramidal geometry, three planar and 120° apart (the equatorial bonds), and the other two at 90° to this plane (called the axial bonds). 2. Add a nucleophile and a LG to the axial valences that are at 180° to each other, and the appropriate groups to the equatorial valences to build each transition structure. Open “Calculations” menu, chose the right charge (-1 for an anion, for example) and run a semi- empirical PM3 “Equilibrium geometry”. This should yield a good starting point for the Transition State search with C-Br distance around 2.2 Å and C-Cl 2.4 Å. If necessary, adjust the bond distances using “Measure distance” option within “Geometry” menu. 3. After the optimization above is complete (you might resubmit “failed” calculations), open the calculations menu and choose “Transition State Geometry”. Check the IR box. Submit the job. 4. If the TS search is successful it should have one and only one negative (or so-called “imaginary”) frequency that could be apparent from the IR spectrum. Click on spectrum icon on the upper panel and chose Calculated IR box in a pop-up window. Clicking on the corresponding frequency on the top of the table will animate the imaginary frequency that leads to collapse of the TS to the products. Hence the name “imaginary” as the common vibrations are reversible and do not collapse a molecule. In this mechanism, the expected movement would be a departure of the leaving group upon an approachment of the nucleophile. 5. Record the energy of the TS in a table below. Safe you file! 6. In a separate file, build the nucleophile and the alkyl chloride together and calculate the energies of the reactants using semi-e.