Solvent effects in the SN2 Cl- + CH3Cl reaction
Nuclephilic subsitution (SN2) X- + CH 3 Y --> CH 3 X + Y- reaction is not only an important class of reactions in organic chemistry but also a model reaction to demonstrate the importance of solvent effects on reaction rate. It is known that the SN2 ...
Nuclephilic subsitution (SN2) X- + CH3Y --> CH3X + Y- reaction is not only an important class of reactions in organic chemistry but also a model reaction to demonstrate the importance of solvent effects on reaction rate. It is known that the SN2 charge transfer reaction such as Cl- + CH3Cl has a rather small barrier in the gas phase. However, experimentally it was observed to have a substantial barrier in aqueous solution.
In this experiment, you will determine the barrier heights for the Cl- + CH3Cl reaction in both the gas phase and in water and find the rationale for such a large solvent effect.
Procedure: Using tools in Avisto. You can download Avisto and its tools at Astonis.
1. Use MolDesign tool to build 3D structures for Cl anion, CH3Cl, and Cl--CH3--Cl- transition state. You can make a guess of the TS structure to have the HCCl angles to be 90 degrees and the C-Cl bonds to be elongated by 20% from its normal value.
2. User Basic QChem Edu or Basic QChem to find stable structures for Cl-, CH3Cl and transition state structure in both the gas phase and in water. There will be 6 calculations. Record the heats of formation of each species.
| Properties | Gas phase | In water | ||||
| Cl- | CH3Cl | TS | Cl- | CH3Cl | TS | |
| Heats of formation H(kcal/mol) |
Barrier Height in the gas phase
= H(TS, gas) - { H(Cl-, gas) + H(CH3Cl, gas)}
Barrier Height in water = H(TS, aq) - { H(Cl-,aq) + H(CH3Cl, aq)}
Free energy of solvation of species A, DGaq (A) = H(A, aq) - H(A, gas)
3. Plot the electrostatic potential ESP (with isovalue = 0.4) for the transition state. You will see the charge is redistributed over the two Cl atoms leading to smaller solvation free energy.
4. Use the TS structures for both the reactions in the gas phase and in water to perform IRC runs. Animate both IRC runs to see how the geometry of the reaction complex changes as it proceeds from the reactant to the product. What can you say about the motions of the incoming Cl anion, methyl group, and outgoing Cl anion as the reaction proceeds?
5. You can repeat the calculations for OH- reacting with CH3Cl.
