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-empirical PM3 calculations. Remember to choose

the correct charge in the Calculations menu for this van der Waals complex where the

nucleophile and RX are bonded by weak intermolecular forces (London, ion-dipole,

dipole-dipole, etc.)

7. In another separate file, calculate the energy of the complex of the alkyl bromide with

chloride anion (the product).

8. For each reaction, create a table in your lab notebook like the one below and work with

your lab partner if you like to run the calculations to fill in each table. The Eaq calculations

account for the solvation effects and provides an estimate of the energy profile in an

“aqueous” environment. Running the calculations in water as a solvent is easy. Open the

saved files with gas phase results and safe as new files with “aq” suffix. Then change the

state from in “gas” to in “water” as shown below. Run the calculations, safe your files and

record the energy in the tables below. You also can use Excel spreadsheet to facilitate the

calculations of the Ea.

Data

CH3Cl + Br- TS CH3Br + Cl- Ea, kJ/mol

E,

kJ/mol

Eaq,

kJ/mol

?E = E(Product)-E(Reactant) ?Eaq = E(P)aq – E(R)aq

PhCH2Cl + Br- TS PhCH2Br + Cl- Ea, kJ/mol

E

Eaq

EtCl + Br- TS EtBr + Cl- Ea, kJ/mol

E

Eaq

i-PrCl + Br- TS i-PrBr + Cl- Ea, kJ/mol

E

Eaq

9. After calculating all of the activation energies, summarize them in a table like the one

shown below.

MeCl + Br- EtCl + Br- i-PrCl + Br- PhCH2Cl + Br-

Ea

Ea(aq)

Discussion Points

1. Briefly discuss your results in terms of consistency with your expectations for these

reactions. Did you expect the Ea to depend on the structure? Why?

2. How does solvation affect the activation energies for these reactions?

3. How does the reactivity of benzyl chloride compare with the various alkyl chlorides? Can

you find a rational top explain high reactivity of allylic and benzylic substrates in SN2?

4. Draw the energy diagram for MeCl + Br- reaction including the reactants, the TS and the

products. Assign the energy of the reactant the zero value. Repeat for the reaction in

water, using a blue pen or crayon.

5. The benzylic and allylic halides are very reactive in SN1 reaction as well. Why? Draw the

mechanism of the reaction between benzyl chloride and water and use it to illustrate your

point.