Experiment 3 : Computational Chemistry experiment

Kerry Jones, Niamh Bayliss, Maria Calvo, Liam Edwards

Chemistry, School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK

Introduction

The aim of this experiment is to use HyperChem

software to build molecules and study their

conformations , orbital energy levels and rotational

energy plots and therefore deduce their reactivity.

Conformations

K � ���

���

K � 506.1

The Diels-Alder Reaction

The Diels-Alder adducts are formed through the

reaction between furan, which is the diene

component and maleic anhydride as the dienophile to

give products DA1 and DA2.

DA1 Exo-

ΔHf = -72.13 kcal/mol

DA2 Endo-

ΔHf = -85.71 kcal/mol

The Endo- adduct is preferred as

the ΔHf value is smaller.

To determine whether the reaction

occurs readily: ΔE = LUMOdienophile – HOMOfuran

ΔE = -1.55 – (-9.38)

ΔE = 7.83 kcal/mol l

Frontier-Molecular Orbitals

FMOs are the highest occupied molecular orbital

(HOMO) and the lowest unoccupied molecular orbital

(LUMO) . They play a key part in the reactivity of

molecules. We can use HyperChem to plot the HOMO

and LUMO of a molecule.

e.g. Butadiene

LUMO HOMO

Wagner-Meerwein rearrangements

This reaction proceeds through a WM rearrangement from

intermediate X to Y.

X - 1⁰ carbocation

ΔHf = 181.1 kcal/mol

Y - 2⁰ carbocation

ΔHf = 167.2 kcal/mol

The major product derived from Y would be the one that

goes through a lower energy transition state: in this case,

B’s intermediate (X) is more stable (lower heat of

formation) A’s intermediate (Y). Therefore, B is the major

product.

Conclusions

Rotational energy plot for butadiene can be used to

calculate equilibrium constant for the rotation between

s- cis and s- trans conformations which shows that the

trans conformation is preferred.

Using HOMO and LUMO energy values, the Diels-Alder

reaction of furan and maleic anhydride was determined

to occur readily, since the ΔE value calculated is

relatively low, indicating a favourable reaction.

Dehydration can occur through a WM rearrangement,

and using values for the heat of formation of the

intermediates, we can deduce the major product is the

more substituted alkene.

Figure 1. Rotational energy plot of butadiene.

(T=298K)

DG