experiment 3 : computational chemistry experiment · 2019-10-31 · experiment 3 : computational...
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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 � ���
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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