You will use the Gaussian-03W program to calculate the Gibbs energies of the reactants and products and to determine which product, 3-chlorobutene or 1-chloro-2-butene -- and by how much -- is expected to be thermodynamically more stable.
COLLEGE OF SAINT MARY PHYSICAL CHEMISTRY FALL 2008
INSTRUCTOR: DR. PETER ILICH YOUR NAME ___________________________
PROJECT # 1: THERMODYNAMIC STABILITIES DETERMINED BY GAUSSIAN 03W PROGRAM
The project consists of three parts:
| (A) Addition of
hydrogenchloride to butadiene can give two products, the 1,2-adduct or
direct (kinetic) product (A1) and 1,4-adduct, or thermodynamic product, (A2)
in the
scheme at right. According to the majority of sophomore organic
textbooks, the1,4-product is considered more stable.
You will use the Gaussian-03W program to calculate the Gibbs energies of the reactants and products and to determine which product, 3-chlorobutene or 1-chloro-2-butene -- and by how much -- is expected to be thermodynamically more stable. |
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(B) Diels-Alder
1,4-cycloaddition reactions with substituted dienes and dienophiles can give
a number of products, the scheme at left. The experimental observation
suggest that 1,2- or ortho and 1,4- or para products are more stable and
more likely to form.
You will use the Gaussian-03W program to calculate the Gibbs energies of the reactants and products and to determine which product, 2-chloro-3-methyl-1,3-cyclohexadiene (B1) or 1-chloro-3-methyl-1,,3-cyclohexadiene (B2) -- and by how much -- is expected to be thermodynamically more stable. |
| (C) Brønsted
acidity is defined as the likelihood by which a molecule gives away a proton
in an acid-base reaction. In the following scheme (at right) are shown
the acid-base reactions of ethanol, EtOH (C1) and thioethanol, EtS (C2). You will use the Gaussian-03W program to calculate the Gibbs energies of the reactants and products and determine which "acid' ethanol or thioethanol is expected to be stronger, by comparing the ΔGө298 of the two reactions. |
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General remarks:
When a problem is posed so that you have to compare two molecules you will find the difference of the corresponding Gibbs free energies. So, for the addition of hydrogen chloride to butadiene, the quantitative answer will be given by ΔG = G(A2) - G(A1) and for the comparison of the two Diels-Alder cycloaddition products, your answer will be based on the value of the following expression: ΔG = G(B2) -G(B1). In the third case, since we cannot calculate the Gibbs energy of free proton (it has not electrons and therefore no "electronic energy"), you will first calculate partial ΔG for the dissociation reaction: ΔG(C1) = G(CH3CH2O-) - G(CH3CH2OH). Then, repeat the same procedure for the thioethanol to obtain ΔG(C2). To estimate which alcohol is likely to be more acidic you need a DIFFERENCE of DIFFERENCES, i.e. ΔΔG = ΔG(C2) - ΔG(C1).
