This laboratory experiment is focused on theoretically predicting the acidity of several
molecules. A software package called “Spartan” will be used to predict the structure and the
electrostatic potential of each molecule. The electrostatic potential at a point P is defined as the
Coulomb energy of interaction between the electron-charge cloud of the molecule and a positive
point charge placed at the point P. The electrostatic potential can be used to distinguish regions
that are electron deficient from those that are electron rich. A positive value of the electrostatic
potential at point P indicates that the positive charge is repelled by the cloud. Such a condition
occurs when an area is deficient in electron density. A negative electrostatic potential value at
point P indicates that the positive point charge is attracted to the cloud. Such a condition exists
with the area has excess electron density.
Molecules that are strong acids should have a highly positive electrostatic potential near
their acidic hydrogen(s). This leads to a repulsion of the hydrogen nucleus (i.e., the proton) that
will cause it to be more easily transferred to molecules that can act as bases.
In this laboratory experiment you will use the Spartan program to determine the
electrostatic potential of the hydrogens on the following molecules…
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Department of Chemistry
Experiment #6: Correlation of Acidity with Electrostatic Potential
Background:
This laboratory experiment is focused on theoretically predicting the acidity of several
molecules. A software package called “Spartan” will be used to predict the structure and the
electrostatic potential of each molecule. The electrostatic potential at a point P is defined as the
Coulomb energy of interaction between the electron-charge cloud of the molecule and a positive
point charge placed at the point P. The electrostatic potential can be used to distinguish regions
that are electron deficient from those that are electron rich. A positive value of the electrostatic
potential at point P indicates that the positive charge is repelled by the cloud. Such a condition
occurs when an area is deficient in electron density. A negative electrostatic potential value at
point P indicates that the positive point charge is attracted to the cloud. Such a condition exists
with the area has excess electron density.
Molecules that are strong acids should have a highly positive electrostatic potential near
their acidic hydrogen(s). This leads to a repulsion of the hydrogen nucleus (i.e., the proton) that
will cause it to be more easily transferred to molecules that can act as bases.
In this laboratory experiment you will use the Spartan program to determine the
electrostatic potential of the hydrogens on the following molecules…
A. carbonic acid - H CO E. acetic acid - CH COOH
2 3 3
B. phosphoric acid - H PO F. chloroacetic acid - CH ClCOOH
3 4 2
C. sulfuric acid- H SO G. dichloroacetic acid - CHCl COOH
2 4 2
D. nitric acid - HNO H. trichloroacetic acid - CCl COOH
3 3
You will then be asked to explain on the link between molecular structure and electrostatic
potential and the link between molecular structure and acidity. Each student is strongly
encouraged to review the information in their...