CHEM 1212 Notes: Acidities and Structure with Spartan

Acids and Bases: Fundamental concepts in chemistry regarding how molecules react with chemical species.
Acidity Measurement: Refers to the degree of ionization in a solution, specifically the relative number of hydronium ions produced by an acid.
Classification of Acids: Acids are categorized into two groups based on acidity:
- Strong Acids: Approximately 7 common strong acids that are considered to ionize 100% in solution.
- Weak Acids: Acids that ionize to a lesser degree, their acidity can vary greatly.

Definition of Protic Bond: The bond that holds the proton from a Brønsted-Lowry acid to the molecule.
Weak vs. Strong Protic Bonds:
- Weak Protic Bond: Easier to break, leading to more complete ionization and producing a greater number of hydronium ions; thus, the acid is stronger.
- Strong Protic Bond: Harder to break, leading to less ionization and fewer hydronium ions; thus, the acid is weaker.

Equilibrium Constant (KA):
- A large KA indicates a stronger acid.
- A small KA indicates a weaker acid.
pKa Scale: Used to better represent acidic strength:
- Defined as: $pK<em>a = - ext{log}(K</em>A)$
- A smaller pKa value indicates a stronger acid, as it is inversely related to acidity.

Protic Bond in Focus: The carboxyl group, as depicted in Figure 1.
- Structure of Carboxyl Group: (R-O-H) where R represents any substituent.
Research Questions:
- (a) Orientation: How does the carboxyl group orient to achieve minimal energy structure?
- (b) Estimating Acidity: How can we estimate differences in acidity among various acids by examining their structure?
List of Acids to Investigate:
- Formic Acid (pKa = 3.75)
- Acetic Acid (pKa = 4.75)
- Chloroacetic Acid (pKa = 2.85)
- Dichloroacetic Acid (pKa = 1.48)
- Trichloroacetic Acid (pKa = 0.7)
- Benzoic Acid (pKa = 4.19)
- Pivalic Acid (pKa = 5.03)

Structure-building: Use Spartan to create a formic acid molecule.
Orientation Consideration:
- Two orientations as shown in Figure 2; initially build the one on the left.
Energy Calculation: Perform equilibrium geometry optimization using EDF2/6-31G* and record the energy in atomic units (a.u.).
Re-orientation Process: Adjust the carboxyl orientation:
- Click on "Geometry" > "Measure dihedral angle" and select atoms O, C, O, then H in order.
- Change the dihedral angle value to 0.00 and hit enter.
Re-computation: Complete another EDF2/6-31G* optimization and record the new energy.
Compute Energy Difference:
- Convert energy difference from a.u. to kJ/mol using the conversion:
- 1 a.u. = 2625.50 kJ/mol.
Results Validation: Confirm that the computed results match experimental data.

Using Spartan: Build each remaining acid without requiring adjustment:
- Each acid is present in the Spartan Library, allowing immediate access to optimized structures without recalculations.
Replace Function: For each named acid, click on it and select “Replace” to obtain the best equilibrium geometry.

Definition: An electrostatic potential map visualizes the force a test charge would feel near a molecule:
- Red areas indicate attraction (positive interaction).
- Blue areas indicate repulsion (negative interaction).
Polar Bonds: Partial charges due to unequal sharing of electrons in bonds contribute to acid strength and polarity.
Visual Analysis: Generate and explore electrostatic potential maps for each acid:
- Access via “surfaces” icon and add an electrostatic potential map.
- View color-coded attractions/repulsions and adjust the surface style to “transparent” for better visibility.
Data Collection: Record maximum values and trends observed in electrostatic maps for the acids listed in Table 1.

Ensuring that structural asymmetries and bond polarities are considered can lead to insights into acid strengths.
Final analysis should report visual and quantitative findings regarding acidity comparisons amongst the explored carboxylic acids based on electrostatic potential and structural considerations.