Thermodynamics, Acids, Bases, and Reaction Mechanisms

Electron-Pushing Arrows

Electron-pushing arrows are a fundamental tool in organic chemistry for illustrating the movement of electrons during chemical reactions and resonance structures.

• Definition: Electron-pushing arrows (curved arrows) show the flow of electrons from a source (lone pair or bond) to a destination (atom or bond).

• Rules: Arrows always start at electrons (lone pairs or bonds) and point toward where the electrons are moving.

• Application: Used to depict resonance structures and reaction mechanisms.

• Example: In the resonance of the acetate ion, an arrow starts at the lone pair on oxygen and moves toward the carbon-oxygen bond.

Bronsted-Lowry Acids and Bases

The Bronsted-Lowry theory defines acids and bases based on their ability to donate or accept protons (H+).

• Bronsted-Lowry Acid: A species that donates a proton.

• Bronsted-Lowry Base: A species that accepts a proton.

• Example: In the reaction between HCl and H2O, HCl is the acid (donates H+), and H2O is the base (accepts H+).

Acid Dissociation Constant (Ka) and pKa

Ka and pKa are quantitative measures of acid strength.

• Ka: The equilibrium constant for the dissociation of an acid in water.

• pKa: The negative logarithm of Ka; lower pKa indicates a stronger acid.

• Equations:

• Example: Acetic acid has a Ka of and a pKa of 4.76.

Using pKa Values to Predict Reaction Direction

pKa values help determine which side of an acid-base equilibrium is favored.

• Rule: The reaction favors the formation of the weaker acid (higher pKa).

• Application: Compare pKa values of acids on both sides; equilibrium lies toward the side with the higher pKa.

• Example: If the acid on the left has pKa 5 and the acid on the right has pKa 15, equilibrium favors the right.

Factors Influencing Acidity

Several structural factors affect the acidity of a molecule.

• Bond Energies: Stronger bonds to hydrogen make acids weaker.

• Electronegativity: More electronegative atoms stabilize negative charge, increasing acidity.

• Inductive Effects: Electronegative groups near the acidic proton withdraw electron density, stabilizing the conjugate base.

• Resonance: Delocalization of negative charge via resonance stabilizes the conjugate base, increasing acidity.

• Example: Carboxylic acids are more acidic than alcohols due to resonance stabilization of the carboxylate ion.

Arranging Species by Acidity or Basicity

Ranking acids and bases requires consideration of structural and electronic factors.

• Acidity: Rank by pKa, electronegativity, resonance, and inductive effects.

• Basicity: Rank by the ability to accept protons, often the inverse of acidity.

• Example: Phenol (pKa ~10) is more acidic than ethanol (pKa ~16) due to resonance.

Electron-Pushing in Acid/Base Reactions

Electron-pushing arrows are used to show the mechanism of acid/base reactions.

• Application: Draw arrows from the base's lone pair to the acidic proton, and from the acid's bond to the conjugate base.

• Example: In the reaction of ammonia with water, an arrow from ammonia's lone pair to H+ on water, and from O-H bond to oxygen.

Drawing Products and Labeling Acids/Bases

It is important to identify acids and bases on both sides of a reaction and predict the predominant direction.

• Label: Mark acid and base on each side of the equation.

• Predict Direction: Use structural features and pKa values to determine equilibrium.

• Example: In the reaction of acetic acid with hydroxide, acetic acid is the acid, hydroxide is the base; products are acetate and water.

Lewis Acids and Bases

The Lewis theory expands the definition of acids and bases to include electron pair transfer.

• Lewis Acid: Electron pair acceptor.

• Lewis Base: Electron pair donor.

• Example: BF3 is a Lewis acid; NH3 is a Lewis base.

Recognizing Functional Groups

Functional groups are specific groups of atoms within molecules that determine chemical reactivity.

• Common Functional Groups: Alcohols, carboxylic acids, amines, alkenes, alkynes, ethers, aldehydes, ketones, etc.

• Importance: Identifying functional groups is essential for predicting chemical behavior.

• Example: The -OH group in ethanol is an alcohol functional group.