Organic Chemistry Chapter 3: Acids and Bases (Study Notes)

Acids and Bases in Organic Chemistry

Bronsted-Lowry Acids and Bases

The Bronsted-Lowry theory is a fundamental concept in acid-base chemistry, defining acids and bases by their ability to donate or accept protons (H+ ions).

• Bronsted-Lowry Acid: A substance that donates a proton (H+).

• Bronsted-Lowry Base: A substance that accepts a proton (H+).

• Example: The reaction between hydrochloric acid and water: $\ce{HCl} + \ce{H2O} \rightleftharpoons \ce{Cl^-} + \ce{H3O^+}$

Conjugate Acids and Bases

Every acid-base reaction involves the formation of conjugate acid-base pairs. When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.

• Conjugate Acid: Formed when a base accepts a proton.

• Conjugate Base: Formed when an acid loses a proton.

• Example: In the reaction: $\ce{CH3COOH} + \ce{H2O} \rightleftharpoons \ce{CH3COO^-} + \ce{H3O^+}$ - CH3COOH is the acid, H2O is the base, CH3COO- is the conjugate base, and H3O+ is the conjugate acid.

Curved Arrows in Reaction Mechanisms

Organic chemists use curved arrows to depict the movement of electrons during chemical reactions, especially in acid-base mechanisms.

• Electron Movement: Curved arrows show the flow of electron density, indicating bond formation and bond breaking.

• Single-Step Mechanism: All acid-base reactions occur in one step, with the base "attacking" the acid to remove a proton.

• Multistep Mechanism: More complex reactions may involve several proton transfers, each shown with curved arrows.

• Example: In a single-step acid-base reaction: $\ce{B:} + \ce{H-A} \rightarrow \ce{B-H} + \ce{:A}$ The base uses its lone pair to form a bond with the proton, while the bond between H and A breaks.

Quantifying Acidity and Basicity

Quantitative Analysis: $K_a$ and $pK_a$

The strength of acids and bases can be measured quantitatively using equilibrium constants and logarithmic scales.

• Acid Dissociation Constant ($K_a$): Measures the extent to which an acid donates protons in water. $\ce{HA} + \ce{H2O} \rightleftharpoons \ce{A^-} + \ce{H3O^+}$ $K_{eq} = \frac{[\ce{H3O^+}][\ce{A^-}]}{[\ce{HA}][\ce{H2O}]}$ $K_a = \frac{[\ce{H3O^+}][\ce{A^-}]}{[\ce{HA}]}$

• $pK_a$: The negative logarithm of $K_a$. $pK_a = -\log K_a$ Lower $pK_a$ values indicate stronger acids. Range: $pK_a$ values typically range from -10 to 50.

Qualitative Analysis: Stability of Conjugate Bases

When $pK_a$ values are unknown, acid strength can be compared by evaluating the stability of their conjugate bases.

• More stable conjugate base: Corresponds to a stronger acid.

• Factors affecting stability: Type of atom, resonance, induction, and orbital type.

Factors Affecting Acidity: The ARIO Principle

Atom

The identity of the atom bearing the negative charge in the conjugate base is crucial.

• Size: Down a group, larger atoms stabilize negative charge better.

• Electronegativity: Across a period, more electronegative atoms stabilize negative charge better.

• Example: Oxygen stabilizes negative charge better than carbon, making alcohols more acidic than alkanes.

Resonance

Resonance delocalizes negative charge over multiple atoms, increasing stability.

• Example: Acetic acid is more acidic than ethanol because its conjugate base is resonance stabilized.

Induction

Electron-withdrawing groups stabilize negative charge by inductively pulling electron density away.

• Example: Trichloroacetic acid is more acidic than acetic acid due to the inductive effect of chlorine atoms.

Orbital

The type of orbital holding the negative charge affects stability. More s-character (as in sp orbitals) holds electrons closer to the nucleus, increasing stability.

• Example: Acetylene (sp) is more acidic than ethylene (sp2) and ethane (sp3).

Using $pK_a$ Values to Predict Equilibria

Acid-base equilibria favor the formation of the weaker acid and weaker base. The direction of equilibrium can be predicted using $pK_a$ values.

• Equilibrium favors: The side with the higher $pK_a$ (weaker acid).

• Example: If $pK_a$ of reactant acid is lower than product acid, equilibrium favors products.

• Calculation: The difference in $pK_a$ values can be used to estimate the ratio of products to reactants: $\text{Ratio} \approx 10^{\Delta pK_a}$

Solvent Effects and Leveling Effect

Leveling Effect

Water can act as both an acid and a base, limiting the strength of acids and bases that can be used in aqueous solution.

• Acids stronger than H3O+: Cannot exist in water; they are leveled to the strength of H3O+.

• Bases stronger than OH-: Cannot exist in water; they are leveled to the strength of OH-.

Solvating Effects and Sterics

The ability of a solvent to stabilize ions affects acidity. Steric hindrance can reduce solvation and thus decrease acidity.

• Example: Ethanol is more acidic than tert-butanol because its conjugate base is better solvated.

Counterions

Counterions (spectator ions) balance charge in solution but do not participate directly in acid-base reactions.

• Example: In the reaction $\ce{NaNH2} + \ce{H2O} \rightarrow \ce{NH3} + \ce{NaOH}$, Na+ is a counterion.

Lewis Acids and Bases

Lewis Definition

The Lewis definition broadens the concept of acids and bases to include electron pair transfer.

• Lewis Acid: Accepts a pair of electrons.

• Lewis Base: Donates a pair of electrons.

• Relationship: All Bronsted-Lowry acids/bases are also Lewis acids/bases, but not all Lewis acid/base reactions involve proton transfer.

• Example: $\ce{BF3}$ (Lewis acid) reacts with $\ce{NH3}$ (Lewis base) to form $\ce{F3B-NH3}$.

Summary Table: Factors Affecting Acidity (ARIO)

Factor	Effect on Conjugate Base Stability	Example
Atom	More electronegative/larger atom stabilizes negative charge	O vs. C (Alcohol vs. Alkane)
Resonance	Delocalization of charge increases stability	Acetic acid vs. Ethanol
Induction	Electron-withdrawing groups stabilize charge	Trichloroacetic acid vs. Acetic acid
Orbital	More s-character increases stability	Acetylene (sp) vs. Ethylene (sp2)

Additional info: These notes expand on the provided slides and text, adding definitions, equations, and examples for clarity and completeness.