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 foundational concept in acid-base chemistry, defining acids and bases by their ability to donate or accept protons (H+ ions).

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

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

• Example: In the reaction between hydrochloric acid and water, HCl acts as the acid (proton donor) and H2O as the base (proton acceptor).

Conjugate Acids and Bases

Every acid-base reaction involves the formation of conjugate acid-base pairs. The conjugate acid and base are related to the original acid and base by the transfer of a proton.

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

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

• Example: In the reaction CH3COOH + H2O → CH3COO- + H3O+, acetic acid is the acid, water is the base, acetate is the conjugate base, and hydronium is the conjugate acid.

Curved Arrows in Reaction Mechanisms

Curved arrows are used in organic chemistry to depict the movement of electrons during chemical reactions, especially in mechanisms involving acids and bases.

• Electron Movement: Making and breaking bonds involves the movement of electrons.

• Curved Arrows: Indicate the flow of electron density from a donor (usually a lone pair or bond) to an acceptor (usually an atom or bond).

• Mechanisms: Drawing mechanisms with curved arrows is essential for understanding reaction pathways.

Single-Step Mechanisms

• All acid/base reactions occur in one step, with the base "attacking" the acid using a pair of electrons.

• Two arrows are used: one for bond breaking (acid losing a proton), one for bond making (base gaining a proton).

Multistep Mechanisms

• Some reactions involve multiple steps, each with its own proton transfer and electron movement.

• Identifying which steps are proton transfers is key to understanding the overall mechanism.

Quantifying Acidity and Basicity

Quantitative Analysis: Acid Dissociation Constant ($K_a$)

The strength of an acid is measured by its acid dissociation constant ($K_a$), which quantifies the extent to which an acid donates protons in water.

• Definition: $K_a$ is the equilibrium constant for the reaction: $HA + H_2O \rightleftharpoons A^- + H_3O^+$

• Formula:

$K_{eq} = \frac{[H_3O^+][A^-]}{[HA][H_2O]}$

$K_a = \frac{[H_3O^+][A^-]}{[HA]}$

• Interpretation: A strong acid has a large $K_a$ (greater than 1), indicating extensive proton donation.

Quantitative Analysis: $pK_a$

Because $K_a$ values can be extremely large or small, chemists use $pK_a$ as a more manageable measure of acidity.

• Definition: $pK_a = -\log K_a$

• Range: $pK_a$ values typically range from -10 to 50.

• Interpretation: Lower $pK_a$ values indicate stronger acids.

• Example: If $K_a = 10^{-5}$, then $pK_a = 5$.

Additional info: The notes continue with qualitative analysis, ARIO factors, and applications of acid-base theory in organic mechanisms, which are essential for understanding reactivity and stability in organic chemistry.