Chapter 2: Acids and Bases

Introduction

This chapter explores the fundamental concepts of acids and bases in organic chemistry, focusing on their definitions, properties, and roles in chemical reactions. Understanding acid-base behavior is essential for predicting reaction mechanisms and the stability of organic molecules.

Lewis Acids and Bases

Definitions and Concepts

• Lewis acid: A species that accepts a share in an electron pair.

• Lewis base: A species that donates a share in an electron pair.

• When a Lewis base donates its electron pair to a Lewis acid, a new covalent bond is formed.

• All Brønsted acids are Lewis acids, and all Brønsted bases are Lewis bases.

Example Reaction

• The lone pair on nitrogen forms a new bond with the proton.

Examples of Lewis Acid-Base Reactions

• Aluminum chloride (Lewis acid) + Dimethyl ether (Lewis base): (new bond formed between Al and O)

• Ferric bromide (Lewis acid) + Ammonia (Lewis base): (new bond formed between Fe and N)

How Chemists Use the Terms

Brønsted vs. Lewis Acids

• Brønsted acid: A proton-donating acid.

• Lewis acid: A non-proton-donating acid (accepts electron pairs).

• Most organic bases are Lewis bases because they possess a pair of electrons available for sharing.

The Acid Dissociation Constant ()

Definition and Calculation

The acid dissociation constant () measures the extent of dissociation of an acid in water.

• General reaction:

• Equilibrium expression:

• Acid dissociation constant:

Acid Strength

pKa and Classification

• pKa is defined as

• The stronger the acid, the larger the .

• The stronger the acid, the smaller the pKa.

Carboxylic Acids

Common Organic Acids

• Acetic acid (): pKa = 4.76

• Formic acid (): pKa = 3.75

Alcohols and Amines

pKa Values

Amines

Basicity

• Methylamine (): pKa = 40

• Ammonia (): pKa = 36

Alcohols as Acids and Bases

Dual Behavior

• Alcohols can act as acids by donating a proton, or as bases by accepting a proton.

• Mechanism involves the movement of electron pairs, shown by curved arrows from electron donor to electron acceptor.

Example Reactions

• As an acid:

• As a base:

A curved arrow always points from the electron donor to the electron acceptor in reaction mechanisms.

Summary Table: pKa Values of Common Organic Compounds

Key Takeaways:

• Acid and base strength can be compared using pKa values.

• Lewis and Brønsted definitions are both important for understanding organic reactions.

• Alcohols and amines are common organic bases, while carboxylic acids are common organic acids.

• Curved arrows in mechanisms indicate the flow of electrons during bond formation and breaking.