Chapter 2: Acids and Bases; Functional Groups – Study Notes

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Acids and Bases; Functional Groups

Introduction

This chapter introduces the foundational concepts of acid-base chemistry and the identification of functional groups in organic molecules. Understanding these topics is essential for predicting chemical reactivity and properties in organic chemistry.

Bond Dipole Moments

Definition and Measurement

Bond dipole moments arise from differences in electronegativity between atoms in a bond, resulting in a separation of charge.
The magnitude of a dipole moment depends on the amount of charge and the distance between charges.
Measured in debyes (D).

Examples

Ethane (C–C): nonpolar, μ = 0.0 D
Methylamine (C–N): μ = 1.31 D
Methanol (C–O): μ = 1.51 D
Chloromethane (C–Cl): μ = 1.87 D
Methylammonium chloride (ionic): μ = 3.6 D

Bond	Dipole Moment (D)	Bond	Dipole Moment (D)
C–N	0.22	H–C	0.3
C–O	0.86	H–N	1.31
C–F	1.51	H–O	1.5
C–Cl	1.56	H–S	0.7
C–Br	1.48	H–O	1.5
C–I	1.29	H–N	3.6

Additional info: Dipole moments can be used to infer molecular geometry and charge distribution.

Intermolecular Forces

Types and Effects

The strength of intermolecular attractions influences melting point (m.p.), boiling point (b.p.), and solubility.
Types of forces:
- Dipole-dipole forces: Attraction between polar molecules.
- London dispersion forces: Temporary dipoles in all molecules, main force in nonpolar molecules.
- Hydrogen bonding: Strong dipole-dipole attraction in molecules with N–H or O–H groups.

Effect of Branching on Boiling Point

Long-chain isomers (e.g., n-pentane) have higher boiling points due to greater surface area.
Increased branching decreases surface area and boiling point (e.g., neopentane).

Hydrogen Bonding

Requires N–H or O–H groups.
O–H is more polar than N–H, so alcohols have stronger hydrogen bonding than amines.
Hydrogen bonding raises boiling points (e.g., ethanol b.p. = 78°C vs. dimethyl ether b.p. = –25°C).

Acid/Base Chemistry

Arrhenius Definition

Arrhenius acids: Substances that dissociate in water to give H3O+ ions.
Arrhenius bases: Substances that dissociate in water to give OH– ions.
Stronger acids/bases dissociate more completely.

Brønsted–Lowry Definition

Brønsted–Lowry acids: Proton (H+) donors.
Brønsted–Lowry bases: Proton acceptors.

Conjugate Acid-Base Pairs

Conjugate acid: Formed when a base gains a proton.
Conjugate base: Formed when an acid loses a proton.

Acid and Base Strength

Acid strength is measured by the extent of ionization in water.
Equilibrium expression:
A stronger acid has a weaker conjugate base, and vice versa.
Acid-base reactions favor the formation of the weaker acid and base.

Equilibrium Positions

Equilibrium favors the side with the weaker acid and weaker base (larger pKa and pKb).

Factors Affecting Acidity

1. Electronegativity

More electronegative atoms stabilize negative charge better, increasing acidity.
Across a period, acidity increases with electronegativity (e.g., CH4 < NH3 < H2O < HF).

2. Size

Larger atoms can better stabilize negative charge by spreading it over a larger volume.
Down a group, acidity increases with size (e.g., HF < HCl < HBr < HI).

3. Inductive Effects

Electron-withdrawing groups stabilize the conjugate base, increasing acidity.
Multiple electron-withdrawing groups have a stronger effect than a single group.

4. Hybridization

Greater s-character in the atom holding the negative charge increases acidity.
Order: sp > sp2 > sp3 (e.g., alkynes > alkenes > alkanes).

5. Resonance

Delocalization of negative charge over multiple atoms stabilizes the conjugate base, increasing acidity.
Example: Acetic acid is more acidic than ethanol due to resonance stabilization of the acetate ion.

Lewis Acids and Bases

Definitions

Lewis base (nucleophile): Donates a pair of electrons to form a new bond.
Lewis acid (electrophile): Accepts a pair of electrons to form a new bond.

Nucleophiles and Electrophiles

Nucleophile: Electron-rich species that attacks electron-poor centers.
Electrophile: Electron-poor species that accepts electrons.
Curved arrows in mechanisms show the movement of electron pairs from nucleophile to electrophile.

Functional Groups

Hydrocarbons

Alkanes: Only single bonds (sp3 carbons).
Cycloalkanes: Alkanes in ring form.
Alkenes: Contain C=C double bonds (sp2 carbons).
Cycloalkenes: Double bond in a ring.
Alkynes: Contain C≡C triple bonds (sp carbons).
Aromatic: Contain a benzene ring.

Alkane Naming

Alkane Name	Number of Carbons
Methane	1
Ethane	2
Propane	3
Butane	4
Pentane	5
Hexane	6
Heptane	7
Octane	8
Nonane	9
Decane	10

Compounds Containing Oxygen

Alcohols: Contain the hydroxyl group (–OH).
Ethers: Two alkyl groups bonded to an oxygen atom.
Aldehydes and Ketones: Contain the carbonyl group (C=O).
Carboxylic acids: Contain the carboxyl group (–COOH).

Compounds Containing Nitrogen

Amines: Alkylated derivatives of ammonia (R–NH2, R2NH, R3N).
Amides: Carboxylic acid derivatives with nitrogen attached to the carbonyl group.
Nitriles: Contain the cyano group (–C≡N).

Summary Table: Functional Groups

Functional Group	Structure	Example
Alcohol	R–OH	Ethanol
Ether	R–O–R'	Diethyl ether
Aldehyde	R–CHO	Acetaldehyde
Ketone	R–CO–R'	Acetone
Carboxylic Acid	R–COOH	Acetic acid
Amine	R–NH2	Methylamine
Amide	R–CONH2	Acetamide
Nitrile	R–C≡N	Acetonitrile

Additional info: Mastery of functional groups is essential for understanding organic reactivity and nomenclature.