Unit 2

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Acids and Bases; Functional Groups

Introduction

This chapter provides a foundational understanding of acids and bases, intermolecular forces, and the classification of organic compounds by functional groups. These concepts are essential for predicting molecular behavior, reactivity, and physical properties in organic chemistry.

Bond Dipole Moments

Definition and Measurement

Bond dipole moments arise from differences in electronegativity between atoms in a covalent bond.
The magnitude depends on the amount of charge separation and the distance between charges.
Measured in debyes (D), with typical values ranging from 0 to 3.6 D.

Table: Bond Dipole Moments for Common Covalent Bonds

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	C=O	1.53
C–Cl	1.56	C=O	2.4
C–Br	1.48	C≡N	3.6
C–I	1.29

Molecular Dipole Moment

Concept and Calculation

The molecular dipole moment is the vector sum of all bond dipole moments in a molecule.
It depends on both bond polarity and bond angles.
Lone pairs of electrons also contribute to the overall dipole moment.

Example:

Formaldehyde (H2CO): μ = 2.3 D
Carbon dioxide (CO2): μ = 0 D (linear geometry cancels dipoles)

Lone Pairs and Dipole Moments

Effect of Lone Pairs

Lone pairs can significantly affect the dipole moment by adding to or partially canceling the vector sum of bond dipoles.
Examples:
- Ammonia (NH3): μ = 1.5 D
- Water (H2O): μ = 1.9 D
- Acetone: μ = 2.9 D
- Acetonitrile: μ = 3.9 D

Intermolecular Forces

Types and Effects

Also known as van der Waals forces.
Influence melting point, boiling point, and solubility.
Types:
- 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.

Interesting Fact:

Gecko toes adhere to surfaces via millions of van der Waals interactions from tiny hair tips.

Dipole–Dipole Forces

Nature and Orientation

Result from the approach of two polar molecules.
Attractive if positive and negative ends align; repulsive if like charges align.
In solids and liquids, molecules tend to orient for net attraction.

London Dispersion Forces

Temporary Dipoles

Temporary dipoles induce dipoles in neighboring molecules, leading to weak, short-lived attractions.
Main force in nonpolar molecules.
Larger atoms are more polarizable, increasing dispersion forces.

Effect of Branching on Boiling Point

Surface Area and Boiling Point

Long-chain isomers (e.g., n-pentane) have greater surface area and higher boiling points due to stronger intermolecular forces.
Increased branching makes molecules more spherical, reducing surface area and boiling point.
Neopentane (most branched) has the lowest boiling point.

Hydrogen Bonding

Requirements and Strength

Occurs in molecules with N–H or O–H groups.
O–H bonds are more polar than N–H, so alcohols have stronger hydrogen bonding than amines.
Hydrogen bonds are responsible for high boiling points and unique properties (e.g., DNA structure).

Boiling Points and Intermolecular Forces

Comparative Examples

Ethanol (O–H): b.p. = 78°C (hydrogen bonding)
Dimethyl ether (no O–H): b.p. = –25°C
Ethylamine (N–H): b.p. = 17°C
Alcohols have higher boiling points than amines due to stronger hydrogen bonding.

H-bond Donors and Acceptors

Definitions

Donor: Molecule with an O–H or N–H group.
Acceptor: Molecule with a lone pair that forms a weak partial bond to the hydrogen atom from the donor.

Polarity Effects on Solubility

Like Dissolves Like

Polar solutes dissolve in polar solvents; nonpolar solutes dissolve in nonpolar solvents.
Polar solutes cannot dissolve in nonpolar solvents and vice versa.
Solubility is enhanced when intermolecular forces are similar.

Hydration and Solvation

Hydration (water) or solvation (other solvents) increases entropy and releases energy.
Salts dissolve due to hydration and entropy increase.

Hydrophobic and Hydrophilic

Definitions and Examples

Hydrophobic: Nonpolar molecules (e.g., oil) do not mix with water.
Hydrophilic: Polar molecules (e.g., ethanol) are miscible with water.
Vitamins A and D are nonpolar and stored in fat, making them potentially toxic in large doses.

Acids and Bases

Arrhenius Definition

Acids: Substances that dissociate in water to give H3O+ ions.
Bases: Substances that dissociate in water to give OH– ions.

Brønsted–Lowry Definition

Acids: Proton donors.
Bases: Proton acceptors.

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.

Acid Strength and Constants

Quantitative Measures

Acid strength is measured by the extent of ionization in water.
Key equations:
- at 25°C
- Acid dissociation constant:
- Base dissociation constant:
- For conjugate pairs:

Table: Relative Strength of Some Common Acids and Bases

Acid	Conjugate Base	pKa
HCl	Cl–	–7
H2SO4	HSO4–	–3
CH3COOH	CH3COO–	4.76
H2O	OH–	15.7
NH4+	NH3	9.25

Sample Problems and Applications

Dipole Moments

Explain why NF3 has a smaller dipole moment than NH3 despite a more polar N–F bond.
Predict dipole moments for CF4 and HCN.

Isomer Dipole Moments

Draw isomers of 1,2-dichloroethene and explain why one has zero dipole moment (cis/trans isomerism).

Boiling Point Ranking

Rank neopentane, n-hexane, 2,3-dimethylbutane, pentan-1-ol, and 2-methylbutan-2-ol by boiling point.

Hydrogen Bonding Comparison

Explain hydrogen bonding in ethanol vs. dimethyl ether.

Solubility

Identify which of a pair of compounds is more soluble in water based on polarity and hydrogen bonding.

Conclusion

Understanding the principles of dipole moments, intermolecular forces, and acid-base behavior is crucial for predicting the physical and chemical properties of organic molecules. Mastery of these concepts provides a strong foundation for further study in organic chemistry.