Organic Chemistry Study Notes: Hybridization, Electronegativity, and Acids & Bases (Ch. 1 Part 2 & Ch. 2 Part 1)

Hybridization in Organic Molecules

Review of Hybridization

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding in molecules. The geometry and bond angles around an atom are determined by the number of bonded atoms and lone pairs.

• Step 1: Count the number of bonded atoms plus lone pairs to determine electron geometry and bond angles.

• Step 2: Assign the appropriate hybridization for the geometry:

Bonded atoms + lone pairs	Electron geometry	Bond angle	Hybridization
2	Linear	180°	sp
3	Trigonal planar	120°	sp2
4	Tetrahedral	109.5°	sp3

• Warning: Skeletal structures often omit lone pairs and C-H bonds. Double/triple-bonded atoms still count as one for geometry.

• Hint: #(bonded atoms + lone pairs) = # hybridized orbitals

Application to Research

Hybridization concepts are essential in advanced organic synthesis, such as the formation of Dewar intermediates and analysis of bond angles in complex molecules.

• Example: In the synthesis of a bicyclic lactone (Dewar intermediate), determining the hybridization of the central carbon in O–C=O and in CO2 helps predict bond angles and reactivity.

Electronegativity and Its Role in Organic Chemistry

Review of Electronegativity

Electronegativity (EN) is the tendency of an atom to attract electrons in a chemical bond. It affects molecular polarity, acid/base strength, and drug excretion.

• Key EN values: Learn the electronegativities of H, C, N, O, F, Cl, Br, I for exams.

• Importance: Polarity influences solubility and excretion of drugs; more polar (charged) forms are more water-soluble.

Acids and Bases: Central Concepts

Definitions

Acids and bases are defined by their ability to donate or accept protons (Brønsted-Lowry definition):

• Acid: Species that can lose a proton (H+).

• Base: Species that can gain a proton.

• These are roles, not compound classes.

Reversibility of Acid-Base Reactions

Most acid-base reactions are reversible, meaning products can revert to reactants.

• Equilibrium arrows: The longer half-arrow indicates which side (products or reactants) predominates at equilibrium.

Conjugate Acid-Base Pairs

Every acid has a conjugate base, and every base has a conjugate acid.

• When an acid loses a proton: It forms its conjugate base.

• When a base gains a proton: It forms its conjugate acid.

Acid Strength and Dissociation

Acids vary in strength, which is measured by their tendency to lose a proton.

• Strong acid: Products favored at equilibrium.

• Weak acid: Reactants favored at equilibrium.

Acid Dissociation Constant ($K_a$)

The extent of acid dissociation is quantified by the acid dissociation constant:

• $HA + H_2O ightleftharpoons H_3O^+ + A^-$

• $K_a = rac{[H_3O^+][A^-]}{[HA]}$

pKa and Acid Strength

• $pK_a = - ext{log} K_a$

• The stronger the acid, the smaller its pKa.

Acid strength	pKa range
Very strong acids	pKa < 1
Moderately strong acids	pKa = 1–3
Weak acids	pKa = 3–5
Very weak acids	pKa = 5–15
Extremely weak acids	pKa > 15

pH and Its Significance

pH measures the concentration of protons in solution:

• $pH = - ext{log}[H^+]$

• Lower pH = higher acidity.

• Small amounts of strong acid can have higher pH than large amounts of weak acid.

Common Organic Acids and Bases

Carboxylic Acids

• Most common organic acids (e.g., acetic acid, formic acid).

• pKa values: acetic acid (4.76), formic acid (3.75).

• Many organic compounds can act as acids even if not named as such.

Alcohols

• Weaker acids than carboxylic acids (e.g., methyl alcohol pKa = 15.5, ethyl alcohol pKa = 15.9).

• Can lose a proton to a strong enough base.

Protonated Alcohols and Carboxylic Acids

• Protonation increases acidity (e.g., protonated methyl alcohol pKa = –2.5).

• Protonated carboxylic acids are much stronger acids.

Amines and Protonated Amines

• Amines are weak acids (e.g., methylamine pKa = 40).

• Protonated amines are stronger acids but still weaker than carboxylic acids.

Acid-Base Behavior of Organic Compounds

Alcohols, Carboxylic Acids, and Amines as Acids/Bases

• Alcohols, carboxylic acids, and amines can act as acids or bases depending on the reaction context.

• Curved arrows in mechanisms show electron flow from donor to acceptor.

Factors Affecting Acid Strength

Stability of Conjugate Base

• The stronger the acid, the more stable its conjugate base.

• Stable bases are weak and less reactive.

Electronegativity

• Higher electronegativity of the atom attached to the acidic proton increases acid strength.

• Relative acidities: CH4 < NH3 < H2O < HF

Atomic Size

• Increasing atomic/ionic radius down the periodic table increases acidity.

• Relative acidities: HF < HCl < HBr < HI (HI is strongest due to largest size).

Hybridization

• Greater s-character (sp > sp2 > sp3) increases acidity.

Inductive Electron Withdrawal

• Electronegative substituents near the acidic proton increase acid strength.

• Effect decreases with distance from the proton.

Electron Delocalization

• Delocalization of electrons (resonance) stabilizes the conjugate base, increasing acid strength.

Summary Table: Approximate pKa Values

pKa < 0	pKa ~ 5	pKa ~ 10	pKa ~ 15	pKa ~ 40
Protonated alcohol	Carboxylic acid	Protonated amine	Alcohol, water	Amine

Additional info:

• These notes cover foundational concepts in organic chemistry relevant for Chapters 1 (Electronic Structure and Bonding) and 2 (Acids and Bases).

• Understanding these principles is essential for predicting reactivity, mechanisms, and properties of organic molecules.