Acids, Bases, and Introduction to Organic Compounds: Structured Study Notes

Ch. 2 – Acids and Bases: Central to Understanding Organic Chemistry

2.1 – Brønsted–Lowry Acids and Bases

Organic chemistry relies on understanding acid-base behavior. The Brønsted–Lowry theory defines acids as proton donors and bases as proton acceptors.

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

• Base: A substance that accepts a proton.

• Examples: HCl, H2SO4, H3O+ (acids); H2O, NH3, OH- (bases).

Acid-base reactions involve the transfer of a proton from the acid to the base.

2.2 – pKa and pH

The pKa value measures acid strength; the lower the pKa, the stronger the acid. The pH measures solution acidity.

• pKa: $pK_a = -\log K_a$

• pH: $pH = -\log [H^+]$

• Relationship: The stronger the acid, the smaller its pKa value.

2.3 – Organic Acids and Bases

Common organic acids and bases include carboxylic acids, alcohols, and amines. Their pKa values help predict reactivity.

Compound	Functional Group	pKa
Carboxylic acid	R–COOH	~5
Alcohol	R–OH	~16
Amine	R–NH2	~38
Hydronium ion	H3O+	–2

Arrow Pushing: Curved arrows show the flow of electrons in acid-base reactions. The arrow starts at the electron pair and points to where the electrons are moving.

2.4 – Predicting the Outcome of Acid-Base Reactions

Two approaches are used:

1. Quantitative Approach: Compare pKa values to predict equilibrium direction. The reaction favors the side with the weaker acid (higher pKa).

2. Qualitative Approach: Analyze the structure and stability of conjugate acids and bases.

Example: $CH_3CH_2OH + NH_2^- \rightarrow CH_3CH_2O^- + NH_3$ Compare pKa values: $pK_a$ (ethanol) = 16, $pK_a$ (ammonia) = 38. The equilibrium favors the formation of ammonia (weaker acid).

2.5 – Factors Affecting Acid and Base Strength

The stability of the conjugate base (C.B.) determines acid strength. Factors include:

• The Atom: Electronegativity and size of the atom bearing the negative charge.

• Resonance: Delocalization of charge stabilizes the conjugate base.

• Induction: Electron-withdrawing groups stabilize negative charge by inductive effect.

• Orbitals: Greater s-character (e.g., sp vs. sp3) stabilizes negative charge.

Example: Acidity increases as the atom bearing the negative charge becomes more electronegative or as resonance delocalizes the charge.

2.6 – Resonance and Inductive Effects

Resonance: If the negative charge can be delocalized by resonance, the conjugate base is more stable, making the acid stronger.

Induction: Electronegative atoms near the negative charge stabilize it by pulling electron density away (inductive effect). The closer and more electronegative the atom, the greater the effect.

2.7 – Summary of Acid-Base Equilibria

The direction of acid-base equilibrium can be predicted using the Henderson-Hasselbalch equation:

$pH = pK_a + \log \left(\frac{[A^-]}{[HA]}\right)$

When $pH > pK_a$, the base form is favored; when $pH < pK_a$, the acid form is favored.

Ch. 3 – Introduction to Organic Compounds

3.1 – Nomenclature and Functional Groups

Organic compounds are classified by functional groups. Nomenclature rules allow systematic naming of alkanes, alkenes, alkynes, cycloalkanes, ethers, alcohols, and amines.

• Alkanes: Saturated hydrocarbons (single bonds only).

• Alkenes: Contain at least one C=C double bond.

• Alkynes: Contain at least one C≡C triple bond.

• Cycloalkanes: Ring structures with only single bonds.

Example: 2-butene (alkene), cyclohexane (cycloalkane).

3.2 – Isomers

Isomers are compounds with the same molecular formula but different structures or connectivity.

• Constitutional Isomers: Differ in the connectivity of atoms.

3.3 – Carbon Classification

Carbons are classified by the number of other carbons attached:

• Primary (1°): Attached to one other carbon.

• Secondary (2°): Attached to two other carbons.

• Tertiary (3°): Attached to three other carbons.

• Quaternary (4°): Attached to four other carbons (not hydrogens).

3.4 – Alcohols and Alkyl Halides

Alcohols: Compounds with an –OH group. Classified as 1°, 2°, or 3° based on the carbon to which the –OH is attached.

Alkyl Halides: Compounds with a halogen (F, Cl, Br, I) attached to a carbon chain. The C–X bond length increases and strength decreases as the halogen size increases (C–F shortest/strongest, C–I longest/weakest).

3.5 – Ethers and Amines

Ethers: Compounds with an R–O–R' structure. The oxygen is sp3 hybridized, and ethers are named as alkoxy derivatives of alkanes.

Amines: Compounds with a C–N single bond (R–NH2, R2NH, R3N). Classified as 1°, 2°, or 3° based on the number of alkyl groups attached to nitrogen.

3.6 – Conformational Analysis: Rotation Around C–C Bonds

Rotation around single (σ) bonds leads to different conformers (rotational isomers). The most stable conformer is the staggered conformation, while the least stable is the eclipsed conformation.

• Staggered Conformer: Lowest energy, atoms are as far apart as possible.

• Eclipsed Conformer: Highest energy, atoms are aligned.

Potential Energy Diagram: Shows energy changes as the molecule rotates about the C–C bond.

3.7 – Factors Affecting Conformational Stability

• Hyperconjugation: Delocalization of electrons stabilizes staggered conformers.

• Steric Hindrance: Repulsion between bulky groups increases energy in eclipsed conformers.

Summary Table: Key Functional Groups and Properties

Functional Group	General Structure	Example	pKa
Carboxylic Acid	R–COOH	Acetic acid	~5
Alcohol	R–OH	Ethanol	~16
Amine	R–NH2	Methylamine	~38
Alkyl Halide	R–X	Chloroethane	–
Ether	R–O–R'	Diethyl ether	–

Additional info: These notes cover foundational concepts from Ch. 2 (Acids and Bases) and Ch. 3 (Introduction to Organic Compounds), including acid-base equilibria, functional group classification, and conformational analysis, all of which are central to first-semester Organic Chemistry.