Organic Chemistry Exam Study Guide: Acids, Bases, Alkenes, and Reaction Mechanisms

Exam Instructions and Structure

Overview of Exam Format

This section outlines the rules and structure for the upcoming Organic Chemistry exam. Understanding these instructions is essential for proper preparation and exam conduct.

• Exam Duration: 2 hours, in-person only.

• Materials Allowed: Periodic table handout, scientific calculator, blank scratch paper (provided).

• Prohibited Items: Electronic devices, personal notes, scantron sheets.

• Question Types: Multiple-choice, fill-in-the-blank, drawing products of reactions, and mechanism questions.

• Scoring: Total points: 200, distributed across 29 questions.

Question	Pts	Question	Pts
1	11	21	8
2	11	22	8
3	14	23	8
4	14	24	8
5	14	25	8
6	14	26	8
7	14	27	8
8	14	28	8
9	14	29	8
10	14
Total = 200

• Exam Retake Policy: Four versions of Exam II exist; retake scores are averaged or kept as the higher score.

Chapter 4: Acids, Bases, and Equilibrium

Types of Acids and Bases

Organic chemistry recognizes several definitions for acids and bases, each with unique criteria and applications.

• Arrhenius Acid/Base: Acid produces H+ in water; base produces OH-.

• Bronsted-Lowry Acid/Base: Acid donates a proton (H+); base accepts a proton.

• Lewis Acid/Base: Acid accepts an electron pair; base donates an electron pair.

• Amphoteric Compound: Can act as both an acid and a base (e.g., water).

Strength of Acids and Bases

The strength of acids and bases is determined by their ability to donate or accept protons or electron pairs.

• Strong vs. Weak Acid/Base: Strong acids/bases dissociate completely; weak acids/bases only partially dissociate.

• pKa and pKb: Quantitative measures of acid and base strength.

Key Equations:

• $K_a = \frac{[A^-][H^+]}{[HA]}$

• $K_b = \frac{[BH^+][OH^-]}{[B]}$

• $pK_a = -\log K_a$

• $pK_b = -\log K_b$

Acid/Base Conjugate Pairs

Every acid has a conjugate base, and every base has a conjugate acid. The strength of one is inversely related to the other.

• Conjugate Acid/Base: Formed when an acid loses a proton or a base gains a proton.

• Example: $\text{NH}_3$ (base) $\rightarrow$ $\text{NH}_4^+$ (conjugate acid)

Equilibrium and Reaction Direction

Acid-base reactions reach equilibrium, favoring the formation of the weaker acid/base pair.

• Equilibrium Constant ($K_{eq}$): Indicates the extent of reaction.

• Direction: Reaction favors side with weaker acid/base (higher pKa).

Gibbs Free Energy and Reaction Favorability

Gibbs free energy ($\Delta G$) determines whether a reaction is spontaneous.

• Exergonic Reaction: $\Delta G < 0$ (favorable, spontaneous)

• Endergonic Reaction: $\Delta G > 0$ (unfavorable, non-spontaneous)

• Equation: $\Delta G = \Delta H - T\Delta S$

Factors Affecting Acidity

Several structural factors influence the acidity of a molecule.

• Inductive Effect

• Resonance

• Hybridization

• Size of Atom

• Electronegativity

Example: Acidity increases with more electronegative atoms or resonance stabilization of the conjugate base.

Identifying Acids and Bases in Reactions

When analyzing a chemical reaction, it is important to identify the acid, base, nucleophile, and electrophile.

• Nucleophile: Electron-rich species (often a base).

• Electrophile: Electron-poor species (often an acid).

• Arrow Pushing: Draw arrows from nucleophile to electrophile.

Chapter 5: Alkenes and Their Reactions

Structure and Bonding in Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond, which affects their geometry and reactivity.

• Bond Angles: Approximately 120° due to sp2 hybridization.

• Double Bond: Consists of one sigma and one pi bond; rotation is restricted.

• Electron Orbitals: Pi bonds result from side-by-side overlap of p orbitals.

Degree of Unsaturation

The degree of unsaturation indicates the number of rings and multiple bonds in a molecule.

Degree of Unsaturation (DU)	Formula
Number of pi bonds + number of rings	$DU = \frac{2C + 2 + N - H - X}{2}$
C = number of carbons
N = number of nitrogens
H = number of hydrogens
X = number of halogens (F, Cl, Br, I)

Alkene Nomenclature and Classification

Alkenes are named according to IUPAC rules, considering the position and geometry of the double bond.

• Alkene: Hydrocarbon with C=C double bond.

• Cycloalkene: Cyclic structure with a double bond.

• Diene/Triene: Contains two or three double bonds.

• E/Z Isomerism: Describes the relative positions of substituents around the double bond.

Carbocation Stability

Carbocation intermediates are common in alkene reactions; their stability affects reaction pathways.

• Order of Stability: $3^ ext{°} > 2^ ext{°} > 1^ ext{°} >$ methyl

• Rearrangement: Carbocations may rearrange to more stable forms (e.g., hydride or alkyl shifts).

Chapter 6: Alkene Reactions and Mechanisms

Nucleophiles and Electrophiles

Understanding nucleophiles and electrophiles is essential for predicting reaction outcomes.

• Nucleophile: Donates electrons; attacks electrophile.

• Electrophile: Accepts electrons; is attacked by nucleophile.

• Factors Affecting Nucleophilicity: Periodic trend, resonance, steric hindrance.

Major Alkene Reactions

Alkenes undergo a variety of addition reactions, often following Markovnikov or anti-Markovnikov rules.

• Hydrohalogenation: Alkene + HX → Alkyl halide (Markovnikov addition)

• Hydration: Alkene + H2O (acid catalyst) → Alcohol (Markovnikov addition)

• Halogenation: Alkene + X2 → Dihalide (anti addition)

• Oxymercuration-Demercuration: Alkene + Hg(OAc)2, H2O/CH3OH, NaBH4 → Alcohol (Markovnikov, no rearrangement)

• Hydroboration-Oxidation: Alkene + BH3, H2O2, NaOH → Alcohol (anti-Markovnikov)

• Epoxidation: Alkene + peracid → Epoxide

Reaction Mechanisms

Mechanisms show the stepwise movement of electrons during a reaction. Drawing mechanisms is crucial for understanding organic reactions.

• Arrow Pushing: Use curved arrows to show electron flow.

• Intermediates: Identify carbocations, bridged intermediates, and rearrangements.

• Major Product: Predict based on Markovnikov/anti-Markovnikov rules and stability of intermediates.

Examples of Mechanisms

• Hydrohalogenation: Alkene + HBr/HCl → Alkyl halide (Markovnikov product)

• Halohydrin Formation: Alkene + Br2/H2O → Halohydrin (anti addition)

• Bromination: Alkene + Br2 → Dibromide (anti addition, bridged intermediate)

• Oxymercuration: Alkene + Hg(OAc)2, H2O/CH3OH, NaBH4 → Alcohol (Markovnikov, no rearrangement)

Example: For hydrohalogenation, the mechanism involves protonation of the alkene to form a carbocation, followed by nucleophilic attack by the halide ion.

Drawing and Predicting Products

Students must be able to draw intermediates, predict major products, and provide step-by-step mechanisms for reactions.

• Identify the type of addition (Markovnikov or anti-Markovnikov).

• Show all intermediates and electron flow.

• Provide reagents and conditions for each reaction.

Summary Table: Common Alkene Reactions

Reaction	Reagents	Product	Mechanism Notes
Hydrohalogenation	HX	Alkyl halide	Markovnikov, carbocation intermediate
Hydration	H2O, acid	Alcohol	Markovnikov, carbocation intermediate
Halogenation	X2	Dihalide	Anti addition, bridged intermediate
Halohydrin Formation	X2, H2O	Halohydrin	Anti addition, bridged intermediate
Oxymercuration-Demercuration	Hg(OAc)2, H2O/CH3OH, NaBH4	Alcohol	Markovnikov, no rearrangement
Hydroboration-Oxidation	BH3, H2O2, NaOH	Alcohol	Anti-Markovnikov, syn addition

Additional info:

• Some context and definitions have been expanded for clarity and completeness.

• Mechanism questions require detailed stepwise electron movement and identification of intermediates.

• Students should be familiar with drawing reaction mechanisms and predicting products for all major alkene reactions.