Organic Chemistry Exam 2 Study Guide: Acids, Bases, Reaction Mechanisms, and Alkene/Alkyne Chemistry

Acid-Base Chemistry

Bronsted and Lewis Acids/Bases

Understanding acid-base reactions is fundamental in organic chemistry. Two main theories are used to classify acids and bases: Bronsted-Lowry and Lewis.

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

• Bronsted Base: A species that accepts a proton.

• Lewis Acid: A species that accepts an electron pair.

• Lewis Base: A species that donates an electron pair.

Example: In the reaction of HCl with NH3, HCl acts as a Bronsted acid (proton donor), and NH3 acts as a Bronsted base (proton acceptor).

Curved Arrow Mechanisms for Acid/Base Reactions

Curved arrows are used to show the movement of electrons during reaction mechanisms. In acid/base reactions, two-electron curved arrows indicate the transfer of electron pairs.

• Two-electron arrows: Used for showing electron pair movement in acid/base reactions.

• Single-electron arrows: Used for radical mechanisms (see below).

Predicting Acid Strength and Conjugate Base Stability

Several factors influence acid strength and the stability of the conjugate base:

• Periodic Trend: Acidity increases down a group and across a period (left to right) in the periodic table.

• Induction: Electronegative atoms near the acidic proton stabilize the conjugate base by withdrawing electron density.

• Resonance: Delocalization of negative charge in the conjugate base increases stability and thus acid strength.

• Hybridization: Greater s-character in the atom holding the negative charge increases acidity (sp > sp2 > sp3).

Example: Acetic acid is more acidic than ethanol due to resonance stabilization of its conjugate base.

Important pKa Values

Memorizing key pKa values helps predict reaction outcomes:

• C-H (alkane): ~50

• N-H (ammonia): ~38

• O-H (water): ~15.7

• F-H (HF): ~3.2

• Cl-H (HCl): ~-7

• Acetic acid: ~4.8

• Alkyne (terminal): ~25

• Alkene: ~44

• Alkane: ~50

• H3O+: ~-1.7

Predicting Reaction Direction at Equilibrium

The direction of acid-base reactions is determined by comparing the pKa values of the acid and conjugate acid:

• Reaction favors formation of the weaker acid (higher pKa).

Equation:

$\text{Equilibrium favors the side with the weaker acid (higher pKa)}$

Radical Reactions and Halogenation

Single-Electron ("Single Hook") Arrow Mechanisms

Radical reactions involve movement of single electrons, shown by single-headed arrows.

• Initiation: Formation of radicals, e.g., Cl2 → 2Cl•

• Propagation: Radicals react with molecules to form new radicals and products.

Example: Halogenation of methane with Cl2 involves initiation and propagation steps.

Predicting Major Products of Halogenation

Halogenation of alkanes with Cl2 and Br2 produces different products based on radical stability and selectivity.

• Chlorination: Less selective, more products.

• Bromination: More selective, favors formation of the most stable radical.

Radical Stability Trend

Radical stability increases with substitution:

• 3° (tertiary) > 2° (secondary) > 1° (primary) > methyl

Calculating Enthalpy Changes in Halogenation

Given bond dissociation energies, you can calculate the enthalpy change for each step:

• ΔH (step): = Bonds broken - Bonds formed

Equation:

$\Delta H = \sum \text{Bond energies broken} - \sum \text{Bond energies formed}$

Carbocation Chemistry and Alkene Reactions

Carbocation Stability Trend

Carbocation stability is crucial for predicting reaction pathways:

• 3° (tertiary) > 2° (secondary) > 1° (primary) > methyl

Reactions of Alkenes with Acids, Water, and Alcohols

Alkenes react with strong acids, water, or alcohols to form various products via carbocation intermediates.

• With strong acid: Alkene undergoes electrophilic addition to form alkyl halide.

• With acid in water: Alkene forms alcohol (hydration).

• With acid in alcohol: Alkene forms ether.

Curved Arrow Mechanisms for Alkene Addition

Two-electron curved arrows are used to show the movement of electron pairs during addition reactions.

• Step 1: Protonation of alkene to form carbocation.

• Step 2: Nucleophilic attack by water or alcohol.

• Step 3: Deprotonation to yield product.

Identifying Reactants from Products

Given a product, you should be able to deduce the alkene and acid used in its synthesis.

Carbocation Shape and Stereochemistry

Carbocations are planar (sp2 hybridized), which affects the stereochemistry of products, especially in cyclic systems like substituted cyclohexenes.

Alkene Addition Reactions

Addition of Bromine (Br2) to Alkenes

Alkenes react with Br2 to form vicinal dibromides via a bromonium ion intermediate.

• Anti-addition: The two bromine atoms add to opposite faces of the alkene.

Addition of Br2 in Water

When Br2 reacts with alkenes in the presence of water, a bromohydrin is formed.

• Mechanism: Formation of bromonium ion, followed by nucleophilic attack by water.

Hydration of Alkenes Using Hg(OAc)2 and NaBH4

This is the oxymercuration-demercuration reaction, which hydrates alkenes without carbocation rearrangement.

• Step 1: Alkene reacts with mercuric acetate to form a mercurinium ion.

• Step 2: Water attacks, followed by reduction with NaBH4.

Epoxidation and Epoxide Reactions

Peracids convert alkenes to epoxides, which can then react with acid in water to form diols.

• Epoxidation: Alkene + peracid → epoxide

• Epoxide opening: Epoxide + acid/water → trans-diol

Hydrogenation of Alkenes

Alkenes can be hydrogenated to alkanes using H2 and a metal catalyst (e.g., Pd/C).

Hydroboration-Oxidation of Alkenes

This reaction converts alkenes to alcohols with anti-Markovnikov selectivity.

• Step 1: Hydroboration (BH3 adds to alkene)

• Step 2: Oxidation (H2O2, NaOH)

Alkyne Chemistry

Reactions of Alkynes with Strong Acids

Alkynes react with strong acids to form carbocation intermediates and addition products.

• Example: Addition of HBr to a terminal alkyne yields a bromoalkene.

Mechanisms and Product Prediction

Be able to draw mechanisms and predict products for alkyne reactions with acids.

Summary Table: Stability Trends

Species	Stability Trend	Example
Radical	3° > 2° > 1° > methyl	Alkyl radical formation in halogenation
Carbocation	3° > 2° > 1° > methyl	Alkene addition reactions
Conjugate Base	Stabilized by resonance, induction, hybridization	Acetate ion from acetic acid

Additional info:

• Some details (e.g., specific pKa values, mechanism steps) have been expanded for academic completeness.

• Other topics discussed in class may also be relevant for the exam.