Key Organic Reaction Mechanisms: Alkenes, Alkynes, and Aromatics (Chapters 13–15)

Organic Reaction Mechanisms: Alkenes, Alkynes, and Aromatic Compounds

Overview

This guide summarizes essential reaction mechanisms and concepts from Organic Chemistry Chapters 13, 14, and 15, focusing on the chemistry of alkenes, alkynes, and aromatic compounds. Understanding these mechanisms is crucial for predicting reaction outcomes and mastering organic synthesis.

Addition Reactions of Alkenes

Addition of HX to Alkenes (Including Rearrangements)

• Mechanism: Electrophilic addition of hydrogen halides (HX, where X = Cl, Br, I) to alkenes proceeds via a carbocation intermediate.

• Markovnikov's Rule: The hydrogen atom adds to the carbon with more hydrogens, and the halide adds to the more substituted carbon.

• Carbocation Rearrangements: Hydride or alkyl shifts may occur to form a more stable carbocation.

• Equation: $\ce{RCH=CH2 + HX -> RCHX-CH3}$

• Example: Addition of HBr to propene yields 2-bromopropane as the major product.

Hydration of Alkenes

• Mechanism: Acid-catalyzed addition of water (H2O) across the double bond forms alcohols.

• Markovnikov's Rule Applies.

• Equation: $\ce{RCH=CH2 + H2O \xrightarrow{H^+} RCH(OH)-CH3}$

• Example: Hydration of 1-butene yields 2-butanol.

1,2 vs 1,4 Additions (Kinetic vs Thermodynamic Control)

• 1,2-Addition: Addition occurs at adjacent carbons (kinetic product, forms faster at lower temperatures).

• 1,4-Addition: Addition occurs at the terminal carbons of a conjugated diene (thermodynamic product, favored at higher temperatures).

• Equation: $\ce{CH2=CH-CH=CH2 + HBr \rightarrow}$ 1,2- and 1,4-addition products

Addition Reactions of Alkynes

Addition of HX to Alkynes

• Mechanism: Similar to alkenes, but can add one or two equivalents of HX, leading to geminal dihalides.

• Equation: $\ce{RC#CH + 2HX -> RCHX2-CH3}$

Addition of Cl2 and Br2 to Alkenes and Alkynes

• Mechanism: Anti addition via a cyclic halonium ion intermediate for alkenes; alkynes react similarly but less readily.

• Equation: $\ce{RCH=CH2 + Br2 -> RCHBr-CH2Br}$

Formation of Alkynes

• Preparation: Typically via elimination reactions (e.g., double dehydrohalogenation of vicinal dihalides).

• Equation: $\ce{RCHBr-CH2Br + 2NaNH2 -> RC#CH}$

Rearrangement and Isomerization

Epoxidation

• Mechanism: Reaction of alkenes with peroxy acids (e.g., mCPBA) forms epoxides (three-membered cyclic ethers).

• Equation: $\ce{RCH=CH2 + R'CO_3H -> RCH(O)CH2}$ (epoxide)

End-to-End Isomerization

• Definition: Migration of double or triple bonds within a molecule, often catalyzed by acids or bases.

Electrophilic Aromatic Substitution (EAS)

General Mechanism

• Step 1: Generation of an electrophile (E+).

• Step 2: Electrophile attacks the aromatic ring, forming a sigma complex (arenium ion).

• Step 3: Loss of a proton restores aromaticity.

• Equation: $\ce{C6H6 + E^+ -> C6H5E + H^+}$

Types of EAS Reactions

• Halogenation: Introduction of Cl or Br using FeCl3 or FeBr3 as catalysts.

• Nitration: Introduction of NO2 using HNO3/H2SO4.

• Sulfonation: Introduction of SO3H using fuming H2SO4.

• Friedel-Crafts Alkylation: Introduction of alkyl groups using alkyl halides and AlCl3.

• Friedel-Crafts Acylation: Introduction of acyl groups using acyl halides and AlCl3.

Activators and Deactivators

• Activators: Electron-donating groups (e.g., -OH, -OCH3) increase ring reactivity and direct substitution to ortho/para positions.

• Deactivators: Electron-withdrawing groups (e.g., -NO2, -CF3) decrease ring reactivity and direct substitution to meta positions.

Other Key Mechanisms

Oxidation of Alkenes and Alkynes

• Dihydroxylation: Addition of two hydroxyl groups (e.g., using OsO4 or KMnO4).

• Oxidative Cleavage: Cleavage of C=C or C≡C bonds to form carbonyl compounds (e.g., using KMnO4 or O3).

• Equation: $\ce{RCH=CHR' + O_3 -> RCHO + R'CHO}$ (ozonolysis)

Formation and Reaction of Acetylide Ions

• Formation: Terminal alkynes are deprotonated by strong bases (e.g., NaNH2) to form acetylide ions.

• Reactivity: Acetylide ions are strong nucleophiles and can react with alkyl halides in SN2 reactions to form new C–C bonds.

• Equation: $\ce{RC#CH + NaNH2 -> RC#C^- Na^+ + NH3}$

Summary Table: Key Reaction Types

Reaction Type	Key Reagents	Main Product	Notes
Addition of HX to Alkene	HX (HCl, HBr, HI)	Alkyl halide	Markovnikov orientation; possible rearrangement
Hydration of Alkene	H2O, acid	Alcohol	Markovnikov orientation
Halogenation of Alkene	Br2, Cl2	Dihalide	Anti addition
Electrophilic Aromatic Substitution	Various (Br2/FeBr3, HNO3/H2SO4, etc.)	Substituted aromatic	Ortho/para or meta directing
Oxidative Cleavage	KMnO4, O3	Carbonyl compounds	Cleavage of double/triple bonds
Formation of Acetylide Ion	NaNH2	Acetylide ion	Strong nucleophile for C–C bond formation

Additional info:

• Some mechanisms (e.g., 1,2- vs 1,4-addition) are especially relevant for conjugated dienes and are covered in detail in the chemistry of conjugated molecules.

• Electrophilic aromatic substitution includes several specific reactions (halogenation, nitration, sulfonation, Friedel-Crafts alkylation/acylation) and is influenced by substituent effects.

• Oxidative cleavage and dihydroxylation are important for the structural determination and synthesis of complex molecules.