Advanced Reactions of Alkenes: Mechanisms, Regiochemistry, and Stereochemistry

Unit 12: More Reactions of Alkenes

Overview

This unit covers advanced reactions of alkenes, focusing on addition mechanisms, thermodynamic and kinetic considerations, regioselectivity, stereochemistry, and the use of specialized reagents. The material is based on Chapter 8 of a standard Organic Chemistry I curriculum.

Thermodynamic Favorability in Alkene Reactions

Thermodynamics of Addition Reactions

Addition reactions to alkenes are generally thermodynamically favored due to the relative instability of pi bonds compared to sigma bonds.

• Key Reactions and Enthalpy Changes:

Reaction	ΔH (kcal/mol)
$\ce{H2C=CH2 + H2 \rightarrow H3C-CH3}$	-33
$\ce{H2C=CH2 + Cl2 \rightarrow H2CIC-CClH2}$	-43
$\ce{H2C=CH2 + HBr \rightarrow H3C-CH2Br}$	-19
$\ce{H2C=CH2 + H2O \rightarrow H3C-CH2OH}$	-11

• Pi bonds are less stable than sigma bonds, making addition reactions thermodynamically favorable even if not favored by entropy.

Kinetic vs. Thermodynamic Statements

Precision in Chemical Language

• Kinetic statement: "Compound A is more reactive than compound C" (depends on conditions).

• Thermodynamic statement: "Compound A is less stable than compound B" (independent of conditions).

Elimination vs. Addition Reactions

Mechanistic Predictions

• Elimination Reaction: Removal of atoms/groups to form a double bond.

• Addition Reaction: Addition of atoms/groups across a double bond.

• Mechanistic details explain and predict the products of both reaction types.

Electrophilic Addition Mechanism

Markovnikov Addition

• Markovnikov's Rule: In the addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogens already present.

• Product formation is explained by carbocation stability and mechanistic details.

Resonance and Carbocation Stability

Resonance Stabilization

• Positive charges (carbocations) can be stabilized by neighboring pi bonds or lone pairs, including those in aromatic rings.

• Breaking aromaticity costs energy, reducing resonance stabilization.

• The best resonance structure has octets on all atoms.

Electrophilic Addition Possibilities

Product Diversity

• Alkenes can react with various electrophiles: $\ce{H^+}$, $\ce{Br_2}$, $\ce{Cl_2}$, $\ce{I_2}$, $\ce{H_2O}$, etc.

• Finkelstein reaction is often preferred for converting alkyl halides to iodides.

Finkelstein Reaction

Halide Exchange

• Mechanism: $\ce{RBr + NaI \rightarrow RI + NaBr}$ (in acetone, NaBr precipitates, driving equilibrium).

• Le Châtelier's principle applies due to solubility differences.

Hydration of Alkenes

Mechanistic Details

• Alkene reacts with water (acid-catalyzed) to form alcohols.

• Formation of R/S enantiomeric mixture due to planar carbocation intermediate.

Enantiomers in Alkene Addition

Optical Activity and Selectivity

• Single starting material (SM) can yield two enantiomers (e.g., (R)- and (S)-2-phenylbutan-2-ol).

• If one isomer is favored, the reaction is enantioselective.

Carbocation Rearrangements

Unexpected Behavior

• Carbocations may rearrange to form more stable intermediates, affecting product outcome.

Preventing Rearrangement: Oxymercuration-Demercuration

Mechanism and Reagents

• Oxymercuration: Alkene reacts with $\ce{Hg(OAc)_2}$ and water to form a mercurinium ion intermediate.

• Demercuration: $\ce{NaBH_4}$ reduces and removes mercury, yielding alcohol.

• No carbocation rearrangement occurs.

Mercurinium Ion Bonding

Resonance Structures and Attack

• Mercurinium ion has non-bonded resonance structures, with favored nucleophilic attack at the more substituted carbon.

• Bond angles in three-membered rings reduce steric hindrance.

Acid-Base Definitions

Arrhenius, Brønsted, and Lewis Acids/Bases

• Arrhenius: Acids produce $\ce{H^+}$, bases produce $\ce{OH^-}$ in water.

• Brønsted: Acids donate $\ce{H^+}$, bases accept $\ce{H^+}$ (have lone pairs).

• Lewis: Acids accept electron pairs, bases donate electron pairs.

Lewis Acid Examples

Bonding and Charge Representation

• Lewis acids (e.g., $\ce{BF_3}$, $\ce{AlCl_3}$) form dative bonds with electron pair donors (e.g., THF, alkyl chlorides).

• Charges and dative bond arrows must be shown in mechanisms.

Advantages of Oxymercuration-Demercuration

Key Features

• Net Markovnikov addition.

• No carbocation rearrangement.

• Steric hindrance is not a significant problem.

• Nucleophile does not have to be water.

Boron Reagents in Organic Chemistry

Boron Chemistry

• Boron has three valence electrons and forms three covalent bonds.

• Common Lewis acid: $\ce{BF_3}$ (often in ether solution).

• $\ce{NaBH_4}$ is a widely used reducing agent.

• Other important boron reagents: $\ce{RBH_2}$, etc.

Borane ($\ce{BH_3}$) Structure and Properties

Bonding and Reactivity

• Exists as a monomer in dilute gas phase, dimer ($\ce{B_2H_6}$) in liquid.

• Pyrophoric (bursts into flame in air).

• Features unusual three-center, two-electron bonds.

Hydroboration of Alkenes

Alcohol Synthesis

• Hydration with acid/water yields Markovnikov alcohol.

• Hydroboration-oxidation yields anti-Markovnikov alcohol with no rearrangement.

Mechanism of Hydroboration

First Step

• Boron and hydrogen add to the same side of the alkene (syn addition).

• Stoichiometry depends on steric hindrance.

Oxidation of Alkyl Borane

Alcohol Formation

• Alkyl borane is oxidized with $\ce{H_2O_2}$ and $\ce{OH^-}$ to yield alcohol.

• Retention of configuration occurs; no carbocation rearrangement.

Regiochemistry of Hydroboration

Electronic and Steric Factors

• Electronics: Boron adds to less substituted carbon.

• Sterics: R groups orient away from bulky alkene ends.

• Specialized reagents (e.g., 9-BBN, disiamylborane) offer greater selectivity.

Stereochemistry of Hydroboration/Oxidation

Syn Addition and Stereospecificity

• Boron and hydrogen add from the same side (syn addition).

• Oxidation places -OH in the same orientation as -BH2.

• Reaction is stereospecific, controlling E/Z isomers and diastereomers.

Isomer Type	Product
E/Z Isomers	Yield different diastereomers
Enantioselective Hydroboration	Possible with chiral boron reagents

*Additional info: The notes also reference related topics such as carbene chemistry, cyclopropanation, and advanced oxidation reactions, which are covered in later chapters of organic chemistry.*