Chapter Overview

Focus: Reactions of Alkenes and AlkynesImportance: Key reactions that characterize organic compound behavior.

Learning Objectives

• Characterize main reactions of alkenes and alkynes:

  • Halogenation

  • Hydrohalogenation

  • Hydration

  • Hydroboration

  • Catalytic reduction

• Contrast behaviors of different carbocations:

  • Secondary carbocations can undergo structural rearrangements.

  • Comparison to primary and tertiary carbocations.

• Articulate via chemical equations:

  • Electrophilic substitution reaction mechanism.

  • Reaction coordinate diagrams.

• Use acetylide anion for new Carbon-Carbon bonds.

Reactions of Alkenes

Addition to Carbon-Carbon Double Bond

This is the core functional behavior that defines the reactions of alkenes.

Polymerization

Multiple methods lead to products like polyethylene, which has large industrial applications.

Reaction Mechanism

A detailed description of how reactants are converted to products, emphasizing several tools:

Curved Arrows

Illustrates electron movement during bond formation and breaking, crucial for understanding reaction mechanisms.

Reaction Coordinate Diagrams

Energy diagrams show energetics through various reaction stages:

• Types of Diagrams:

  • Single Transition State:

    • Represents a simple reaction without intermediates.

  • Multiple Transition States:

    • Involves one intermediate between the states, as seen in stepwise reactions.

• Thermodynamics:

  • Enthalpy (ΔH) defining reactions:

    • Exothermic: Produces heat; e.g., combustion reactions.

    • Endothermic: Consumes heat; e.g., certain polymerization reactions.

• Transition State:

  • The maximum energy point during a reaction—old bonds are partially broken, and new ones are partially formed.

• Kinetics and Activation Energy (Ea):

  • Refers to energy needed to reach a transition state, indicative of rate-determining steps.

• Intermediates:

  • Represent energy minima between transition states; often unobservable, yet critical for reaction pathways.

Examples in Energy Diagrams

• Draw diagrams for two-step exothermic reactions, illustrating product stability and energy assessments.

Developing a Reaction Mechanism

Utilizes kinetic experiments; this does not prove a mechanism but helps eliminate possibilities.

Significance in Fuel Production

Example of isomerization in gasoline production: the transformation of straight-chain hydrocarbons to branched forms enhances combustion efficiency.

Common Patterns in Reaction Mechanisms

1. Add a Proton (Brønsted-Lowry Acid-Base Reactions)

   • Identify acids and bases that can donate and receive protons, e.g., HCl acting as an acid to protonate an alkene.

2. Remove a Proton (Brønsted-Lowry Acid-Base Reactions)

3. Nucleophile and Electrophile reactions to form new bonds

   • Example: Nucleophilic attack on an electrophilic carbon in alkyl halides or carbonyls.

4. Rearrangement of existing bonds

   • Carbocation rearrangements lead to more stable products, as seen in hydride shifts.

5. Bond breaking to form stable intermediates

   • Intermediate generation can lead to more favorable reaction pathways.

Carbocation Generation

Carbocations are often the first steps in mechanism development, acting as essential intermediates.

Mechanism of Electrophilic Addition to an Alkene

Alkenes interact through electrophile-nucleophile mechanisms due to their electron-rich π-bond.

• Addition of Hydrogen Halides:

  • Relevant for HCl, HBr, and HI, showing regioselectivity and Markovnikov’s Rule.

  • Mechanism outline: Proton addition followed by nucleophile addition.

  • Example:

    • HBr adds to propene, with initial protonation of the double bond to form a secondary carbocation, followed by bromide attack.

Acid-Catalyzed Hydration Mechanism

• Steps:

  1. Protonation of the C-C double bond.

  2. Nucleophilic addition of water.

  3. Proton removal regenerates hydronium ion.

  • Example Products:

    • Conversion of methylenecyclohexane to 1-methylcyclohexanol.

Addition of Bromine and Chlorine

• Mechanism:

  • Involves electrophilic addition with significant stereochemistry effects.

  • Example: Cyclohexene adds bromine, showing no cis-products indicating strong stereoselectivity.

Carbocation Rearrangements

• Explanation of 1,2-shifts:

  • Rearrangements increase stability; for example, a 2° carbocation may shift to a 3° carbocation to favor more stable products.

  • Relate observed product distributions to reaction rates based on core stability.

Hydroboration-Oxidation of Alkene

• Mechanism:

  • Anti-Markovnikov hydration involves a step-wise hydroboration followed by oxidation.

  • Highlight mechanistic understanding with emphasis on regioselectivities during each step.

Reduction of Alkene to Alkane

• Catalytic reduction:

  • Utilizes various catalysts (such as Pt, Pd, or Ni) and solvents to shift equilibria for desired product distributions.

  • Mechanism typically involves hydrogenation across the double bond.

Heats of Hydrogenation

• Evaluations address the exothermic nature, substitution effects on heats, and comparisons of trans vs. cis alkenes—emphasizing stability variations.

Acetylide Anion as a Nucleophile

• Details of producing acetylide anions:

  • Alkynes serve as sources for these anions, showcasing their nucleophilic behavior.

  • Two-step reaction examples illustrate carbon-carbon bond formation with acetylide attacks on carbonyls or other electrophiles.

Retrosynthetic Analysis

• Techniques for synthesis involving bond dissections and necessary components analysis—facilitating a clearer pathway for synthesis planning.

Assignments & Tasks

• End-of-Chapter 5 Problem Set:

  • Review specific problems to reinforce learned concepts and facilitate a deeper understanding of reaction mechanisms.