Organic Chemistry Lecture Notes - Reactions of Alkenes and Alkynes

The lecture focuses on four key addition reactions involving double bonds (alkenes).
Alkyne reactions will be briefly addressed at the end, noting their similarity to alkene reactions.

Introduction: Adding a halogen (Cl, Br, I) across a double bond.
Reagents: React the alkene with a halogen acid (e.g., HBr).
Mechanism:
- Pi electrons from the double bond attack the hydrogen in HBr.
- This forms a new bond between a carbon and the hydrogen, leaving $Br^-$ behind.
- A carbocation intermediate is formed.
- The $Br^-$ attacks the carbocation, forming a halogenated compound.
Markovnikov's Rule: Determines which side of the double bond the H and Br add to.
- The hydrogen adds to the carbon that already has more hydrogens, called "the rich get richer" rule.
- Carbocation Stability: The more substituted carbocation (tertiary > secondary > primary) is more stable.
  - Carbons donate electron density to stabilize the positive charge.
Example: Propene reacting with HBr.
- Two possibilities:
  - H adds to carbon 3 (more substituted), forming a secondary carbocation.
  - H adds to carbon 2 (less substituted), forming a primary carbocation.
- The secondary carbocation is preferred, leading to the major product.

Reacting propene with HBr to yield either 2-bromopropane (major) or 1-bromopropane (minor).
Reacting a tertiary substituted alkene with HBr to yield a tertiary halide.

1-Pentene + HCl → 2-chloropentane (major product due to Markovnikov's rule)
2-methyl-2-pentene + HCl → 2-chloro-2-methylpentane (major product due to Markovnikov's rule)
Importance of drawing out the molecules to visualize the hydrogen count at each carbon in the double bond.

Introduction: Addition of water (H and OH) across a double bond.
Reagents: Dilute acid ( $H2SO4$ in water) or water with an acid catalyst ( $H^+$ ).
Mechanism:
- Similar to halogenation, but water adds in the second step.
- $H^+$ adds to the double bond, forming the more stable carbocation.
- Water molecule attacks the carbocation.
- The resulting oxonium ion loses a proton to form an alcohol.
Markovnikov's Rule applies: The hydrogen adds to the carbon with more hydrogens, and the OH group adds to the more substituted carbon.
Example: Water and acid to alkene, yielding an alcohol.
- NOTE: The oxygen in the product alcohol comes from the water molecule.

Mention of anti-Markovnikov addition, where the positions are swapped (halogen or OH group adds to the more "rich" carbon).
Shown later how to make this happen

Reacting an alkene with water and $H^+$ , the major product will have the hydroxyl group on the more substituted carbon.

Introduction: Adding two halogens (e.g., Br and Br) across a double bond.
Reagents: $Br2$ , $Cl2$ , or $I_2$ in a non-aqueous solvent (to prevent side reactions with water).
Stereochemistry: Anti-addition occurs exclusively. One halogen adds from each face of the double bond (one up, one down).
No syn addition: The product where both bromines are on the same side is not formed/doesn't exist.
Diastereomers are formed.
Mechanism:
- $Br_2$ approaches the double bond.
- A cyclic bromonium ion intermediate is formed above the double bond.
- $Br^-$ attacks from the opposite side (underneath).
- This results in anti-addition of the two bromine atoms.
Example: Addition of $Br_2$ to cyclohexene results in trans-1,2-dibromocyclohexane.
- The product will be diaxial or diequatorial; diequatorial is favored.

Emphasis on the stereospecificity of the reaction. It's NOT a mix of syn/anti product.

Introduction: Adding two hydrogen atoms across a double bond.
Reagents: Hydrogen gas ( $H_2$ ) and a metal catalyst (Pd, Pt, Ni).
Stereochemistry: Syn-addition occurs exclusively; both hydrogens are added on the same side of the double bond.
Mechanism:
- $H_2$ is adsorbed onto the surface of the metal catalyst.
- The alkene approaches the catalyst surface.
- Both hydrogen atoms are added to the same face of the double bond.
Stereochemistry Control for Drug Synthesis:
- Importance of enantiomerically pure drugs (only one enantiomer is desired—the other may be inactive or toxic).
- Chiral Catalysts: Used to control the stereochemistry of hydrogenation reactions.
- Example: Hydrogenation of a prochiral alkene using a chiral catalyst to selectively form one enantiomer of a drug precursor.

Four main reactions:
- Hydrogenation ( $H_2$ /catalyst): Syn-addition of two H atoms.
- Hydration ( $H_2O$ /acid): Addition of H and OH, Markovnikov's rule applies.
- Hydrohalogenation (HX): Addition of H and X, Markovnikov's rule applies.
- Halogenation ( $X_2$ ): Anti-addition of two halogen atoms.
Mention of a reverse reaction: elimination.

Introduction: Polymers are formed by linking many monomers (small units with double bonds) together.
Mechanism: Addition of initiator causes reaction of a pi bond's electrons, causing stringing together of alkenes into a large chain.
Polymerization Reaction: Initiator donates electrons to the double bond, and this process repeats with other monomers.
Examples:
- Superglue: Anionic polymerization initiated by water; rapid chain growth.
- Rubber: Polymer of polystyrene and isoprene; cross-linked with sulfur (vulcanization) to form tires.
- Chewing Gum: Contains butadiene-styrene rubber, the same material used in car tires.
- PVC (polyvinyl chloride), Polypropylene, Plexiglass, Polystyrene.
Ubiquity of Polymers: Found everywhere (medicine, clothing, etc.).

Problem: Plastics are not biodegradable and accumulate in the environment.
Environmental Accumulation: Massive amounts of plastic waste in the ocean.
- Food Preservation: Plastics are essential for reducing food wastage.
Lightweight Materials: Plastics reduce fuel consumption in transportation (cars, planes).
Durability: Plastics improve the lifespan of many products (clothing, paints).

Alkynes: Triple bonds undergo similar addition reactions as alkenes.
HX Addition: Addition of H and X across the triple bond, following Markovnikov's rule; can occur twice, resulting in two X atoms on same carbon.
Halogen Addition: Addition of $Br2$ or $Cl2$ occurs in two steps; first addition yields cis- or trans-dibromoalkene (trans preferred), second addition yields tetrahalogenated product.
Hydrogenation: Using $H_2$ over platinum catalyst leads to complete reduction to alkane. Lindlar's catalyst (poisoned platinum) can stop the reaction at the cis-alkene.

Summary Slide: Review of the four major reactions on alkenes (hydrogenation, hydration, hydrohalogenation, halogenation).