🥼Organic Chemistry Unit 8 – Alkenes – Reactions and Synthesis

Key Concepts and Definitions

• Alkenes contain a carbon-carbon double bond ($C=C$) consist of two $sp^2$ hybridized carbon atoms
• Functional group in alkenes is the carbon-carbon double bond ($C=C$) determines chemical reactivity and physical properties
• Planar geometry around the double bond results from the $sp^2$ hybridization of the carbon atoms
  • Leads to restricted rotation and distinct cis/trans isomers
• $\pi$ bond formed by the sideways overlap of the unhybridized p orbitals on each carbon atom in the double bond
• Allylic positions are the carbon atoms directly adjacent to the double bond (1 and 3 positions) exhibit unique reactivity
• Markovnikov's rule predicts regioselectivity in addition reactions across the double bond
  • Hydrogen adds to the carbon with more hydrogen substituents, and the electrophile adds to the carbon with fewer hydrogen substituents
• Zaitsev's rule predicts the major alkene product in elimination reactions will be the most stable, typically the most substituted

Structure and Properties of Alkenes

• Alkenes can be classified as terminal (double bond at the end) or internal (double bond within the carbon chain)
• Cis-trans isomerism arises from the restricted rotation around the double bond
  • Cis isomers have identical groups on the same side of the double bond
  • Trans isomers have identical groups on opposite sides of the double bond
• E-Z system used to designate the stereochemistry of alkenes with different substituents
  • E (entgegen) denotes higher priority groups on opposite sides
  • Z (zusammen) denotes higher priority groups on the same side
• Degree of substitution affects stability and reactivity of alkenes
  • Stability increases with increasing substitution (tertiary > secondary > primary)
  • Reactivity decreases with increasing substitution due to steric hindrance
• Alkenes are nonpolar and hydrophobic due to the absence of polar functional groups
• Lower boiling points and melting points compared to corresponding alkanes due to weaker intermolecular forces (van der Waals)
• Alkenes are more reactive than alkanes due to the electron-rich double bond susceptible to electrophilic addition reactions

Common Alkene Reactions

• Hydrogenation adds hydrogen ($H_2$) across the double bond in the presence of a metal catalyst (Pt, Pd, Ni) to form an alkane
• Halogenation adds a halogen ($X_2$) across the double bond to form a vicinal dihalide (1,2-dihalide)
  • Bromination and chlorination are common examples
• Hydrohalogenation adds a hydrogen halide ($HX$) across the double bond to form an alkyl halide
  • Markovnikov's rule predicts the major product
• Hydration adds water ($H_2O$) across the double bond in the presence of an acid catalyst to form an alcohol
  • Markovnikov's rule predicts the major product
• Halohydrin formation adds a halogen and a hydroxyl group ($OH$) across the double bond in the presence of $X_2$ and $H_2O$
• Oxymercuration-demercuration adds a hydroxyl group ($OH$) and a hydrogen across the double bond using mercury(II) acetate and $NaBH_4$
• Epoxidation adds an oxygen atom across the double bond to form a three-membered cyclic ether (epoxide) using a peroxyacid ($RCO_3H$)
• Ozonolysis cleaves the double bond using ozone ($O_3$) to form carbonyl compounds (aldehydes or ketones) upon workup

Reaction Mechanisms

• Electrophilic addition is the most common mechanism for alkene reactions involves the addition of an electrophile and a nucleophile across the double bond
  • Proceeds through a carbocation intermediate
  • Regioselectivity is determined by the stability of the carbocation intermediate (Markovnikov's rule)
• Concerted addition involves the simultaneous addition of an electrophile and a nucleophile across the double bond without a discrete intermediate
  • Occurs in halogenation and epoxidation reactions
• Free-radical addition involves the addition of radicals across the double bond
  • Initiated by light or heat
  • Anti-Markovnikov regioselectivity due to the stability of the more substituted radical intermediate
• Carbocation rearrangements can occur in electrophilic addition reactions leading to unexpected products
  • Hydride shifts and alkyl shifts are common examples
• Stereochemistry of addition reactions depends on the mechanism
  • Syn addition occurs in concerted reactions (halogenation, epoxidation) with retention of stereochemistry
  • Anti addition occurs in stepwise reactions (hydrohalogenation, hydration) with inversion of stereochemistry

Synthesis Strategies

• Retrosynthetic analysis involves working backwards from the target molecule to identify simpler precursors and reactions
  • Disconnection of the carbon-carbon double bond is a key strategy in alkene synthesis
• Elimination reactions are used to synthesize alkenes from alkyl halides or alcohols
  • Dehydrohalogenation eliminates $HX$ from an alkyl halide using a strong base ($NaOH$, $KOH$)
  • Dehydration eliminates water ($H_2O$) from an alcohol using an acid catalyst ($H_2SO_4$, $H_3PO_4$)
• Wittig reaction forms a carbon-carbon double bond by reacting an aldehyde or ketone with a phosphonium ylide
  • Produces alkenes with defined stereochemistry (Z or E) depending on the ylide and reaction conditions
• Partial reduction of alkynes using Lindlar's catalyst ($Pd/CaCO_3$, quinoline, $H_2$) selectively forms cis alkenes
• Partial reduction of alkynes using sodium in liquid ammonia ($Na/NH_3$) selectively forms trans alkenes
• Cross-coupling reactions (Suzuki, Heck, Sonogashira) form carbon-carbon double bonds by coupling an alkene with an organometallic reagent
• Olefin metathesis rearranges carbon-carbon double bonds using a metal-carbene catalyst (Grubbs, Schrock)
  • Ring-closing metathesis (RCM) forms cyclic alkenes
  • Cross metathesis (CM) exchanges alkene substituents between two molecules

Stereochemistry in Alkene Reactions

• Stereochemistry is crucial in alkene reactions due to the planar geometry and restricted rotation around the double bond
• Cis-trans isomers have distinct physical and chemical properties
  • Cis isomers generally have lower melting points and boiling points than trans isomers
  • Trans isomers are typically more stable than cis isomers due to reduced steric strain
• E-Z system is used to designate the stereochemistry of alkenes with different substituents
  • Cahn-Ingold-Prelog (CIP) rules are used to assign priorities to the substituents
• Stereospecific reactions produce products with a specific stereochemistry determined by the stereochemistry of the starting material
  • Examples include halogenation, epoxidation, and Lindlar hydrogenation
• Stereoselective reactions produce one stereoisomer preferentially over another
  • Examples include Wittig reaction and alkyne reductions
• Enantiomeric excess (ee) and diastereomeric excess (de) are used to quantify the stereoselectivity of a reaction
• Chiral catalysts and ligands can induce enantioselectivity in alkene reactions
  • Examples include asymmetric hydrogenation and asymmetric epoxidation

Industrial Applications and Real-World Examples

• Alkenes are important raw materials for the production of polymers (polyethylene, polypropylene, PVC)
  • Polymerization reactions involve the addition of alkene monomers to form long-chain macromolecules
• Alkenes are used in the synthesis of various chemicals (alcohols, aldehydes, ketones, acids)
  • Ethylene oxide produced by epoxidation of ethylene is a key intermediate for ethylene glycol and surfactants
• Alkenes are found in natural products and pharmaceuticals
  • Terpenes (limonene, pinene) are alkene-containing compounds found in essential oils and fragrances
  • Steroids (cholesterol, testosterone) contain multiple carbon-carbon double bonds
• Alkenes are used in the production of fuels and lubricants
  • Cracking of alkanes produces a mixture of alkenes used in gasoline blending
  • Oligomerization of alkenes produces higher molecular weight lubricants
• Alkenes are used in the synthesis of fine chemicals and specialty materials
  • Conjugated alkenes (butadiene, isoprene) are used in the production of synthetic rubbers
  • Fluorinated alkenes (tetrafluoroethylene) are used in the production of Teflon and other fluoropolymers

Practice Problems and Study Tips

• Practice drawing and naming alkenes using IUPAC nomenclature
  • Identify the longest carbon chain containing the double bond
  • Number the chain to give the double bond the lowest possible number
  • Indicate the position of the double bond and any substituents
• Practice predicting the products of alkene reactions using Markovnikov's rule and stereochemical considerations
  • Identify the electrophile and nucleophile in the reaction
  • Determine the regioselectivity based on the stability of the carbocation intermediate
  • Determine the stereochemistry based on the reaction mechanism (syn or anti addition)
• Practice retrosynthetic analysis of alkenes by identifying disconnections and potential precursors
  • Consider functional group interconversions (FGIs) and carbon-carbon bond forming reactions
  • Analyze the stereochemistry of the target molecule and plan accordingly
• Practice solving stereochemistry problems using the E-Z system and CIP rules
  • Assign priorities to the substituents based on atomic number and mass
  • Determine the configuration (E or Z) based on the relative positions of the highest priority substituents
• Use molecular models or online visualization tools to understand the 3D structure and stereochemistry of alkenes
• Review reaction mechanisms and electron flow to understand the underlying principles of alkene reactivity
• Work through practice problems from textbooks, online resources, and past exams to reinforce concepts and problem-solving skills
• Collaborate with classmates to discuss difficult concepts and share study strategies
• Seek help from the instructor or teaching assistants for clarification on challenging topics