Organic Chemistry (CHM 222) Chapter10

Chapter 10: Alkenes - Addition Reactions

• Alkenes are also called olefins.
• Addition reactions are methods to add substituents to multiple bonds, primarily double bonds (alkenes). Other types of double bonds have similar basic principles.
• Slides up to 16 cover previously discussed concepts.
• Naming: ...ene denotes a chain with a double bond. Cyclo... has a double bond in a ring.
• Cis-Trans Isomers: Alkenes having one alkyl group bonded to each carbon atom can be differentiated using cis/trans prefixes.
• Cis: alkyl groups on the same side of the double bond.
• Trans: alkyl groups on opposite sides of the double bond.
• Cyclic Alkenes: Cycloalkenes with fewer than eight carbon atoms have a cis geometry.
• Alkenes Summary:
• Restricted Rotation: Rotation around the C=C double bond is restricted. Unfavorable bond breaking and reforming is required for rotation.
• Stereoisomerism: Different groups on a C=C double bond result in cis/trans isomers.
• Stability: Trans alkenes are generally more stable than cis alkenes, and stability increases with the number of R groups on the C=C.
• Prefixes E and Z: Step 1: Assign priorities to substituents on each end using Cahn-Ingold-Prelog rules. Divide the double bond into two parts, and assign 1 and 2 to indicate relative priority. Also assign priority separately to each side.
• E-isomer: Higher priority groups are on opposite sides.
• Z-isomer: Higher priority groups are on the same side.
• Common Names: Some alkenes have common names (e.g., ethylene for ethene). Also alkenyl substituents have common names: methylene, vinyl, allyl.
• Useful Products from Ethylene: Ethylene is an important industrial starting material for various products like polyethylene, alcohols (ethanol, ethylene glycol), PVC, styrene, and poly(vinyl acetate).
• Naturally Occurring Alkenes: Some alkenes are naturally produced, like β-carotene (pigment in carrots) and zingiberene (oil of ginger).
• Fatty Acids and Alkenes: Increasing double bonds in fatty acids decreases melting points.
• Preparation of Alkenes: Alkyl halides, tosylates, and alcohols are used for preparation via elimination reactions. These processes create a suitable leaving group.
• Regioselectivity and Stereoselectivity of Alkene Formation: The most stable alkene predominates (Zaitsev product), and this usually includes a higher number of alkyl group substitutions.
• Addition Reactions: Addition reactions of alkenes generally involve a π bond breaking and two new σ bonds forming. Alkenes act mainly as nucleophiles and react with electrophiles (but not nucleophiles themselves).
• Syn Addition: Both groups add to the same side of the double bond.
• Anti Addition: Groups add to opposite sides of the double bond.
• Addition Reactions of Cyclohexene: Addition to cyclohexene demonstrates similar principles of π-bond breaking and new σ bond formation. This shows general principles for any alkene.
• Hydrohalogenation: Electrophilic addition of HX (HCl, HBr, HI) to the double bond, with the H bonding to the less substituted carbon. This depends on the preference for forming carbocations.
• Examples of Hydrohalogenation: Reaction examples are shown with different starting materials, illustrating the concept.
• Carbocation rearrangements: Less stable carbocations transform into more stable ones through hydrogen and alkyl shifts.
• Markovnikov's Rule: In unsymmetrical alkenes, the H atom in HX adds to the less substituted carbon, leading to a more stable carbocation in the rate-determining step of the mechanism.
• Carbocation Stability and Markovnikov's Rule: The stability of carbocations (primary, secondary, tertiary) reflects why Markovnikov addition occurs. The mechanism of the reaction favors the more stable carbocation as a product,
• Stereochemistry of Electrophilic Addition: Trigonal planar atoms react equally in two directions. Hydrohalogenation sometimes creates new stereogenic centers.
• Stereochemistry of Carbocation Formation: Initial H⁺ addition to a double bond may occur from either side. Both modes yield the same achiral carbocation.
• Hydration: Adding water to alkenes forms alcohols. Addition of water to an alkene creates an alcohol. The mechanism involves a carbocation intermediate with the acid catalysing the reaction.
• Hydration Example & Mechanism: Examples of hydration are shown, and the detailed mechanism is explained.
• Mechanism of Hydration: Step-by-step mechanism of the hydration reaction, with an emphasis on the rate-determining step and the formation of the carbocation intermediate.
• Use of Alkenes in Synthesis: Retrosynthetic analysis is used to determine the starting material needed.
• Halogenation: Addition of X₂ to an alkene forms a vicinal dihalide. This reaction follows a different mechanism than hydrohalogenation; the double bond creates a dipole in the halogen and addition occurs.
  • Halogenation Details: Halogens are polarizable, so X₂ forms a dipole facilitating the reaction. This also shows how addition is different than the Markovnikov rule, due to the mechanism.
  • Halogenation Mechanism: This is a two-step process involving a bridged halonium ion, followed by a nucleophilic attack of halide.
  • Stability of Cation Intermediates: Carbocations are unstable due to lack of a full octet on the carbon. Bridged halonium ions are unstable due to ring strain.
  • Stereochemistry of Halonium Formation: Chlorination of cyclic alkenes produces only trans-dihalides because the intermediate chloronium ion is achiral,
  • Stereochemistry of Halonium Ring Opening: Substitution occurs via a backside attack resulting in a trans product.
• Halohydrin Formation: Addition of X₂ and H₂O to an alkene yields a halohydrin (an alcohol with a halogen). This is an electrophilic addition of the electrophilic molecule halogen followed by a nucleophile (OH).
  • Mechanism of Halohydrin Formation: Mechanism of halohydrin formation shows three steps, with the rate-determining step generating a bridged halonium ion.
  • Anti Stereochemistry in Halohydrin Formation: Bridged halonium opening is a backside attack and favors a trans product.
  • Regiochemistry of Halohydrin Formation: Mechanism preferences the more stable carbocation intermediate. The nucleophile (water) will preferentially attack the more substituted carbocation.
  • Summary of Halohydrin Formation: Halohydrin mechanism involves three steps: forming a bridge, adding the nucleophile, and forming the product. The electrophile attacks the less substituted carbon. Addition is anti.
• Borane and Hydroboration: Hydroboration-oxidation adds BH₃ to the alkene, creating an alkylborane.
  • Hydroboration Mechanism: The reaction proceeds in one step without any carbocation formation. Borane is added to the less substituted carbon, and the product follows syn addition.
  • Regiochemistry of Hydroboration: Borane attacks the less substituted carbon. This is called "anti-Markovnikov addition."
  • Electronic Factors of Hydroboration: More stable transition state occurs when partially positive charge is on the less substituted carbon, which is favored due to alkyl groups stabilizing the positive charge.
• Oxidation Following Hydroboration: Alkylboranes are oxidized to the alcohol without isolating the intermediate. The reaction follows syn addition, retaining the original stereochemical properties.
• Summary of Hydroboration-Oxidation: Alkylboranes are oxidized. OH group joins the less substituted carbon in the syn addition.