Alkene Preparation, Properties

• Topics covered include MOP (Methods of Preparation) of Alkenes and the chemical properties of Alkenes.

Elimination Reactions of Alkyl Halides to Form Alkenes

• Alkyl halides undergo elimination reactions in the presence of a base, like alcoholic KOH, to form alkenes.
• Different alkenes can result, including major and minor products based on the reaction conditions and substrate structure.
• Zaitsev's rule suggests the major product would be the more substituted alkene.
• The major product formed depends on factors like the bulkiness of the base.
• Bulky bases favor the less substituted alkene due to steric hindrance.

Dehydrohalogenation Reactions

• Dehydrohalogenation reactions involve the removal of a hydrogen and a halogen from adjacent carbon atoms.
• Results in the formation of a double bond and an alkene
• Reaction conditions and substrate structure affect the distribution of products

Preparation of Alkenes in the Presence of Metal Catalysts

• Alkenes are prepared from alkynes using metal catalysts like H2/Ni, H2/Pd-Basov, or Na/liq. NH3.
• Syn addition occurs with D2/Ni.

Elimination Reactions with Bulky Bases

• Bulky bases tend to abstract protons from less hindered positions
• The product distribution can be altered when compared to reactions with smaller bases like ethoxide.

Dehydration of Alcohols

• Alcohols undergo dehydration to form alkenes, requiring a strong acid catalyst (H2SO4 or H3PO4) and heat.
• The reaction proceeds through a carbocation intermediate.
• Rearrangements can occur to form more stable carbocations.
• The major product is typically the more stable alkene.
• Reactions can be either E1 or E2 mechanisms.
  • E1 reactions: unimolecular
  • E2 reactions: bimolecular
• E1 reactions follow Zaitsev's rule, favoring the more stable, more substituted alkene.

E1 vs E2 Dehydration

• E1 reactions involve a carbocation intermediate and favor stable alkenes, involving rearrangements if possible.
• E2 reactions are one-step, bimolecular reactions.
• Dehydration reactions follow a specific order of reactivity
• Reactivity with alcohols: 3° > 2° > 1°

Elimination Reaction Considerations

• Sterically hindered sites are less favored in elimination reactions.
• The stability of the resulting alkene plays a significant role in determining the major product.
• Partial double bond character and steric interactions influence the product distribution.

Methods of Alkene Preparation (MOP)

• Dehydrohalogenation of alkyl halides, using reagents like alcoholic KOH or NaOEt.
• Dehalogenation of vicinal dihalides, using Zn dust.
• Dehydration of alcohols.
• Controlled hydrogenation of alkynes to alkenes, using Lindlar's catalyst or dissolving metal reduction.

Chemical Properties of Alkenes: Hydrogenation

• Hydrogenation of alkenes involves the addition of hydrogen across the double bond.
• Requires a metal catalyst: Ni, Pt, or Pd.
• This is a syn addition.
• The reaction takes place at room temperature.
• Raney nickel is a common catalyst used.

Electrophilic Addition Reactions (EAR)

• Electrophilic addition reactions are a key characteristic of alkenes, with the first step involving the attack of an electrophile (E+) on the π bond of the alkene.
• EARs occur via classical or non-classical carbocation intermediates.
• Classical carbocations are open carbocations, while non-classical carbocations involve bridged structures.
• Classical EARs involve carbocation rearrangements, while non-classical EARs do not.
• Stability based of Alkynes during EAR follows this trend: Alkene>Alkyne

Markovnikov's Rule

• In the addition of HX to unsymmetrical alkenes, the hydrogen adds to the carbon with more hydrogens.
• The X adds to the carbon with fewer hydrogens.
• The rule is based on the formation of the more stable carbocation intermediate.

Hydrohalogenation in Addition Reactions

• Hydrohalogenation is the addition of HX (HCl, HBr, etc.) to an alkene.
• Markovnikov's rule applies, where the hydrogen atom becomes attached to the carbon that has the greatest number of hydrogen atoms.
• The reaction proceeds via an electrophilic addition mechanism.
• Carbocation rearrangements are possible.

Halogenation Reactions - Addition of X2

• Halogenation reactions involve the addition of halogens (X2) to alkenes or alkynes.
• Typically anti-addition, where the two halogen atoms add to opposite sides of the double or triple bond.
• Proceeds through a halonium ion intermediate (a three-membered ring containing the halogen).
• The major product has the anti configuration.
• Reaction media include CCl4 or CS2.