Organic Chemistry: Alkenes - Reactions and Synthesis

Overview of Alkenes

• Alkenes react with various electrophiles to produce molecules with functional groups.

Reactions of Alkenes

• Preparation of alkenes is commonly through elimination reactions.
• Key reactions with alkenes include:
  • Dehydration
  • Dehydrohalogenation
  • Hydrohalogenation
  • Halogenation
  • Halohydrin formation
  • Acid-catalyzed hydration
  • Oxymercuration
  • Hydroboration
  • Addition of alcohol
  • Simmons-Smith reaction
  • Hydrogenation
  • Epoxidation
  • Hydroxylation of epoxides and alkenes
  • Ozonolysis
  • Oxidation with KMnO4
  • Oxidation with dilute KMnO4

Preparation of Alkenes

• Dehydrohalogenation: Involves the loss of HX from an alkyl halide using a strong base (E2 mechanism).
• Dehydration: Produces alkenes by removing water from alcohols, occurs under strong acid conditions (e.g., sulfuric acid).
  • Reactivity order: 3º > 2º > 1º (tertiary alcohols are most reactive).

Mechanism of Dehydration

1. Protonation: Alcohol’s OH group turns into a better leaving group (H2O).
2. Formation of Carbocation: Water leaves leading to carbocation formation.
3. Deprotonation: Weak base abstracts a proton, yielding alkene.

Carbocation Rearrangement

• Carbocations can rearrange to more stable forms, shifting towards stability if nearby positions offer greater stabilization.

Markovnikov vs Non-Markovnikov Addition

Markovnikov Addition

• Involves the formation of more stable carbocation, where the electrophile adds to the more substituted carbon.

Non-Markovnikov Addition

• A radical intermediate results in the electrophile adding to the less substituted carbon, mainly in reactions with peroxide.

Hydrohalogenation Mechanism with Peroxide

1. Initiation: Formation of a radical from the peroxide.
2. Br Addition: Addition to the least substituted carbon.
3. Hydrogen Abstraction: Resulting in an anti-Markovnikov product.

Halogenation of Alkenes

• Reaction with X2 (e.g., Br2 or Cl2) produces 1,2-dihalides with anti stereochemistry, favoring trans products.

Mechanism of Halogenation

• Involves stereochemical inversion during the bromine attack.

Halohydrin Formation

• Reaction of an alkene with X2 and H2O produces a halohydrin.
  • OH adds to the more substituted carbon (Markovnikov), X to the less substituted.
  • Anti sterochemical addition.

Mechanism for Halohydrin Formation

1. Alkene reacts with halogen to form a bromonium ion.
2. Nucleophilic water attacks the more substituted carbon.
3. Resulting in racemic alcohols with inversion.

Acid-Catalyzed Hydration

• Adds water across an alkene in a Markovnikov manner, forming a carbocation which may rearrange.

Mechanism of Acid-Catalyzed Hydration

1. Protonation: The double bond is protonated to form carbocation.
2. Water Attack: A nucleophilic attack occurs with water.
3. Deprotonation: Produces the alcohol.

Hydroboration-Oxidation

• A two-step reaction where BH3 adds to the alkene in a non-Markovnikov fashion, followed by oxidation to form alcohols.
  • Syn addition occurs during hydroboration.

Oxymercuration-Demercuration

• Adds water in a Markovnikov fashion without carbocation rearrangement.

Epoxidation and Hydrolysis

Epoxidation

• Adds an oxygen atom across the double bond to form an epoxide (syn addition).

Hydrolysis of Epoxides

• Leads to trans 1,2-dialcohols with anti addition.

Oxidative Cleavage of Alkenes

• Achieved with KMnO4 or O3. Results in carbonyl compounds based on the number of substituents on the alkene.

Key Points in Alkene Cleavage

• Cleavage with KMnO4 yields carboxylic acids/ketones based on substituents.
• Ozonolysis forms aldehydes or ketones depending on substitution.

Multistep Reactions

• Often, chemists carry out multiple reactions in sequence for synthesis goals. Understanding each step is crucial for predicting products.

Practice Problems

• Engage with practice questions and multistep syntheses to strengthen understanding and application of concepts.