Organic Chemistry: Chapter 8 - Addition Reactions of Alkenes

Chapter 8: Addition Reactions of Alkenes

8.1 Introduction to Addition Reactions

• Addition reactions are the opposite of elimination reactions.
• Involve the conversion of a C=C π bond into two new sigma bonds.
• The π bond acts as an electron-pair donor.

8.2 Alkenes in Nature and Industry

• Alkenes, particularly ethylene and propylene, are fundamental building blocks in the petrochemical industry.
• Ethylene is used to produce polyethylene, ethanol, ethylene dichloride, ethylene oxide, vinyl chloride, acetic acid, and ethylene glycol.
• Propylene is used to produce polypropylene, isopropyl alcohol, propylene oxide, acetone, propylene glycol, and cumene.
• Vinyl chloride is a precursor to PVC (polyvinyl chloride).

8.3 Alkene Nomenclature

• Alkenes are named using IUPAC nomenclature, similar to alkanes, with modifications to account for the C=C double bond.
• Steps for naming alkenes:
  1. Identify the parent chain, which must include the C=C double bond. The parent chain name ends in "-ene" instead of "-ane".
  2. Identify and name substituents.
  3. Assign a locant to each substituent, giving the C=C double bond the lowest possible number. The double bond locant indicates where the double bond starts.
  4. List numbered substituents before the parent name in alphabetical order, ignoring prefixes (except "iso").
  5. Place the C=C double bond locant either just before the parent name or just before the "-ene" suffix.
• Configuration around the double bond (E or Z) must be indicated in the name, for example, (E)-5,5,6-trimethylhept-2-ene.

8.4 Addition vs. Elimination: Enthalpy and Entropy

• Addition reactions are favored by enthalpy because sigma bonds are stronger and more stable than pi bonds.
• ∆H = \text{Bonds broken} - \text{Bonds formed}
• Example: ∆H = 166 \text{ kcal/mol} - 185 \text{ kcal/mol} = -19 \text{ kcal/mol}
• Addition reactions are generally not favored by entropy because two molecules combine to form one, decreasing entropy.

8.5 Hydrohalogenation

• Hydrohalogenation: addition of H-X (HCl, HBr, HI) to an alkene.
• If the alkene is asymmetrical, two regioisomers are possible.

Regioselectivity – Markovnikov Addition

• Hydrohalogenation is regioselective and follows Markovnikov's rule: the H atom tends to add to the carbon already bearing more H atoms.
• The halogen (X) is generally installed at the more substituted carbon.

Anti-Markovnikov Addition with Peroxides

• In the presence of peroxides (ROOR), HBr addition exhibits the opposite regioselectivity (anti-Markovnikov).
• The reaction mechanism differs when peroxides are present.

Mechanism

• Two-step process involving a carbocation intermediate.
  • Markovnikov pathway leads to the more stable carbocation (more substituted).
  • Anti-Markovnikov is possible but less favorable due to carbocation stability.

Stereochemistry

• Hydrohalogenation can result in the formation of a chiral center.
• Two enantiomers are formed in equal amounts, resulting in a racemic mixture, due to the carbocation intermediate being attacked from either side of the empty p orbital with equal probability.

Rearrangements

• Carbocations can rearrange via hydride or methyl shifts if they can become more stable (e.g., 2° to 3°).
• When carbocation rearrangements are possible, they generally occur.

8.6 Acid-Catalyzed Hydration

• Addition of water (H and OH) across the π bond.
• Follows Markovnikov regioselectivity.
• Sulfuric acid (H2SO4) is a commonly used acid catalyst.

Mechanism

• Similar to hydrohalogenation, beginning with protonation of the alkene to form a carbocation.
• Nucleophilic attack by water produces an oxonium ion, which is then deprotonated to yield the alcohol product.

Thermodynamics

• Reactants and products are in equilibrium.
• Le Chatelier's principle can be used to control the equilibrium:
  • Excess water favors alcohol formation from an alkene.
  • Concentrated acid and removal of water favor alkene formation from an alcohol.

Stereochemistry

• Analogous to hydrohalogenation.
• If a new chiral center is formed, a racemic mixture (equal amounts of R and S enantiomers) is obtained.

8.7 Oxymercuration-Demercuration

• An alternative to acid-catalyzed hydration that avoids rearrangements.
• Markovnikov addition of H and OH.

Reagents

• Mercuric acetate [Hg(OAc)2] in water, followed by sodium borohydride (NaBH4).
• The mercuric cation (Hg2+) acts as a Lewis acid.

Mechanism

• The π bond attacks the mercuric cation, forming a stabilized mercurinium ion, which prevents carbocation rearrangements.
• The mercurinium ion is attacked by water, followed by deprotonation.
• NaBH4 replaces the —HgOAc group with a —H group via a free radical mechanism.

Comparison with Hydration

• Provides the same product as acid-catalyzed hydration but without rearrangements.
• If a chiral alcohol is produced, a racemic mixture results.

8.8 Hydroboration-Oxidation

• Adds H and OH with anti-Markovnikov regioselectivity.
• Two-reaction sequence: hydroboration followed by oxidation.

Stereoselectivity

• Syn addition of H and OH.
• Anti addition is not observed.
• If the OH group lands on a chiral carbon, a racemic mixture is produced.

Mechanism

• Hydroboration: BH3 adds to the alkene, with boron adding to the less substituted carbon. This step is repeated for each B-H bond, creating a trialkylborane.
• Oxidation:
  1. Hydroxide ion (OH-) deprotonates hydrogen peroxide, forming a hydroperoxide.
  2. The hydroperoxide attacks the trialkylborane (nucleophilic attack).
  3. An alkyl group migrates, expelling a hydroxide ion.
  4. The first three steps of oxidation are repeated, converting the trialkylborane into a trialkoxyborane.
  5. Hydroxide ion attacks the trialkoxyborane, leading to the formation of an alkoxide ion.
  6. The alkoxide ion is protonated, forming the alcohol and regenerating the hydroxide ion.

8.9 Catalytic Hydrogenation

• Addition of H2 across a C=C double bond, reducing an alkene to an alkane.
• Requires a metal catalyst (e.g., Pt, Pd, Ni).

Stereoselectivity

• Stereospecific syn addition of H2 occurs.
• If two chiral centers are formed, only the stereoisomers resulting from syn addition are obtained.

Mechanism

• The metal surface binds H2 and the alkene, facilitating the addition of both H atoms to the same face of the alkene (syn addition).
• Without the metal catalyst, the addition of H2 is too slow due to a very high activation energy.

8.10 Halogenation

• Addition of two halogen atoms (Cl2, Br2) across a C=C double bond.
• Key step in the production of polyvinyl chloride (PVC).

Stereoselectivity

• Anti addition of the halogen atoms occurs.
• Halogenation with I2 is poor; halogenation with F2 is too violent.

Mechanism

• The alkene acts as a nucleophile, attacking Br2, which is polarizable.
• A bromonium ion intermediate is formed, consistent with anti addition.
• Br– attacks the bromonium ion in an SN2 process, giving anti addition.

8.10 Halohydrin Formation

• Occurs when halogenation is conducted in water.
• Water acts as the nucleophile that attacks the bromonium ion.
• More H2O molecules are present compared to Br– ions, making attack by H2O more likely.

Regioselectivity

• Water attacks the more substituted carbon of the bromonium ion (faster than it attacks the less substituted carbon).
• After water attacks and is deprotonated, a neutral bromohydrin or chlorohydrin product is formed.

8.11 Anti Dihydroxylation

• Addition of OH and OH across the π bond with anti stereochemistry.
• Two-step procedure:
  1. Conversion of alkene to an epoxide using a peroxyacid (RCO3H).
  2. Reaction of the epoxide with H2O and an acid catalyst to form the anti diol.

Mechanism

• Epoxide formation proceeds through a butterfly mechanism, resulting in anti addition.
• Epoxides are reactive due to ring strain and a +1 formal charge, making them good electrophiles.
• The nucleophile must attack from the side opposite the leaving group (SN2-like process), yielding anti products.

8.12 Syn Dihydroxylation

• Addition of OH and OH across the π bond with syn stereochemistry.
• OsO4 (osmium tetroxide) is used as a reagent.
• NMO (N-methylmorpholine N-oxide) or an alkyl peroxide is used as a co-oxidant, so only a catalytic amount of OsO4 is needed due to its toxicity and expense.
• Potassium permanganate (KMnO4) can also achieve syn dihydroxylation but only under mild (cold) conditions; its synthetic utility is limited because it reacts with many other functional groups.

8.13 Oxidative Cleavage: Ozonolysis

• C=C double bonds are also reactive toward oxidative cleavage.
• Ozonolysis: reaction with ozone (O3) followed by a reducing agent.
• Common reducing agents include dimethyl sulfide (DMS) and Zn/H2O, cleaving the double bond and yielding carbonyl compounds (aldehydes or ketones).

Chapter 8 Review of Alkene Reactions

• Summary of reactions, including reagents and stereochemical/regiochemical outcomes.

8.15 One-Step Syntheses / Planning

• To plan a synthesis, assess the reactants and products to see what changes need to be made.
• Consider addition, substitution, and elimination reactions.

Changing Position of a Halogen or OH

• Transformations that cannot be done with a single reaction can be accomplished in a two-reaction sequence (elimination-addition).
• Carefully choose the base for the elimination reaction; a non-bulky base favors the Zaitsev product (more substituted alkene).
• Decide on the reagents needed to add H and Br, choosing conditions to achieve Markovnikov or anti-Markovnikov addition as required.

More Complex Examples

• Transformations that are not simple substitution, addition, or elimination require combining two or more processes.
• To obtain the Hofmann product, the elimination must be done via an E2 mechanism using a bulky base.
• If the alcohol must be changed to a good leaving group, use a bulky base to afford the Hofmann product.
• The addition reaction must give anti-Markovnikov addition of H and OH.

Changing the Position of a π Bond

• Two processes must be combined: Anti-Markovnikov addition of H and Br, followed by elimination to give the Hofmann product.

Summary of Reagents

• Anti-Markovnikov addition of H and Br: HBr, ROOR
• Elimination to give the Hofmann product: t-BuOK (bulky base)