Chapter 8: Addition Reactions of Alkenes

8.1 Introduction to Addition Reactions

  • Addition reactions are the opposite of elimination reactions.
  • The carbon-carbon π bond is converted to two new σ bonds.

Description

  • The π bond acts as an electron-pair donor.

8.2 Alkenes in Nature and Industry

  • Alkenes can be acyclic, cyclic, or polycyclic.
  • Carbon-carbon double bonds are often found in pheromone structures.

8.3 Alkene Nomenclature

  • Alkenes are named using IUPAC nomenclature, similar to alkanes, but with modifications.
  • Steps:
    • Identify the parent chain, which includes the C=C double bond.
    • Identify and name substituents.
    • Assign a locant to each substituent, giving the C=C double bond the lowest number possible.
    • List numbered substituents before the parent name in alphabetical order (ignore prefixes except "iso").
    • Place the C=C double bond locant just before the parent name or the -ene suffix.

Step One

  • Identify the parent chain, which must include the C=C double bond.
    • The parent chain name ends in -ene instead of -ane.

Steps Two and Three

  • Identify and name the substituents.
  • Assign a locant to each substituent. Give the C=C double bond the lowest number possible.
    • The locant of the double bond is the number indicating where the double bond starts.

Steps Four and Five

  • List numbered substituents before the parent name in alphabetical order (ignore prefixes except iso).
  • The C=C double bond locant is placed either just before the parent name or just before the -ene suffix.
  • Indicate the E/Z configuration in the name, e.g., (E)-5,5,6-trimethylhept-2-ene.

8.4 Addition vs. Elimination

  • Addition and elimination are equilibrating reactions.
  • The favored side depends on temperature.
  • Higher temperature favors elimination due to increased entropy.

Enthalpy

  • Addition reactions are favored by enthalpy.
  • Sigma (σ) bonds are stronger than pi (π) bonds.
  • ΔH=Bond broken−bonds formed\Delta H = \text{Bond broken} - \text{bonds formed}
  • ΔH=166 kcal/mol−185 kcal/mol=−19 kcal/mol\Delta H = 166 \text{ kcal/mol} - 185 \text{ kcal/mol} = -19 \text{ kcal/mol}

Entropy

  • Addition reactions are not favored by entropy.
  • Two molecules combine to form one product; entropy decreases.

Enthalpy vs Entropy

  • At lower temperatures, enthalpy dominates, favoring addition reactions.
  • At higher temperatures, entropy dominates, favoring elimination reactions.
  • Lower temperatures are typically used for addition reactions.

8.5 Hydrohalogenation

  • Hydrohalogenation involves the addition of H-X to an alkene (X = Cl, Br, or I).
  • If the alkene is unsymmetrical, two regioisomers are possible.

Regioselectivity

  • Hydrohalogenation is regioselective for Markovnikov addition.
  • Markovnikov observed that H atoms tend to add to the carbon already bearing more H atoms.
  • The halogen typically installs at the more substituted carbon.

Adding HX in ROOR

  • When peroxides (ROOR) are used with HBr, anti-Markovnikov regioselectivity is observed.
  • The reaction mechanism differs when peroxides are present.

Modulating HX Addition

  • The regioselectivity of HBr addition can be controlled.

Mechanism

  • The mechanism is a two-step process.
  • The first step (formation of the carbocation intermediate) is the rate-determining step, having the highest activation energy (Ea).

Two Pathways

  • Markovnikov and anti-Markovnikov products are possible.
  • Markovnikov product is formed due to carbocation stability.

Forming the Markovnikov Product

  • The Markovnikov product forms through a lower energy transition state, resulting in a faster reaction.

Stereochemistry

  • Hydrohalogenation may result in the formation of a chiral center.
  • Two enantiomers are formed in equal amounts.

Intermediate

  • The carbocation intermediate can be attacked from either side of the empty p orbital with equal probability.

Rearrangements

  • Carbocations can rearrange (hydride or methyl shift) to become more stable.
  • A secondary (2°) carbocation may rearrange to a more stable tertiary (3°) carbocation.

Common Outcome

  • Carbocation rearrangements do occur when possible.

8.6 Acid-Catalyzed Hydration

  • The components of water (H and OH) are added across the π bond.
  • Acid-catalyzed hydration follows Markovnikov regioselectivity.
  • Sulfuric acid (H2SO4) is a commonly used acid catalyst.

More Substituted Carbon

  • The OH group adds to the more substituted carbon of the alkene.
  • The more substituted the carbon atom, the faster the reaction, suggesting a carbocation intermediate.

Mechanism

  • The mechanism begins similarly to hydrohalogenation.
  • Nucleophilic attack produces an oxonium ion, which is 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, without added water, favors alkene formation from an alcohol.

Stereochemistry

  • The stereochemistry is analogous to hydrohalogenation.
  • If a new chiral center is formed, a racemic mixture is obtained.

8.7 Oxymercuration-Demercuration

  • Oxymercuration-demercuration provides Markovnikov addition of H and OH without rearrangements.

Lewis Acid

  • Mercuric cation (Hg2+) acts as the Lewis acid instead of H+.

No Rearrangements

  • A stabilized mercurinium ion is formed, preventing carbocation rearrangements.

Nucleophilic Attack

  • The mercurinium ion is an electrophile susceptible to nucleophilic attack.
  • Sodium borohydride (NaBH4) is typically used to replace the —HgOAc group with a —H group via a free radical mechanism.

Comparison with Hydration

  • This two-step sequence yields the same product as acid-catalyzed hydration but without rearrangements.

8.8 Hydroboration-Oxidation

  • Hydroboration-oxidation adds H and OH with anti-Markovnikov regioselectivity.
  • This is a two-reaction sequence.

Stereoselective

  • Hydroboration-oxidation is stereoselective; H and OH are added in a syn fashion.
  • Anti addition is not observed.

MO Viewpoint

  • Geometry/hybridization of BH3 is analogous to a carbocation.

Filling Boron’s Valence Shell

  • The boron atom does not have an octet. BH3 forms a dimer (B2H6) to help fulfill the boron atom's octet.
  • Resonance hybrid with three-center, two-electron bonds.

Ether Solvent

  • B2H6 can be stabilized by using an ether solvent, such as THF, so that an appreciable amount of BH3 is present.
  • The active reagent is BH3•THF.

Regioselectivity

  • Hydroboration follows anti-Markovnikov regioselectivity.
  • The less-substituted carbon attacks the boron, and the more-substituted carbon develops a δ+ charge, triggering a hydride shift.
  • The more-substituted carbon better stabilizes a partial positive charge.

Three Alkene Equivalents per Borane

  • One BH3 reacts with three equivalents of alkene.

Sterics

  • Steric factors also influence the regioselectivity.

Stereoselectivity

  • Hydroboration is stereospecific: only syn addition occurs.
  • If only one chiral center is formed, a pair of enantiomers is formed by addition to either face of the alkene.

Enantiomers

  • If two chiral centers are formed, a pair of enantiomers is obtained.

8.9 Catalytic Hydrogenation

  • Hydrogenation is the addition of H2 across a C=C double bond.
  • It requires a metal catalyst.
  • An alkene is reduced to the corresponding alkane.

Stereoselectivity

  • Syn addition is observed.
  • If two chiral centers are formed, only the stereoisomers resulting from syn addition are obtained.

Graphical Interpretation

  • Without the metal catalyst, H2 addition is too slow due to a high activation energy (Ea).

Metal Surface

  • The metal surface binds the H2 and the alkene, explaining why both H atoms are added to the same face of the alkene (syn addition).

Symmetrical Alkenes

  • Syn addition of H2 to a symmetrical alkene will not produce a pair of enantiomers; a meso compound is produced instead.

Types of Catalyst

  • Heterogeneous catalyst: does not dissolve in the reaction medium (e.g., Pt or Pd metal).
  • Homogeneous catalyst: does dissolve in the reaction medium, using ligands with the metal.

8.10 Halogenation

  • Halogenation involves the addition of two halogen atoms across a C=C double bond.
  • This is a key step in the production of polyvinyl chloride (PVC).

Anti Addition

  • Halogenation is practical with Cl2 and Br2.
  • Iodination with I2 is poor; fluorination with F2 is too violent.
  • Stereoselectivity: halogenation occurs with anti addition.

Bromine

  • Br2 is nonpolar but polarizable. Approach of a nucleophile induces a dipole.
  • The alkene acts as the nucleophile.

No Syn Addition

  • Only anti addition is observed, inconsistent with a true carbocation intermediate.

Anti Addition

  • The formation of a bromonium ion intermediate is consistent with anti addition.
  • This intermediate is similar to the mercurinium ion.

Final Step

  • Br– attacks the bromonium ion in an SN2 process, giving anti addition.

Stereoselectivity

  • Halogenation is stereospecific; the stereochemistry of the starting alkene determines the stereochemistry of the product(s).

8.10 Halohydrin Formation

  • Halohydrins are formed when halogenation is conducted in water.
  • Water acts as the nucleophile that attacks the bromonium ion because there are many more H2O molecules compared to Br– ions.

Bromohydrin and Chlorohydrin

  • After water attacks, it is deprotonated to yield the neutral halohydrin product (bromohydrin or chlorohydrin).

Regioselectivity

  • Halohydrin formation is regioselective.
    • The halide adds to the less-substituted carbon.
    • The OH adds to the more-substituted carbon.

Origin

  • Regioselectivity results from H2O attacking the more-substituted carbon faster than the less-substituted one.

Cationic Character

  • The more-substituted carbon has more cationic character.

8.11 Anti Dihydroxylation

  • Dihydroxylation involves the addition of OH and OH across the π bond.
  • Anti dihydroxylation of an alkene is a two-step procedure.

Step One

  • Conversion of alkene to an epoxide using a peroxyacid (RCO3H).

Step Two

  • The epoxide is reacted with H2O and an acid catalyst to form the anti diol.

Similar Intermediates

  • Ring strain and a +1 formal charge make these structures good electrophiles.
  • All three yield anti products because the nucleophile must attack from the side opposite the leaving group (SN2-like process).

8.12 Syn Dihydroxylation

  • Syn dihydroxylation adds OH and OH across the π bond in a concerted, syn fashion.

Potassium Permanganate Has Limited Use

  • Syn dihydroxylation can also be achieved with KMnO4 under mild conditions (cold temperatures).
  • However, the synthetic utility of KMnO4 is limited because it reacts with many other functional groups as well.

8.13 Oxidative Cleavage

  • C=C double bonds are also reactive toward oxidative cleavage.
  • Ozonolysis is one such process.
  • Ozone exists as a resonance hybrid.

Elaboration

  • Common reducing agents include dimethyl sulfide (DMS) and Zn/H2O.

8.14 Predicting Products of Addition Rxns

  • Analyze the reagents to determine what groups will be added across the C=C double bond.
  • Determine the regioselectivity (Markovnikov or anti-Markovnikov).
  • Determine the stereospecificity (syn or anti addition).
  • Familiarity with mechanisms and reagents makes product prediction easier.

8.15 One-Step Syntheses

  • Assess reactants and products to see what changes need to be made.

Practice

  • Give reagents and conditions for addition reactions that add H and OH with Markovnikov regiochemistry.

Changing Position of a Halogen or OH

  • Changing the position of a halogen requires a two-reaction sequence: elimination followed by addition.

Carefully Choose the Base

  • For elimination, a non-bulky base is used to obtain the desired alkene.

Choose Reagents

  • Reagents for Markovnikov addition of H and Br are needed.

More Complex Example

  • Some transformations are not simple substitution, addition, or elimination and require combining two processes.

Hoffmann Product

  • Elimination to give the Hofmann alkene via an E2 elimination.
  • Convert the alcohol must be changed to a good leaving group so we can use a bulky base to afford the Hofmann product

Summary

  • The addition reaction must give anti-Markovnikov addition of H and OH.

Changing the Position of a π Bond

  • Changing the position of a π bond requires two combined processes

Summary

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

Ch. 8 Review of Alkene Reactions

  • Hydrohalogenation (Markovnikov)
  • Hydrohalogenation (anti-Markovnikov)
  • Acid-catalyzed hydration and oxymercuration-demercuration
  • Hydroboration-oxidation
  • Hydrogenation
  • Bromination
  • Halohydrin formation
  • Anti dihydroxylation
  • Syn dihydroxylation
  • Ozonolysis