Addition Reactions of Alkenes and Alkynes Note

Overview of Electrophilic Addition Reactions

• In addition reactions of alkenes, the carbon of the alkene that possesses fewer alkyl groups becomes bonded to the less electronegative atom.

• This section serves as the foundational principle for the regioselectivity behaviors explored in subsequent reaction types.

Reactions of Alkenes with Halogens

• Chlorine ($Cl_2$) and bromine ($Br_2$) undergo addition to alkenes to form vicinal dihalides.

• The term "vicinal" refers to the fact that the halogens bond to adjacent carbons, specifically the two carbons that previously comprised the double bond.

• Mechanism of Halogen Addition:

  • The mechanism is dependent on the solvent used.

  • The key intermediate in this process is a halonium ion. Depending on the halogen used, this can be a bromonium ion, a chloronium ion, or an iodonium ion.

• Step-by-Step Mechanism of Bromine Addition:

  • Step 1: The alkene reacts with $Br_2$. This single mechanistic step is complex and involves three curved arrows to show the movement of electron pairs. This step results in the formation of a cyclic bromonium ion and a bromide ion ($Br^-$).

  • Step 2: The bromide ion ($Br^-$) acts as a nucleophile and donates an electron pair to one of the ring carbons of the bromonium ion. This opens the three-membered ring to yield the vicinal dibromide.

• Stability Comparison:

  • A bromonium ion is significantly more stable (and thus more likely to form) than a traditional carbocation intermediate. This is because the bromonium ion has more covalent bonds and all atoms possess complete octets.

• Formation of Halohydrins:

  • Halohydrins are compounds that contain both a hydroxyl group ($OH$) and a halogen ($Cl$, $Br$, or $I$).

  • These form when the halogen addition reaction is conducted in a nucleophilic solvent, such as water ($H_2O$), used in large excess.

  • Vicinal halohydrins (where the $OH$ and halogen are on adjacent carbons) are the most common products. Iodohydrins are noted to be typically unstable.

• Regioselectivity in Halohydrin Formation:

  • Regioselectivity is observed when using unsymmetrical alkenes.

  • The reaction is highly regioselective when one carbon of the double bond contains two alkyl substituents (a tertiary carbon).

  • Explanation: In the bromonium ion intermediate for these highly regioselective cases, approximately $90\%$ of the positive charge resides on the tertiary carbon. The $C-Br$ bond at this position is so long and weak that the intermediate effectively mimics the character of a carbocation, directing the nucleophilic attack of water to that specific carbon.

Writing Organic Reactions

• Catalysts are typically written over the reaction arrow.

• Equations may show only the principal organic reactant and the major organic product.

• Additional reactants and specific reaction conditions (like temperature or solvent) may also be written over the arrow.

• Organic equations are rarely balanced, and inorganic by-products are usually omitted for clarity.

Oxymercuration–Reduction of Alkenes

• This is a two-step process to convert alkenes into alcohols.

• Step 1: Oxymercuration:

  • Alkenes react with mercuric acetate ($Hg(OAc)_2$) in an aqueous solution to produce addition products.

  • Tetrahydrofuran ($THF$) is commonly used as a solvent in this step.

  • The reaction is highly regioselective.

  • Mechanism: A reactive intermediate called a mercurinium ion forms. This ion features a three-center, two-electron bond.

  • Resonance: The mercurinium ion has resonance structures. The structure representing a secondary carbocation is the most important contributor and accounts for the regioselectivity of the reaction.

  • Water reacts as a nucleophile, attacking the carbon with the greatest amount of positive charge. The equilibrium of the final step in the oxymercuration phase lies heavily in favor of the products.

• Step 2: Reduction:

  • The oxymercuration adduct is treated with a basic solution of sodium borohydride ($NaBH_4$).

  • The $C-Hg$ bond is replaced by a $C-H$ bond. The hydrogen atom comes directly from the borohydride reagent.

• Summary and Advantages:

  • The process is highly regioselective (Markovnikov orientation: $OH$ goes to the more substituted carbon).

  • No rearrangements occur because traditional carbocation intermediates are not formed.

  • Compared to acid-catalyzed hydration, oxymercuration–reduction is free of side reactions and rearrangements.

Hydration of Alkynes and Enols

• The addition of water to alkynes is catalyzed by either a strong acid or a dilute acid combined with mercuric ion (Hg^{2+}).

• Unlike alkenes, the final product is a ketone, not an alcohol.

• Mechanism of Acid-Catalyzed Hydration:

  • Protonation: The process begins with the protonation of the $\pi$ bond to form a vinylic cation.

  • Association: A Lewis acid–base association reaction occurs (nucleophilic attack by water).

  • Deprotonation: A Brønsted acid–base reaction follow to form an enol.

• Enol-Keto Tautomerism:

  • Enols are unstable and readily convert into the more stable carbonyl form (aldehyde or ketone).

  • Mechanism: The $\pi$ bond of the enol is protonated to form a resonance-stabilized carbocation. Subsequent loss of a proton from the oxygen yields the ketone.

• Hg^{2+} Catalysis:

  • The mercuric ion acts as a catalyst and is regenerated in the second step of the process.

Hydroboration–Oxidation of Alkenes

• Borane ($BH_3$) adds $-H$ and $-BH_2$ regioselectively to alkenes.

• Stoichiometry: Each borane molecule can react sequentially with three alkene molecules to produce trialkylboranes ($R_3B$).

• Mechanism:

  • The accepted mechanism is a concerted process, meaning the reaction occurs in one step without an intermediate.

  • Regioselectivity is explained by two factors in the transition state: electron deficiency and van der Waals repulsions (steric effects).

• Conversion to Alcohols:

  • Organoboranes are oxidized using hydrogen peroxide ($H_2O_2$) and aqueous sodium hydroxide ($NaOH$).

• Regioselectivity Comparison:

  • Oxymercuration–reduction and hydroboration–oxidation are complementary.

  • Oxymercuration–reduction places the $OH$ group on the more highly substituted carbon.

  • Hydroboration–oxidation places the $OH$ group on the less substituted carbon (anti-Markovnikov hydration).

Hydroboration–Oxidation of Alkynes

• This reaction is analogous to the hydroboration of alkenes.

• The resulting enols undergo tautomerization to produce aldehydes or ketones.

• Steric Control: In principle, two additions of $BH_3$ could occur across the triple bond. To limit the reaction to a single addition, organoboranes with branched substituents are used to leverage steric effects.

• Example: Hydroboration–oxidation of 1-octyne results in an aldehyde.

• Comparison: Hydroboration–oxidation of alkynes is complementary to Hg^{2+ }-catalyzed hydration; one typically produces aldehydes from terminal alkynes, while the other produces ketones.

Ozonolysis of Alkenes

• Ozonolysis involves the addition of ozone ($O_3$) to alkenes, which breaks the carbon–carbon $\pi$ bond to form an unstable cycloaddition product.

• This unstable product spontaneously converts into an ozonide, effectively breaking the second carbon–carbon ($\sigma$) bond.

• Mechanism Details:

  • Step 1: Concerted cycloaddition of ozone.

  • Step 2: Spontaneous rearrangement. Cyclic electron flow forms an aldehyde and an aldehyde oxide intermediate. A second cycloaddition then forms the stable ozonide.

• Workup with Dimethyl Sulfide (DMS):

  • Treatment with $DMS$ ($CH_3-S-CH_3$) splits the ozonide.

  • If the starting alkene is symmetric, only one organic product results.

  • If the alkene is unsymmetric, a mixture of products (aldehydes and/or ketones) results.

• Workup with Water:

  • Reaction of ozonides with water forms hydrogen peroxide ($H_2O_2$).

  • This causes any resulting aldehydes (but not ketones) to be further oxidized into carboxylic acids.

Summary of Additions to Alkenes and Alkynes

• Summary of Alkenes:

  1. $HBr$, $HCl$, $HI$: Halogen goes to the more substituted carbon.

  2. Acid-catalyzed hydration: $OH$ goes to the more substituted carbon.

  3. Addition of $Cl_2$ and $Br_2$: Forms vicinal dihalides.

  4. Halohydrin formation: $OH$ goes to the more substituted carbon if it is tertiary.

  5. Oxymercuration–reduction: Adds $H$ and $OH$; $OH$ goes to the more substituted carbon.

  6. Hydroboration–oxidation: Adds $H$ and $OH$; $OH$ goes to the less substituted carbon.

  7. Ozonolysis: Double bond is broken to form carbonyl compounds.

  8. Catalytic hydrogenation: Addition of $H_2$ across the bond.

• Summary of Alkynes:

  1. $HBr$, $HCl$, $HI$: Can occur once (vinylic halide) or twice (geminal dihalide).

  2. Hg^{2+}-catalyzed hydration: Adds $H$ and $OH$ to the more substituted carbon; enols convert to ketones.

  3. Hydroboration–oxidation: Adds $H$ and $OH$ to the less substituted carbon; enols convert to aldehydes/ketones.

  4. Catalytic hydrogenation: Using a poisoned catalyst (like Lindlar's) adds one equivalent of $H_2$; no poison leads to two equivalents and a saturated alkane.

• Nature of the Reaction: All these additions (except catalytic hydrogenation) are electrophilic additions where alkene $\pi$ electrons react with an electrophile. $\pi$ electrons are more accessible than $\sigma$ electrons because $\pi$-type overlap is inherently weaker.

Alkenes in the Chemical Industry

• Ethylene ($CH_2=CH_2$):

  • It is the most produced organic compound globally.

  • It is the most widely used starting material for chemical production.

• Industrial Production:

  • Produced via thermal cracking of hydrocarbons.

  • "Green" ethylene production methods are also being developed.

• Polymers: Many common polymers (plastics) are synthesized directly from alkenes.

• Petroleum: Petroleum remains a critical source of raw materials (feedstocks) for the chemical industry to generate these alkenes.