10.3.2. Addition Reactions

Electrophilic Addition to Alkenes and Alkynes

• General pattern: An alkene (C=C) or alkyne (C≡C) acts as a nucleophile because its pi electrons are easily accessible. The first step is attack on an electrophile, breaking the pi bond and creating a carbocation (or a more stabilized vinyl cation in the case of alkynes). The second step is nucleophilic attack (often by a halide or anion of the electrophile), yielding the addition product.

Addition reactions are processes where alkenes or alkynes react with reagent molecules to form new compounds. In these reactions, the double or triple bond in alkenes and alkynes acts like a nucleophile, meaning it can donate electrons. When a reagent approaches, the pi bond breaks, leading to the formation of a positive ion called a carbocation. This intermediate can then be attacked by other nucleophiles, such as halides or water, resulting in the addition of new atoms or groups to the original molecule.

Think of an alkyne or alkene as a VIP club with a very exclusive entrance (the double or triple bond). When someone (an electrophile) knocks at the door, they get in and break the exclusive barrier, leading to a new friend (the carbocation). This new friend can then invite their friends (other nucleophiles) in, resulting in a bigger party (addition product) inside the club.

1. Hydrogen halides (HX, where X = Cl, Br, I):
   • Markovnikov’s rule: H adds to the carbon bearing more hydrogens, X adds to the carbon bearing fewer hydrogens, because the more substituted carbocation intermediate is more stable.
   • Example: CH3–CH=CH2 + HBr → CH3–CH(Br)–CH3 (2-bromopropane).
   • Rearrangement can occur if the initially formed carbocation can rearrange to a more stable carbocation (hydride or alkyl shift).

When hydrogen halides like HCl, HBr, or HI react with alkenes, they follow Markovnikov’s rule. This rule states that in the addition of HX to an alkene, the hydrogen atom will bond to the carbon with the greater number of hydrogen atoms, while the halide (X) will attach to the carbon with fewer hydrogen atoms. This happens because the more substituted carbocation is more stable, leading to favorable reaction pathways. Additionally, if the resulting carbocation can rearrange into a more stable form through processes like hydride or alkyl shifts, the product formed can change as a result of this rearrangement.

Imagine a group of friends deciding to share a game console. The friend with the most games (more hydrogens) gets to keep the controller (H), while the friend with fewer games (fewer hydrogens) receives the less important accessory (X). If a more popular friend who is willing to share comes along (stable carbocation rearrangement), they may switch the accessories around, leading to a different gaming setup at the end.

2. Halogenation (X2 addition):
   • X2 (Br2 or Cl2) adds across C=C via a cyclic halonium ion intermediate. Stereochemistry is anti (the two halogen atoms add to opposite faces of the former double bond).
   • Example: CH2=CH2 + Br2 → Br–CH2–CH2–Br (1,2-dibromoethane). Bromine adds in anti orientation.

In halogenation reactions, molecules of bromine (Br2) or chlorine (Cl2) react with alkenes. The double bond opens up and forms a cyclic intermediate, known as a halonium ion. In this process, the halogen atoms are added in an anti fashion, meaning they will attach to opposite sides of the double bond. This anti addition leads to stereochemical outcomes where the final product contains two different configurations depending on how the halogens are oriented.

Imagine two dancers performing a unique routine. As one dancer moves to the left (adding one halogen), the other dancer must follow to the right (adding the second halogen) while maintaining their distance in a perfectly coordinated manner (anti addition). Their synchronized movements create a visually stunning performance on the dance floor (the final product).

3. Hydration (addition of water):
   • Acid-catalyzed hydration: alkene + H2O (H2SO4 catalyst) → alcohol. Follows Markovnikov’s rule (alcohol on more substituted carbon).
   • Oxymercuration-demercuration: alkene + Hg(OAc)2, H2O → organomercury intermediate → NaBH4 yields alcohol without rearrangement. Markovnikov product.
   • Hydroboration-oxidation: alkene + BH3 or B2H6 → organoborane intermediate → H2O2, OH– yields alcohol with anti-Markovnikov regiochemistry and syn stereochemistry.

Hydration reactions involve the addition of water to alkenes, leading to the formation of alcohols. One pathway is acid-catalyzed hydration, which follows Markovnikov’s rule to determine the position of the alcohol. In another method called oxymercuration-demercuration, an organomercury intermediate helps to add water in a way that avoids rearrangement, also following Markovnikov’s rule. The hydroboration-oxidation route offers a contrasting outcome, adding water in the opposite orientation (anti-Markovnikov) and leading to syn addition of the alcohol product.

Think of hydration as watering two different types of plants. One type (Markovnikov pathway) prefers to have more water at the roots (more substituted carbon), while the other type (anti-Markovnikov pathway) absorbs the water evenly throughout. By understanding how much water each plant needs, you can give them the best chance to grow (yield the correct alcohol).

4. Hydrogenation:
   • H2 with a metal catalyst (Pt, Pd, or Ni) adds hydrogen across C=C to give alkane (syn addition). Alkynes hydrogenate stepwise to alkenes then to alkanes unless poisoned catalyst (e.g., Lindlar’s catalyst) yields syn cis-alkene.

Hydrogenation is the reaction where hydrogen (H2) is added across the double bond of an alkene in the presence of a metal catalyst like platinum (Pt), palladium (Pd), or nickel (Ni). This reaction results in the formation of an alkane and involves syn addition, meaning the hydrogen atoms are added to the same side of the double bond. Alkynes can undergo hydrogenation in a stepwise manner first converting to alkenes and then to alkanes. However, if a poisoned catalyst like Lindlar’s catalyst is used, the reaction can stop at the alkene stage, yielding a cis-alkene instead.

Imagine adding two scoops of ice cream (hydrogen) into a bowl of mixed fruit (alkene). If you dump both scoops in at the same time (syn addition), they sit together on one side of the bowl, creating a delicious combination (alkane). If you only want to mix some ice cream but keep the fruity flavors (catalysts controlling the reaction), a special scooper (poisoned catalyst) can help you balance flavors without fully mixing everything together.