Lecture Notes: Dissolving Metals and Alkynes, and Addition Reactions to Alkynes

Dissolving Metals and Alkynes

Reactions with Sodium and Ammonia

• Dissolving sodium metal in liquid ammonia with an alkyne forms a trans alkene.
• Contrast: Treating the same alkyne with Lindlar's catalyst produces a cis alkene.
• Mechanism explains the different outcomes using radical vs. catalyst mechanisms.

Mechanism Concepts

• Before diving into the mechanism, understanding mechanism arrows is essential.
• Heterolytic cleavage: Both electrons in a bond go to one species, forming a cation and an anion.
• Homolytic cleavage: Each electron in the bond goes to a different atom, forming two radicals.

Dissolving Metal Reduction Mechanism

• Sodium donates an electron to the alkyne, which splits the triple bond homolytically.
• Liquid ammonia acts as a proton source.
• The radical intermediate forms the trans alkene due to electronic repulsion favoring lower energy states.
• Radical reactions proceed through a mechanism involving fish hook arrows (single-headed arrows).
• Trans alkene favored due to lower electronic repulsion compared to cis.

Addition Reactions of Alkynes

Hydrohalogenation of Alkynes

• Similar to alkenes but consider molar ratios of reactants.
• Equal molar (1:1) adds once: forms a vinyl halide.
• Excess HX adds twice: forms a geminal dihalide (both halogens on the same carbon).
• Mechanism involves formation of a carbocation (not entirely accurate due to vinylic carbocation instability).
• Reaction follows Markovnikov's rule.

Anti-Markovnikov Addition

• Treating alkenes/alkynes with HBr and peroxides follows anti-Markovnikov addition.
• For alkynes: results in a mix of E (trans) and Z (cis) products.

Hydration of Alkynes (Acid-Catalyzed)

General Reaction

• Alkyne + H\textsubscript{2}O + H extsubscript{2}SO extsubscript{4} + HgSO extsubscript{4} produces a ketone.
• Intermediate enol forms, then tautomerizes to a ketone (keto-enol tautomerism).

Mechanism

• Mercury acetate forms a three-membered ring intermediate.
• Water attacks the more substituted carbon.
• Proton transfer steps lead to the keto form from the enol (acidic conditions).
• Keto form favored at equilibrium.

Hydroboration-Oxidation of Alkynes

• Anti-Markovnikov addition of water across the triple bond.
• Uses BH extsubscript{3} / THF then H extsubscript{2}O extsubscript{2}.
• Yields an enol which tautomerizes to an aldehyde (terminal alkyne).
• Diethylborane and 9-BBN are bulky alternatives to BH extsubscript{3}, enhance anti-Markovnikov selectivity.

Basic Conditions for Tautomerization

• Mechanism differs under basic conditions compared to acidic conditions.
• Base deprotonates the enol, leading to tautomerization to the more stable keto form.

Review and Closing Notes

• The lecture will continue with further reactions in the next video.
• Reach out with any questions or clarifications needed.